Section from the PhD hhesis of Cyril Moulin - U. of Paris, June 1997. Rough translation
by C. Moulin. For discussion purposes only.
1. Seasonal climatology of the optical depths
Figure 1 shows the seasonal maps of the mean dust optical depths over the
Mediterranean and the Atlantic. The major features of the seasonal cycle of the dust transport
are summarized here: from year to year, the transport occurs regularly enough, so that marked
structures appear on these maps; maximum transport zone shows a regular shift from winter
to summer, from the south to the north over the Atlantic and from the east to the west over the
Mediterranean.
Over the Atlantic, African dust transport occurs during the whole year. Maximum dust
transport is shifted from the south to the north between winter and summer, following the
ITCZ movement. Minimum transport, both in intensity and in geographical extension, takes
place during fall, whereas high optical depths are shown during winter near the southern limit
of the analyzed zone (around 5°N). During this wintertime period, dust is transported toward
the South American coasts (Swap et al., 1992). The more intense dust transport occurs during
spring and summer, with mean optical depths higher than 0.6 and 0.8 near the African coast,
respectively. During these two seasons, the principal transport direction shifts from southwest to west, and dust reachs the Caraîbes and the Central America (Prospero et Carlson,
1977). In summer, optical depths at a longitude of 40°W are strong, about 0.3-0.4 between 10
and 20°N. This climatology of the African dust transport over the Atlantic is in good
agreement with the one obtained using four years of AVHRR images by Swap et al. (1996).
African dust transport over the Mediterranean is only poorly known, mainly because it
is generaly very sporadic, and thus very difficult to monitor (Prodi et Fea, 1978; Chester et al.,
1984; Dayan et al., 1986; Bergametti et al., 1989a). Figure 1 shows clearly that satellite
images enable to observe well defined structures of the spatial distribution of dust over this
basin: the shift of the maximum transport from the east to the west between spring and
summer; the huge difference between the wet period (fall and winter) characterized by very
small optical depth (< 0.1), and the dry period (spring and summer) with relatively high
optical depths (0.15-0.2).
These results show that processes controling dust transport are the same from year to
year, and that they are very different over the Mediterranean and the Atlantic. It is particularly
clear on figure 1, where optical depth gradients in the principal direction of transport (toward
the north for the Mediterranean and the west for the Atlantic) are stronger over the
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Mediterranean: in summer, the mean optical depth decreases by a factor of 2 between the
north African and southern Europe coasts (from 0.25 to 0.12 over about 1000 km), whereas
such a decrease (from 0.9 to 0.4) occurs only after 2000 km toward the west over the Atlantic.
In addition, dust sources for the Mediterranean region are generaly closer to the coast (above
30°N for the Western Mediterranean; Bergametti et al., 1989a) than those for the Atlantic
(Marticorena, 1995). These results thus suggest that the residence time of particles in the
atmosphere is lower over the Mediterranean, certainly in relationship with the mean
precipitations field (discussed in another section; see also the paper submitted to JGR).
Figure 1. Seasonal maps of the dust optical depth (daily Météosat images 1984 - 1994).
2. Variability of the climatology
Figure 2 shows the maps of the standard deviation of the seasonal mean optical depths
(see figure 1), computed from the individual seasonal mean of the 11 years (1984 - 1994).
Standard deviations over mid-latitude zones are relatively lowAtlantic and the Mediterranean
(0.03 - 0.05). However, the relative variations are stronger over these zones than over the
tropical and sub-tropical zone where the mean optical depth are two to five times higher.
Over the Mediterranean, figure 1 shows a small and relatively homogenous variability
of the mean optical depth during winter and fall. During spring and summer, standard
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deviations increases with increasing optical depths, and mainly over the central basin. During
this dry period, standard deviation reachs values above 0.08 over the southern part of the
central basin.
Over the Atlantic, standard deviation also increases with increasing mean optical
depth. However, very high values are shown during the whole year near the African coast,
between 5 and 10°N. The variability over this zone ranges between 0.1 and 0.14 for every
season, and during fall, may be higher than the variability found over the zone of maximum
transport. Figure 2 shows clearly the specific features of the retrieved optical depth over this
southern Atlantic zone, particularly in summer and fall when the maximum transport is
remote. This point is discussed in the next section.
Figure 2. Seasonal maps of the standard deviation of the mean seasonal dust optical depth shown
in figure 1 (daily Météosat images 1984 - 1994).
Comparison with the climatology of Husar et al. (1997)
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I have computed, from Meteosat imagery, the seasonal mean optical depths for the
period studied by Husar et al. (1997) with AVHRR, from June 1989 to June 1991. The two
types of seasonal maps are shown in figure 3. It shows a good general agreement, in term of
intensity of the dust transport as well as in term of its location over the two basins. The most
important difference concerns the maximum optical depths obtained during spring and
summer over the Atlantic: AVHRR optical depth seems to reach a « saturation value » above
0.5 over the zone of maximum dust transport, whereas Meteosat maps show relatively strong
gradients with values above 0.6. This difference is discussed later.
Another major difference appears during summer in the south of the Atlantic
zonebetween 5 and 10°N, where Meteosat optical depths are higher than those obtained with
AVHRR. This difference is certainly related to the method used to analyse data, and
particularly to the interpolation of the optical depth performed with Meteosat under cloudy
conditions. Indeed, this zone is affected during summer by the ITCZ, which generates a strong
cloud coverage: mean summer cloud frequency was found to be within 30-50% during the
Meteosat processing. In that case, the two methods generate their own problem: AVHRR
optical depths beeing not interpolated, the representativity of their seasonal mean is not
obvious since a lot of values are not available; at the opposite, Meteosat optical depths are
computed using a lot of interpolated data which strongly increase the uncertainties on the
results (Moulin et al., 1997b). It is not possible to determine simply which is the best method
for this specific problem, but in a near future, the use of 3-D dust transport model taken into
account the dust scavenging and washout will improve our knowledge.
