International Dust module
Chapter 6 – Synoptically forced dust storms (outbreaks)
All graphics done or speced for cw [swfs]
Marianne: Metric [+ American?] JK says both, but WA is ok with metric
Section Outline
Subsection 1: Introduction
Page 1: Introduction
Subsection 2: Prefrontal Dust Events
Page 1: Introduction
Page 2: Prefrontal Example: North Africa
Page 3: Prefrontal Exercise
Page 4: Cloud Microphysics
Subsection 3: Postfrontal Dust Events
Page 1: Postfrontal Dust Storms
Page 2: Surface Features
Page 3: Middle and Upper Levels
Page 4: Dust Squall over Riyadh
Page 5: Postfrontal Dust over Saudi Arabia and Kuwait
Page 6: Dust Crossing the Gulf of Aden
Page 7: Determining Dust Cloud Height
Page 8: Example of Frontal Rain and Dust
Page 9: Australian Example
Page 10: American Southwest Example
Page 11: East Asia Example
Subsection 4: Dust Events Caused by Large-Scale Trade Winds
Page 1: Global Trade Wind Patterns
Page 2: Synoptic Pattern: Northern Africa
Page 3: Synoptic Pattern: Middle East
Page 4: Summer Shamal Exercise
Page 5: Winter Dust Storms in Northern Africa
Page 6: Major Transport of Dust from West Africa to the Lesser Antilles
Page 7: Dust Transport Animation
Subsection 1: Introduction
Page 1: Introduction
Dust storms are typically classified by the broad meteorological conditions that create them. In this section, we’ll
examine large-scale dust outbreaks that occur in the arid and semi-arid areas of the world. These are caused by
frontal winds that primarily occur in winter and spring, and non-frontal persistent trade winds that occur in summer.
Synoptically forced dust outbreaks are rare in autumn when the trade winds are weaker and cold air outbreaks from
higher latitudes do not reach deserts of northern Africa and the Middle East.
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major_deserts.jpg
29578 where get image from?] [are desert names ok?]
Since each region has its own weather patterns that lead to dust storms, forecasters should be familiar with the local
atmospheric conditions that lead to strong winds under dry conditions in their areas.
Subsection 2: Prefrontal Dust Events
Page 1: Introduction
Prefrontal dust storms occur when a band of winds generated by and ahead of low-pressure areas moves across a
dust source region such as the Sahara.
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. Towards the west, southwesterly or Suhaili winds pick up
dust from western Arabia and move it northeast in advance of the cold front.
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prefrontal_map.jpg
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This MODIS true color 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.
sharqiPlumes.jpg
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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
ahead of the upper-level trough. In addition, the overlapping of these jet cores and the coupling of secondary
circulations between the jets enhance mid-level upward vertical velocities and increase the lifting of blowing dust.
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prefrontal.gif 29573
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.
Page 2: Prefrontal Example: North Africa
Here’s an example of a prefrontal dust outbreak that occurred on 22 March 2008 over Algeria, Tunisia, and Libya. The
synoptic chart for 0600 UTC shows a deep, long-wave trough over Central Europe with the related 300-hPa polar
frontal jet (PFJ) crossing the northern parts of the Mediterranean Sea from the Gulf of Biscay to Croatia. The
subtropical jet (STJ) is close to the PFJ over Algeria and Tunisia and ahead of a short-wave trough over Algeria.
prefrontal_noafrica_108_geohight_isotachs.jpg
29580 . GET ECMWF CR!!!!
Next Part
[left image] Starting at around 1000 UTC, prefrontal southwesterly winds ahead of the upper-level short-wave trough
over Algeria strengthen, lifting large amounts of dust into the air around Illizi in eastern Algeria. (The dust clouds
appear in bright magenta in these MSG dust RGBs.) The prefrontal winds responsible for the blowing dust are
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referred to as Scirocco in Tunisia, Ghibli in Libya, and Khamsin in Egypt. They can have speeds of up to 100 km/h
and are most common during autumn and spring.
msg_dust_rgb_3panel_22march08.jpg
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[Assuming that we’ve explained how to interpret dust rgbs in a previous section, we might want to have an
‘Interpreting Dust RGBs’ link that pops info]
As the images in the middle and on the right show, prefrontal winds continue strengthening during the following hours
and dust particles from different source regions in Algeria and Libya are lifted into the air, enlarging the dust cloud.
