The use of TOMS and MODIS to identify
dust storm source areas: the Tokar Delta (Sudan)
and the Seistan Basin (south west Asia)
ben hickey, andrew s. goudie
Abstract
Two major Northern Hemisphere source regions for dust storm generation are the Seistan basin
on the borders between Iran and Afghanistan, and the Tokar delta region on the Red Sea coast
of Sudan. MODIS images of these two areas clearly show the locations from which dust plumes
are derived, and TOMS permits the tracing of such dust plumes. e Seistan basin is an arid
basin of internal drainage with active deflation of lake and deltaic sediments. e Tokar delta
is a large alluvial system produced by the Baraka River, and is also in an arid area. Both
areas are active in the summer months, display the importance of topographic funneling of
wind, and contribute substantially to the formation of dust clouds over northern Pakistan and
the Red Sea respectively.
key words: Dust, TOMS, MODIS, Afghanistan, Sudan
1. Introduction
Dust storms, dust raising events that reduce visibility to less than 1 km, are common
phenomena in many drylands. !ey are important because they have numerous
implications for human societies, cause substantial erosion and deflation of desert
surfaces, are an important control on soil development, can have an impact on regional and global climates, and are a major component of biogeochemical systems.
!e largest and most persistent sources of desert aerosol are located in the Northern
Hemisphere, especially in hyper-arid regions and in proximity to major closed depressions (Washington et al. 2003; Prospero et al. 2002). In this paper we use two
main types of remote sensing, TOMS (the Total Ozone Mapping Spectrometer) and
MODIS (the Moderate Imaging Spectroradiometer) to identify two particular hot
spots of deflation and dust generation: the Tokar Delta in Sudan and the Seistan
Basin in south west Asia.
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2. The Total Ozone Mapping Spectrometer (TOMS)
!e Total Ozone Mapping Spectrometer (TOMS) began taking measurements
using the NIMBUS 7 satellite in 1978. Since 1996, TOMS, which provides daily
coverage of most of the Earth’s surface, has been carried on the Earth Probe satellite. TOMS can detect dust in the largest UV wavelengths (340–380 nm), and the
amount of dust in the atmosphere can be expressed in terms of an Aerosol Index
(AI), with the most positive values representing the highest dust loadings (Herman
et al. 1997; Hsu et al. 1999). TOMS detects absorbing aerosols over land surfaces as
well as water and so enables the observation of large dust events over the continents
and the tracing of the movement of large plumes over the oceans. A limitation of
TOMS is that it is not an effective means of detecting dust activity below an altitude
of 1–2 km (Mahowald, Dufresne 2004). On the other hand, various studies have
provided evidence of a good correlation between ground climatological data and
TOMS AI for dust storm distribution (e.g. Chiapello et al. 1999; Goudie, Middleton
2000). TOMS data are available from http://toms.gsfc.nasa.gov/aerosols/aerosols.
html. !ey have been used to study dust storm generation at quite local scales (see,
for example, Bryant 2003).
3. The Moderate Imaging Spectroradiometer (MODIS)
!e MODIS instruments are carried aboard the Earth Observing System (EOS)
Terra and Aqua polar orbiting satellites. !e high spatial and spectral resolution of
MODIS enhances the visibility of airborne dust over land and water, allowing for
detailed analysis of dust activity at selected hotspot areas (Ichoku et al. 2004). Good
images are presented on the Visible Earth web site (http://visibleearth.nasa.gov).
4. The Seistan Basin
!e Seistan (Sistan) Basin (Figure 1) is located on the border between southeastern
Iran and western Afghanistan (lat. 31.3° N, long. 61.4° E). Within this region lies
the Hamoun lakes complex. !ese shallow lakes (e.g. Hamun-i-Sabari, Hamun-eHelmand, Hamun-e-Puzak and Gaud-i-Zirch), which are fed by the Helmand River,
have from time to time been more extensive than today, covering an area ranging
from 2,000–4,000 km², but equally they have a history of almost complete desiccation. For example, when the British Commissioners were there in 1872, the Hamoun
lakes were dry, whereas when they were visited by members of the Russo-Afghan
Boundary Commission in 1885/6 they formed an immense lake (Curzon 1892).
