Table of Contents
- Physical Processes
- Dust Source Regions
- Synoptically Forced Dust Storms
- Dust Storms Caused by Mesoscale Systems
- Satellite Detection of Dust
- Forecasting Dust Storms
- Case: Long-Range
- Case: Medium-Range
- Case: Short-Range
- Dust Onset
- Why Dust Model Forecasts Differ
Dust Storm Hazards
There are countless examples of how dust storms can impact military and civilian life. One such example occurred on the evening of 24 April 1980, when helicopters en route to rescue U.S. hostages in Iran encountered a haboob (that's a dust storm generated by a convective downburst). Unable to fly through it at night, several helicopters turned back, leading to the mission's termination. Dust raised by one of the withdrawing helicopters obscured visibility, leading to a collision with a C-130 transport plane, in which eight servicemen lost their lives.
On 12 August 2005, blowing dust in the U.S. state of Washington led to several chain reaction accidents. More than 50 cars and trucks were involved and seven people were killed.
Intense dust storms reduce visibility to near zero in and near source regions, with visibility improving away from the source. From the edge of blowing dust to within 150 miles (241 km) downstream, visibility can range from ½ to three miles (800 to 4800 meters).
Dust settles when winds drop below the speed necessary to carry the particles, but some level of dust haze will persist for longer periods of time. For example, dust haze may remain at four to six miles downstream (5000 to 9000 meters) for days after a dust storm.
Note that air-to-ground or slant-range visibility is more reduced than surface visibility. This may make it impossible to, for example, pick out an airfield from above, even when the reported horizontal surface visibility is three miles (or about 5 km) or more.
Dust hazards include more than just limits on visibility. Take the case of dust from Africa. Every year, wind carries hundreds of millions of tons of dust from the Sahara and Sahel across the Atlantic to the Caribbean and southeastern United States. The dust transports various microorganisms and chemicals that latch onto the small particles.
These dust storms have increased in frequency and intensity since the 1970s, with many ill effects.
- In the Caribbean, roughly 30% of the bacteria isolated from airborne soil dust are known pathogens, able to infect plants, animals, and people
- Caribbean and Florida coral reefs have been declining since the late 1970s
- The incidence of asthma in Barbados has increased 17-fold since 1973
- Dust events correlate with toxic red tides off the coast of Florida
- Fungal outbreaks affecting commercial crops have occurred within days of dust events
Clearly, airborne dust can carry a potentially toxic brew. The average soil particle size within a dust cloud decreases as the cloud travels and the larger particles settle. Eventually, the remaining particles become so small that our lungs cannot readily expel them. Accurate dust forecasts may allow people to take protective actions to mitigate the health effects of airborne particulates.
Dust also impacts equipment such as aircraft and automobile engines and electro-optical systems.
Before you can forecast dust storms, it's important to understand basic information about them such as
- The various type of dust sources (some of which are shown in the montage)
- The characteristics of dust particles, particularly their size
- The process of dust storm initiation, transport, and dissipation
This module will help you understand dust storm processes and more accurately forecast dust storms. We'll look at examples of dust storms from around the world, focusing primarily on Southwest Asia and the Middle East. Regardless of location, though, the lessons can be applied to any region of the world given some knowledge of local climatology and dust source regions.
Dust moves through several processes, described below. The latter two are integral to the formation of dust storms since they loft dust into the air.
- By saltation, where small particles move forward through a series of jumps or skips, like a game of leap-frog. The particles are lifted into the air, drifting approximately four times farther downwind than the height that they attain above ground. If saltating particles return to the ground and hit other particles, they jump up and forward, continuing the process.
- By creep, where sediment moves along the ground by rolling and sliding. Large particles and/or light winds favor creep.
- By suspension, where sediment materials are lifted into the air and held aloft by winds. If the particles are sufficiently small and the upward air currents are strong enough to support the weight of the individual grains, they will remain aloft. The larger particles settle more quickly, although increases in wind speed keep progressively larger particles aloft. Note that strong winds can lift suspended dust particles thousands of meters upward and thousands of kilometers downwind, with turbulent eddies and updrafts holding them in suspension.
Particle Size and Settling Velocity
Dust particles remain suspended in the air when upward currents are greater than the speed at which the particles fall through air. This graphic shows the fall speed, or settling velocity, as a function of particle size.
Dust particle size is usually measured in micrometers, which are 1/1000 of a millimeter or 1/1,000,000 of a meter. Particles capable of traveling great distances usually have diameters less than 20 micrometers (much smaller than the width of a human hair).
Of the following types of particles, which fit this description? (Choose all that apply.)
The correct answers are a) and b).
Appropriate source regions for dust storms have fine-grained soils rich in clay and silt.
Returning to the graph, dust particles fall at a speed of about 100 millimeters/second or roughly four inches per second. Particles larger than 20 micrometers in diameter fall disproportionately faster: 50-micrometer particles fall at about 500 mm/s or half a meter per second. Particles smaller than 20 micrometers settle very slowly. Ten-micrometer particles fall at only 30 millimeters/second while 2-micrometer particles fall at only 1 millimeter per second. The finest clay particles settle so slowly that they can be transported across oceans without settling.
Sources of Dust: Desert
Precipitation binds soil particles together and promotes plant growth. Plant growth, in turn, binds the soil even more and shields the surface from wind. Consequently, dust storms occur in regions with little vegetation and precipitation. These conditions most often occur in deserts—when it hasn't rained recently. The rule of thumb is that dust is unlikely within 24 to 36 hours of a rainstorm.
A thin veneer of stones called desert pavement covers many desert regions. This veneer results from the process of deflation where wind removes the finer-grained material, leaving only stones on the surface, which suppress blowing dust. If the pavement is disrupted by human activities, such as farming or off-road driving, the fine-grained material will be exposed to the wind again, raising the likelihood of dust storms. Studies show that large-scale military operations in the desert increase the likelihood of dust storms at least five-fold.
When seasonal rains occur over desert and near-desert environments, runoff water can create flash floods. The resulting erosion washes soil particles downstream. This continues until the velocity of the water slows to a point where it can no longer carry the load of sediment. The heaviest particles are deposited first, the lightest particles last. Once the water evaporates, the stream bed becomes a prime source for blowing dust.
Other Dust Sources
This montage shows other sources of dust. Are you surprised to see ocean sediments and glacial deposits in the list? Click each image to view more information on that topic or simply scroll down to view all.
Agricultural areas: Agricultural land that’s fallow, recently tilled, or has a marginal growing climate is a potential source area for dust. The mechanical breaking of soil creates an environment rich in fine-grained soil that is picked up and moved by seasonal winds.
We see this in the grain belts of northern Syria and Iraq, where seasonal rain is relied upon to water the crops. When drought occurs, the area becomes an active dust source region.
The same occurs in Colorado, New Mexico, and western Texas. An extreme example occurred in the American mid-west during the 1930s, known as the Dust Bowl.
Coastal areas: These MODIS images show well-defined dust plumes extending from the coastal area of the United Arab Emirates near Abu Dhabi. The dust plumes were generated by prefrontal southerly winds in advance of a cold front to the north.
River flood plains (alluvial plains): The flood plain of the Tigris and Euphrates Rivers in southern Iraq serves as the source region for many dust storms, particularly during shamal events. (A shamal is a northwesterly wind that blows over Iraq and the Persian Gulf states. It is often strong during the day and decreases at night.)
While river channels carry fairly sandy sediment, they deposit mud in the flood plain when they rise and flood. When the area dries out and desertifies, the rapid evaporation results in the formation of a salty white crust.
Ocean sediments: Ancient ocean sediments in the Baja California peninsula are the source region for the prominent dust plumes in this SeaWiFS image. The desiccated sediments were once a muddy sea floor that was lifted up above sea level.
Glacial deposits: Dust storms occur outside of the world’s deserts. This satellite image shows a dust storm blowing out from Iceland's southern coast. The bright white areas are glaciers. The melt water that emerges from beneath them carries a tremendous load of pulverized rock, or glacial flour. This material gets deposited on large mud flats referred to as outwash plains. The harsh climate and constantly shifting channels prevent vegetation from becoming established. During dry periods, the dust is picked up and carried offshore by high winds associated with storms in the North Atlantic. Similar glacial deposits can be found at high latitudes or high elevations around the world. For example, ancient deposits of wind-blown glacial flour, referred to as loess, fuel the prodigious dust storms of the Gobi desert in northwest China.
Dry lake beds: Dry lake beds are called playas in the U.S. and sabkhas in the Middle East. They arise as water erodes rocks and forms fine-grained soils. The erosion can occur over long periods of time or can happen quickly as a result of recent precipitation events.
When lakes dry up, the fine-grained deposits inhibit plant growth, which further contributes to dust availability. The salty deposits tend to be much lighter in color than the surrounding ground on satellite imagery, making them easy to detect.
The MODIS true color image above shows long plumes of dust coming off of dry lake beds and dry wetlands in the Sistan Basin. Straddling southern Afghanistan and eastern Iran, it’s one of the world’s driest basins. High-resolution (250-m) images like this show that entire flood plains, dried lakes, and agricultural areas do not erode. Rather, numerous small point sources with diameters of one to tens of kilometers erode to produce numerous individual dust plumes. It is these individual plumes that merge downstream to form mesoscale dust clouds and dust fronts.
Point Sources of Dust
As we've seen, most dust comes from a number of discrete areas that are small enough to be regarded as point sources, much like smokestacks. Many of the point areas are much lighter than the surrounding ground on satellite imagery, indicative of salt or gypsum-type compounds vs. the reddish-brown coloration of desiccated river flood plains (alluvial dust).
The black plus marks on this map are dust source areas in Iraq. Many of the red pluses are areas that were active before 2005 but are no longer so. The larger black pluses are additional dust source areas that were located by the Naval Research Laboratory (NRL).
