To be able to use altimetry measurements, we first have to correct them for the effects of atmospheric water - either rain or vapour. One of the side benefits of altimetry satellites is therefore acquiring information about these meteorological parameters, especially rain, for the entire ocean where there are very few meteorological stations. Such studies enable us to gain a better understanding of rain mechanisms and improve altimeter corrections, thus providing increasingly precise data.

Intense storms developping off the Coast of West Africa seen by several instruments onboard Envisat: background colour map from AATSR (infrared sensor, initially for sea surface temperature), rain rate from the altimeter (RA-2, black along-track curves) and liquid water content from the radiometer (MWR, red along-track curves). The optical depth is a measure of cloud thickness. With all three instruments operating simultaneously, it can be noted that rain occurs where the optical depth is greatest, but the active "rain cells" are narrower than the expanse of dense clouds. (Credits National Oceanography Centre, Southampton/Rutherford Appleton Laboratory).


Each frequency of a dual-frequency altimeter responds differently to rain. This not only makes it possible to accurately detect rain events, but can also be used to yield quantitative values. However these techniques are limited in time and space sampling of nadir-pointing instruments.

Monthly mean rain rate computed from Topex/Poseidon, for November 1997 (top, at the height of the 1997-1998 El Niño) and November 1999 (bottom, during La Niña). During El Niño rains are more abundant over the warm water pushed toward the South American coasts, whereas during La Niña most rains are over Indonesia and the 'Warm Pool'.
(Credits Ifremer / Cersat).

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