Improved Radiative Transfer Calculations from Information Provided by Bulk Microphysical Schemes

Abstract

Bulk microphysical schemes are providing increasingly detailed information of hydrometeor profiles both within and below clouds. This information can be used to improve radiative transfer calculations with little increase in computation time. In the simple context of a single column, the work described in this paper uses a relatively complex radiation code and a five-category bulk microphysical scheme to investigate simple and computationally efficient methods of utilizing microphysical information in radiative transfer calculations. The bulk microphysical scheme used here is typical of many and predicts mixing ratios of liquid water droplets, rain, ice crystals, snow, and graupel. When all hydrometeors are treated separately in the radiation scheme, improvements can be made to the radiative transfer calculations. First, the effective radii of the various hydrometeors can be calculated from information provided by the microphysical scheme. Also, adjustments can be made to the radiation scheme to allow for the nonspherical shape of ice and snow. The calculation of the effective radius of the various hydrometeors was simple and fast, and it gave integrated in-cloud thermal infrared heating rates that differed by up to 16% from a scheme that used fixed effective radii. Adjusting for the shape of ice crystals and snow was also simple and increased the clouds? albedo, reducing integrated in-cloud solar heating by ?10%. All the links between the bulk microphysical scheme and the radiation code are physically based and do not significantly increase the complexity of the radiation code.