Maps of their extent at the start of the twentieth century appear in Tate (1909) and
show large bodies of water. A Landsat image for November 1976 indicates that there
was a large amount of water in the basin, whereas in September 1987 the basin was
once again dry. !e basin was wet in September 1998. Now, however, due to a run
the use of toms and modis to identify dust storm source areas
Fig. 1
The location of the Hamoun Lakes and the Seistan basin.
of drought years and water abstraction from the Helmand, the lakes have all but
disappeared (as they had in 1902) exposing large areas of lake bed and the delta of
the Helmand, to wind erosion.
In the 1990s and 2000s the silt-laden Helmand experienced substantial declines
in water flow, with a major drought being experienced between 1999 and 2003. In
2001, for example, the river ran at 98 % below its annual average and failed to reach
the Hamoun lakes altogether. In addition, the damming of the Helmand by the Ar-
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Fig. 2a
Landsat images of the Hamoun Lake region taken in 1976 (from Partow 2003).
the use of toms and modis to identify dust storm source areas
Fig. 2b
Landsat images of the Hamoun Lake region taken in 2003 (from Partow 2003).
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qhandab and Kajaki dams and the abstraction of water for irrigation since the 1970s
have contributed to low inputs from that river. LANDSAT images [http://earthobservatory.nasa.gov/study/Hamoun/] show the difference in water and vegetation
in the Hamoun region between 1976 and 2003. Water and vegetation have largely
disappeared, exposing silty, salt flats (Figure 2).
In addition to possessing a lake regime that fluctuates almost yearly between being wet or dry, the area is characterized by very rapidly altering river courses and
depocentre locations for the Helmand River delta (Rawlinson 1873). !e Helmand
also carries a very large silt load, which McMahon (1906, p. 218) found on occasions ‘to be as much as one part silt to 127 parts of water, a figure which very few
rivers in the world can surpass’.
!e Hamoun region is one with an arid climate. !e hottest and driest months
occur from June to August (Figure 3). !e mean annual precipitation at Zahedan
is only 82 mm. In addition the area is affected by the high velocity Bad-i-sad-o-bist
roz (‘wind of 120 days’) (Middleton 1986), particularly in May, June and July. Early
travelers to the region (e.g. Huntington 1905) have described the ferocity of the
dust generating winds. McMahon (1906, p. 224), for example, wrote: ‘It sets in
at the end of May or the middle of June, and blows with appalling violence, and
with little or no cessation, till about the end of September. It always blows from one
direction, a little west of north, and reaches a velocity over 70 miles an hour. It creates a pandemonium of noise, sand and dust.’ He noted that it had le6 old irrigation
canal beds, which are more resistant than surrounding sediment, standing above
the level of the surrounding land, and that there were wind scour features some
6 m or so deep. !ese winds are associated with high level pressure building across
Uzbekistan, giving rise to a pressure gradient across the mountain ranges north of
Afghanistan. !is creates the strong winds which flow through mountain passes to
the Hamoun lowlands.
Fig. 3
Monthly mean temperatures and rainfall for Zahedan, Iran.
the use of toms and modis to identify dust storm source areas
5. The Tokar Delta, Sudan
!e Tokar Delta, fed by the Baraka River, is located on the Red Sea coast of Sudan, approximately 170 km south of Port Sudan at lat. 18.5° N and long. 37.7° E.
!e climate of the region is arid, with little rainfall between March and September
(Figure 4). !e mean annual total is 74 mm. Strong haboobs blow from the end of
Fig. 4
Monthly mean temperature and rainfall for Tokar, Sudan.
Fig. 5
The location of the Tokar Delta, Sudan (from Cressman et al. 1999).
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May onwards. !ese ‘may persist for days on end without a lull and cover the delta
with a dust pall which effectively curbs all outdoor activities’ (Tothill 1948, p. 709).
!e Delta (Figure 5) consists of alluvial silts produced by floodwater coming
from the Red Sea hills and Eritrean Highlands, and at its widest spreads for some
80 km either side of the main Baraka channel. In all it covers 2,150 km². !e silt load
of the Baraka is very high, with its waters carrying as much as 10.6 % of their weight
as suspended silt (Tothill 1948, p. 708). Inland from the Delta there is a major gap
(ca. 110 km wide) in the Red Sea hills, which may serve to channel high velocity
winds into the area.
6. The overall pattern of dust activity in the Middle East and southwest Asia
Figure 6 shows the annual pattern of dust storm activity in the Middle East and
southwest Asia as derived from TOMS. Broadly speaking there appear to be four
main areas with high dust loadings (i.e. AI values greater than 1.3):
(i)
An area in eastern Sudan and over the southern part of the Red Sea. !is
includes the Tokar Delta region, which has an AI value of 1.7 located just
offshore.
(ii) !e south and east of Arabia.
(iii) !e south of Afghanistan and the west of Pakistan down to the Makran
coast. !is includes the Seistan Basin, where AI values of 1.6 are detected.
(iv) !e !ar Desert of northwest India and Pakistan.
Two of these areas have been discussed elsewhere – (ii) by Goudie, Middleton
(2002) and (iv) by Goudie, Middleton (2000).
7. Case Study: the Seistan Basin
Figure 7 is an example of a MODIS image of a dust storm event over the Seistan
basin. !is image, recorded by the Terra satellite on 23rd September 2003, clearly
shows long plumes of dust coming off the dry lake beds pictured in the Landsat
image (see Figure 2). From this, it appears that the dry lake beds serving as point
sources for the dust storm are located specifically in the northern half of Lake Sabari
and at separate points on the northern boundary of Lake Puzak.
Comparison with other MODIS images reveals that these dry lake beds repeatedly provide the source material for dust storms in this region. Dense plumes of dust
originating from the dry lake beds can be seen to travel in a south/southwest direction along the Iran–Afghanistan border for hundreds of kilometers before moving
eastward over Pakistan. !e plumes initially extend in a narrow swath immediately
downwind from the source region before spreading out laterally as dust is lo6ed up
into the troposphere. !e dust plumes remain well defined until reaching the Chagai
the use of toms and modis to identify dust storm source areas
Fig. 6 Annual pattern of dust storm activity (1998–2000) in the Middle East and Southwest Asia,
derived from TOMS (Ai values × 10).
Hills, which straddle the border between the Pakistan and Afghanistan. Here, the
elevated topography (seen more easily in a SeaWifs image of a separate dust event
recorded on 18tʰ May 2001, see Figure 8) appears to break up the plume flow, causing the dust to disperse to the south and west. !e orientation of the Chagai Hills
and Seistan Mountains further to the west appear to channel the dust plume east
over the Registan desert plains in Afghanistan.
!e MODIS image of September 23rd 2003 can be compared with the TOMS AI
values at the same period (Figure 9). !e TOMS image for 22ⁿd September (Figure
9a) shows no great concentration of dust in the upper atmosphere close to the source
region, with all AI values below 1.1. Small concentrations of dust are present in the
atmosphere on the Pakistan coast and Indus plains. !e TOMS image for 23rd September (Figure 9b) shows a significant spread of atmospheric dust approximately
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Fig. 7
MODIS Image of dust storm in Afghanistan for 23rd September 2003.
Fig. 8
SeaWifs image of dust storm in Afghanistan for 18th May 2001.
the use of toms and modis to identify dust storm source areas
130 km southeast of the source region. With the dust being carried by northerly
winds, the disturbed dust was expected to appear visible south of the source. !ere
appears to be no dust in the upper atmosphere for 130 km around the source itself.
!is can perhaps be explained by the fact that TOMS is unable to detect aerosol
concentrations at surface level. It is likely that dust will initially be transported
further away from its source. !e highest concentrations of dust (AI = 1.5) appear
to be concentrated just north of the Chagai Hills. !e TOMS dust plume is spread
parallel to the Pakistan border, with the majority of the dust remaining north of the
Chagai Hills. However, some dust appears to have moved south over the mountains, reaching as far south as lake Hamoun-i-Mashkel (28.3° N, 62.9° E). !e TOMS
data compare well with the MODIS satellite image recorded on the same date. In
the MODIS image, several narrow dust plumes can be seen originating from the
dry lake beds in the Seistan basin and from additional point sources further to the
north. !e dust blows southeast to the Chagai Hills with some passing over the high
terrain moving south. !is is in agreement with the description of dust movement
detailed by TOMS with both images showing a large area of high dust concentration
on the north side of the Chagai Hills. !e TOMS image also shows dust activity has
increased along the Pakistan coast although this is too distant to be linked to the
Seistan dust event.
