Synergy Between Dual-Frequency Altimeters and Radiometers for Precipitation Studies

Dual-frequency altimeter measurements can accurately detect rain events and can also be used to infer quantitative values. However, the accuracy of the rain rate estimate has been limited by the uncertainty in the height of the freezing level (FL) necessary to infer the surface rain rate from the measured signal attenuation. Altimetric satellites also carry microwave radiometers designed to correct for atmospheric water effects. Using a radiative transfer model and simplified rainy atmospheres, the microwave brightness temperatures can be inverted in terms of FL height. The surface rain rate is then computed from the altimeter attenuation and the radiometer FL. The rain climatology is computed for the three altimeters currently in operation using a mixed log-normal distribution. Comparison with the GPCP and SSM/I climatologies shows that the use of FL greatly improves the altimeter climatology which is of the same quality as SSM/I one for annual mean. The merging of the three altimeters is investigated. The resulting monthly mean rain rates are comparable to those derived from SSM/I