5.0.0 Dust Storms Caused by Mesoscale Systems
5.1.0 Downslope Winds
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.
5.2.0 Gap flows
5.2.1 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.
5.2.2 COAMPS Simulation
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.
5.2.3 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.]
5.3.0 Haboob
5.3.1 Description
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.
5.3.2 Properties of a Haboob
Click to view animation.
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.
5.3.3 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.
Click to view animation.
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.
5.3.4 Forecasting Haboobs
Click to view animation.
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
1. Look for signs of instability aloft. Use the Best Lifted Index (the Most Unstable Lifted Index).
2. 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).
3. 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.
4. Determine if your forecast area is located in or near a dust source region.
Forecasting haboobs from collapsing thunderstorms
1. At what time of day is the thunderstorm occurring? Thunderstorm collapse is most likely after sunset.
2. 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.
3. Check for rapidly warming cloud tops in looped geostationary infrared imagery, which are indicative of
thunderstorm collapse.
4. Determine if the thunderstorm is occurring over a dust source region.
5.4.0 Inversion Downburst Storms
Click to view animation.
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.]
5.5.0 Dust Devils
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.
Click to view animation.
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.