Retrieval of water vapor profiles from microwave radiometric measurements near 90 and 183 GHz

Upwelling radiometric measurements at 90 GHz and three side bands near 183 GHz are used to retrieve water vapor profiles over the ocean surface. An algorithm incorporating a new technique of handling moderate cloud cover is illustrated for the profiling of both relative humidity and water vapor burden. It is shown that the retrieved relative humidity profiles reflect gross features of the corresponding profiles recorded by the radiosondes. However, the retrieval generally cannot produce fine details of the observed profiles at altitudes where a rapid change in relative humidity occurs. For this reason, comparison of retrieved and observed values at a given altitude often yields an appreciable rms error. Profiling of water vapor burden, a parameter equivalent to total integrated water vapor above a certain altitude, results in much better agreement, as expected. The rms error obtained from the results of the retrieval at the surface is comparable to that derived from the combination of measurements at 18 GHz and 21 GHz channels of the Scanning Multichannel Microwave Radiometer aboard the Nimbus 7 satellite.