A comparison of two bulk microphysical schemes and their effects on radiative transfer using a single-column model

Increasingly, numerical models in climate studies are using prognostic bulk microphysical schemes to predict grid-scale cloud cover and properties. These schemes provide information which can lead to improved calculations of radiative transfer, and a better understanding of the interaction of radiation with cloud microphysics on the large scale. In this study a one-dimensional, hydrostatic column model with fixed vertical velocities includes two different bulk microphysical schemes to investigate how the type of scheme influences the hydrometeor content in a cloud, and the effect of this on the radiative heating rates through the cloud. Two test cases are performed, one representing the stratiform region of a tropical cloud cluster, the other a dissipating tropical cirrus cloud. Each test is first performed using a microphysical scheme that carries only one variable for solid water (MS1), and then using a scheme that separates solid water into ice crystals, snow, and graupel (MS2). Further sensitivity tests are made using MS2 to examine the effects of excluding graupel, and of allowing the ice crystals to fall. The influence of these modifications on the hydrometeor contents of the clouds and the corresponding radiative heating rates is considered. In a simulation of the stratiform region of a tropical cloud cluster, MS1 is shown to produce significantly larger hydrometeor contents than MS2. However, in the simulation of a dissipating cirrus cloud, the ice content predicted by MS2 remains much larger than the ice content predicted by MS1, throughout a 24-hour integration. This is because there is a non-precipitating ice category in MS2 which is very slow to convert to snow at low ice-water contents. The use of a non-precipitating ice variable is shown to have a major impact on both the solar and the infrared radiative heating rates at the cloud top, and in some cases to give unrealistic predictions of cloud ice contents.