By 24tʰ September (Figure 9c) TOMS data show a huge increase in atmospheric
dust loading, in terms of both intensity and spatial distribution. !e dust plume
has spread out to cover much of Pakistan and southern Afghanistan. Average AI
values within the plume now exceed 1.5, reaching a maximum value of 3.2 over the
Pakistan coast. Significant values (AI > 1.1) are also present over much of eastern
Pakistan. Comparison with the relief map for this region reveals a good correlation
between low relief and the path taken by the most concentrated parts of the dust
plume. !e most intense part of the plume appears to follow a path eastward along
the Chagai hills before moving south through the valleys of the Siahan range in Pakistan. Having blown south to the Pakistan coast the dust then accumulates over
the Gulf of Oman. It is not clear from the TOMS image to what extent dust activity
on the Pakistan coast is contributing to the peak concentrations observed in this
region. However, there is a clear spread of peak AI values corresponding to high
dust concentrations in the atmosphere that can be traced back to the source region.
!ere is also a noticeable absence of any dust activity in the Baluchistan province
of western Pakistan, an area approximately 100,000 km² in size. !is is most likely
due to the fact that this area is shielded from dust flow by the high mountains of the
Siahan range to the east and the Chagai hills to the north.
!e TOMS image for September 25tʰ (Figure 9d) shows that dust in the upper
atmosphere is now largely restricted to a concentrated area above the Makran coast
of Iran and Pakistan. Although the spread of dust has contracted and peak AI values
have dropped to below 2.5, the area of high dust loading is still significantly large
(approximately 160,000 km²). !e only dust activity to appear outside the concentrated zone is located in the northeast of Pakistan and appears to relate to a distinctly
separate dust source.
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Fig. 9
a
b
c
d
e
f
The TOMS AI values × 10 for 22nd–27th September 2003.
the use of toms and modis to identify dust storm source areas
Table 1
Seasonality of dust storms in the Hamoun Lakes region of Afghanistan, 2002–3.
Time of year
Number of major dust events recorded by MODIS
Dec, Jan, Feb
1
Mar, Apr, May
2
Jun, Jul, Aug
9
Sep, Oct, Nov
2
By 26tʰ September (Figure 9e), three days a6er the dust plume first formed, there
is no longer any significant dust present in Afghanistan close to the source region.
Most of the dust above the Pakistan coast has dissipated and the spots of high dusts
concentrations have disappeared. Maximum AI values have dropped from 2.1 to 1.3
over the last 24 hours. !e dust over the Makran coast appears to have moved northeast, with greater concentrations (AI > 1.5) now visible over northeastern Pakistan.
!e final TOMS image in this sequence for 27tʰ September (Figure 9f) shows
that there are no longer any significant levels of dust remaining in the upper troposphere. Aerosol Index values have dropped to below 1.1 throughout Afghanistan
and Pakistan. !e only dust present at all can be seen in northeast Pakistan close to
where peak values were recorded 24 hours previously.
By looking at the dates of all MODIS images taken of dust storms in this region
from 2001–2003, it is possible to get an idea of the seasonal nature of the dust storms
observed in the Hamoun Lake region (assuming all dust storm events have been
recorded by MODIS) (Table 1).
It appears from the above data that most dust storm activity in the Seistan basin
occurs during the summer months of June, July and August during which nine of the
fourteen recorded dust events took place. No dust activity was observed during the
winter months of January and February. !is substantially confirms the picture determined by analysis of climatological data for Zabol (Iran), which shows that June,
July and August are the three main dust storm months (Middleton 1986, Table 1).
8. Case study: the Tokar Delta
!e nature of dust storms blowing from the Tokar Delta was studied using daily
TOMS AI data (Figure 14). In order to compare the results of TOMS with other
satellite imagery, data were selected over a time period for which several MODIS
satellite images were recorded: the 21st–30tʰ June, 2003 (Figures 10, 11, 12, 13). For
every TOMS map (Figure 14) AI values between 4.5 and 5.5 appear as grey and represent moderate levels of dust loadings in the atmosphere. Higher dust levels (> 5.5)
appear as a dark tone. !e exact locations of the highest TOMS AI values have been
labeled on each daily TOMS map created in order to help trace the movement of the
most concentrated part of the dust plume.