These photographs show how the wetlands of southern Iraq have been restored, eliminating some source areas for blowing dust. However, the most prevalent ones between the Tigris and Euphrates rivers remain.
After an appropriate source, the next key ingredient for dust storm generation is wind from the surface through the depth of the boundary layer that’s strong enough to move and loft dust particles.
The first sand and dust particles to move are those from 0.08 to 1 mm in diameter. This occurs with wind speeds of 10 to 25 knots.
As a rule of thumb, winds at the surface need to be 15 knots or greater to mobilize dust. The table shows the wind speeds required to lift particles in different source environments.
Once a dust storm starts, it can maintain the same intensity even when wind speeds slow to below initiation levels. That’s because the bond between the dust particles and the surface is broken and saltation allows dust to lift.
Lofting of dust typically requires substantial turbulence in the boundary layer. This image shows dust being mobilized during a downslope windstorm on the lee slope of the Sierra Nevada mountains in California.
Laminar flow in the right half of the photograph carries the dust close to the valley floor. Further left, the flow slows down and quickly becomes extremely turbulent. During the transition, the dust is lofted approximately 10,000 feet (3000 m).
Typically, wind shear creates the turbulence and horizontal roll vortices that loft dust up and away from the surface. As a rule of thumb, if the wind at the surface is blowing 15 knots, the wind at 1,000 feet (305 meters) must be about 30 knots to keep the dust particles aloft.
Because vertical motions are required to loft dust particles, it stands to reason that dust storms are favored by an unstable boundary layer. In contrast, stable boundary layers suppress vertical motion and inhibit dust lofting.
With the lack of vegetation in dust-prone regions, the ground can experience extreme daytime heating, which creates an unstable boundary layer. As the amount of heating increases, the unstable layer deepens.
As we’ve seen, it’s not enough to have strong wind; the wind must be sufficiently turbulent to loft dust and must occur in a reasonably unstable environment. Wouldn’t it be nice to have a single parameter that expresses wind speed, turbulence, and stability? We do. It’s called the friction velocity.
In more technical terms, dust mobilization is proportional to the flux of momentum, or stress, into the ground. A friction velocity of 60 centimeters per second is typically required to raise dust.
Friction velocity is computed by many NWP models. This NOGAPS analysis for northwest Africa on 7 January 2003 at 12Z shows surface winds, ground wetness, topography, and friction velocity values greater than 60 cm/s.
Note the high friction velocities plotted in red and magenta across the Sahara, particularly near the west coast.
These parallel the area of blowing dust in this SeaWiFS true color image. Since both are from January, the dust in both cases is probably being lifted by the remnants of frontal boundaries manifested as shear lines across equatorial Africa. (Note: The image is from a year prior to the friction velocity chart but is still relevant.)
The whiter plumes are clearly visible in the center of the satellite image, as is an area of higher friction velocities to their north. The plumes are oriented northeast-southwest and are enhanced by the funneling of winds between two areas of higher terrain to the north and the south of the area.
The remnants of the cold front appear as cloud cover over the Red Sea, with cold air cumulus over the northern part.
Notice how the plume of dust blowing out to sea lines up nicely with the region of high friction velocity on land.
Dry desert air has a wide diurnal temperature difference. Strong radiative cooling leads to rapid heat loss after sunset. This quickly cools the lowest atmosphere, resulting in a surface-based inversion that can have a strong impact on blowing dust.
While a 10-knot wind can raise dust during the day, it may have little impact at night. This effect accounts for much of the diurnal variation in summer shamal dust storms, which we will discuss later.
The formation of a surface-based inversion has little effect on dust that’s already suspended higher in the atmosphere. Furthermore, if winds are sufficiently strong, they will inhibit the formation of an inversion or even remove one that has already formed.
If you’ve heard that dust storms always go away at night, that’s not necessarily true; occasionally they persevere. Dust RGB products enable us to detect dust storms at night, something that was not possible with earlier surface and satellite observations.
If you’re not familiar with RGBs, the acronym stands for Red, Green, Blue processing. The products are made from several spectral channels or channel differences and highlight specific features, such as dust. For more information, see the COMET module Multispectral Satellite Applications: RGB Products Explained.
When you are evaluating the potential for dust lofting, be aware of when the boundary layer has a dry adiabatic lapse rate, for the strongest winds aloft can be brought down to the surface, creating gusty conditions.
Be sure to examine winds at 925 mb (approximately 2,500 feet or about 750 meters above the surface when at sea level) where stronger winds allow more dust to be suspended aloft and persist for longer periods due to turbulent mixing.
This section addresses the fate of suspended dust once it’s been lofted high into the atmosphere. Eventually that dust will settle, although it may travel half way around the globe before doing so. As a forecaster, you need to be concerned about the processes that lead to lower dust concentrations, improved visibility, and reduced hazards. (But you should continue to look for conditions that can lower visibility again.)
On the following pages, we will discuss three processes that remove dust:
- Entrainment in precipitation
Gravity also plays a role, although we will not discuss it.
Life Cycle of a Dust Storm
This animation depicts the life cycle of a typical summer dust storm in Iraq, called a shamal.
The initial dust plume extends in a narrow swath immediately downwind from a relatively small source region. As the wind continues to blow, the plume expands laterally and also continues to move downwind.
Sometime later, the wind starts to diminish, eventually falling below the threshold required to continue raising dust. Although no new dust is being raised, the existing dust remains in suspension. The plume detaches from the source region and continues to move downstream and spread. Eventually the dust concentration diminishes through lateral dispersion and settling.
In the shamal example, the dust dissipated through two processes: dispersion and advection. We’ll start by looking at dispersion.
The fanning of a dust plume as it moves downstream from its source region is caused by dispersion, which is a diluting process. Basically, the more air you mix with a dust plume, the more it dilutes, spreads out, and disperses. This is similar to what you see if you pour dye into a river and watch how the color fades as the water moves downstream. Dispersion processes always act to dilute; plumes never re-concentrate.
This figure shows a highly idealized view of a plume dispersing as it moves downstream from a point source. Note that the concentration is not uniform throughout the plume. The highest concentration remains in the center and falls off away from it.
Dispersion and Turbulence
Dispersion is primarily governed by turbulence, which mixes ambient air with the plume. Any increase in turbulence increases the rate at which the plume disperses.
Three kinds of turbulence act to disperse a plume:
- Mechanical turbulence
- Turbulence caused by shear
- Turbulence caused by buoyancy
Mechanical turbulence is caused by air flowing over rough features, such as hills or buildings.
Turbulence from shear can result from differences in wind speed and/or direction.
Buoyancy turbulence can be caused by something as dramatic as an explosion or as simple as parcels of air rising during the diurnal heating of the surface. Particularly in the latter case, buoyancy is governed by the stability of the atmosphere.
Turbulence acts to disperse dust plumes and keep the dust particles in suspension. Without turbulence, dust generally settles at a rate of 1,000 feet (305 meters) per hour. However, this is highly dependent on environmental conditions. Any turbulence will slow the settlement rate.
Dispersion and Stability
We've seen how unstable conditions favor the lofting of dust and formation of dust storms. Stability also has a strong influence on how dust disperses.
This graphic shows dust plumes dispersing under both stable and unstable conditions.
When the local environment is unstable, how do dust plumes disperse? (Choose the best answer.)
The correct answer is c).
Dust disperses in both directions although the effect is significantly more pronounced for the vertical component.
When the atmosphere is stable, dust disperses: (Choose the best answer.)
The correct answer is a).
A stable atmosphere tends to suppress the vertical dispersion of dust, but horizontal transport is still possible.
Under neutral stability conditions, dust plumes spread: (Choose the best answer.)
The correct answer is c).
When the atmosphere has neutral stability, dust plumes disperse roughly equally in both directions because neither one is favored.
Our initial shamal schematic showed the dust plume detaching from the source area when the winds dropped below the threshold to loft dust. Visibility would be expected to improve substantially in the source area soon after this happened. The dust that was lofted simply moved away from its source. Where does the dust go? Recall that dust storms are typically several thousand feet high and frequently extend up to 15,000 feet (4600 meters), and that wind shear contributes to the turbulence needed to loft dust. Therefore, winds aloft may very well carry dust in a direction that’s different from the wind direction on the ground.
When predicting where a dust plume will travel, you should check the vertical wind profile. As this animation shows, dust that leaves the ground going one direction can rise to a level where it travels in an entirely different direction. Fortunately, dust forecast models can do the hard work for you, accounting for the complex evolution of dust plumes in a three-dimensional framework.
Settling of Dust
Particle size plays an important role in both lifting and settling thresholds. Longer suspension times for smaller particles result in long periods of dust haze in arid areas.
Particles between 10 and 50 micrometers fall at about 1,000 feet (305 meters) per hour. Using that rate, if dust is lifted to 5,000 feet (1500 meters) and the wind ceases, the dust will settle in about 5 hours.
Over how large an area? If winds are 10 knots and there’s little to no vertical motion, the dust will typically settle up to 50 nautical miles downstream from the source. Settling is by particle size, with the largest particles falling out first and the smallest ones falling out last. Therefore, the larger, heavier particles will settle near the source area, with the smaller ones settling farther away.
Most dust particles are hygroscopic, or water-attracting. In fact, they usually form the nucleus of precipitation. Because of this affinity to moisture, precipitation very effectively removes dust from the troposphere.
Dust Source Regions
Dust-Prone Regions from Land Cover Types
Dust storms can only form if there’s an appropriate source region. In this section, we’ll look at where these are located, starting with a global view and then focusing on the Middle East and Southwest Asia.
Drawing on a USGS global database of land cover types, eight land covers are thought to produce dust: low sparse grassland in Mongolia; bare desert equatorward of 60° latitude; sand desert; semi-desert shrubs equatorward of 60° latitude; semi-desert sage; polar and alpine desert; salt playas/sabkhas; and sparse dunes and ridges.
