SECTION 7: Dust Storms Caused by Mesoscale Systems
Section Menu
Subsection 1: Downslope Winds
Page 1: Overview
Page 2: North Africa
Page 3: Argentina
Subsection 2: Gap Flow
Page 1: Tokar Gap
Page 2: Bodélé Depression
Subsection 3: Diurnal Cycle
Page 1: The Diurnal Cycle of Winds and Dust Release
Subsection 4: Haboobs
Page 1: Description
Page 2: Properties of a Haboob
Page 3: Haboobs in Different Regions
Page 4: Forecasting Haboobs
Subsection 5: Inversion Downburst Storms
Page 1: Inversion Downburst Storms
Subsection 1: Downslope Winds
Page 1: Overview
This section examines dust storms generated and influenced by mesoscale forcing, focusing on examples from
northern Africa, the Middle East, and South America.
The mesoscale phenomena that excite dust storms include downslope winds, gap flow, convection, and inversion
downburst storms. As you’d expect, these are more difficult to forecast than synoptically forced dust storms related to
frontal winds and other large-scale phenomena. We'll begin by examining downslope winds.
Will make a montage of the different types. For upper left image, only show area of Atlas mountains with dust
generated by downslope winds
Page 2: North Africa
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(2006_02_23_1200_m8_rgb_dust.png MSG (M-8) Dust RGB 23 February 2006 1200 UTC EUMETSAT
label first duststorm and countries
This MSG dust RGB shows two separate dust outbreaks over Algeria, Tunisia, and Libya on 23 February 2006.
What’s the large outbreak over Tunisia and Libya?
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A pre-frontal dust storm
A post-frontal dust storm **
A dust storm from gap winds
Feedback: This is a post-frontal dust storm related to a cyclone over Tunisia. Note the strong colour difference
between the moist pre-frontal air and the dry post-frontal air further to the west. That’s due to the influence of water
vapour on the product. (In cloud-free areas [do we need to say that?], dry air is redder than moist air.)
[IS THAT OK? ORIG FROM JOCHEN: The water vapour content has a strong impact on the Dust RGB
product for cloud free areas, i.e. it modulates/changes the red component of the RGB product from little red
(moist air) to more red (dry air).]
NEXT PART [mark 2nd storm on the image]
The second, smaller dust outbreak is over Algeria, south of the Atlas Mountains. These dust plumes are caused by
downslope winds on the leeward side of the mountains. The plumes appear as dust streaks oriented northwestsoutheast, with their source regions at the northern tips.
NEXT PART
To identify a dust storm generated by downslope winds, look for the following:
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A mountain range that’s sufficiently high (at least how high?)
Strong synoptic-scale winds moving perpendicular to the range
A dust cloud with a “streaky” structure
As you can see, these conditions are met in our case. IJK:, I can retrieve winds from ECMWF model for this case and
overlay the wind info on the dust RGB image – yes, please do that!
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Page 3: Argentina
2009_07_21_1600_m9_rgb_dust.png MSG (M9) Dust RGB 21 July 2009 1600 UTC EUMETSAT
Here’s another example of a dust outbreak caused by strong downslope winds. In Argentina, the downslope wind
from the Andes Mountains is called “viento zonda” (“Zonda” in English). The Zonda is a dry wind that develops when
polar maritime air descends from the Andes Mountains to the plains of the Argentinean Pampa. The Zonda blows
from May to November and often carries dust.
This daytime dust RGB is from 21 July 2009 when large amounts of dust were carried across Bolivia and Argentina.
The polar front is visible over Bolivia, Paraguay, and Southern Brazil, while the centre of the cyclone is located in the
Uruguay-Argentina border region. The source region for the dust is the Salar de Uyuni, a dry lake bed in the Andes in
Northern Argentina and Southern Bolivia. TOMS identifies this area as a “dust hotspot.”
Looking at the source region and the structure of the dust cloud, how high do you think the dust cloud is?
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0 to 1 km (low-level)
2 to 3 km (mid-level)
4 to 5 km (high-level) **
Feedback: The correct answer C. We can tell that it’s mainly a high-level dust cloud (5 km) because the dust
originated in the Andes Mountains and is oriented northwest to southeast. Overlaying ECMWF surface wind barbs on
the dust RGB confirms that the high-level dust is not following the post-frontal southerly surface winds.
