Microwave radiometric calibration of airborne weather radars under rain-free conditions over the ocean

Because of the growing interest in the effect of the global distribution of rainfall on climate, the space agencies of Japan and the United States have initiated the development of a Tropical Rain Measuring Mission (TRMM) satellite. The satellite will carry a 14 GHz radar along with 10, 19, 22, 37 and 86 GHz radiometers. In order to simulate these measurements, a major effort has been initiated to measure precipitation from airborne radars and microwave radiometers. In the present study, an aircraft was fitted with nadir pointing radars and radiometers operating at nearly the same frequencies as those that will be used on the TRMM satellite. Deploying 10, 19, 34 and 92 GHz microwave radiometers permitted the measured brightness temperatures to be used to compute the ocean surface wind speed and the total path attenuation of a rain-free atmosphere. The normalized surface radar backscattering cross section, at 10 and 35 GHz, was then computed using the surface wind speed. The effective surface radar backscattering cross section per unit area, at these frequencies, was calculated by applying the knowledge of the transmission of the rain-free atmosphere. Adding measurements of the returned surface radar power, allowed the determination of the radar constants. The reflectivity of small ice particles at the tap of clouds measured with the 10 and 35 GHz radars was used to validate the ratio of both retrieved radar constants