A fast and accurate cloud overlap parameterization for microwave data assimilation

Accurate cloud overlap parameterizations are required for assimilation of satellite radiances of partially cloudy areas and areas of overlapping clouds. Currently, most weather services attempting assimilation of cloudy radiances use a simplistic approach to treat the profile of cloud fraction in the radiative transfer. In this work, cloud overlap assumptions are examined and used to develop an accurate reference model. A fast cloud overlap algorithm is presented that allows for the accurate simulation of microwave radiances with a small number of radiative transfer calculations. The performance of this algorithm in terms of both error and complexity is compared to the usual simple approach. Finally, it appears that the algorithm may be easily extended into the thermal infrared.