2009_07_21_1800_m9_rgb_dust_swind_2.png
NEXT PART
What do you expect to happen to the high-level dust cloud over the next two hours? How will it grow and move? For
each statement, select the option that completes it. When you are finished, click Done.
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The size of the dust cloud should [increase**/decrease/stay the same].
The dust cloud should move in a [northward/southward/eastward**/westward] direction.
The dust cloud should move [very quickly **/quickly/slowly/very slowly].
Feedback: Strong post-frontal winds have blown the dust very quickly in a southeasterly direction. It’s reached far into
Argentina and covers southern Bolivia nearly up to the Paraguay border. In addition, new dust is being lifted over the
Andes Mountains, increasing the size of the dust cloud.
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2009_07_21_1800_m9_rgb_dust.png MSG (Meteosat-9) Dust RGB 21 July 2009 1800 UTC EUMETSAT
Subsection 2: Gap Flow
Page 1: Tokar Gap
This SeaWiFS true color image shows a dust storm 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.
Note that the Tokar Gap is a low-elevation break in the mountains that flank the west side of the Red Sea.
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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 helps trap the dust in the center of the sea. The Zone is formed by air flowing in
from the north and south, creating an area of convergence that ensnares 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 10 kilometers wide, high-resolution mesoscale models should be able to capture the
flow. For more information on gap winds, see COMET's Gap Winds module at
http://www.meted.ucar.edu/mesoprim/gapwinds/index.htm.
Page 2: Bodélé Depression
2005_01_05_1210_amo_rgb_truecol.jpg Aqua MODIS True Colour RGB product on 5 January 2005 at 12:10
UTC. NASA, image created by Martin Setvak
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For geographical reference use this image (2007_01_02_amo_rgb_truecol.jpg) NASA Earth Observatory:
http://earthobservatory.nasa.gov/IOTD/view.php?id=7279
The Bodélé Depression of northern Chad contains a series of dry lake beds that form the world’s largest single source
of wind-blown dust. The large dust events that frequently arise in this area are primarily caused by a high-pressure
system over North Africa that drives the northeasterly Harmattan winds over the Sahara.
The gap between the Tibesti Mountains (3415 m) and the Ennedi Mountains (1450 m) plays a role as well. Although
it’s not as narrow as some, the gap creates a natural wind tunnel that focuses and intensifies winds across the Bodélé
Depression. That’s evident in this MODIS image from 5 January 2005, which shows bright streaks of dust arcs across
the Bodélé Depression toward Lake Chad. Notice how the dust veils the lower elevations, with the higher elevations of
the Jos Plateau and Adamaoua Mountains peeking out as if through fog.
NEXT PART
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2005_01_05_0800-1600_m8_rgb_dust_loop.avi 8-hour sequence of MSG (Meteosat-8) Dust RGB products
5 January 2005, 08:00 to 16:00 UTC EUMETSAT
This MSG dust RGB animation shows how the evolution of a dust storm from 0800 UTC to 1600 UTC 5 January
2005. As you play the animation, notice how the dust acts as a passive tracer for the channeled flow in the Borkou
Gap between the Tibesti and Ennedi Mountains.
How many dust storm regions are evident in the animation?
WRITE CHOICES, INDICATING WHICH IS CORRECT. THEN WRITE FEEDBACK!
NEXT PART
Show the rgb ani again
You can deduce which part of a dust cloud is thickest by looking for areas where no surface features shine through.
You can confirm your assessment with the natural colour RGB. [Jochen: should we say anything about what we see
in the natural colour ani?]
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Show 2005_01_05_0800-1600_rgb_03-02-01_loop.avi [why does the 2nd half go backwards?]
Subsection 3: The Diurnal Cycle
Page 1: The Diurnal Cycle of Winds and Dust Release
2009_11_13_1200_m9_rgb_dust.jpg MSG (M-9) Dust RGB 13 November 2009 1200 UTC EUMETSAT
What happens to winds at night? Does new dust continue being released? During very strong Harmattan events, dust
often continues blowing at night. But normally, there’s a pronounced diurnal cycle to surface wind speeds and dust
release, with wind minima at night and wind maxima in late morning/noon. The latter is due to the mixing of
momentum from the low-level jet down to the surface by radiative heating [clarify that sentence].