!e first map made using TOMS data for 21st June (Figure 14) shows two distinct
dust clouds over the Middle East. !e first appears over land immediately west of
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Fig. 10
MODIS image of Tokar Delta dust plume, 21st June 2003.
the Tokar Delta trending southwest-northeast in agreement with expected surface
wind flow direction in this area for this time of year. Peak AI values recorded over
the Red Sea reach 6.9. !e dust appears to be moving south over the Red Sea, with
no significant dust levels recorded north of Port Sudan either on land or over sea.
Comparison with the MODIS image recorded over this area on the same date (Fig-
the use of toms and modis to identify dust storm source areas
Fig. 11 MODIS image of Tokar Delta dust plume, 25th June, 2003.
Fig. 12 MODIS image of Tokar Delta dust plume, 26th June, 2003.
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Fig. 13
MODIS image of Tokar Delta dust plume, 30th June, 2003.
ure 10) helps to confirm the direction of dust movement with dust clearly visible
trailing south for several hundred kilometers. !e second dust cloud seen by TOMS
over the southeast Arabian Peninsula appears to be separate from the dust movement visible over the Red Sea and is therefore unlikely to have originated from the
Tokar Delta region.
the use of toms and modis to identify dust storm source areas
On 22ⁿd June (Figure 14b), TOMS shows a significant increase in dust levels over
much of the Red Sea south of Tokar. An intense dust cloud where AI values reach 8.6
now exists over the southern Red Sea and parts of Eritrea and Yemen. In total the
intense dust covers an area of 251,000 km². High but less intense dust levels can be
seen near Tokar. !ere is also notably less dust west of Tokar now than the previous
day. Moderate dust indicated by AI values above 4.0 can be seen extending east and
then north up to latitude 29° N over Saudi Arabia.
By 23rd June (Figure 14c) dust levels have dropped significantly with AI values
falling below significant levels almost everywhere. Some moderate dust loading
remains over the coast of Eritrea, where AI values have dropped to 5.2. !e TOMS
image for 24tʰ June (Figure 14d) shows the dust cloud of highest intensity to have
moved approximately 670 km to the southeast over Somalia and the Gulf of Aden.
In total since 21st June dust emanating from the Tokar Delta has traveled some
1,450 km from its source. !e dust cloud has now increased in intensity to AI 5.8.
Much lower dust values (AI of 4.0–5.0) trail back north toward Tokar. A moderate AI value is picked up over Tokar, suggesting a relatively weak dust event at the
source.
On 25tʰ June TOMS records lower levels of dust in the upper troposphere (Figure
14e). Dust distribution is similar for that seen on 21st June. !ere is no longer any
noticeable dust over the southern Red Sea east of Tokar, where AI values peak at 4.5.
Low levels of dust can also be seen running southwest of Tokar for at least 300 km.
!e TOMS data for 25tʰ June can be compared with the MODIS image recorded
on the same date (Figure 11). Although it is difficult to identify dust movement over
land in this image, MODIS clearly shows strong dust plumes emanating from the
Tokar Delta and also from a source approximately 150 km further north along the
coast near Port Sudan. Dust from these sources can be seen blowing out most of the
way across the Red Sea, moving in a direction curving south in agreement with the
geometry of the dust plume shown by TOMS.
!e MODIS image recorded on 26tʰ June (Figure 12) shows that dust activity
along the Sudan coast has significantly increased, with dust blowing into the Red
Sea from several sources all along the coast as far north as Dunqunab. !e single
most productive dust source still appears to be the Tokar Delta with its plume extending out into the Red Sea for over 250 km.