When these land cover types are combined with wetness values, we get a bulk measure of erodibility. The figure shows how the world's deserts dominate the resulting pattern.
Source Regions from TOMS Aerosol Index
Identifying dust-prone regions based on land cover characteristics can be refined by incorporating satellite data. The Total Ozone Mapping Spectrometer Aerosol Index (TOMS AI) provides a near-real-time measurement of aerosols in the atmosphere.
This plot bases the dust productivity of the earth surface on the observed frequency of high aerosol values and results in a much more refined view of global dust source regions. Clearly, the majority of the world’s dust storms arise in relatively few areas, in particular, the Sahara, Middle East, Southwest Asia, China, Mongolia, and Southwestern North America.
Using TOMS AI to identify dust source regions, Prospero et. al., 2001 hypothesized that dust sources are associated with topographical lows and depressions.
This graphic shows the dust sources used in the Air Force Weather Dust Transport Application (DTA) model. The oranges and reds indicate strong dust source areas.
Source Regions from DEP
In 2001, NRL started identifying and locating dust emission areas in Southwest Asia using the satellite-derived NRL Dust Enhancement Product (DEP). DEP’s 1-km resolution allows for the identification of individual plume heads that often measure 10 km or less across.
The MODIS true color image and the NRL DEP image show southern Afghanistan, northwestern Pakistan, and eastern Iran on 20 August 2003. By comparing the images, we see the benefit of DEP for identifying small dust plumes. The one in the white rectangle is barely visible in the true color image while it is readily apparent in the dust enhancement product in shades of pink.
What does the close-up view of the white rectangle in the dust enhancement product indicate about the dust plume? (Choose the best answer.)
The correct answer is b).
The close-up view of this localized dust plume indicates that it originated from many point sources to the north and merged into a single small plume as it dispersed to the south.
Cataloguing the individual point sources in dust enhancement products has led to the development of the NRL high-resolution (1-km) Dust Source Database (DSD).
Here we see the 1-km dust sources plotted in red for the 10°X10° tile covering Iraq. Each red area identifies land that has eroded and produced a dust plume.
This plot shows the NRL 1-km dust sources averaged on an 18-km grid where the grid erodible fraction varies from 0 (non-erodible or non-dust producing) to 1.0 (completely erodible and dust producing). Note the many dust-prone areas in eastern portions of the Arabian Peninsula and the spotty source regions in Iran and Afghanistan.
Middle East/Southwest Asia
Soil Types in the Middle East
Some areas of the Middle East are much more prone to dust storms than others. Why might this be? (Choose all that apply.)
All the responses are correct.
Even in bare desert, sandy areas, such as those found on the Arabian Peninsula, generally do not generate dust storms. It is the areas with silt- and clay-rich soils, most common in Iran and Iraq, that are responsible for most dust storms. In this region, these fine-grained soils are found in areas with dry lake beds and river flood plain deposits. The low-lying regions of the eastern Arabian Peninsula, southern Syria, and western Iraq are particularly prone to dust storm generation because prevailing west/northwesterly winds are unimpeded by higher terrain. An area's potential for dust storm generation is also indicated by its climate, that is, its precipitation patterns, prevailing wind direction and speed, and normal location of low- and high-pressure centers.
Dust Source Climatology
You can frequently identify potential dust source regions with satellite imagery. If you’re forecasting for a new region, you can build a dust storm climatology from archived satellite imagery to establish the most prevalent source areas. This is similar to the TOMS Aerosol Index climatology discussed earlier except that it can be much more precise.
For example, this sequence of images reveals that the same light-colored areas in western Afghanistan repeatedly serve as the source for dust storms.
Once you know the color characteristics of source areas in a given region, you should look for other potential areas with a similar appearance.
Interannual Variations in Dust Source Regions
Periods of extended drought dry out lakes, wetlands, and otherwise productive agricultural land, often resulting in new and expanded dust sources. The opposite occurs with wet winters, when numerous storms, heavy rains, and/or above-average snowfall can flood lakes, rivers, and streams and shut off active dust sources.
For example, Southwest Asia experienced an extended drought from 1998 to 2005. Then in 2005, heavy rain and melting snow led to numerous floods in southern Afghanistan.
This MODIS true color image shows the Sistan Basin, one of the world’s driest basins, as it was on 21 February 2005 before it experienced heavy rains and snow melt.
The false color image from 7 March 2005 shows how much the basin changed. The dark blue indicates clear, deep water, the light blue mud-laden water.
The oval in this MODIS true color image from 12 October 2005 shows Lake Saberi. Notice that it is still filled with muddy, brown water after the long, hot summer.
When the Hamoun Lakes and wetlands are filled with water, the production of dust plumes and storms decreases.
These NRL DEP images of Pakistan and Afghanistan on 2 May 2003 and 12 October 2005 demonstrate the difference between a drought-ravaged basin and one that has experienced a wet period.
Areas of Highest Occurrence
This map shows the areas with the highest occurrence of dust storms around the northern Persian Gulf. Maps like this are compiled over several seasons of observations and are invaluable for helping forecasters anticipate dusty conditions. Note that these areas correspond relatively well with the dust source regions that we looked at previously.
Synoptically Forced Dust Storms
In most areas, we can classify dust storms by the broad meteorological conditions that cause them. In this section, we will examine the most common events that occur in the Middle East. These are dust storms caused by prefrontal and postfrontal winds that primarily occur in winter, and summer dust storms caused by persistent northerlies.
Note that whenever dust is a forecast consideration in your area, you should become familiar with the local atmospheric conditions that lead to strong winds under dry conditions. Each region has its own weather patterns that lead to dust storms.
Prefrontal Dust Storms
Prefrontal dust storms occur across Jordan, Israel, the northern Arabian Peninsula, Iraq, and western Iran as low-pressure areas move across the region. Antecedent factors include a band of winds generated by, and ahead of, the low-pressure area that presses against a stationary high-pressure center in Saudi Arabia or the western slopes of Iran’s Zagros Mountains.
This chart depicts prefrontal winds as a low-pressure area migrates into Iraq. The southeasterly or sharqi winds that blow northward up the Tigris/Euphrates River basin are intensified as low-level flow is funneled between the Zagros Mountains to the east and the pressure gradient to the west. Toward the west, southwesterly or suhaili winds pick up dust from western Arabia and move it northeast in advance of the cold front.
This image shows sharqi prefrontal dust plumes emanating from dry lake beds and fluvial deposits in southeastern Iraq. (Fluvial deposits are associated with rivers and streams.)
The polar jet stream behind the cold front and the subtropical jet stream in front of it often interact dynamically to strengthen the front east of the upper-level trough. The strengthened cold front induces stronger prefrontal winds out ahead of the upper-level trough. In addition, the overlapping of these jet cores and the coupling of secondary circulations in the right rear of the polar jet and the left front of the subtropical jet enhance mid-level upward vertical velocities and increase the lifting force for blowing dust.
Under these conditions, westerly winds mobilize dust and sand across Jordan, Syria, and northwestern Saudi Arabia, transporting it east and northeastward across the Arabian Peninsula, Iraq, and the Persian Gulf countries.
Let’s look at an example. A prefrontal dust storm occurred on 25 March 2003. Winds in front of a powerful Mediterranean cyclone whipped up a thick dust storm that significantly impacted movement on the ground.
This graphic shows a 24-hour prognostic weather map valid on 25 March at 12Z as the dust storm moved across Iraq. We see the convergence of the polar front jet and the subtropical jet, indicated by the red arrows. The shaded areas indicate where dense dust storms with visibilities of less than 1 km were predicted along the associated cold front.
This SeaWiFS true color image taken during the morning hours shows the extent of the unfolding dust event. Areas of prefrontal dust cover northeastern and eastern Saudi Arabia and extend into Iraq where cloud cover makes it difficult to observe its full extent from satellite. In addition, postfrontal dust is forming behind the advancing cold front from Egypt and Sudan northeastward into northern Saudi Arabia and northwestern Iraq.
The upper-level trough moved east into the northern Arabian Peninsula, resulting in fairly cloud-free skies along the front from the Red Sea. From there to the west, the building high pressure to the north resulted in convergence (a shear line) that provided lift for blowing dust and sand across Sudan and equatorial Africa.
Over the Red Sea itself, satellite imagery can be used to help identify the front as the associated low-level convergence and moisture typically lead to the formation of stratiform clouds. This SeaWiFS image taken on the morning of 26 March shows cloud cover along the cold front in the southern Red Sea extending northeast along the remainder of the frontal zone to the main area of low pressure.
Arabian Peninsula Prefrontal Dust Event
Here is another example of a prefrontal dust event. (There's also some post-frontal activity.) The weather chart shows that behind an advancing cold front, southern Iraq and northern Saudi Arabia are experiencing blowing dust or shamal conditions.
At the same time, prefrontal dust plumes south of the low are being lifted from the United Arab Emirates northward across the Persian Gulf due to increasing low-level southerly flow.
Now we'll zoom in on the area in the red box.
This MODIS true color image highlights the type of prefrontal dust conditions that affected large portions of the Middle East, including Iraq, northern Saudi Arabia, and the United Arab Emirates during this event.
North Africa Sirocco Winds and Prefrontal Dust Event
This MODIS true color image shows another case of prefrontal dust over North Africa. It is associated with a frontal system over the Mediterranean, which extends southward into Libya. The prefrontal winds responsible for the blowing dust are known as the sirocco in this region.
Plumes of dust can be seen moving from northwestern Egypt and northeastern Libya across the Mediterranean Sea.
The leading edge of the advancing cold front is indicated by the middle- and high-level cloud band oriented north to south.
The weather chart shows the synoptic features and low-level wind flow associated with the dust outbreak as the low-pressure system over the southern Mediterranean moves eastward.
Sirocco wind events can have speeds of up to 100 km/h (55 knots) and are most common during the fall and spring.
