Radiometric determination of water vapor and liquid water and its validation with other techniques

Abstract

For remote sensing applications in the field of atmospheric physics it is necessary to know the highly variable part of the tropospheric water vapor absorption. With a dual‐channel microwave radiometer, operating at 21.3 and 31.5 GHz, the estimation of the water vapor and liquid water content is possible. The retrieval is performed iteratively with an algorithm using simplified model profiles of the troposphere and a millimeter wave absorption model. This site‐independent profile algorithm changes a model atmosphere until the calculated and measured sky brightness temperatures do agree. Integrated precipitable water vapor (IWV) and integrated liquid water (ILW) are obtained by integrating over the corresponding profiles. By the use of the absorption model sky brightness temperatures, opacities or propagation delays at other frequencies up to 1000 GHz can be calculated. Simulations with a few thousand test profiles showed that the rms error, excluding that of the absorption model, is about 3% for the IWV and between 10% and 20% for the ILW (ILW > 0.1 mm), while for the predicted opacities at 90, 142, and 204 GHz the error ranges between 15% and 25%. The performance of the algorithm and the water vapor radiometer has been checked with independent brightness temperature measurements at 142 and 204 GHz, respectively, IR spectroscopic measurements and radiosonde data. The independent measurements showed all a good agreement with the retrieved quantities of the profile algorithm.