The effects of rain on TOPEX/Poseidon altimeter data

The presence of rain in the sub-satellite track can significantly degrade altimeter measurements by causing an attenuation of the backscattered signal, a change in its path length through the atmosphere and a change in the mean square slope of the sea surface. This can cause errors, not only in the measurement of the satellite altitude, but also in the determination of wind speed and wave height. TOPEX/Poseidon dual-frequency altimeter data (cycles 3 and 8) were searched for instances where the data were possibly degraded by the presence of rain over the North and inter-Tropical Atlantic. A subjective analysis of the data, similar to the one used in previous studies was conducted on the backscatter coefficient, wind speed, significant wave height, sea surface height, TOPEX Microwave Radiometer (TMR) brightness temperatures, liquid water content and data quality flags to identify the orbits possibly affected by rain. From the 105 probable rain events identified, the effects of rain on the TOPEX measurements and data quality parameters were characterized. The strong differential effect of rain on the Ku and C band measurements was then used to define a new rain flag based on a departure from the normal relationship between the C and Ku band backscatter. This new rain flag was shown to detect all the identified rain events, as well as new ones. The TMR rain flag, used operationally, was shown to flag too many altimeter samples and too few rain events, mainly because of its large resolution (few tens of kilometers compared to few kilometers for the altimeter). An estimation of the rainfall rate from the attenuation of the Ku band backscatter was proposed.