SECTION 7: Dust Storms Caused by Mesoscale Systems
Hi Jochen. Tom reviewed this file and has some comments for you in green. Please address them!
Marianne: WHEN MSDB CAN ACCEPT ACCENTS, GO THRU FILES, ADDING ACCENTS FOR BODELE.
Marianne: Still need the following? 2009_07_21_1800_m9_rgb_dust_swind_2.png. And a higher-quality image for
2005_06_07_1300_m8_rgb_dust.jpg? If possible, leave off the time/date stamp (I’ll put it on).
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 induce dust storms include downslope winds, gap flow, convection, and inversion
downburst storms. As you would 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.
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mesoscale_composite.jpg 29320
change gap flow if needed [add martin?]
Page 2: North Africa
msg_dustrgb_12utc_23feb2006_q1.jpg 29281
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 (blue) and the dry post-frontal air (pink) further to the west. That’s due to the
influence of water vapour on the product. Dry air is has a stronger red component than moist air in the dust RGB.
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NEXT PART
msg_dustrgb_12utc_23feb2006_q2.jpg 29284
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 in the lee of the Atlas Mountains.
NEXT PART
To identify a dust storm generated by downslope winds, look for the following:
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A mountain range at least 2 km high
Strong synoptic-scale winds moving perpendicular to a range of mountains
A dust cloud with a “streaky” structure
As you can see, these conditions are met in our case. Strong synoptic 700-hPa winds are perpendicular to the Atlas
Mountains, which are 4,167 metres high.
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msg_dustrgb_12utc_23feb2006_ecmwf700windheights.jpg 29285 verify the streaky structure applies!!!
Page 3: Argentina
msg_dustrgb_16utc_21jul2009.jpg 29286
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
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Northern Argentina and Southern Bolivia. Repeated satellite observations from the Total Ozone Mapping
Spectrometer (TOMS) over this area have identified this source region 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 over Argentina.
(Tom’s note: The dust does follow northwesterly surface winds (actually mountains winds) over Bolivia.)
msg_dustrgb_18utc_21jul2009_ecmwf_sfcwind jpg
29288
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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/ southeasterly**/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 has 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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msg_dustrgb_18utc_21jul2009.jpg
29287
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.
tokar_seawifs.jpg
29572
The Tokar Gap is a low-elevation break in the mountains that flank the west side of the Red Sea.
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tokar_topo.gif 29648
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. [Tom’s note: Isn’t it northwesterly winds over the Red Sea that
turns the dust southeastward?]
tokar_seawifs.jpg
29647
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
modis_tc_1210utc_5jan2005.jpg 29290 Martin sent CR info. The image is on the EU site. Marianne: Send CR
letter if needed. Change credit if needed Martin Setvak setvak@chmi.cz
[for Marianne only: Who created and owns it, NASA or Martin? Did he create it while working at NOAA or
where he works, using NOAA imagery? Did he change anything in the original imagery? If Martin worked on
the imge and "owns" it, not NOAA, I need to contact him and get copyright permission to use it in the module.
The image is: 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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aqua_image_bodele_dep_2jan2007.jpg 29291
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
not as narrow as some, the gap creates a natural wind tunnel that focuses and intensifies winds across the region,
forming a low-level jet. 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. [Jochen: Can you mark the Jos
Plat and Ad. Mtns on the map? I’m not sure where they are!]
NEXT PART
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This MSG dust RGB animation shows the evolution of a dust storm from 0800 UTC to 1600 UTC 5 January 2005. As
you watch 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.
msg_dustrgb_5jan2005_8to16utc.swf 29294 [speced for cw]
This graphic shows the dust source regions (the circles). The most prominent one is the dark spot in the middle of the
Gap, which is probably a dry lake bed. (Jochen: Please annotate on the image!!!) Other dust hot spots are hidden
under the dust cloud (between Borkou Gap and Lake Chad).
msg_dustrgb_5jan2005_dustsources.jpg 29297
NEXT PART
msg_dustrgb_5jan2005_8to16utc_natcolour.swf 29299 speced for cw
jk or pd: are both of these images from MSG or is the one on the right a MODIS natural color image? [mw:
update msdb] [mw: right NC, left dust enhancement] Do you have the webpage that it came from? (I need to
verify the source re: copyrights.) 2005_01_05_0800Synoptic Section
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1600_m8_rgb_dust_natcol_loop.avi.
This animation shows the dust and natural colour RGBs for the same period (8 to 16 UTC). In the natural colour RGB,
the dust clouds appear in a lighter colour, tending towards white. As you watch the animation, can you tell which dust
clouds are the thickest (optically speaking)?
To identify thick dust clouds in the natural colour RGB, look for areas in which no surface features shine through. In
our case, the thickest dust cloud is the one in the Bodélé Depression. It’s harder to see the dust cloud west of the
Tibesti Mountains because it’s much thinner optically.
Tom added this new question: Based on this comparison, why is the dust enhancement generally better for tracking
dust plumes? Select the option(s) that apply, then click Done.



It shows thin dust better**
It show dust events that the natural color image does not detect **
Dust tends to blend in with the background too much on natural colour images **
Feedback: The dust extending to Lake Chad is evident in the dust enhancement but not the natural colour loop. The
same goes for the dust event west of the Tebesti Mountains. The problem with natural colour imagery is that dust
resembles the surface from which it was scoured, creating ambiguity.
Subsection 3: The Diurnal Cycle
Page 1: The Diurnal Cycle of Winds and Dust Release
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msg_dustrgb_13nov_to18nov2009.swf speced for cw 29302
This MSG dust RGB animation shows the Bodélé area from 00 UTC 13 November to 23 UTC 18 November 2009,
when several dust clouds (dust “pulses”) were released and drifted westward over the Sahara/Sahel. As you watch
the animation, notice the diurnal cycle of winds and dust release. At what time(s) of day is new dust released? Select
the correct options, then click Done. (Note that in this part of the world, GMT or Z time is roughly the same as local
time. Therefore, 12 Z is near noon, and 0 Z is near midnight.)
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During the day ***
During the afternoons
During nighttime
Feedback: The correct answer is A. There’s a pronounced diurnal cycle of surface wind speeds and dust release in
the Bodélé area, with both at their minimum at night and maximum in late morning/noon. Surface winds are weak at
night because they decouple from the Bodélé low-level jet, which flows above a near-surface temperature inversion.
During the day, intense surface heating induces turbulence in the lowest layers and the jet mixes down to the surface,
turning up dust.
NEXT PART
Did you notice that the high-level cirrus clouds moved in the opposite direction from the dust, from west to east? This
is explained by strong wind shear above the surface layer in which the dust is trapped.
Finally, did you notice how far the Bodélé dust was transported? It moved over Niger, Nigeria, and Mali, which is not
unusual for dust coming from the Bodélé Depression. In fact, large outbreaks of dust from that area can cross the
Atlantic Ocean, reaching Central and South America.
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. TOM says that he doesn’t
understand the last sentence – what “other areas?”
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haboob_ransom_canyon_texas_2009.jpg 29646
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.
haboob_swf.jpg and haboob.swf
29649
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.
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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.
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. TOM asks:
Why the difference?
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 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.
msg_dustrgb_13utc_7jun2005.jpg 29303 Annotate these locations.
Sudan
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msg_dustrgb_29apr_to_30apr07.swf 29304 speced for SD
This MSG dust RGB animation shows a large nighttime Haboob over Sudan on 29 to 30 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 (very bright magenta).
Tom’s comment: I don’t have the animation, but I’m wondering how they’ll know when it is day, and when it is night.
The gust front in our animation 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.
Did you notice how the arc-shaped gust front traveled significantly faster when it encountered a strong temperature
inversion near the ground? (Annotate the time that this occurred.)
West Africa
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msg_dustrgb_9jun_to_10jun2010.swf [speced for SD 29305
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 scene. 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.
Morocco/Algeria
msg_dustrgb_12to17utc_25may2006.swf [speced for SD, 29306]
Easterly flow in the Sahel moves haboobs westward and northward. In contrast, Haboobs over northern Africa, like
the one shown in this case, are steered by westerly winds southward and eastward into the desert.
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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 blends 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
msg_dustrgb_09to2345utc_5jun2005.swf [speced for SD 29307]
This MSG animation shows the path of a Haboob that originated 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 frontal boundary. At this point, the
original storms have dissipated and drifted off to the northeast. Only the Haboob is left within the slowly decaying
outflow.
Page 4: Forecasting Haboobs
msg_dustrgb_1915utc_28jul08_0830utc_28jul08.swf speced for SD 29308
ADD IMAGE
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 Most Unstable Lifted Index.
2. Look for high relative humidity between 700 hPa and 500 hPa and/or high values of reflectivity from a nearby
weather radar. Also look for steep lapse rates between the surface and approximately 5 km.
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3. Find the strongest wind at any level aloft where the wet bulb potential temperature is less than the (surface
potential temperature + 4°C) [what’s missing here???]. It’s possible that this wind [should this say wind
velocity?] 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)
[TOM says he doesn’t understand this?], the warmer the resultant outflow at the surface due to adiabatic
compression, and the weaker the potential Haboob.
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 (can delete if needed)
Page 1: Inversion Downburst Storms
Inversion downburst windstorms 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.swf and inversion_downburst_swf.jpg 29650 [should we show this? If not, x msdb]
Click to view animation.
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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.
modis_tc_4may09_dust_seabreeze.jpg 29314
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 common along the Iranian and Pakistani coasts where strong sea
breezes are favored. 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 at http://www.meted.ucar.edu/mesoprim/seabreez/index.htm.
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