Postfrontal Dust Storms
As you’ve seen, widespread dust can also occur following pre-frontal events. Especially in winter months, cold frontal passage leads to strong northwesterly winds on the backside of the front. The resulting dust storm is referred to as a shamal from the Arabic word for north. Shamals produce the most widespread hazardous weather known to the region.
The RGB animation shows a cold front-generated sandstorm stretching to the west of the Persian Gulf. The front has passed and lies to the south of the dust front. Strong northwesterly postfrontal flow has picked up dust along the front and appears to be moving to the south and east.
Winter shamals generally last for either 24 to 36 hours or three to five days.
The shorter-period shamals typically begin with passage of the front. When the associated upper-level trough or rapidly moving short waves move eastward, winds diminish after 24 to 36 hours. Such cases are relatively common, occurring two to three times a month. Sustained winds typically reach 30 knots, with stronger gusts to 40 knots.
The longer-term (three- to five-day) shamal occurs one to three times a winter and produces the strongest winds and highest seas in the Persian Gulf. Over the exposed gulf waters, sustained wind speeds have reached 50 knots and produced 10- to 13-foot seas. This type of shamal arises either from:
- The temporary stagnation of a 500-mb shortwave over or just east of the Strait of Hormuz, or
- The establishment of a mean longwave trough over the same area
Persistent dust and sand storms occur throughout the life spans of both types of shamals.
Dry conditions enhance dust during the first few cold frontal passages of the season. In fact, widespread dust often occurs with the first passage, restricting visibility to less than three nautical miles. Subsequent fronts bring precipitation that binds soil particles together. In these circumstances, winds above 25 knots are often needed to raise dust.
Here’s a typical synoptic-scale surface chart for a winter frontal event similar to the one seen in the previous satellite image. Strong pressure gradients develop behind a moderate to strong cold front due to upper-level subsidence and rapidly building surface high pressure over northwestern Saudi Arabia and Iraq. The strong northwesterly low-level winds are quickly reinforced by west-to-northwesterly upper-level winds behind the mid-level trough. Shamal winds and postfrontal blowing dust develop behind the cold front over southern Iraq and northeastern Saudi Arabia. Farther to the west, another area of post-frontal blowing dust forms to the north of the surface trough and shearline over southern Egypt.
In this example, a low is centered over Iraq and Kuwait, with a strong pressure gradient to the southwest. As a result, there is strong northwesterly flow to the west of the low, with winds reaching 20 to 25 knots near the Persian Gulf. When this flow is combined with unstable boundary layer stratification in the postfrontal environment, conditions are ripe for a dust storm.
The synoptic chart highlights the major frontal features as the dust event unfolds. Notice that in addition to the areas of postfrontal dust, pre-frontal dust also forms over northeastern Saudi Arabia, southern Iraq, and Kuwait.
Let’s focus on the cold front as it moves southward over the Red Sea. In this MODIS true color image, the dust cloud is readily visible over the water as dust is advected southward by strong northerly postfrontal winds.
A mid-level trough centered over Saudi Arabia and extending northward to the eastern Mediterranean Sea is associated with the surface trough over Iraq, Kuwait, and Saudi Arabia. Strong northwesterly flow exists on the backside of the trough with 80-kt winds being reported over Egypt (in the cyan area).
Deep vertical mixing is helpful in generating dust storms. Strongdownward vertical motion, which is likely to be associated with a middle- to upper-level front within the upper-level trough shown, does two things:
- It helps to prevent cloud formation or evaporate preexisting cloudiness. This increases solar warming in the lower troposphere and enables strong winds to mix into the low levels from above. This requires that middle and lower tropospheric lapse rates be unstable, which is more likely to occur when strong solar heating is present. Although subsidence itself can stabilize lapse rates, even the wintertime Middle Eastern sun can often offset this stabilizing effect.
- Strong winds dynamically accompany the downward intrusion of upper tropospheric air into the mid-levels and near the surface. This momentum and dry air then mix to the surface, leading to clearing conditions and little precipitation on the back side of a trough.
Examples from Other Regions
Excitation of dust storms by frontal passage is not limited to the Middle East. This MODIS true color image shows a massive dust storm that struck Sydney, Australia on 23 October 2002.
In this image, the dust extends 932 miles (1500 km) north-northwest from near the southeastern corner of Australia. The source region for the dust was an enormous dry lake bed in south-central Australia. Eastern Australia had been in the grip of a drought for six months, which made the soil much easier to loft. The red dots mark active bushfires. Smoke plumes clearly show the low-level wind direction ahead of the advancing dust front.
This weather map shows the location of the cold front and surface trough at about the same time as the MODIS image. Notice the close correlation between the frontal location and the dust cloud.
Postfrontal dust storms are also common across the American southwest. This MODIS true color image shows one that originated in northern Mexico and western Texas.
In this case, surface winds were very strong. In this mid-afternoon image, the jet maximum had rounded the base of an upper-level trough and was transporting momentum to the surface from strong winds at middle and upper levels.
Finally, post frontal dust storms also affect large portions of East Asia as dust is lifted by strong winds from the arid regions of northwestern China and Mongolia. The dust is transported across China and the Yellow Sea, often impacting Korea and Japan. During the winter and spring, the westerly jet stream sometimes transports the dust over the North Pacific and as far east as North America.
This NASA SeaWiFS true color image shows an expansive dust storm circulating around low pressure over northeastern China.
The summer shamal is a wind that blows with persistence over Iraq and the Persian Gulf from late May to early July. Compared to winter events, summer dust storms have greater vertical motion due to high temperatures and resultant convective currents.
The summer shamal results from a characteristic synoptic pattern in which the most prominent features are:
- A semi-permanent high-pressure cell extending from the eastern Mediterranean to northern Saudi Arabia
- A low-pressure cell over Afghanistan
- Thermal low pressure associated with the monsoon trough extending into southern Saudi Arabia
As the surface pressure analysis shows, the cyclonic circulation around the region of low pressure combines with the anti-cyclonic circulation around the high-pressure cell to increase the winds over the northern Persian Gulf region. These winds are normally confined from the surface up to 5,000 feet (1500 meters). The shamal is particularly strong at ground level during daytime but weakens at the surface overnight. Shamal events can be quite long-lived, lasting several weeks. These are known as the "40-day shamal."
This Meteosat Second Generation (MSG) infrared image overlaid with surface pressure for 7 August 2004 shows the thermal low over southern Afghanistan and northern Pakistan. Higher surface pressure is located over the eastern Mediterranean, with lower surface pressure in the northern Persian Gulf. This produces strong northerly winds in Afghanistan and Iraq.
This NRL MODIS DEP overlaid with surface winds confirms the presence of strong surface northerly winds, with a mesoscale dust storm taking place over western and southern Afghanistan. Dust plumes are evident in Central Iraq.
Upper-level high pressure is often found over the Saudi Peninsula during the summer shamal. The location of the center of the high can vary, as we see in the MSG infrared image and the 500-mb height composite for 7 August 2004.
For the larger-scale events that we’ve been describing, it’s critical to forecast the following:
- Where the wind will be sufficiently strong to mobilize dust
- Where there’s a sufficiently unstable boundary layer
- Where there’s an appropriate source region to excite a dust storm
In general, NWP models do a very good job of forecasting winds and atmospheric stability due to synoptic-scale weather events. As a forecaster, you need to integrate this information with your knowledge of local conditions to accurately forecast blowing dust.
Dust Storms Caused by Mesoscale Systems
This section examines dust storms generated and influenced by mesoscale forcing, focusing on examples from the Middle East and Southwest Asia.
The range of mesoscale phenomena known to excite dust storms includes downslope winds, gap flow, and convection. We'll start with downslope winds.
This MODIS true color image shows two separate dust storms that occurred along the northern Afghan border on 25 March 2009. One was in the Termez Valley, the other in the northern Herat Province. Notice that the dust plumes of the two storms are orientated in different directions. The wind flow in the Termez Valley is easterly, while there is southwesterly, downslope flow in the northern Herat Province. The difference is due to terrain and its impact on the direction of the wind flow.
A surface low located to the north-northwest near the Aral Sea is causing a southeast-to-northwest surface pressure gradient.
Himalayan Gap Flow and Afghan Dust Storms
This MODIS dust product shows dust storms in Afghanistan and to the south in Iran and Pakistan.
Looking at Afghanistan, we see plumes of dust coming off numerous dry lake beds that lie immediately south of the high terrain of the Hindu Kush. The strong flow is a result of gap flow through mountain passes and down to the lowlands where the lake beds lie. The image also shows plumes of dust coming off Iran and Pakistan and blowing over the Arabian Sea.
These graphics show another view of the Afghan dust event. The image on the left is a MODIS true color image, the one on the right a plot of surface winds from a COAMPS simulation run with 9-km grid spacing. By comparing the dust plumes in the satellite image to the streamlines predicted by the model, you can see that COAMPS does a good job with the circulation. The north-northwest gap flows over Afghanistan are well represented, as are the northeasterly flows over the coast. This suggests that a good mesoscale model can give you a handle on the flow that sets up dust events.
The Afghan dust storms are associated with upper-level lows and highs propagating across central Asia. In particular, these events are associated with high pressure building across Uzbekistan, which gives rise to a very strong pressure gradient across the mountains. The pressure gradient results in ageostrophic gap flow that raises dust storms. This is very different than the synoptic-scale geostrophic flow that gives rise to shamal-type events further to the west, over the Persian Gulf region.
Red Sea Dust Storm
This SeaWiFS true color image shows a dust storm event that occurred around the Red Sea in July 1999.
The large thermal contrast between the interior of Sudan and the Red Sea resulted in a strengthened pressure gradient that helped generate the dust storm. The lower terrain of the Tokar Gap provided a path for the dust to move over the Red Sea.
(The Tokar Gap is a low-elevation break in the mountains that flank the west side of the Red Sea.)
