Modelling study of beamfilling effects in passive microwave remote sensing

The effect of the inhomogeneity of clouds on the brightness temperatures (TBs) measured by the passive microwave radiometer SSM/I (Special Sensor Microwave/Imager) is investigated. The results of a numerical cloud model are used as input for a fully three-dimensional Monte Carlo radiative transfer model to calculate the brightness temperature in the radiometers field of view (FOV) for each SSM/I frequency. These brightness temperatures are compared with those for plane-parallel homogeneous clouds. The strongest reduction of brightness temperatures due to these cloud inhomogeneities occurs in the 37 GHz channel. It is well correlated with the variance of the total cloud water and ice path in the FOV. The high resolution of the 85 GHz channel (15×13 km) and the doubled sampling rate (Bscan) compared to the other channels provide the data to estimate cloud variance, but caused by the highly non-linear relation between rain/ice content and the 85 GHz brightness temperature some uncertainties arise in the application of simple detection schemes