Oceanic rainfall retrievals from the QuikSCAT radiometer

The SeaWinds instrument, launched June 19, 1999 on the QuckSCAT satellite, measures surface backscatter at 13.4 GHz to derive sea surface wind vectors. It also includes a radiometer function (QRAD) at the same frequency. While the radiometer is very noisy on a single pixel basis, when the data are averaged over 0.5° squares, the noise is reduced to a manageable level.

A technique was developed to predict brightness temperatures at one set of viewing parameters (frequency, angle, polarization) from multifrequency observations with different viewing parameters. The approach works over both open ocean for a cold end calibration and a nearly black surface such as the Amazon basin for a warm end calibration. This technique was used to check the TRMM Microwave Imager (TMI) data with the Windsat observations. In turn, the TMI data were then used to check the calibration of QRAD. Given that the QRAD was never intended for quantitative measurements, the calibration is surprisingly good.

The excess brightness temperature relative to the non-raining background is used to derive rain rates from the QRAD observations. The results compare reasonably with retrievals from other radiometer systems and provide additional sampling.

There will be a number of challenges in the Global Precipitation Measuring mission in deriving consistent rain fields from a disparate set of radiometers. This effort allows us to explore approaches to these challenges.