The dense plume of dust entering the Red Sea disperses and casts a pall over the area. The mountains to the east appear to block and turn the winds southeastward.
The Red Sea Convergence Zone also helps to keep dust trapped in the center of the Sea. The zone is formed by air flowing in from the north and south, which creates an area of convergence that traps the transported dust.
Accurately forecasting gap flows generally requires a mesoscale model with several grid cells inside the gap. Since the Tokar Gap is approximately 68 miles (110 kilometers) wide, high-resolution mesoscale models should sufficiently capture the flow.
[Note: For more information on gap winds, see COMET's Gap Winds module.]
Here we see a haboob, which is a dust storm caused by convective downbursts. Haboobs are the true walls of dust and sand that most people think of as strong dust storms. Most of the dust particles range from 10 to 50 micrometers, but larger particles (up to several millimeters in size) can be blown about. The larger particles settle rapidly after the wind subsides, whereas the finer ones settle at about 1,000 feet (305 meters) per hour when the haboob finally dissipates. Other areas clear rapidly as the dust is advected out of the area.
Properties of a Haboob
Winds associated with the gust front of a dry downburst from a convective storm average 35 to 50 knots and can easily excite a dust storm when they encounter an appropriate source area. Haboobs tend to be rather small, on the order of 60 to 90 miles (100 to 150 km). Their average height extends from 5,000 to 8,000 feet (about 1500 to 2500 meters) at the peak of the event. However, heights up 15,000 feet (4500 meters) have been recorded when exacerbated by convergent outflow boundaries. The average haboob tends to be short-lived, about three hours. Visibility usually begins to improve soon after the gust front passes.
Although haboobs can be seen approaching a location from afar, they move in very quickly, typically at about half the velocity of the winds within the storm. So a haboob packing 50-knot winds will move at about 25 knots.
Haboobs in Different Regions
This visible satellite image shows a haboob near the Persian Gulf that's associated with a thunderstorm system to the north. The convection is related to summer conditions, with moist inflow from the Persian Gulf.
These MODIS satellite images show additional gust fronts that probably had surface haboobs associated with them. This first image is over Iraq on 31 Mar 2010 at 0730Z.
This second image is over Iran at the same time.
This infrared loop shows a haboob that occurred in the early morning hours of 01 Aug 2001 in the Western Sahara Desert. Over the next 6 hours, the haboob moved west, eventually reaching the Canary Islands. This shows how long a distance strong haboobs can propagate.
The haboob shown above was associated with a large convective complex over central Australia. It illustrates the clearing behind the gust front due to the subsidence and cooler air accompanying the downburst.
Haboobs are much more difficult to forecast than synoptically forced dust storms and rely largely on nowcasting (determining if the environment is right for haboobs). The following procedures will help you forecast haboobs from both ongoing and collapsing thunderstorms.
Forecasting haboobs from ongoing thunderstorms
- Look for signs of instability aloft. Use the Best Lifted Index (the Most Unstable Lifted Index).
- Look for high environmental relative humidity between 700 and 500 mb and/or high values of simulated radar reflectivity from WRF/COAMPS or actual reflectivity from a nearby EWR radar if it’s available. Also look for steep lapse rates between the surface and approximately 18,000 feet (5 km).
- Find the strongest wind at any level aloft where the wet bulb potential temperature is less than the (surface potential temperature + 39 °F or 4 °C). It’s possible that this wind may be brought to the surface.
- Determine if your forecast area is located in or near a dust source region.
Forecasting haboobs from collapsing thunderstorms
- At what time of day is the thunderstorm occurring? Thunderstorm collapse is most likely after sunset.
- Determine the cloud base height of the thunderstorm. The higher it is (greater than 10,000 feet or 3 km above ground level), the warmer the resultant outflow at the surface due to adiabatic compression, and the weaker the potential haboob. Downdraft acceleration will mitigate the warming issue to a limited extent.
- Check for rapidly warming cloud tops in looped geostationary infrared imagery, which are indicative of thunderstorm collapse.
- Determine if the thunderstorm is occurring over a dust source region.
Inversion Downburst Storms
Inversion downburst storms are windstorms that occur on sloping coastal plains with a strong sea breeze. As the sea breeze intensifies, convergence along the sea breeze front can generate sufficient lift to break a capping inversion. This potential instability results in the downward mixing of cool air aloft, which flows downslope and out over the water. The descending air produces roll vortices and potentially severe local dust storms along the coast. Then the inversion is reestablished and the event dies out.
Inversion downburst storms form in coastal terrain where slopes are at least 20 feet (6 m) per mile, such as those found along the Red Sea and Persian Gulf. They occur when the sea breeze exceeds 15 knots and there's an inversion aloft, but not a particularly strong one. The downburst winds last 15 to 45 minutes and reach speeds of 90% of the gradient flow immediately above the inversion, typically 20 to 25 knots. These storms are limited in size, although they can still reduce visibility to less than one mile depending on local surface soil conditions.
Inversion downburst storms typically lead to a very narrow streamer of dust out over the Persian Gulf. Although they occur on both sides of the Gulf, they are more commonly associated with the eastern side, along the Iranian coast. That's probably because the climatologic synoptic flow favors a stronger sea breeze there. Predicting their location is very difficult, but you should look for places where coastal curvature favors stronger sea breezes or sea breeze convergence. Variations in the strength of the inversion also impact where the event is located. And, like all dust events, they require an appropriate source region.
[For more information on sea breezes, see COMET's Sea Breeze module.]
Dust devils are a common wind phenomenon that occur throughout much of the world. These dust-filled vortices are created by strong surface heating and are generally smaller and less intense than tornados. Their diameters typically range from 10 to 300 feet (3 to 90 m), with an average height of approximately 500 to 1,000 feet (150 to 300 m). Dust devils typically last only a few minutes before dissipating. However, when conditions are optimal, they can persist for an hour or more. Wind speeds in larger dust devils can reach 60 mph or greater.
Dust devils form in areas of strong surface heating. This typically occurs under clear skies and light winds when the sun can warm the air near the ground to temperatures well above those just above the surface layer.
Once the ground heats up enough, a localized pocket of air will quickly rise through the cooler air above it. Hot air rushes in to replace the rising air at the bottom of the developing vortex, intensifying the spinning effect. Once formed, the dust devil is a funnel-like chimney through which hot air moves both upwardly and circularly. If a steady supply of warm, unstable air is available, the dust devil will continue to move across the ground. However, once that supply is depleted or the balance is broken in some other way, the dust devil will break down and dissipate.
Dust devils can vary greatly in size, both in diameter and vertical extent. Notice how aggressive the interaction with the surface can be.
Dust Storm Seasonality and Frequency
Climatologies tell us what happened in the past, which helps us anticipate future events and improve our forecasting. Climatology provides several types of data that help with forecasting the location, seasonality, frequency, and severity of dust storms.
We’ve seen how data from the TOMS Aerosol Index helps us map dust source regions. That same data can help us determine seasonal variations in dust storms.
This animation shows the seasonal variability of dust storms in the dust belt that stretches from western Africa up through the Taklamakan Desert in central Asia. Note the strong seasonal dependence of dust storm frequency. For example, dust storms in this desert show a pronounced peak in May while the maximum values for West African dust storms shift northward from winter to summer.
Dust Storm Frequency and Severity
Climatologies compiled by the Air Force Weather Agency Metsat Applications Branch show the monthly frequency of dust storms. Note how the number of storms in the Gobi Desert spikes in March and April and tapers off from May through July.
When we categorize the dust storms by visibility, the picture becomes clearer. Not only is the highest frequency in the early spring, but the majority of severe dust storms occurs in March and April, more than the rest of the year combined.
This graph of dust storm climatology for Iraq reveals some important information. Dust storms tend to be most frequent in the summer, although severe storms can occur from spring through autumn.
Dust Storm Frequency and Precipitation
If you are forecasting in a region and don't have access to information on the frequency of dust storms, you may be able to infer a climatology by examining other climatologic data such as the frequency of dust events vs. annual precipitation rates (PR).
Obviously, drier, hotter conditions favor more dust storms. Here we see a minimum for precipitation events and a maximum for temperature in central Iraq through the summer months, the dustiest time of the year.
Satellite Detection of Dust
Defining the Problem
Using satellites to detect dust has been difficult historically. A dust cloud that's visible in one time and place can suddenly seem to disappear, only to reappear somewhere or sometime else. Furthermore, dust that's prominent during the day can suddenly seem to disappear at night.
Much of the problem stems from the use of single channel visible and infrared images. While they are beneficial in some situations, a number of issues limit their overall usefulness.
In this section, we will show how satellite detection of dust has dramatically improved through the use of multispectral products, which are increasingly available to forecasters.
Specifically, you will:
- See how visible and infrared images are used to detect dust
- See how animating single channel imagery can improve dust interpretation
- Learn how a scientifically based Aerosol Optical Depth product can be helpful to forecasters
- Learn about the improvements made possible with RGB products
Detecting Dust During Midday
In general, it's easier to detect dust during the day than at night although there are differences in daytime performance based on the time of day. Surface type also has a large impact.
Midday visible images depict dust better over water than desert land surfaces. That's because the dust disappears into the sandy, dusty land background while it contrasts distinctly against water (dark) surfaces.
The reverse occurs with infrared images. Dust clouds that are cooler than the underlying hot surface show up distinctly over land. But when they drift over water, the dust usually disappears against the relatively cool waters.
Let's look at these effects in MODIS imagery. The visible image above shows the dust front moving from north to south over the Red Sea. However, the dust is not nearly as easy to detect over the bright land. Still, we can faintly see some plumes emanating from source regions on the east side of the Red Sea.
The infrared imagery does a poor job of detecting dust fronts over water and a much better job over land where the thermal contrast between the dust and surface is enhanced. Note the prominent dust plumes over land emanating from the source regions.