The dust is transported upwards in a type of conveyor belt to higher levels in the troposphere, probably to the level of
the subtropical jet.
Next Part
This image from the next day shows that as the short-wave trough moves eastward over Libya and the cold front
intensifies, the dust cloud gets stretched along the cold front, turning this into a frontal dust event. The cold front is
marked by the blue lines, which represent the thermal front parameter (TFP). At this stage, large amounts of dust get
entrained into the southern part of the cloud system that stretches from Crete to the Black Sea.
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msg_23mar08_dustrgb_tfp.jpg
29553 need permission from ecmwf
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This natural color product shows that six hours later, at 1200 UTC, the cold front and embedded dust cloud have
reached Crete. Since the dust cloud is optically thin, we can just make out some thin (brownish) clouds south of Crete
but it’s hard to tell if they are cirrus or dust.
msg_natcol_12utc_23mar08.jpg
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The dust cloud is clearly visible in magenta in the dust RGB. Notice the large dust cloud over the Sahara, which was
not visible in the natural color RGB.
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msg_dustrgb_12utc_23mar08.jpg 29556 LET USERS TOGGLE B/T THEM AT THE END
Other things of note:


During the dust storm, a research flight to measure aerosols and cloud microphysics was carried out around
Istanbul, Turkey. The findings confirmed that the dust was most abundant just below cloud base (3660
m/12,000 ft) and that the dust particles acted as very efficient ice nuclei. The cloud only contained small
amounts of supercooled water—just above cloud base—but lots of ice needle hydrometeors.
The Bulgarian and Turkish Meteorological Services used satellite information to correctly forecast "coloured
rain" for both days of the dust storm. Red rain was reported from Varna on 23 March and other places in
Bulgaria received coloured rain or snow. The coloured rain continued the next day but the aerosols were
gradually washed out as the precipitation strengthened.
Page 3: Prefrontal Exercise
This MSG Dust RGB from 16 March 2007 is overlaid with 925-hPa winds and shows both pre and postfrontal dust.
Use the white drawing tool to outline the areas with prefrontal dust. (If you want to identify the postfrontal areas
as well, outline them with the red tool.) When you are finished, click Done.
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msg_rgb_925wind_12utc_16mar07.jpg 29557 NEED ECMWF CR APPROVAL
Feedback: The advection indicated by the prefrontal dust streamers in darkish magenta are well aligned with the 925hPa wind field in the southern parts of Oman. Behind the cold front, in the upper left part of the image, strong Shamal
(northwesterly) winds are driving dust clouds (in brighter magenta) southwards toward Yemen and Oman. The preand postfrontal dust clouds are thin enough to offer a faint view of underlying land surface structures.
[Incorporate any of this information? Another case of prefrontal and postfrontal dust can be seen in this
MSG dust RGB product from 16 March 2007. The 925-hPa wind field indicates the strong pre- and postfrontal
winds and marks the position of the cold front that crosses the Arabian Peninsula from 15 to 18 March in
conjunction with a slow-moving trough from the Mediterranean Sea. Low-level pre-frontal winds are pushing
moist, maritime air northwards from the Arabian Sea across southern Oman and eastern Yemen towards the
approaching cold front.
