Section 3: Dust Source Regions & Section 4: Dust Climatology (at end of file)
Section 3: Dust Source Regions
Page 1: Dust-Prone Regions from Land Cover Types
Page 2: Source Regions from TOMS Aerosol Index
Page 3: World’s Major Dust Source Regions
Page 4: Characteristics of Dust-Prone Areas
Page 5: Inter-annual Variations in Dust Source Regions
Page 6: Identifying Dust Source Regions
Page 7: Source Regions from DEP
Page 1: Dust-Prone Regions from Land Cover Types
Dust storms can only form if there’s an appropriate source region. In this section, we’ll look at where these
regions are located and the key factors that make them prone to dust storms.
Eight land covers are thought to produce dust: low sparse grassland in Mongolia; bare desert equatorward of 60°
latitude; sand desert; semi-desert shrubs equatorward of 60° latitude; semi-desert sage; polar and alpine desert;
salt playas/sabkhas; and sparse dunes and ridges.
dust_land_cover.gif
When these land cover types are combined with wetness values, we get a bulk measure of erodibility. The map
shows how the world's deserts dominate the resulting pattern.
Page 2: Source Regions from TOMS Aerosol Index
Identifying dust-prone regions based on land cover characteristics can be refined by incorporating satellite data.
The Total Ozone Mapping Spectrometer Aerosol Index (TOMS AI) plot provides a near-real-time measurement of
aerosols in the atmosphere.
This plot bases the dust productivity of the earth surface on the observed frequency of high aerosol values and
results in a much more refined view of global dust source regions than the land cover type database seen
previously. Clearly, the majority of the world’s dust storms arise in relatively few areas, in particular, the Sahara,
Middle East, Southwest Asia (notably Iran, Pakistan, and India??), China, Mongolia, Southwestern North
America, west coast of South America, and Australian interior. Some believe that poor management of the Earth’s
dry lands is increasing dust storms from desert margins and changing global and local climates.
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Page 3: World’s Major Dust Source Regions
Let’s take a closer look at the world’s major dust source regions. Click each tab for a brief description.
Tabs for: North American, Thar, Turkestan, Iranian, Patagonian, Kalahari, Namib, Arabian, Antarctica, Sahara,
East Asia, Atacama
http://www.powayusd.com/teachers/blees/desert_locations.htm
NORTH AMERICAN DESERT: The North American Desert includes all of the deserts on the continent, and
stretches down the western side of the North American continent from southern Oregon and Idaho to northern
Mexico. The individual deserts vary based on differences in latitude, elevation, climate, topography, vegetation,
and soil. The four primary deserts are the cold Great Basin Desert and the warm Sonoran, Mojave, and
Chihuahuan deserts.
THAR DESERT: The Thar Desert, also known as the Great Indian Desert, is a large, arid region that forms a
natural boundary along the border of India and Pakistan.
TURKESTAN DESERT: This desert is in Turkmenistan, south of the Aral Sea.
IRANIAN DESERT: Iran’s arid interior plateau contains the Kavir and Lut deserts, which are some of the most
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arid and hot areas in the world. The Kavir Desert is a great salt desert whose salt crust is caused by little to no
precipitation and intense surface evaporation.
kavirnp_l7_aug00sep02.jpg Landsat-7 natural colour image combines data from two satellite
overpasses, the left half from 28 August 2000, the right half from 7 September 2001.
http://earthobservatory.nasa.gov/IOTD/view.php?id=5733 NASA image created by Jesse Allen, Earth
Observatory, using data obtained from University of Maryland’s Global Land Cover Facility.
PATAGONIAN DESERT: This is the largest desert in South America and the seventh largest in the world. It
covers 673,000 square kilometers and is located primarily in Argentina, with small portions in Chile.
AUSTRALIA DESERTS: Thirty eight percent of Australia is desert, with most of it in the central and north-western
parts. The largest deserts include the Great Victoria Desert, Great Sandy Desert, Tanami Desert, and Simpson
Desert.