Next, we'll see what it's like to observe dust during the late afternoon and early morning hours using visible images.
Detecting Dust at Sunset and Sunrise
The rules for interpreting dust with visible imagery are different both before the sun sets and after it rises. If the satellite is looking in the general direction of the sun and the dust, the forward scattering of dust particles heightens the reflection from dust.
Backscattering occurs when the satellite is looking away from the sun. Since less solar energy is being reflected back to the satellite, it reduces our ability to see the dust.
Let's look at an example. This MSG natural color RGB product is derived from visible and other solar wavelengths. It shows the Arabian Peninsula at dawn, when the forward scattering of a large dust cloud reveals an advancing dust plume. It would be difficult to detect the cloud with this RGB product in the middle of the day.
DMSP polar-orbiting satellites pass over a location in the early evening and early morning local time, providing a favorable sun-satellite viewing geometry for observing clouds and dust.
In this morning example, more of the scattered sunlight reaches the satellite because the satellite is looking in the direction of the sun's rays. We can see the dust and surface of the water (sun glint), which are more evident against the darker land surface. Notice the dust front approaching Kuwait. The low sun angle even allows us to see wave structure within it.
The second image was taken at the same time from the geostationary Meteosat 7 satellite, which is located further east. Notice how the dust is much less evident and how dark the water surface is. That's because the satellite is viewing from an angle similar to the sun's, meaning that it is seeing less scattered solar energy.
It's evening now and the same dust front has pushed southward to about 25 degrees north. How would you expect the dust to appear with the DMSP satellite that's now to the east? (Choose the best answer.)
The correct answer is a).
The DMSP satellite is viewing the dust from the east, looking in the direction of the sun. Therefore, it sees more of the energy scattered in the forward direction.
From this discussion, it should be clear that some of the best dust viewing on visible images occurs in the early morning and evening with the DMSP satellite.
Aerosol Optical Depth
Now we'll examine a product that's based on the solar channels from MODIS midday data. The visible image is hard to interpret…
…but the aerosol optical depth (AOD) product shows color-coded optical depth (a strong indicator of dust) over the region.
AOD is a unitless measure of the amount of light that airborne particles, such as dust, smoke, haze, and pollution, prevent from passing through a column of atmosphere. AOD does not translate directly into surface visibility estimates because the location of the dust in the vertical is not known: it could be mostly aloft or near the surface. However, AOD serves as a first-order indicator of how dusty the atmosphere is. It is increasingly being assimilated into numerical dust forecast models and forecasters are using it as a nowcast tool to help quantify dust information near and over deserts.
In this example, we see a gathering dust storm over Syria in shades of orange, indicating an optical depth of up to about 1.5, which would likely impact visibility.
About 24 hours later, the dust storm has spread into the Persian Gulf. There are large values of AOD over Saudi Arabia, Iraq, and Kuwait, and lower values elsewhere.
Animating Satellite Images
We've seen the problems that can arise when trying to detect dust in most visible images over land. Animating daytime images can be a useful solution.
In this daytime animation, the motion of the dust cloud helps us identify its location. The cloud is moving southward from Saudi Arabia, Iraq, and Kuwait into the Persian Gulf. Notice how distinct the dust cloud is against the dark ocean background of the Persian Gulf. Just as a second surge of dust moves in from the northwest, darkness descends.
The corresponding infrared animation starts at about 0Z (0300 LST) and continues until 0Z the next night (0300 LST). Before starting the animation, notice that we can see clouds from a cold front but no dust in the Iraq and Kuwait region. Dust is actually present but it's invisible since it blends into the cool surface at night.
At dawn, solar heating starts to create thermal contrast, enabling us to see the dust flooding into the Persian Gulf region. By midday, the dust contrasts well against the heated Saudi Arabia land mass but poorly against the nearby Persian Gulf. By evening, however, the land cools down, the contrast between the elevated dust and surface decreases, and the dust disappears. Although we cannot see it, copious amounts of dust are still there.
This animation illustrates the inherent limitation in using infrared satellite loops to view dust. Detection diminishes as the temperature contrast between the dust and the background decreases.
This MSG dust RGB animation occurs over the same period but depicts dust at night when visible data are not available and infrared data are not very useful for this purpose.
By partially relying on multispectral channel differencing rather than thermal contrasts, the RGB product lets us detect dust at night, an unprecedented capability for satellite dust products.
The first frame of the animation, at about 0Z (0300 LST), shows a nighttime dust cloud in violet moving through southern Iraq. As you may recall, this feature was not apparent in the infrared animation.
We also see how several dust clouds progress through the 24-hour period, including a new outbreak the next evening.
Comparing Dust Products
Now we'll compare two dust RGBs, one from EUMETSAT, the other from the U.S. Naval Research Lab (NRL). The EUMETSAT dust RGB is based on three infrared channels and is available 24 hours a day. Notice the dust squall over Kuwait and the surrounding countries, which appears as pink or violet. Wind barb and weather symbols have been overlaid, including the symbol for suspended dust (S) in southern Iran and Saudi Arabia.
NRL's MODIS dust RGB is only available during daytime hours and portrays dust in orange or pink. Since it uses both visible and infrared data as inputs, it often reveals more dust over water than the EUMETSAT dust RGB.
In the extreme southern portion of this MSG dust RGB animation over Saudi Arabia, what is causing the dust outbreak in violet? View the dust RGB color scheme. (Choose the best answer.)
The correct answer is d).
Thunderstorms, which appear in deep red, create outflows of cool air and gusty winds, which pick up dust. The dust fronts then move away from the thunderstorms that created them.
What happens to the dust fronts? (Choose the best answer.)
The correct answer is b).
The two dust fronts, one from the north and one from south, collide and create a convergence line, along which fresh convection develops.
Forecasting Dust Storms
The Forecast Process
This section presents a general process for forecasting dust storms that incorporates the wide array of tools currently available to help forecasters predict dust storms. These tools include satellite imagery and RGB products, surface and upper-air observations, NWP models, and a new generation of dust/aerosol models.
The dust forecast process is divided into three parts defined by the forecast lead time:
- Long range, 72 to 180 hours
- Medium range, 24 to 72 hours
- Short range, 0 to 24 hours
We'll describe the process and then apply it to a case. Note that the forecast process refers to U.S. Department of Defense (DoD) models and tools but is general enough to easily adapt to other forecast requirements and data sources.
Before starting to develop a dust forecast, you should be familiar with your area of responsibility and local rules of thumb. In particular, you should know:
- The types and locations of local dust source regions; for example, if there are lake beds, salt flats, or newly developed drought regions
- The types of soil present
- The impact of local terrain on wind speeds
- The wind direction with respect to local dust source regions
- How the winds align from the upper levels down to the surface (vertical wind shear), especially during winter
The use of model forecasts depends on the time range of your forecast. Short-range dust forecasts tend to rely on real-time analyses, while medium- and long-range forecasts rely far more on model output from:
- Mesoscale models such as the DoD's COAMPS, DTA-MM5, and DTA-WRF
- Global-scale models such as the DoD's DTA-GFS, NAAPS, and NOGAPS
Here are some tips to keep in mind when viewing model guidance. When possible:
- Consult different dust products from the same dust model or a different model since each product provides slightly different information
- Animate forecast products to identify mesoscale dust features and their movement, extent, and location
On the following pages, we'll examine the three forecast periods and then apply the process to a case in Southwest Asia. As you go through it, a Notes window will be available for tracking information about each time period. You'll need to refer to the information as you proceed through the case.
Long-Range Forecast Process
The long-range (72 to 180 hr or 3 to 5 day) dust storm forecast process has two steps.
Step 1: Look for large-scale, synoptically driven dust events in the 3 to 7.5-day range in global models, such as DTA-GFS and NAAPS.
Step 2: Look for model-forecast midlatitude troughs that drive pre- and post-frontal dust storms in winter and that can amplify the large-scale wind patterns associated with summer events, such as the northerly winds that create shamals. These large-scale waves are resolved by global NWP models such as GFS and NOGAPS, while the associated dust outbreaks are modeled by the global dust models DTA-GFS and NAAPS.
Medium-Range Forecast Process
For the 24- to 72-hr forecast, use mesoscale dust model output from models such as COAMPS, DTA-MM5, and/or DTA-WRF, and larger-scale dust forecasts from the global DTA-GFS and/or NAAPS models. Guidance from NOGAPS shows the evolution of larger-scale atmospheric features and is helpful for identifying conditions favorable for a blowing dust event.
Here are the steps in the medium-range dust storm forecast process.
Step 1: Examine the following charts from the DTA-WRF, COAMPS, NOGAPS, and/or DTA-GFS models.
- 300-mb height and wind forecast charts to track troughs and jet streaks; briefly examine upper-tropospheric winds to identify the presence of any jet streaks, especially for cool-season dust storms; jet streaks within a pronounced upper-level trough are indicative of an intensifying low-pressure system with stronger surface fronts and associated winds
- 500-mb height and relative vorticity forecasts to identify and track troughs and vorticity maxima
- MSLP and surface wind forecast charts for fronts and potentially strong wind conditions
Step 2: Looking at the forecast soundings from WRF or COAMPS, determine the forecast stability and wind profile at your forecast time of interest.
Step 3: Check the 6-hrly precipitation and 700-mb relative humidity forecast charts to determine where increased moisture and precipitation are anticipated since they decrease the probability of dust lofting.
Step 4: Combine COAMPS forecasts of surface friction velocity, surface winds, and soil wetness from WRF and/or COAMPS with your knowledge of dust source areas to see if the criteria for a potential blowing dust event are met. Recall that friction velocity incorporates atmospheric stability and wind speed into one variable.
Step 5: Examine DTA-WRF and COAMPS forecasts of surface visibility due to dust. Compare them to WRF and COAMPS forecasts of winds through the mixed layer and dust optical depth to help assess changes in geographical extent and intensity with each successive model run.