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msg_rgb_925wind_12utc_16mar07_fb.jpg
29558 NEED ECMWF CR APPROVAL
Later during the day, the pre- and postfrontal dust clouds actually converge over southern Saudi Arabia and the
United Arab Emirates.
msg_rgb_925wind_6mar07_ani.swf [make from 2007_03_16_0300-2400_m8_rgb_dust.mpg [0300 to 2400 UTC] –
spec for brannan or steve
Page 4: Cloud Microphysics [put info into 3 tabs]
TAB 1: Cloud Phase and Cloud Particle Size
We’ve seen how satellites can detect and trace dust clouds in cloud-free situations. They can also detect the
presence of dust particles inside ice clouds, albeit indirectly. Since this mainly occurs with pre-frontal dust storms [is
that the right way to say it?], we’ll explore it in this section. Note that the information is optional and will not be
covered in the module quiz. [is that ok to say? Patrick: Do you think we should even include it?]
Dust aerosols have a large impact on cloud microphysics such as cloud phase and cloud particle size. In general,
clouds that have ingested dust glaciate more quickly, forming a large number of very small ice particles. These have a
higher reflectance than large ice particles in the “microphysical” channels (NIR1.6 and IR3.9) and appear in bright
orange in this day microphysics RGB. The dust cloud isn’t so visible over the highly reflective desert background,
where the dust RGB product does a much better job of depicting it.
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msg_daymicro_12utc_23mar2008.jpg
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The high-level cirrus shield over the Mediterranean and Black Seas is bright orange due to the high reflectance of the
cold ice cloud in the IR 3.9r µm channel. This indicates that the cloud has ingested a lot of dust particles, resulting in
the formation of small ice particles.
TAB 2: Macro-Physical Structure of Clouds
Dust aerosols impact the macro-physical structure of clouds, such as their texture. We know that aerosols strongly
affect the structure of stratocumulus cloud fields over the ocean through the formation of open and closed Bénard
cells. (For information on Bénard cells, access the following link:
http://oiswww.eumetsat.org/WEBOPS/iotm/iotm/20010823_benard_cells/20010823_benard_cells.html.) Likewise,
when a synoptic-scale conveyor belt transports dust particles to high levels in the troposphere, the cirrus shield often
assumes a fine-scale cellular/granular structure similar to that observed in closed Bénard cells over the oceans. This
can best be seen in early morning or late afternoon visible images, when the three-dimensional structure of the cloud
tops is clearly visible due to shadows. It can also be seen in infrared images.
Notice the high-level cloud shield in the lower part of this HRV image, where many fine-scale cellular elements are
aligned in a north-south direction. Further north, the cellular elements are more irregular, similar to closed Bénard
cells. Preliminary research suggests that this cellular structure typically develops in cloud shields that have ingested
large amounts of dust aerosols.
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msg_hrv_06_23mar08.jpg
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Tab 3: Examples of Cellular Elements [Do we need any or all of these examples?]
This early morning HRV/IR 10.8 µm product from 11 February 2010 offers a striking example of cellular elements in a
cirrus cloud shield caused by the ingestion of dust. The dust aerosols that caused the granular structure are evident in
the MSG dust RGB inset from the day before. The synoptic situation is very similar to the March 2008 case where a
pre-frontal dust cloud was lifted upwards in the conveyor belt. The cloud shield in this image developed in the front of
a Mediterranean cyclone and can be analysed as a warm-conveyor belt or warm frontal shield.
msg_hrv_108rgb_11feb2010.jpg
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Next Part
This NOAA AVHRR IR image from 11 February shows the cellular structure of the warm frontal cloud shield caused
by dust aerosols. Only clouds with cloud top temperatures below -55°C are included. The extreme cold cloud top
temperatures (between -75 and -80°C) and the cellular structure of the cloud top may be due to changes in the
radiation processes of the clouds, namely the increased longwave radiation flux that leads to cooling of the cloud top
and enhanced instability in the upper part of the cloud layer.
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noaa_ch4_0753utc11feb2010.jpg
29562 [PD: noaa credit?]
