Tuesday, 11 February 2003
Advance and withdrawal of the African monsoon flow deduced from Meteosat-7 data
Major difficulties remain in the tracking of low-level clouds over land due
to their size and lifetime moreoften below the spatial and temporal
resolutions of current imagers, and from the underlying surface features.
Cloud motion winds (CMWs) have been determined in the African monsoon flow
during the summer months of 1998. Several conditions have been achieved to
improve the retrieval of monsoon winds by the tracking of low-level clouds
: the VIS imagery, a grid with closed grid-points (40 km), the maximal
duration of efficient daylight (8h), and ten-day periods from 1st June 1998
to 30th September 1998. This retrieval has been done over areas
characterized by the scarcity in time and space of conventional
measurements and shows the interest of satellite measurements in such
remote areas. So we used a cloud classification to validate the level of
low CMWs retrieved by the Euclidean distance method as resulting from the
detection of low-level clouds in the classification.
The southwesterly flow has been observed penetrating into the westwards
propagating cells on their southern side or turning around these cells. It
can become southerly between two successive large cells. Monsoon CMWs are
more numerous when the ITCZ is at its northernmost position than before or
after in connection with the extension of the area covered by the rear part
of the monsoon flow. The mean direction of CMWs has been measured around
220° and the mean speed around 5 ms-1 as for climatological data. Some
differences are observed during the advance and the withdrawal of the
monsoon flow : lower speeds with variable directions during the advance,
higher speeds and more uniform direction during the retreat.
Mesoscale features have been observed thanks to the high density of wind
fields. Wind fields are found to be very different in two neighbouring
valleys on both sides the plateau of Jos. In the Benue valley oriented
south-west/north-east, monsoon CMWs remain south-westerly but can speed up
to about 8 ms-1 whereas less numerous CMWs become southerly and travel at
about 5 ms-1 or less in a part of the Niger valley oriented north-south.
Two similar areas at about 7°N, the first one at about 2°E and the second
at about 8°E, exhibit very different monsoon wind fields. Several reasons
are suggested : a different upwelling in the Guinean gulf, a different
convection (maybe a descending Walker branch), a different orientation of
the coast line.
Supplementary URL: