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Monsoons in the altimetry corrections

Image of the Month - December 2008

Correction of the delay of the altimeter radar wave introduced by water in the atmosphere (Wet tropospreric correction) from Jason-1 data, in July 2002 (cycle 20) and January 2003 (cycle 37). Note that this is a negative correction, so the lower the value (blue), the higher the quantity of water in the atmosphere (and thus the higher the slowing of the altimeter wave). In July, the monsoon is wet (even if in 2002 it was drier than normal), water vapor is closer to the Asian continent than in January, where the higher concentration of water vapour in the atmosphere is located closer to the Equator.

Monsoons are part of the climate of Tropical areas, especially North of the Indian Ocean. In summer (wet monsoon), land is hotter than ocean, so winds blow from the ocean to the land, thus bringing humidity. Moreover, when going North, the wet atmosphere encounter the reliefs of the Indian sub-continent, which trigger cloud precipitation. Wet monsoons bring much-needed rains, but too much or too little rain can be catastrophic, so their study, and forecast, are important. Altimetry can help, by giving information on water masses moves in the ocean. Moreover, since the altimeter radar wave is slowed down by water in the atmosphere, altimetry satellites usually have a radiometer instrument onboard, to measure such things (see Image of the Month, January 2000: Water, water in the air). Thus we can map water content in the atmosphere all along the more than 16 years of altimetry data now available, and look at both seasonal and year-to-year variations. Jason-2 is now continuing the series. In the future, the Saral altimetry mission (Cnes/Isro) will be a bit different, since it will use a different frequency for the altimeter (Ka-band). Concerning non-altimetric mission, Megha-Tropiques (Cnes/Isro) will be specifically dedicated to the measurement of the water content in the tropical zone.

Wet tropospheric correction from Jason-2 cycle 4 (August 2008). This year, the monsoon was rather stronger than usual, with important flooding in India, in particular.


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