The cellular structure of the “dusty” cirrus clouds can also be seen in these radar (Cloudsat) and lidar (CALIPSO)
data. The track goes from Central Turkey (40 deg latitude) to Southern Finland (60 deg latitude). [mw: add track in
previous IR image]
cloudsat_radar_calipso.jpg 29563 Need verify cr info. Keep all of this here???? Fig_011.png A-Train view
of the “dusty” cirrus cloud shield on 11 February 2010 with CALIPSO 532 nm attenuated backscatter
coefficient & CloudSat radar reflectivity plotted together (Cloudsat plotted over CALIPSO). Also the MODIS IR
radiance, scaled between curtain min and max, is shown. Source: Mike Fromm – NRL
mike.fromm@nrl.navy.mil [mw: highlight what’s said in the bullet below] [Jochen: Write clear explanation of
what’s shown in the graphic. Can we remove anything? Label everything! MODIS shouldn’t be overlaid on the
radar scale, right? What’s its scale?
cloudsat_radar_calipso.jpg done, need to spec is the credit ok???? Should I say Mike Fromm?
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mike.fromm@nrl.navy.mil
Notice the following:


Parts of the thick cirrus cloud are completely transparent at the CloudSat frequency, indicating the presence
of very small ice particles
Most of the dusty cirrus shield has no precipitation below, which confirms the theory that dust aerosols
suppress precipitation
Next Part
This image shows an MSG 24-hour microphysics RGB overlaid with a backward trajectory from within the cirrus shield
on the Ukraine/Belarus border back to Africa where the dust plume was detected initially. The trajectory was
computed from ECMWF wind fields (u, v, w). Note that the air parcel ascended more than 5 km within 48 hours,
confirming that Saharan dust aerosols can ascend up to cirrus levels within the pre-frontal conveyor belt of the
Mediterranean cyclone.
msg_microphysics_06utc_11feb10.jpg 29582 [DECIDE IF USE THIS IMAGE. IF SO, NEED PERMISION
FROM ECMWF AND Kornel Kollath of the Hungarian Meteorological Service who created the image.
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SAY UNUSED IF DON’T USE
In addition to backward trajectories, we can also follow the route of the dust aerosols with forward dust model
forecasts. This image shows the 18-hr SKIRON forecast for aerosol optical thickness at 532 nm for 0600 UTC 11
February 2010. There’s is a very good correspondence between the position of granular-structured cirrus shield and
the forecasted dust aerosols. Note that dust forecasting with models will be discussed in more detail in the model
section.
IF USE THIS, SPEC IT!!! Fig_013.png and/or Fig_013.avi) Aerosol optical thickness forecast for 11 February
2010, 06:00 UTC, from the SKIRON model. Source: University of Athens (http://forecast.uoa.gr). Show the
animation? (Fig_013.avi) aod_skiron_model_11feb2010.jpg all done BUT NEED CR PERMISSION!!!!!!!!!!
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DELETE FROM MSDB IF DON’T USE
NEXT SUBSECTION: POSTFRONTAL DUST EVENTS
Page 1: Postfrontal Dust Storms
As we have seen, widespread dust can also occur behind a cold front. Especially in winter months, the passage of a
cold front leads to strong northwesterly winds on its back side. 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 Middle East.
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shamal_msg_dust_rgb_feb2090_swf.jpg
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This dust 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 winter Shamal typically begins 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.
Page 2: Surface Features
[from afwa dust]
Here’s a typical synoptic-scale surface chart for a winter frontal event. 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-tonorthwesterly 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 postfrontal blowing dust forms to the north of the surface trough and shearline over southern Egypt.
postfrontal_surface_prog_chart_12utc_26feb10.jpg
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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.
sfc_anal_shamal_suhaili_26feb2010.jpg
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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 apparent over the water as dust is advected southward by strong northerly postfrontal winds.
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modisTC_coldFrontOverRedSea.jpg
Page 3: Middle and Upper Levels
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[from afwa dust]
In this example, 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).
postfrontal_upperair_prog_12z_26feb10_circle.jpg
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Deep vertical mixing helps generate dust storms. Strong downward vertical motion, which is likely to be associated
with a middle- to upper-level front within the upper-level trough shown, does two things:
1. 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 with strong solar heating.
Although subsidence itself can stabilize lapse rates, even the wintertime Middle Eastern sun can often offset
this stabilizing effect.