A last difference between the two climatologies in figure 3 appears in summer over
the Atlantic zone of maximum transport: AVHRR results show a region of very high optical
depths at long distance from the African coast, between 10 and 30°N. Such an increase do not
appears on Meteosat results, and seems to be relatively unrealistic. This artefact may be due to
remaining clouds or, as discussed above, to the use of non-interpolated data which generate a
local under-representativity.
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Figure 3. Seasonal mean aerosol optical depth obtained from june 1989 to june 1991, using
AVHRR (after Husar et al., sous presse; left) and Météosat (right).
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In order to perform a more accurate comparison of these two climatology, I have
reprojected Meteosat results on the regular grid (1° x 1°) used by Husar et al. (1997) with
AVHRR. I have then compared for each coincident grid point, the two retrieved optical
depths. The AVHRR method, contrary to the Meteosat method, was not specificaly developed
for desert dust studies, but it has been validated using a set of about 20 optical depths
measured in 1989 using Sun photometer (Ignatov et al., 1995). Figure 4 shows this point by
point comparison. Linear regression performed for each season shows significant correlation
coefficient above 0.82: this is a rather good agreement, considering the large differences of
data, sensors and methods.
For optical depths ranging from 0.2 to 0.4, AVHRR optical depths are generaly higher
than those retrieved from Meteosat. This systematic difference of about 0.03 to 0.07 may be
due to the aerosol model used in the AVHRR inversion: a power-law distribution (Junge
type), with a complex refractive index of 1.5 - i 0.01 and an Angström exponent of 0.6
(Ignatov et al., 1995). This model differs notably from the specific dust model that was
developed for the Meteosat inversion (Moulin et al., 1997a and b): whereas the refractive
index is the same than the one used in the Meteosat inversion, the Angström exponent seems
to be too high to be characteristic of African dust. This fact may indicate that the AVHRR
aerosol model do not contained enough large particles, an assumption which is coherent with
the goal of the work presented by Husar et al. (1997), i.e. to obtain a world-wide climatology
of the global climatology including sulfates,... Such a difference in aerosol models yields
significant differences of the aerosol phase function, and thus in the retrieved aerosol optical
depths. This assumption is reinforced by the differences shown from one season to the other
for Meteosat optical depths between 0.2 and 0.4: differences between the two sensors are
maximum in winter and minimum in spring and summer, suggesting that the solar angle, via
the scattering angle and the phase function, may bias the AVHRR results. Figure 4 also shows
that during Fall and Summer, some points appear to be out of the general trend, and create a
second « branch » for which Meteosat optical depths are always equal or higher than the
AVHRR optical depths. These points are located in the southern part of the Atlantic zone and
observed differences are related to the cloudyness, as discussed above.
For Meteosat optical depths lower than 0.15-0.2 and higher than 0.4-0.5, results are
much more different. A large part of the differences may be due to the radiative transfer
models used in the inversion procedure. For low optical depths, the accuracy of the
computation of other contributions (sea surface reflectance, molecular atmosphere,...) has a
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greater relative influence. However, for this range of optical depth, the low sensitivity of
Meteosat measurements might also plays an important role. For high optical depth, AVHRR
values become lower than the optical depths derived from Meteosat data. Once again, the role
of the radiative transfer model should be invoke: the model used in the AVHRR inversion
(Rao et al., 1989) is less elaborated than the 5S model that I used, and might thus be less
accurate under such extrem conditions. Indeed, at my knowledge, its accuracy has not been
tested for optical depths above 0.45 (Ignatov et al., 1995). However, for this range of optical
depth, it is also noticeable that AVHRR optical depths since to reach an assymptotical value
of about 0.6 in winter and 0.7 in summer, which may also be explained by a saturation of the
AVHRR channel, or by un threshold imposed during the data processing.
This discussion remains however too short, and a more detailled comparison between
the two results should yield important improvements of the two methods. Sensor calibration,
cloud discrimination, optical depth interpolation or daily coincident image comparison
(instead of seasonal means) are the major points that should be studied in this frame. It should
also be interesting to test the improvements induced by the use of the AVHRR channel 2 to
constrain the aerosol model via the Angström exponent (Nakajima and Higurashi, 1997).
0.8
Hiver
0.7
Epaisseur optique (AVHRR)
Epaisseur optique (AVHRR)
0.8
0.6
0.5
0.4
0.3
0.2
0.1
Y = 1.25 X + 0.034; R = 0.91
0
0
Printemps
0.7
0.6
0.5
0.4
0.3
0.2
0.1
Y = 0.72 X + 0.124; R = 0.91
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Epaisseur optique (Météosat)
0
7
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Epaisseur optique (Météosat)
0.8
Eté
0.7
Epaisseur optique (AVHRR)
Epaisseur optique (AVHRR)
0.8
0.6
0.5
0.4
0.3
0.2
0.1
Y = 0.57 X + 0.152; R = 0.82
0
0
Automne
0.7
0.6
0.5
0.4
0.3
0.2
0.1
Y = 0.95 X + 0.067; R = 0.88
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Epaisseur optique (Météosat)
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Epaisseur optique (Météosat)
Figure 4. Point by point comparison (1° by 1° grid) between seasonal aerosol optical depths
estimated from Meteosat and AVHRR (Husar et al., 1997) between June 1989 and June 1991. For
each season, 1073 points are used, corresponding to the Meteosat Atlantic and Mediterranean
zones. The 1:1 slope is shown by the dashed line. The linear regression is shown in plain line.
Both regression and correlation coefficients are written on the plots.
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