Step 6: From the model output and your initial analysis, develop a best-guess forecast as to the onset and duration of any dust events in your area of responsibility in the 24- to 72-hr window.
Short-Range Forecast Process
The process for creating short-range (0- to 24-hr) dust forecasts includes the following steps.
Step 1: Analyze the present state of the atmosphere by looking at satellite imagery, upper-air charts, and surface analyses, keeping in mind the location and characteristics of relevant dust source regions.
Step 2: Examine the latest observed and/or forecast soundings from WRF and COAMPS. Note the strength of any inversions (usually during summertime) and determine if they will break due to turbulent mixing and daytime heating that would ripen the environment for a dust outbreak.
A dry adiabatic lapse rate from the surface through a deep mixed layer allows the dust to loft to great heights, especially if winds are from the same direction and increase with height through the layer
- Note that dust storms generally occur in this kind of environment and that the strongest wind speed aloft within the dry adiabatic layer can be brought to the surface
- The height or top of an elevated dust layer can be approximated by determining where the lapse rate becomes less than the dry adiabatic lapse rate
- Dust storms are less likely in a stably stratified boundary layer although narrow plumes of blowing dust are still possible
Step 3: To determine the potential duration and type of dust event, pay special attention to dust lofting in your area of responsibility, local rules-of-thumb about advection, and geographic features such as the location of dust source regions, terrain, vegetation, and water sources. Also note where precipitation has fallen in the past 48 hours and whether it was convective or stratiform.
Step 4: Use satellite dust enhancement products (such as enhanced infrared imagery) and RGB and other multispectral imagery tuned for dust detection. Integrating these products with surface observations can provide information about the current extent and location of existing dust plumes and fronts.
Step 5: Make a best-guess forecast as to the onset, duration, and persistence of any dust events in your area of responsibility in the very short term, using short-range mesoscale model output from DTA-WRF and/or COAMPS as guidance. The global DTA-GFS and NAAPS models can resolve large-scale features that drive smaller-scale dust events in the short term but cannot resolve localized dust features.
The rest of this section examines a dust storm case from Southwest Asia, focusing on the use of model data in the dust forecast process. Since these data play a critical role in the long- to short-range forecast processes, we'll focus on those periods more than the nowcasting stage when real-time observational data are more important.
We will provide a limited set of the data and products normally available in an operational environment. The products presented here highlight salient meteorological features that factor prominently into the making of a good dust storm forecast for the scenario.
Case Study: Long-Range Forecast (21 Feb 2010)
Before looking at the data, take a minute to consider the area’s dust climatology at this time of year.
- Dust storms occur 2 or 3 times a month
- Dust events typically last 24 to 36 hours but are sometimes 3 to 5 days long
- Moderate to strong cold fronts and strong pressure gradients are common, leading to pre- and post-frontal dust storms
Open the Notes window by clicking the link at the top of the page. As you go through each time period, record your findings so you can refer to them later. If you want to keep the file, you’ll need to save it to your computer. (It will not be saved as part of the module.)
Several other resources are also available via the links at the top of each page:
- Various maps of the Middle East
- A summary of the dust storm forecast process
Question & Data
Use the tabs to review the data for this time period, then answer the question below.
Which of the following are evident in the charts? (Choose all that apply.)
The correct answers are a) and b).
According to the models, the upper-level pattern supports the development of surface low pressure and a moderate to strong cold front. This can potentially lead to both:
• A post-frontal shamal over the upper half of the Saudi Peninsula
• A prefrontal event ahead of the cold front over the Saudi Plateau and the southern Persian Gulf region in the 120-hr forecast time frame
The progressive movement of the longwave pattern indicates that this will be a relatively short-lived event. (Remember to record this information in Notes.)
The first indication that a severe dust storm may be on the horizon is seen in the 300-mb and 500-mb GFS forecast charts valid for five days from now, on 26 February 2010.
Both charts show a strong middle- and upper-level trough over the Middle East and Iraq on 26 February, with a 300-mb jet streak exceeding 100 knots in the base of the trough.
If we only consider the large-scale or synoptic forcing (not mesoscale features such as the surface low and associated fronts), the DTA-GFS and NAAPS surface visibility forecasts show a pan-regional dust event impacting north Africa and the Arabian Peninsula on 26 February.
Case Study: Medium-Range Forecast (24 Feb 2010)
Question & Data
It’s three days later, 24 February 2010 at 12Z, and new forecast charts are ready for you to examine. Use the tabs to review the data, then answer the question below.
Based on the model output and your assessment from the long-range period, what is your best guess as to the onset of any dust events in the next 24 to 72 hours? (Choose the best answer.)
The correct answer is b).
The guidance indicates that a potential blowing dust event will begin around 12Z on 26 February.
Forty-eight hours prior to the anticipated onset of the dust event, the NWP models continue to forecast a well-developed, midlatitude trough over the area. In the upper troposphere, the forecasted 300-mb winds show a jet maximum exceeding 100 knots developing over northern Saudi Arabia, the northern Persian Gulf, and western Iran. Recall that the presence of a strong jet streak supports the strengthening of the surface cold front and associated winds, and will increase the potential for blowing dust.
The WRF 45-km, 48-hr, 500-mb chart is similar to the GFS 5-day, 500-mb forecast that we saw in the long-range forecast period.
- WRF 500-mb Hght. & Relative Vorticity 48-hr Fcst.
- GFS 500-mb Hght. & Relative Vorticity 114-hr Fcst.
The WRF 45-km, 700-mb height chart shows that the forecasted trough will extend down the Red Sea and Saudi Peninsula on 26 February. As the green shading indicates, high relative humidities through a deep layer are predicted for most of Syria, Jordan, and Iraq.
Looking at the precipitation forecasts, the WRF also indicates that favorable dynamics and moisture ahead of the trough may lead to convection and rainfall across that region. Any significant precipitation would suppress the lofting of dust.
The surface temperature forecast shows that early on 26 February, the cold front extends from northern Iraq across the Red Sea and Arabian Peninsula and into Egypt.
Several features are noteworthy in the COAMPS plot of forecasted surface friction velocity, streamlines, and ground wetness (see below). (Recall that friction velocity incorporates atmospheric stability and wind speed.) High friction velocities (the shaded areas) are forecast north and south of the Iraqi border, Kuwait, over the Saudi Plateau, and coastal and inland areas of the United Arab Emirates, indicating that dust mobilization is likely in these areas. The streamline forecast shows southerly flow over the southeastern portion of the Saudi peninsula and a surface low in western Iraq.
COAMPS plots of forecast surface visibility, dust surface concentration, and dust optical depth support the idea that conditions will become favorable for a widespread blowing dust event by 12Z on 26 February for these areas.
Comparing the 48-hr and earlier 72-hr forecast plots, we see that the COAMPS model continues to refine both the intensity and areal extent of the anticipated dust event.
There's an increasing probability that blowing dust will become more intense and that surface visibilities will reduce over southern Iraq, Kuwait, and adjacent regions of Saudi Arabia.
Case Study: Long-Range Forecast (21 Feb 2010)
Question & Data
It's 12Z 25 February 2010 and time to check the new data. Examine the Meteosat-7 visible, infrared, and water vapor imagery as well as the charts and surface analyses, noting the positions and progression of middle- and upper-level troughs, wind maxima, and surface features, including fronts and pressure gradients. Then answer the question below.
Based on short-range mesoscale model output, what is your revised best guess as to the onset and duration of any dust events? (Choose the best answer from each group, then click Done.)
The correct answers are b) and g).
The guidance indicates that a potential blowing dust event will begin in the 6Z to 12Z timeframe on 26 February. Given the speed at which the shortwave pattern, low pressure system, and fronts are progressing, it's likely that the event will last up to 12 or possibly 24 hours.
On 25 February (24 hours out), the COAMPS and DTA-WRF mesoscale models are predicting a widespread dust event for East Africa and the Saudi Peninsula. The blue oval over the Red Sea shows the location of a dust front forecasted by both dust models at 12Z 26 February. Notice how the DTA-WRF dust surface concentration makes it easier to see the edge of the predicted dust front over the Red Sea than the DTA-WRF visibility chart. The blue oval over the southern Persian Gulf shows the mobilization of dust from the interior Arabian Peninsula and coastal UAE. Finally, reduced visibilities between 0.5 and 5 miles at 100 m above the surface are forecast for this area and over the waters of the Southern Persian Gulf.
The COAMPS surface friction velocity and GFS 700-mb relative humidity charts indicate that Jordan, Syria, and most of Iraq will not experience low visibilities due to dust storms.
Furthermore, the 24-hr WRF precipitation forecast shows that portions of these countries and western Iran are likely to experience widespread shower activity early on 26 February.
On the other hand, both COAMPS and DTA-WRF are forecasting low visibilities north and south of the southern Iraqi border and over Kuwait as seen in the blue oval over this region. The models differ with respect to dust activity over the Saudi Plateau (the orange boxes). COAMPS is predicting widespread dust mobilization while DTA-WRF shows little, if any, dust activity (in effect, good visibility conditions). In the next section, we'll discuss why dust model forecasts like these can differ.
Dust Event Onset (26 Feburary 2010)
It’s 6Z on 26 February. We’ve been anticipating the arrival of a significant dust event within the 6Z to 12Z timeframe. The surface analysis valid at that time shows an elongated surface low stretching from the eastern Mediterranean Sea east and south into southern Iraq and the northeastern Arabian Peninsula. A strong surface cold front extends from southern Iraq southwest across the Arabian plateau. As we saw during previous forecast periods, blowing dust conditions were anticipated in advance of the front as well as behind it where surface and atmospheric conditions (winds, instability, and no significant precipitation) were favorable for dust lofting.
What tools other than ground-based observations and visible and infrared satellite imagery would help you monitor the onset and development of blowing dust? (Choose the best answer.)