This MSG dust RGB animation shows the Bodélé area from 00 UTC 13 November to 2300 UTC 18 November 2009.
What happens to the dust? Is it released during the afternoons and at night?
Yes / No **
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Feedback: The correct answer is B. You could almost figure this out from the relatively small size of the dust cloud
and the lack of dust streaks in the Borkou Gap. But the animation clearly shows how the diurnal cycle of the surface
winds modulates dust release over the Bodélé Depression. Northeasterly winds are strong enough to pick up dust
from 07 UTC to 13 UTC but weaken in the afternoon and night hours, preventing the release of new dust.
Did you notice the high-level cirrus clouds moving in the opposite direction from the dust, from west to east. By
combining this with information from HRV images (not shown), we can tell that there’s relatively strong wind shear at
the top of the boundary layer, which keeps the dust trapped in that layer.
Did you also notice how far the Bodélé dust was transported? It moved over Niger, Nigeria, and Mali, reaching the
west coast of Africa. [say something about that distance if it’s unusual]
2009_11_130000-182300_m9_rgb_dust_loop.mpg 6-day sequence of MSG (Meteosat-9) Dust RGB 13 to 18
November 2009 EUMETSAT
Subsection 4: Haboobs
Page 1: Description
This dust squall is a haboob, that 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 are between 10 and 50
micrometers, but larger particles (up to several millimeters in size) can be blown about as well. The larger particles
settle rapidly after the wind subsides, whereas the finer ones settle at about 305 metres per hour when the haboob
finally dissipates. Other areas clear rapidly as the dust is advected out of the area.
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Page 2: 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 100 to 150 kilometers, except in the Sahel area where they can extend up to 600 kilometers horizontally.
Click to view animation.
Haboobs tend to be 1500 to 2500 meters high at the peak of the event. However, they can reach 4500 metres when
exacerbated by convergent outflow boundaries. The average haboob tends to be short-lived, lasting about three
hours. Visibility usually begins to improve soon after the gust front passes.
In the Sahel, though, haboobs created by squall lines tend to be much larger (up to 1000 km) and longer-lived (lasting
up to 24 hours). They can keep visibility poor for many hours after the gust front passes.
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Although haboobs can be seen approaching an area 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.
Page 3: Haboobs in Different Regions
Create 5 tabs (Mali, Sudan, West Africa, Morocco/Algeria, Middle East). When each is clicked, display its section
below.
Mali
This MSG dust RGB shows a large daytime haboob over Mali that’s associated with a squall line over Burkina Faso
(the dark red cloud system to the south). The gust front propagates westward, raising a lot of dust and triggering new
convective storms. That same gust front raises much less dust to the south, where there’s more vegetation. Note that
the dissipating dust cloud further to the west is from a previous convective event.
2005_06_07_1300_m8_rgb_dust.jpg) better quality needed? MSG (Meteosat-8) Dust RGB 7 June 2005 1300
UTC EUMETSAT. Image created by Daniel Rosenfeld.
Sudan
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2007_04_29_2200_m9_rgb_dust.png. MSG (Meteosat-9) Dust RGB product from 29 April 2007 at 22:00 UTC
EUMETSAT. Images created by HansPeter Roesli We could also show the loop?
This MSG dust RGB shows a large nighttime haboob over Sudan on 29 April 2007. This is rather early for the
convective season, which normally starts in May.
As you can see, dust clouds have a different colour at night (dark magenta) than during the day (what colour?). The
gust front was triggered by a thunderstorm system over eastern Sudan and moved very quickly, 50 km/hr during the
night.
A second gust front, coming from a storm further south, intersected the haboob gust front without triggering new
convective storms or producing any dust. [CHECK AGAINST THE ORIGINAL: A second non-dust producing gust
front, coming from a storm further to the south, intersected the haboob gust front in the middle of the image without
triggering new convective storms.]