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Show this? 440px-Australia_deserts.PNG http://en.wikipedia.org/wiki/Deserts_of_Australia NASA World
Wind
KALAHARI DESERT: The Kalahari is a large arid to semi-arid sandy area that covers most of Botswana and
parts of Namibia, Angola, Zambia, and South Africa. August is the height of the dry season, when scorching sun
and little to no precipitation parches the vegetation, and winds excite dust storms.
NAMIB DESERT: This desert covers Namibia and southwest Angola, and is caused by descent of dry air from
the Hadley Cell, which is cooled by the cold Benguela current along the coast. The area gets less than 10 mm of
rain a year and is almost completely barren.
ARABIAN DESERT: This huge desert occupies most of the Arabian Peninsula, stretching from Yemen to the
Persian Gulf and Oman to Jordan and Iraq.
ANTARCTICA: Antarctica is the world’s largest desert due to very low precipitation rates (200 mm/yr along the
coast, far less inland).
SAHARA: The Sahara is the world’s largest hot desert and covers most of Northern Africa. Dust clouds from the
Sahara can travel long distances, drifting to the Caribbean in summer, the Amazon in winter, and Europe during
many months of the year. Satellite aerosol products help us observe the movement of these dust clouds. Note
that the Bodélé depression in northern Chad is the world's dustiest spot.
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Use this???? Figure. MISR-retrieved aerosol type global distribution for July of 2006 of sulfates
(spherical-non-absorbing), smoke (spherical absorbing) and dust (non-spherical). Dust outflow over the
Atlantic from the Sahara and biomass burning in the Congo basin are especially prominent. Source:
credit to MISR team.
EAST ASIA: Many great deserts and much semi-arid land lie in East Asia, where dust outbreaks are very
common and especially severe in spring. Asian dust storms have been documented for thousands of years. Since
dust storm frequency and severity affect soil moisture content, air and surface temperatures, rainfall, and
downwind air quality, these records are used to determine past and current climate change. Use the rest and the
figure? The figure shows MODIS Deep Blue monthly-mean AOD for April 2001, a strong period of dust activity in
Asia. Notice how the Taklamakan desert stands out in the satellite observations.
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ATACAMA DESERT: The Atacama Desert lies along the Pacific Coast west of the Andes Mountains and covers
105,000 km of land in Northern Chile. The desert is the one of the driest in the world due to the rain shadow on
the leeward side of the Chilean Coast Range and a coastal inversion layer created by the cold offshore Humboldt
Current.
Page 4: Characteristics of Dust-Prone Areas
Several factors make an area prone to dust storms. These include soil type, topography, and climate.
Soil type
Even in bare desert, sandy areas generally do not generate dust storms. It’s the areas with silt- and clay-rich soils
that are responsible for the majority of dust storms. These fine-grained soils are found in areas with dry lake beds
and river flood plain deposits. As the map shows, these areas are common to the Middle East.
Topography
Low-lying regions are particularly prone to dust storm generation because prevailing winds are unimpeded by
higher terrain. We see this in the low-lying regions of the eastern Arabian Peninsula, southern Syria, and western
Iraq.
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map_middle_east_terrain.jpg [afwa]
Climate
Finally, an area's potential for dust storm generation is related to its climate, that is, its precipitation patterns,
prevailing wind direction and speed, and normal location of low- and high-pressure centers. These graphics show
how the world’s arid desert and semi-arid climate zones correlate with the major deserts.
koppen_dry.jpg [modified from intro to climo]
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This graphic shows the typical climatology for a hot arid climate. Baghdad has a Köppen classification of BW (arid
desert).
annual_cond_prod.gif
Building a Dust Source Climatology
If you’re forecasting for a new region, you can build a dust storm climatology from archived satellite imagery to
establish the most prevalent source areas. This is similar to the TOMS Aerosol Index climatology discussed
earlier except that it can be much more precise.