2. 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 the trough.
Page 4: Dust Squall over Riyadh
On the morning of 10 March 2009, one of the worst dust storms in a decade engulfed Riyadh, Saudi Arabia,
blanketing it in a thick layer of orange dust. The wall of dust was more than 100 metres high. The city was forced to
close the airport.
riyadh_sandstorm.jpg 29598
This MSG dust RGB clearly shows the dust cloud in magenta. Notice how well it contrasts with the desert features in
cyan. The sharp boundary of the dust cloud on its southern edge and its dark magenta colour suggest that the dust
cloud is a low-level cloud with a sharp boundary; that is, a dust squall triggered by the cold front over western Iran.
The cold front had passed over Israel the day before, producing a wind storm and some rain. Notice the patches of
dust over Iraq.
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msg_dustrgb_9utc_10mar2009.jpg 29584
Next Part
Notice the position of the high-level cirrus clouds over the Arabian Peninsula (in black). Use the white drawing tool to
mark the following, then click Done.


The position of the subtropical jet (STJ)
The polar frontal jet (PFJ) related to the frontal system over western Iran
Feedback: To find the subtropical jet, follow the cirrus fibres that stretch from southeastern Egypt across the Arabian
Peninsula to Iran. The jet is on the cold (northern) side of these cirrus clouds. The polar frontal jet, which is harder to
find, is more or less parallel to the frontal system over western Iran and then curves northward in the cloud-free are
over northern Saudi Arabia.
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msg_dustrgb_9utc_10mar2009_fb.jpg 29585
Page 5: Postfrontal Dust over Saudi Arabia and Kuwait
This animation shows another case of postfrontal dust over Saudi Arabia and Kuwait. The dust cloud is only a short
distance north of Riyadh. Watch the movement of the dust cloud between 1200 and 1400 UTC. When do you expect it
to arrive in Riyadh?
shamal_msg_dust_rgb_feb2090.swf 29576 speced for carl]
A: 15 UTC
B: 16 UTC **
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C: 17 UTC
D: 18 UTC
Feedback: Moving at a roughly constant speed, the dust cloud arrives in Riyadh around 16:00 UTC. Notice how its
colour changes from bright to darker magenta, suggesting that it is a low-level (surface) dust cloud.
msg_dustrgb_16utc_23apr2010.jpg 29588
Page 6: Dust Crossing the Gulf of Aden
Postfrontal dust outbreaks caused by Shamal winds typically slow down over the southern part of the Arabian
Peninsula where the diffluent northerly flow encounters southerly winds from the Arabian Sea. However, when
troughs dig deeply down to the south, the dust clouds get blown far out into the Arabian Sea, crossing the Gulf of
Aden and reaching the Yemini island of Socotra and sometimes even Somalia.
That’s what we see in this MODIS true colour RGB. Relatively thick dust plumes are bordered by a thick bank of
clouds, which mark the southern boundary of the dust outbreak. The boundary represents the advancing cold front,
separating dusty continental air from unmodified marine air to the south.
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modis_tc_2feb2008.jpg
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Notice how the true colour image made from solar channels does a better job of depicting the low-level dust cloud
over the ocean than the IR-based dust RGB. However, if you compare the dust cloud over the heated land surfaces,
such as Oman, Iran, and Pakistan, you’ll see that the dust RGB handles it better than the true colour image.
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msg_dustrgb_0915utc_2feb2008.jpg
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Page 7: Determining Dust Cloud Height
Vertical profiles from the Calipso satellite, such as the one in the upper left corner, help us determine the height of
dust clouds. The green line is the Calipso track, the dark red line the outline of the dust cloud. The relatively weak
magenta colour of the dust cloud indicates that it is low-level dust with a top at around 1 km. The stronger Calipso
signal in white shows that the dust in the southern part of the image is embedded in a stratocumulus cloud field.
Further north, close to the Oman coastline, the dust cloud reaches higher levels of up to 2 km. The low-level dust
cloud over southern Iran has a stronger colour due to the higher temperatures of the underlying land surface in this
mid-day image.
