Raindrops keep falling on the sea

Image of the Month - March 2000

Rain rate (in mm per hour, black curve) derived from analysis of Topex radar waveforms at 13.6 GHz (Ku-band) reflected by the surface, overlaid on a brightness temperature map (showing SSM/I measurements, in degrees Kelvin) of tropical cyclone Elsie. Rainfall is heaviest in the branches of the cyclone, whereas all is calm at its center (measurements were acquired by the two satellites about one hour and 30 minutes apart). (Credits Ifremer)

The Topex altimeters, and Poseidon-2 in the near future, operate at two frequencies (13.6 GHz and 5.3 GHz). They first measure electron content in the atmosphere. We can use these measurements to study rainfall by exploiting the fact that one of the altimeter frequencies is more sensitive to liquid water. Rain attenuates the return signal received by the altimeter, and because this attenuation increases with signal frequency we can compare measurements acquired at the two frequencies and analyze Topex radar waveforms reflected by the surface to calculate the rain rate and size of rainfall cells.
This information is very useful for correcting altimetry data and for weather forecasting-indeed, rainfall gages are pretty hard to come by in mid-ocean!