For example, this sequence of images reveals that the same light-colored areas in western Afghanistan
repeatedly serve as the source for dust storms.
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Once you know the color characteristics of source areas in a given region, you should look for other potential
areas with a similar appearance.
Page 5: Inter-annual Variations in Dust Source Regions
Periods of extended drought dry out lakes, wetlands, and otherwise productive agricultural land, often resulting in
new and expanded dust sources. The opposite occurs with wet winters, when numerous storms, heavy rains,
and/or above-average snowfall can flood lakes, rivers, and streams and shut off active dust sources.
For example, Southwest Asia experienced an extended drought from 1998 to 2005. Then in 2005, heavy rain and
melting snow led to numerous floods in southern Afghanistan.
This MODIS true color image shows the Sistan Basin, one of the world’s driest basins, as it was on 21 February
2005 before it experienced heavy rains and snow melt.
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The false color image from 7 March 2005 shows how much the basin changed. The dark blue indicates clear,
deep water, the light blue mud-laden water.
The oval in this MODIS true color image from 12 October 2005 shows Lake Saberi. Notice that it is still filled with
muddy, brown water after the long, hot summer.
When the Hamoun Lakes and wetlands are filled with water, the production of dust plumes and storms decreases.
These NRL DEP images of Pakistan and Afghanistan on 2 May 2003 and 12 October 2005 demonstrate the
difference between a drought-ravaged basin and one that has experienced a wet period.
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Page 6: Identifying Dust Source Regions
[are AI and AOT products used for identifying dust source regions? If not, start with products that are
used to identify dust, then discuss this]
Aerosol Index (AI) and aerosol optical depth (AOD) satellite products are used to identify and classify desert
areas, and constrain numerical weather prediction models that depict and forecast?? dust. . The Aerosol Index
(AI) indicates the presence of elevated absorbing aerosols in the Earth's atmosphere. The aerosol types that are
mostly seen in the AI are desert dust and biomass burning aerosols. The AIs shown at the beginning of this
section are derived from the reflectances measured by TOMS (Nimbus-7, Earth Probe)_and currently is
measured by OMI (Aura) that replaced TOMS instrument at 354 and 388 nm.
At the beginning of this section, we mentioned the aerosol index and how it’s used to identify dust-prone areas.
[define AI] AI is also used to monitor the spatial extent and inter-annual variability of dust.
TOMS data reveal the coverage and transport of aerosols around the globe. For example, the data show how
dust originating in the Gobi desert is transported across the Pacific into the western United States.
Aerosol optical depth or AOD is another commonly used satellite-based aerosol product. Aerosol optical depth is
the degree to which aerosols prevent the transmission of light. It is a quantitative measure of the extinction of
solar radiation by aerosol scattering and absorption between the point of observation and the top of the
atmosphere. It is a measure of the integrated columnar aerosol load. [define aot, stating its value, particular in the
context of dust. One definition: AOT is the degree to which aerosols prevent the transmission of light.
AOT is made from products from multiple satellites including:



The MISR instrument on the Terra satellite; MISR benefits from its multi-angle capabilities to perform
simultaneous retrievals of aerosol and surface properties over deserts. The instrument has a relatively narrow
swath (400 km) and provides full coverage at mid latitudes in six to seven days.
The MODIS instrument on the Terra and Aqua satellites; MODIS has a much wider swath (2330 km) and
provides near-daily global coverage. The MODIS Deep Blue aerosol product over desert regions is currently
available from both satellites as a Level-2 product with 10-km resolution.
DOES OMI PROVIDE AI AND AOT? The OMI instrument on the Aura satellite, which has a 2600-km wide
swath and provides global coverage on a daily basis. [is the rest needed? if so, describe it in simpler, less
technical terms] OMI aerosol products include an aerosol index (AI) and AODs.