The correct answer is c), explained below.
Model guidance is more suitable for information on the onset and duration of an event. Ground-based weather radar can see higher concentrations of blowing dust but has a limited range and can only observe levels near the surface at short range. Both geostationary and polar-orbiting color composite images (RGBs) are excellent tools for monitoring blowing dust under clear sky conditions. Unlike single channel imagery, they have the sensitivity of multiple channels and are typically tuned to highlight dust compared to clouds and other phenomena that may also be present.
For example, the MODIS dust product on 26 February 2010 shows the anticipated large-scale dust event impacting north Africa and the Saudi Peninsula. The white ovals show a dust front over the Red Sea and dust plumes streaming out of the UAE into the southern Persian Gulf. Within the red circle, the clouds exhibit a pinwheel pattern that qualitatively confirms the high relative humidities and cyclonic streamline forecasts. The pink areas indicate that dust is being mobilized and entrained into the low along the Iraqi/Saudi border, as was forecast by both dust models.
In the MSG dust RGB, elevated dust is more difficult to identify over Kuwait due to the presence of low- and mid-level clouds. But the image confirms dust activity over the Saudi Plateau as was forecast by COAMPS.
The following MSG dust RGB animation lets us see when various dust sources become active and monitor the evolution of dust plumes as dust is transported downwind of its source region. In the 0Z to 12Z animation, we see that the event begins in earnest over southern Iraq and the Arabian peninsula within the 6Z to 9Z timeframe (the magenta area).
Later, in the 12Z to 00Z loop, we see the dust event unfolding across the region on either side of the advancing cold front and around the circulation of the surface low as it intensifies and moves eastward from southern Iraq into the northern Persian Gulf region.
At 12Z, surface observations report dust storms ($) and suspended dust (S) over eastern Africa, the western and eastern shores of the Red Sea, southern Iraq, northern and central Saudi Arabia, and the United Arab Emirates. There are no reports of dust storms ($) and suspended dust (S) over Syria, Jordan, or northern Iraq. Reports of precipitation (the green symbols) are seen in Syria, Iraq, and western Iran.
Surface visibilities from reporting stations on this 12Z METAR chart confirm blinding conditions due to blowing dust for the following areas:
- Along the Iraqi and Saudi border with visibilities of less than 0.25 miles (white circle)
- Over Kuwait with visibilities of 0.5 to 1 miles (red circle)
- Over the Saudi plateau with visibilities of 0.5 to 1 miles (red circles)
Why Dust Model Forecasts Differ
Identifying Dust Sources
As you've seen, the process of forecasting dust storms and surface visibility depends largely on model forecasts, which can differ widely. In this section, we'll examine the main factors that account for these differences. These include the models' processes for identifying dust sources, their dust transport dynamics, and their dust removal processes.
The most critical factors in differentiating dust model forecasts are how dust sources are identified and the resolution of the data. Some models get their dust source information only from satellites…
…while others use a combination of satellite, topographic, and land surface data, station data, atlases, and soil samples.
- COAMPS gets its dust sources from 1-km dust enhancement product imagery, atlases, and maps
- DTA WRF, MM5, and GFS dust models locate dust source regions from satellite data, topography, and/or soil moisture, which can vary in precision from 15 to 55 km
- NAAPS uses a combination of satellite data and land surface information to identify dust sources on a global grid with a resolution of 1-degree latitude and longitude
The important thing is how a model determines the number and extent of dust sources in each grid box. If a model 'thinks' that many dust sources cover a significant portion of a grid box, it may predict large, broad plumes for the area. Another model may not show any dust sources for the box. If that's not correct, it may reflect a weakness in how the model determines erodible or dust-producing areas.
The models used in the 26 February case have varied dust source functions. Some forecast broad plumes, others narrow plumes. Some have too many dust sources, others too few.
For example, COAMPS forecasted several refined plumes for the interior of the UAE and its coast...
… while DTA-WRF forecasted a broad dust plume with low visibilities (1 to 0.5 miles or 1.6 to 0.8 km) from Qatar to the Strait of Hormuz. The COAMPS forecast was based on a few, limited dust sources, whereas DTA-WRF had too many.
Over the Saudi Plateau, COAMPS over-predicted surface dust concentrations, leading to a broad area with visibilities from 2 to 0.5 miles (3.2 to 0.8 km). In contrast, DTA-WRF did not forecast any reduced visibility plumes. This suggests that COAMPS had too many dust sources for this area, while DTA-WRF had too few.
Model Dynamics & Dust Removal Processes
NWP models produce different dynamical forecasts due to their sensitivity to initial conditions. This can lead to different atmospheric motions and stability, which can create variations in the strength and location of upper-level short waves, surface lows, associated fronts, and surface winds. Differences in the forecasted strength and location of surface winds account for different dust visibility forecasts among models.
Finally, a model's handling of soil moisture and precipitation impacts its treatment of dust production and removal. Source areas with significant rainfall in previous time steps will have high soil moisture values and suppress dust production in current forecasts. Since rain removes suspended dust particles, differences in forecasted precipitation patterns lead to different visibility forecasts.
You’ve reached the end of the module. Having a process for forecasting dust storms and a better understanding of why model forecasts can differ should make you feel more comfortable about forecasting for dust-prone areas.
Read through the summary, then complete the module quiz and module survey.
Visibility: Intense dust storms reduce visibility to near zero in and near source regions, with visibility improving away from the source.
Dust moves through saltation (small particles jump and skip and are lifted into the air), creep (sediment rolls and slides along the ground) and sedimentation (dust is lifted into the air and held aloft by winds).
Sources of dust: Deserts, agricultural area, coastal areas, river flood plains, ocean sediments, glacial sediments, and dry lake beds; most dust comes from discrete areas (point sources)
Dust storms requirements: An appropriate source of dust, sufficient wind and turbulence, and an unstable atmosphere
Processes that remove dust: Dispersion, advection, entrainment in precipitation, gravity
Prefrontal dust storms: A band of winds generated by and ahead of a low-pressure area that presses against, for example, a stationary high-pressure center or mountains
Post-frontal dust storms: Widespread dust following pre-frontal events; a shamal is a dust storm resulting from strong northwesterly winds on the backside of a cold front
Mesoscale phenomena that cause dust storms: Downslope winds, gap flow, and convection (haboob: a dust storm caused by convective downbursts)
Climatology provides data that help forecast the location, seasonality, frequency, and severity of dust storms
Satellite detection of dust has dramatically improved through the use of multispectral products, such as dust RGBs
Aerosol Optical Depth: Measures the light that airborne particles prevent from passing through the atmosphere; does not translate directly into surface visibility estimates but serves as a first-order indicator of how dusty the atmosphere is
Dust Forecast Process:Long range (72 to 180 hr):
- Look for large-scale, synoptically driven dust events in the 3 to 7.5-day range in global models, such as DTA-GFS and NAAPS.
- Look for model-forecast midlatitude troughs that drive pre- and post-frontal dust storms in winter and that can amplify the large-scale wind patterns associated with summer events, such as the northerly winds that create shamals. These large-scale waves are resolved by global NWP models such as GFS and NOGAPS, while the associated dust outbreaks are modeled by the global dust models, DTA-GFS and NAAPS.
Medium-range (24- to 72-hr):
- Examine the following: 300-mb height and wind forecast charts to track troughs and jet streaks; briefly examine upper-tropospheric winds to identify the presence of any jet streaks, especially for cool-season dust storms. Jet streaks within a pronounced upper-level trough are indicative of an intensifying low-pressure system with stronger surface fronts and associated winds; 500-mb height and relative forecasts to identify and track troughs and vorticity maxima; and MSLP and surface wind forecast charts for fronts and potentially strong wind conditions
- Looking at the forecast soundings from WRF or COAMPS, determine the forecast stability and wind profile at your forecast time of interest.
- Check the 6-hrly precipitation and 700-mb relative humidity forecast charts to determine where increased moisture and precipitation are anticipated since they decrease the probability of dust lofting.
- Combine COAMPS forecasts of surface friction velocity, surface winds, and soil wetness from WRF and/or COAMPS with your knowledge of dust source areas to see if the criteria for a potential blowing dust event are met. Recall that friction velocity incorporates atmospheric stability and wind speed into one variable.
- Examine DTA-WRF and COAMPS forecasts of surface visibility due to dust. Compare them to WRF and COAMPS forecasts of winds through the mixed layer and dust optical depth to help assess changes in geographical extent and intensity with each successive model run.
- From the model output and your initial analysis, develop a best-guess forecast as to the onset and duration of any dust events in your area of responsibility in the 24- to 72-hr window.
Short-range (0- to 24-hr):
- Analyze the present state of the atmosphere by looking at satellite imagery, upper-air charts, and surface analyses, keeping in mind the location and characteristics of relevant dust source regions.
- Examine the latest observed and/or forecast soundings from WRF and COAMPS. Note the strength of any inversions (usually during summertime) and determine if they will break due to turbulent mixing and daytime heating that would ripen the environment for a dust outbreak.
- To determine the potential duration and type of dust event, pay special attention to dust lofting in your area of responsibility, local rules-of-thumb about advection, and geographic features such as the location of dust source regions, terrain, vegetation, and water sources. Also note where precipitation has fallen in the past 48 hours and whether it was convective or stratiform.
- Use satellite dust enhancement products (such as enhanced infrared imagery) and RGB and other multispectral imagery tuned for dust detection. Integrating these products with surface observations can provide information about the current extent and location of existing dust plumes and fronts.
- Make a best-guess forecast as to the onset, duration, and persistence of any dust events in your area of responsibility in the very short term, using short-range mesoscale model output from DTA-WRF and/or COAMPS as guidance. The global DTA-GFS and NAAPS models can resolve large-scale features that drive smaller-scale dust events in the short term but cannot resolve localized dust features.