Microwave Radiometer Measurements in Clouds During the SOFIA/ASTEX Campaign

DRAKKAR is a two-channel, vertically-pointing microwave radiometer (MWR) which operates at the same frequencies as the ERS-1/MWR (23.8 and 36.5 GHz). It was operated during the SOFlA/ASTEX campaign near the Azores in June, 1992, on board the French ship Le Suroit. Using DRAKKAR and satellite microwave radiometer data (ERS-1/MWR, SSM/1), our objective is to retrieve the cloud liquid water content during the campaign.

A first step was to validate the DRAKKAR measurements: we compared retrieved water vapour contents with radiosonde data and we simulated the brightness temperatures using a radiative transfer model and radiosonde profiles in clear air. We concluded that the water vapour algorithm works satisfactorily, and that the 23.8 GHz channel is well calibrated. An uncertainty remains concerning the 36.5 GHz channel calibration due to its high Sensitivity to atmospheric liquid water.

The second step was to evaluate the cloud liquid water content observed above the ship by adjusting the cloud parameters in the radiative transfer model to fit the measured brightness temperatures. We obtained liquid contents ranging between 0.1 and 0.35 kg/m2 in all cases of boundary layer clouds (stratus /stratocumulus clouds). A second cloud layer, which sometimes occurred at high altitude, had very low liquid water contents.

These results will be compared with data from aircraft and other radiometers to validate our approach.