Nighttime haboobs that encounter strong stratification near the ground can transform from a relatively slow-moving
gravity current into a fast-moving, undular bore [explain that better!!]. That’s probably what happened in this case
given the high speed of the arc-shaped gust front and the "ripples" of clouds behind the leading edge.
West Africa
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2010_06_090000-101000_m9_rgb_dust_loop.mpg) High resolution AVI loop is available. 34-hour sequence
of MSG (Meteosat-9) Dust RGB products on 9-10 June 2010. EUMETSAT Created by HansPeter Roesli.
This MSG dust RGB animation shows a large dust squall over Niger, Mali, and southern Algeria, which was triggered
by a thunderstorm system in the lower part of the images. The strong haboob travelled hundreds of kilometres
westwards over the Sahara, showing how far such systems can propagate and how well defined they can be at night.
On 9 June, we see daytime convection lifting part of the low-lying dust higher up (above the boundary layer) where
westerly winds carry it back in an easterly direction. The higher-level dust is readily apparent in bright magenta in the
late afternoon and nighttime hours as compared to the dark magenta of the low-level dust squall. Towards the end of
the animation, the westward propagation of the dust squall slows down as it approaches a deformation zone. [ask
anything about the case??]
Morocco/Algeria
2006_05_25_1200-1700_m8_rgb_hrv_dust_loop.avi 5-hour sequence of MSG (Meteosat-8) blended HRV
and Dust RGB products on 25 May 2006 from 12:00 to 17:00 UTC EUMETSAT. Images created by
HansPeter Roesli
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Haboobs in the Sahel area that are embedded in an easterly flow generally move westward and northward. In
contrast, haboobs over northern Africa, like the one shown in this case, are steered by westerly winds and tend to
propagate southward into the desert and eastward.
This animation shows a typical afternoon haboob in the border area of Morocco and Algeria. The convective storm
that triggered the outflow gust front developed over the Atlas Mountains and moved eastward into the Algerian desert.
The intensity of the storm is not only manifested by the gust front, which indicates strong downdrafts, but also by
gravity waves, which indicate strong updrafts seen on the top of the thunderstorm anvil.
Note that the animation is an HRV / dust RGB "sandwich product." It combines the high-resolution HRV channel and
the lower-resolution dust RGB, letting us spatially co-locate cloud features, such as the storm's overshooting top and
outflow boundaries, with dust clouds. This blended product is probably the best geostationary product to use for
monitoring haboobs during daytime.
Middle East
2005_06_05_0900-2345_m8_rgb_dust_loop.mpg) High resolution AVI loop is available! 15-hour sequence of
MSG (Meteosat-8) Dust RGB products on 5 June 2005, 09:00 – 23:45 UTC. EUMETSAT
This MSG animation shows the start and ensuing trajectory of a haboob originating from a complex of thunderstorms
over Lebanon and Syria. Over the course of the day, the dust front crosses Iraq and moves into northern Saudi
Arabia, a distance of roughly 800 kilometres. During the night, the haboob crosses Iraq with a sharp demarcation at
the frontal boundary. At this point, the parent storms have long dissipated and their remnants drifted to the northeast,
leaving behind only the haboob within the gradually decaying outflow.
See paper from Miller (2008)
[Make sure the text was not copied exactly from that paper. We need to edit it to differentiate it!]
Page 4: Forecasting Haboobs
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Click to view animation.
Haboobs are much more difficult to forecast than synoptically forced dust storms and rely largely on nowcasting, i.e.,
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). [are they the
same things with different names?]
2. Look for high relative humidity between 700 hPa and 500 hPa and/or high values of reflectivity from a nearby
weather radar if one’s available. Also look for steep lapse rates between the surface and approximately 5 km.
3. Find the strongest wind at any level aloft where the wet bulb potential temperature is less than the (surface
potential temperature + 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 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 mitigates the warming issue to a limited extent.
3. Check for rapidly warming cloud tops in looped geostationary infrared imagery since they’re indicative of
thunderstorm collapse.
4. Determine if the thunderstorm is occurring over a dust source region.
Subsection 5: Inversion Downburst Storms (we can delete this if needed)
Page 1: 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.
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Click to view animation.
Inversion downburst storms form in coastal terrain where slopes are at least 4 metres per kilometre, such as those
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 kilometre 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.
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