!e sustained productivity of dust plumes along the Sudan coast over 25tʰ and
26tʰ June is reflected by TOMS data for June 26tʰ which shows a massive increase
in dust intensity over much of the Red Sea, with the highest AI values approaching 8.0 recorded further north of Tokar (Figure 14f), east of Mecca in Saudi Arabia
and all the way down to the Gulf of Aden. AI values have jumped to more than 7.0
for virtually the entire Red Sea, reaching a peak of 8.1 northeast of Tokar, where
several dust plumes appear to merge as seen by MODIS. !e highest concentrations of dust, whilst extending south to the Gulf of Aden, appear to be restricted
to the atmosphere above the Red Sea with the exception of significant high level
dust distribution over the western Arabian Peninsula due east of Tokar. Comparison
with the relief map for this region suggests that dust moving south over the Red
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a
b
c
d
e
f
Fig. 14a–f The TOMS AI values × 10 for 21st–30th June, 2003.
the use of toms and modis to identify dust storm source areas
Sea is contained by the Asir Mountains flanking the coast of the Arabian Peninsula.
!e Asir Mountains run from south of Mecca in Saudi Arabia to the town of Ibb
in west Yemen with an average elevation of over 3,000 m. !e movement of dust
over the western Arabian Peninsula can perhaps be explained by the lower levels of
relief along the Red Sea coast north of Mecca where elevations drop to 500 m, over
2,500 m lower than the mountains further south.
By 27tʰ June the high level of dust activity over the Red Sea has contracted to a
smaller region closer to the Tokar Delta dust source (Figure 14g). AI values below
5.0 still persist over the southern Red Sea. However, peak AI values are now found
only as far south as the border between Saudi Arabia and Yemen. !e higher TOMS
AI values have now moved further north up the Red Sea. !is switch in peak dust
concentration location as recorded by TOMS suggests a change in wind direction
over the Red Sea from prevailing southerly to northerly winds together with a decline in recent dust storm activity on the coast of Sudan.
TOMS data for 28tʰ June (Figure 14h) shows that dust loading in the atmosphere
has reverted to extremely high intensities over most of the Red Sea and western
Saudi Arabia, following the decline in dust concentrations observed on the 27tʰ.
Peak AI values of 8.7, almost double the peak AI reading on 24tʰ June at the start of
this series and the highest recorded values so far, are observed over the Red Sea east
of the Tokar Delta source region. Average AI values of over 7.0 appear west of Tokar
and extend as far south as the Gulf of Aden and east over much of Saudi Arabia. !e
g
h
i
j
Fig. 14g–j The TOMS AI values × 10 for 21st–30th June, 2003.
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total area covered by AI values exceeding 6.0 is now approximately 1,450,000 km²
(of which 460,000 km² is in Arabia). Moderate dust levels can be seen moving away
from the concentrated dust cloud into the Arabian Sea and north of Saudi Arabia
into Iraq.
!e TOMS AI data mapped for 29tʰ and 30tʰ June (Figures 14i and j) show a general repeat of the changes in dust activity seen over the previous two days. Briefly,
AI values for 29tʰ June (peak AI of 6.9) have decreased notably from peak values the
day before (AI of 8.7), only to increase markedly in both distribution and intensity
once more the following day, 30tʰ June (peak AI of 7.7) where the highest AI values
once again exceed an area of over 1,000,000 km². MODIS imagery for 30tʰ June
shows several dust plumes blowing across from the Sudan coast toward the east and
not directly south (Figure 13). !is is supported by the TOMS image for the same
date showing more dust than usual over western Saudi Arabia.
From these images it can be seen that the dust sources on the Sudan coast have
remained fairly persistent over the last several days with dust productivity waxing
and waning on a 48 hour cycle.
As in the Seistan Basin, dust storm activity appears to be highly seasonal. MODIS
images show June and July to be the main months for dust storm activity. !is is
when the haboobs occur.
9. Conclusions
In the Middle East and southwest Asia there are four main dust source regions as
identified by high TOMS AI values. Two of these – the Seistan Basin and the Tokar
Delta – appear to be very specific point sources, with the former being a desiccated
lacustrine/deltaic area of aridity, into which winds are focused by topography from
the north, and with the latter being a hyper-arid alluvial delta, located downwind
from a major gap in the Red Sea Hills. Both areas are associated with rivers that carry
an exceptionally high silt load, and both areas have a highly seasonal and vigorous
dust regime which occurs in the dry, hot, windy summer months. Areas with high
AI values (determined from TOMS) show a good correlation with MODIS images
captured for the same dates and allows dust transport trajectories to be monitored.
MODIS, on the other hand, is effective at identifying the precise sources of dust
plumes.
the use of toms and modis to identify dust storm source areas
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