Section 3: Dust Source Regions & Section 4: Dust Climatology
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ADD INFO FROM CARLOS’ SECTION?
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Section 3: Dust Source Regions
Page 1: Identifying Dust-Prone Regions
Page 2: World’s Major Dust Source Regions
Page 3: Characteristics of Dust-Prone Areas
Page 4: Inter-annual Variations in Dust Source Regions
Page 5: Identifying Dust Source Regions
Page 6: Source Regions from DEP
Page 1: Identifying Dust-Prone Regions
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
degrees latitude; sand desert; semi-desert shrubs equatorward of 60 degrees latitude; semi-desert sage; polar
and alpine desert; salt playas/sabkhas; and sparse dunes and ridges.
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.
dust_land_cover.gif
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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, these include
the Sahara, Middle East, Southwest Asia (notably Iran, Pakistan, and India??), China, Mongolia, southwestern
North America, west coast of South America, and Australian interior.
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toms_dust.gif 30208
Desertification is on the increase due to various factors. For example, some believe that poor management of the
Earth’s dry lands is changing global and local climates and increasing dust storms from desert margins.
Page 2: World’s Major Dust Source Regions
Let’s take a closer look at the world’s major dust source regions. Click each label [or tab??] for a brief description.
Labels/Tabs: North American, Thar, Turkestan, Iranian, Patagonian, Kalahari, Namib, Arabian, Antarctica,
Sahara, East Asia, Atacama
major_deserts.jpg 29578
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: This 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 hot
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and arid areas in the world. The Kavir Desert is a great salt desert whose crust results from little to no
precipitation and intense surface evaporation.
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 northwestern
parts. The largest deserts include the Great Victoria Desert, Great Sandy Desert, Tanami Desert, and Simpson
Desert.
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 the 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. The Bodélé
depression in northern Chad is actually the world's dustiest spot. 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.
EAST ASIA: Many great deserts, such as the Taklamakan and Gobi, 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, helping us determine past and current climate change.
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 3: Characteristics of Dust-Prone Areas
Several factors make an area prone to dust storms, including 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 in the Middle East.
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mideast_soils.gif 30216
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.
map_middle_east_terrain.jpg 30215
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.
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koppen_dry.jpg 30217
major_deserts.jpg 29578
Here’s the climatology for a hot, arid climate. Baghdad has a Köppen classification of BW (arid desert).
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annual_cond_prod.gif 30218
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.
afghan_source.swf 30214
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 4: Inter-annual Variations in Dust Source Regions
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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) on 21 February 2005 before the heavy rains and snow melt.
sistanBasin_trueColor_2005.jpg
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This false color image from 7 March 2005 shows how much the basin changed in just a few weeks. The dark blue
indicates clear, deep water, the light blue mud-laden water.
sistanBasin_falseColor_2005.jpg 30203
After a long, hot summer, Lake Saberi was still filled with muddy, brown water in October. This decreased the
production of dust plumes and dust storms.
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modis_trueColor_lakeSaberi_12oct2005.jpg 30204
These NRL dust enhancement product (DEP) images of Pakistan and Afghanistan on 2 May 2003 and 12
October 2005 also demonstrate the difference between a drought-ravaged basin and one that has experienced a
wet period.
nrlDEP_2may03_12oct05_prePostDrought_afghanPakis.jpg 30205
Page 5: Satellite Products Used to Identify Dust Source Regions
Various technologies are used to identify and classify dust source regions, from ground-based observations to
satellites and numerical models. Satellites alone can provide frequent and reliable local, regional, and global
observations, making them critical for detecting and monitoring dust sources areas as well as the life cycle of dust
storms. Satellite-derived products also 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, they help 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.
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Now we’ll explore several types of satellite products used to identify dust source regions. Click each tab. When
you are finished, move on to the next page.
Which of the following are operational products that forecasters use? Add more operational information, including
an example and info on how to get the product
TABS: AI (Aerosol Index), AOD (Aerosol Optical Depth); AOT (Aerosol Optical Thickness) [include THIS ONE?],
DEP (Dust Enhancement Product).
Aerosol Index Tab:
The Aerosol Index (AI) indicates the presence of elevated absorbing aerosols in the Earth's atmosphere, such as
desert dust and biomass burning aerosols. It is derived from reflectances measured by the Total Ozone Mapping
Spectrometer (TOMS) instrument on the Nimbus-7 and Earth Probe [is the EP another satellite?) satellites, and
the OMI instrument on the Aura satellite. Note that OMI replaced TOMS at 354 and 388 nm. [need that?]
In addition to identifying dust-prone areas, the aerosol index is also used to monitor the spatial extent and interannual variability of dust.
TOM’s data was the first [OR: among the first?] to reveal the coverage and transport of aerosols around the
globe. For example, they showed how dust originating in the Gobi desert was transported across the Pacific into
the western United States.
Aerosol Optical Depth Tab:
Aerosol optical depth or AOD is another commonly used satellite-based aerosol product. AOD is the degree to
which aerosols prevent the transmission of light, and is a measure of the integrated columnar aerosol load [is that
right?].
ORIGINAL: Aerosol optical depth or AOD is another commonly used satellite-based aerosol product.
AOD is the degree to which aerosols prevent the transmission of light. More specifically, it’s 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. AOD is a measure of the integrated columnar aerosol load.
IS THE FOLLOWING INFO ABOUT AOT OR AOD? IF IT’S AOT, WE NEED TO INTRODUCE IT!
AOT products are made from various instruments, including: [Say something of operational interest about each
product and show an example!]
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The MISR instrument on the Terra satellite; MISR’s multi-angle capabilities enable it 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.
The OMI instrument on the Aura satellite, which has a 2600-km wide swath and provides global coverage on
a daily basis. OMI aerosol products include an aerosol index (AI) and aerosol optical thickness (AOT). Verify
that OMI provides AOT
DEP Tab:
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.
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modis_dep_20aug03.jpg
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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.
modis_dep_20aug03_closeUp.jpg
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Next part
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.
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dustSources_iraq_10x10tile.jpg
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This next 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 dust-prone areas in eastern portions of the Arabian Peninsula and the spotty source regions in Iran and
Afghanistan.
coamps_dustSourceDistrib_seAsia.jpg 30202
SECTION: CLIMATOLOGY
Page 1: Dust Storm Seasonality and Frequency
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dust_seasonality.swf 30209
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
gobi_month.gif 30210
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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gobi_vis.gif 30212
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.
iraq_vis.gif
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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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baghdad_climate.gif 30213
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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