Cloudy skies

Image of the month - June 2005

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)

Ocean altimeters are designed for sea surface height measurements. However, with all the ancillary instruments, and with the two-frequency radar altimeters, the possibilities arise of looking at other things. For example, atmospheric liquid water content is a necessary measurement to correct altimeter heights (water in the air slows the radar waves). But it can also be used for that knowledge in itself, and compared to the altimeter measurements, since both frequencies respond differently to rain. On multi-sensor satellites like Envisat, other instruments can add complementary information, too.
Such studies enable us to gain a better knowledge of rain mechanisms, and also to improve altimeter corrections, thus providing better and better data accuracy.