Dual wavelength rain estimates from space

The 13.8GHz radar on board the TRMM satellite has provided an excellent data set of vertical profiles of reflectivity. For the GPM, follow-on mission, it is planned to fly a 35GHz radar in addition to the 13.8GHz. The 13.8GHz radar can be affected by attenuation in the heavy tropical rainfall, so it has been suggested that the differential attenuation between the 13.8 and 35GHz returns would provide a means of estimating rainfall even when there is appreciable attenuation at 13.8GHz. In this paper we have analysed vertical profiles of 3GHz reflectivity measured at the ground in the tropics which should be unaffected by attenuation, to predict the performance of the dual frequency algorithm at the higher attenuating frequency. We conclude that the dynamic range of such a technique will be rather limited. The minimum rainfall that can be detected is limited by the noisy estimate of Z from space because of the short sample time, whilst the upper limit is set by the total loss of signal at 35GHz.