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Cooperative Institute for Meteorological Satellite Studies / University of Wisconsin-Madison
CIMSS Tropical Cyclone Team
Saharan Air Layer (SAL) - Product Description

Meteosat-8 Mid-Level Water Vapor Imagery:
Background: This product uses the 7.3µm channel on the Meteosat-8 satellite, which is sensitive to moisture in the lower to middle levels of the atmosphere.
Uses: The 7.3µm channel is useful for detecting moisture in the lower to middle troposphere and is typically most sensitive to moisture at ~600 hPa or ~4,500 m.
• This channel is more effective than the upper-level water vapor imagery for tracking the Saharan Air Layer (SAL) because it detects moisture (or lack of) more effectively at levels where the SAL is typically found (~500-850 hPa or 6,000-1,500 m).
• Since this channel is able to detect the lower atmosphere more effectively, caution should be taken in interpreting the imagery, particularly in extremely dry areas over land (e.g. the Sahara Desert). In these regions, rapid heating during the day and cooling at night can affect the satellite retrievals in this channel. Specifically, if the lower to middle atmosphere is extremely dry, this channel "sees" more effectively down to the Earth's surface. Therefore, strong surface heating can produce an enhanced false "dry" signal and surface cooling can produce an enhanced false "moist" signal. However, although this effect can be significant over land areas, it is negligible over the ocean.

For more SAL information, please visit the SAL Background Information page.
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