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msg_dustrgb_0915utc_2feb2008_calipso.jpg
29591 MH: Verify that calipso data are from nasa.
Page 8: Example of Frontal Rain and Dust
A spectacular case of frontal rain in the eastern Mediterranean combined with a major dust outbreak occurred on 21
to 23 January 2004. The postfrontal dust cloud developed over eastern Libya on 21 January, crossed Egypt on 22
January, and moved into the Middle East on 23 January.
This night time IR10.8 µm image shows the cold front over northwest Egypt moving eastward towards Cairo. We
cannot see the postfrontal dust cloud since it blends thermally with the cool desert surface.
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msg_ir108_22feb2004.jpg
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However, the dust cloud is obvious in the area behind the cold front in the dust RGB.
msg_dustrgb_22feb2004.jpg
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Next Part
This image shows the situation on 22 January when the dust storm from the west hit Cairo. The magenta dust streaks
over Egypt and Libya indicate diffluent flow behind the cold front.
msg_dustrgb_22feb2004_12utc.jpg
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Question: Is the circled feature over central Libya a dust cloud? Yes / No ***
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msg_dustrgb_22feb2004_12utc_libya.jpg
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Feedback: The correct answer is no. When we compare the satellite image with a topographic map, we see that the
feature is Haruj, a large volcanic field spread across 45000 km2 in central Libya.
msg_dustrgb_22feb2004_12utc_google_haruj.jpg
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Page 9: Australian Example
Initiation 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. The dust extends 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 over that
area much easier to lift. The red dots mark active bushfires. The smoke plumes show the low-level wind direction
ahead of the advancing dust front.
aus_vis.gif
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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.
aus_mslp.gif
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Page 10: American Southwest Example
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.
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modisTC_newMexico_15apr03_0415.jpg 29568
Surface winds were at maximum strength. A jet maximum had rounded the base of an upper-level trough, transporting
momentum to the surface from strong winds at middle and upper levels.
Page 11: East Asia Example
Dust associated with frontal systems affects 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, 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.
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modis_dustrgb_11nov10_ecmwf_geohgt850.jpg 29599 need ecmwf
This MODIS dust RGB from the morning of 11 November 2010 shows a wall of postfrontal dust blowing across
eastern China. The fast-moving dust was blowing east from the Gobi Desert, where a massive storm had originated
the day before. The dust then moved towards the Korean Peninsula, reaching Korea that evening. Such large dust
storms are common in China, but usually occur in spring when fronts from Siberia sweep southeast across the Gobi
Desert. Late autumn and winter dust storms are rare.
NEXT SUBSECTION: DUST EVENTS CAUSED BY LARGE-SCALE TRADE WINDS
Page 1: Global Trade Wind Patterns
The trade winds (or “trades”) are the prevailing surface winds in subtropical areas. They blow predominantly from the
northeast in the Northern Hemisphere and southeast in the Southern Hemisphere. Their direction can change,
though, over the continents due to local thermal lows, such as the summer heat low over Afghanistan and northern
Pakistan. In the Middle East, the summer trade wind comes from the north and is called a Shamal, while in northern
Africa, it comes from the northeast and is called a Harmattan. When trade winds are strong enough, they can lift large
amounts of dust up into the air. The “40-day Shamal” reflects how long these dust storms can last.
Trade and monsoon winds are responsible for transporting huge dust clouds from northern Africa and the Middle East
westward across the Atlantic Ocean into the Caribbean Sea and southeastward across the Indian Ocean towards
India.
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globe_2.jpg 29645
Dust outbreaks caused by strong trade winds can occur throughout the year. However, summer dust outbreaks have
a greater capacity to lift sand and dust due to the high temperatures and resulting strong convection currents.