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Note that satellite-derived products play an important role in initializing and validating regional models, global
transport models, and general circulation models that include dust along with other major aerosol types. For
example, AOT and AI products are used to validate and constrain model simulations of the spatial and temporal
distribution of dust. They also help validate the seasonal cycle and inter-annual variability of dust, especially in
areas where ground-based observations have poor spatial and temporal sampling.
Page 7: Source Regions from DEP
Now we’ll look at other satellite-based products that are used to identify dust source areas.
The U.S. Naval Research Lab (NRL) currently identifies dust emission areas in Southwest Asia from its satellitederived Dust Enhancement Product (DEP). DEP’s 1-km resolution allows for the identification of individual plume
heads that often measure 10 km or less across.
The MODIS true color image and NRL DEP image show southern Afghanistan, northwestern Pakistan, and
eastern Iran on 20 August 2003. By comparing the images, we see the benefit of using DEP to identify small dust
plumes. The one in the white rectangle is barely visible in the true color image while it is readily apparent in the
dust enhancement product in shades of pink.
Question
modis_dep_20aug03_closeUp.jpg
What does the close-up view of the white rectangle in the dust enhancement product indicate about the dust
plume? (Choose the best answer.)


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The plume comes from one point source
Individual point sources merge to form a larger plume
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
The plume is propagating from south to north
Feedback: The correct answer is b). The close-up view of this localized dust plume indicates that it originated
from many point sources to the north and merged into a single small plume as it dispersed to the south.
NEXT PART OF PAGE
NRL maintains a high-resolution (1-km) Dust Source Database (DSD) based on the individual point sources
identified in its dust enhancement products.
Here we see 1-km dust sources plotted in red for the 10°X10° tile covering Iraq. Each red area identifies land that
has eroded and produced a dust plume.
This plot shows the NRL 1-km dust sources averaged on an 18-km grid where the grid erodible fraction varies
from 0 (non-erodible or non-dust producing) to 1.0 (completely erodible and dust producing). Note the many dustprone areas in eastern portions of the Arabian Peninsula and the spotty source regions in Iran and Afghanistan.
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coamps_dustSourceDistrib_seAsia.jpg
USE THIS ANYWHERE? This graphic shows dust source areas identified by the U.S. Air Force Weather Dust
Transport Application (DTA) model. The oranges and reds indicate strong dust source areas.
dta_dust_sources.jpg
SECTION: CLIMATOLOGY
Page 1: Dust Storm Seasonality and Frequency
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Climatologies tell us what happened in the past, which helps forecasters anticipate future events and improve
their forecasting. Climatology provides several types of data that help with forecasting the location, seasonality,
frequency, and severity of dust storms.
We’ve seen how data from the TOMS Aerosol Index helps us map dust source regions. That same data can help
us determine seasonal variations in dust storms.
This animation shows the seasonal variability of dust storms in the dust belt that stretches from western Africa up
through the Taklamakan Desert in central Asia. Note the strong seasonal dependence of dust storm frequency.
For example, dust storms in this desert show a pronounced peak in May while the maximum values for West
African dust storms shift northward from winter to summer.
Page 2: Dust Storm Frequency and Severity
Climatologies compiled by the Air Force Weather Agency Metsat Applications Branch show the monthly
frequency of dust storms. Note how the number of storms in the Gobi Desert spikes in March and April and tapers
off from May through July.
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When we categorize the dust storms by visibility, the picture becomes clearer. Not only is the highest frequency in
the early spring, but the majority of severe dust storms occurs in March and April, more than the rest of the year
combined.
This graph of dust storm climatology for Iraq reveals some important information. Dust storms tend to be most
frequent in the summer, although severe storms can occur from spring through autumn.
Page 3: Dust Storm Frequency and Precipitation
If you are forecasting in a region and don't have access to information on the frequency of dust storms, you may
be able to infer a climatology by examining other climatologic data such as the frequency of dust events vs.
annual precipitation rates (PR).
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Obviously, drier, hotter conditions favor more dust storms. Here we see a minimum for precipitation events and a
maximum for temperature in central Iraq through the summer months, the dustiest time of the year.
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