Page 2: Synoptic Pattern: Northern Africa
March 2010 was a spectacular month for dust storms, with dust blowing across the Sahara Desert and over the
Atlantic Ocean for much of the time. The synoptic situation in this 18 March 2010 image shows the typical pattern that
causes these large dust outbreaks over northern Africa: a high-pressure area over northern Africa, strengthened by
cold air outbreaks from Europe, presses against a low-pressure area over the southern Sahel that’s related to the
northward movement of the ITCZ, with strong Harmattan winds in between. This leads to continental-scale dust
outbreaks over Sudan, Chad, Niger, Mali, and other countries. In most cases, dust forecast models handle these
situations very well.
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msg_dustrgb_slp_sfcwindanal_18mar2010.jpg 29600 need CR permission from ECMWF
Page 3: Synoptic Pattern: Middle East
The synoptic pattern of large-scale summer dust outbreaks in the Middle East (Shamals) is characterized by:

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
A semi-permanent high-pressure cell extending from the eastern Mediterranean to northern Saudi Arabia (a
subtropical high)
A thermal low-pressure cell over Afghanistan / Pakistan (part of the northern branch of the ITCZ)
Thermal low pressure associated with the monsoon trough extending into southern Saudi Arabia
As this 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 1500 m (5,000 ft). The Shamal is particularly strong at
ground level during daytime but weakens at the surface overnight.
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summer_shamal.jpg
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Page 4: Summer Shamal Exercise
This dust RGB shows a typical summer Shamal from June 2008. Use the drawing tools to outline the dust clouds
and the southern boundary of the dust cloud.
msg_dustrgb_07utc_17jun2008.jpg
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Feedback: The image is from late morning local time when the land surfaces are already heated. The dust clouds are
bright magenta, the desert surfaces bright cyan. The dust clouds had been dark magenta the night before, indicating
that the dust had been low-level.
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WHAT IS THIS?? This dust RGB from June 2008 shows a typical summer Shamal with massive dust clouds hovering
over the Middle East from Iraq to Oman/Pakistan and spreading south past the Arabian Peninsula. Note the absence
of any frontal systems.
msg_dustrgb_07utc_17jun2008_fb.jpg
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Page 5: Continental-scale Sahara Dust Outbreak
Wintertime cold air outbreaks from Europe to Northern Africa frequently lead to large-scale dust storms over northern
Africa. The image in the upper left shows the dust outbreak that started over Morocco and Algeria on 5 March 2006
when the cold front of a cyclone over the Balearic Islands reached Northern Africa. During the following days, the
cyclone moved from the western to eastern Mediterranean Sea and the cold front crossed northern Africa from west
to east, with the dust storm following it. The dust storm reached the Middle East on 8 to 9 March.
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msg_dustrgb_4panel_05_to_08mar2006.jpg
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Next Part
If we zoom in on the product from 8 March, we can appreciate the huge, continental-scale dimension of the dust
outbreaks. Bright magenta dust clouds cover northern Africa from Senegal to Sudan and Saudi Arabia.
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msg_dustrgb_12utc_8mar2006.jpg
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Question 1: Looking at the shape of the dust clouds and dust streaks, use the drawing tools to draw surface wind
vectors on the image.
Feedback: Dust is an ideal indicator/tracer for wind speed and direction. The dust plumes and dust streaks are
generally parallel to the wind direction, and the longer the streaks, the stronger the wind. Following this rule, we can
identify northerly winds over Sudan, north-easterly winds over the central Sahara, and easterly winds over West
Africa. Note the strong north-easterly winds in the gap between the Tibesti and Ennedi mountains.
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msg_dustrgb_12utc_8mar2006_fd.jpg
chad_topo.jpg
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Question 2: What do you think caused the huge dust outbreak?
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A: Strong winds due to a cyclone forming over West Africa
B: Strong post-frontal winds related to the cyclone over the Mediterranean
C: Strong winds from thunderstorm gust fronts
D: Strong Harmattan winds due to a pressure increase from cold air over northern Africa **
Feedback: The correct answer is D.
With the build-up of a high-pressure system in the cold air over northwest Africa, strong northeasterly flow over the
central and southern Sahara generated more dust storms in the area of the Bodélé depression and Agadez, where
huge quantities of dust were picked up and carried towards Niger, Mali, Burkina Faso, and Nigeria. The dust was
stopped as it moved towards the Gulf of Guinea by southerly flow around the Ivory Coast, which led to the formation
of a convergence line over Northern Ghana. The strong colour difference across this convergence line (from pink to
dark blue) is due to the different airmass characteristics: cold, dry, dusty air to the north and warm, moist air to the
south of the boundary. On the following day, the dust was blown far out over the Atlantic Ocean where it was "sucked"
into a cyclone between the Canary and the Cape Verde Islands.
msg_dustrgb_12utc_8mar2006_slp_sfcwindanal.jpg
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DECIDE IF IT’S USEFUL TO SHOW THE ANI. IF SO, CREATE FROM 2006_03_052000092000_m8_rgb_dust_loop.mov. SPEC IT!!!
Page 6: Major Transport of Dust from West Africa to the Lesser Antilles
To Jochen: What should we do about this note: This case was the one for which I proposed in last on-line meeting
to use this long loop with dust RGB alone and ALSO create a new one with the MIX of dust RGB and MODIS true
colors RGB when available (until the antilles) – See my current 5.8.2. You can copy from there the MODIS images
(and explaining text) and I simply take it out from chapter 5. Then keep your exercise and not use the exercise (I had)
that is in the Comet RGB CAL, as the solution to the exercise is already in the title!!]
JOCHEN’S RESPONSE: Well, the main focus here is on the height of the dust cloud. I would keep this page as it is
the only example for a long range dust transport in this chapter.
PD: what do you think?
Westward transport of desert dust from Africa to the Caribbean and Amazon region is a frequent feature of the
subtropical circulation. Over West Africa, the dust is either lifted through thunderstorm gust fronts or strong Harmattan
winds. This image shows a plume of Saharan dust that blew off the west coast of Africa on 22 June 2007. Over the
next few days, the dust cloud remained relatively intact as it traveled across the Atlantic Ocean to the Lesser Antilles
and South America.
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msg_dustrgb_0245utc_22jun2007.jpg all done 29611
See also http://earthobservatory.nasa.gov/IOTD/view.php?id=7811
Question (point on the magenta dust cloud): Given the colour of the dust cloud in this night-time dust RGB over West
Africa, what’s the height of the top of the dust cloud? [mww: make sure we’ve mentioned the colour guide in the
menu]
A: 0 to 1 km (low level)
B: 2 to 3 km (mid level)
C: 5 to 6 km (high level) **
Feedback: The correct answer is C. The bright magenta colour of the dust indicates that it’s high level, with a top at
around 5 km. The Calipso vertical profile verifies this.
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msg_dustrgb_calipso_22jun2007.jpg
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Page 7: Dust Transport Animation
This spectacular 6-day, 15-minute sequence of MSG Dust RGB images was the first to show dust crossing the
Atlantic Ocean. The dust front left the African coast at 1600 UTC 20 June 2007 and reached the Lesser Antilles five
days later, at 1900 UTC 25 June. This translates to a distance of 4600 km in 123 hours or an average speed of 10
m/s or 20 knots. The dust cloud traveled at a level close to 5 km.
msg_dustrgb_ani_20_to_26_jun2007.swf
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TO JOCHEN: WHAT DO WE DO WITH THIS? [So here is the place where a second loop (a mix with MODIS dust
RGB could fit!!] FROM JOCHEN: Has to be checked with what is shown in chapter 5, no need to dublicate this
example. I like this animation very much and it is the final highlight of this chapter.
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Many other features are apparent in the animation. For example, towards the end, when the dust front coming from
the east pushes against the moist and unstable air of the Caribbean Sea, convective clouds develop along the dust
front.
Saharan dust brings mixed blessings as it crosses the Atlantic. It can bring pathogens that harm Caribbean corals and
aggravate human health. But it also supplies the Caribbean islands with valuable nutrients, without which they might
be little more than hulks of barren rock.
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