Cloud microphysical schemes fall into two categories:
Due to the very high computational cost of applying explicit schemes, NCAR–RAL developers focus most of their attention on the BMP schemes for most applications. So–called "single–moment" schemes predict only the mass mixing ratio of the hydrometeors and then diagnose the number concentration by making various assumptions. A "two–moment" scheme also predicts the number concentration and provides more degrees of freedom for representing a size distribution. A recent scheme by Thompson et al, 2008 takes a hybrid approach in order to be computationally efficient while reproducing measurements from various field experiments. The scheme was implemented into the Weather Research and Forecasting (WRF) model and is regularly tested and improved based on results from cloud and precipitation measurements from a variety of convective and stratiform precipitation events.
One of the biggest challenges that NCAR–RAL scientists face when developing these schemes is the intricate and complex interaction between microphysics and other physical processes. For instance, the amount and size of cloud water and ice greatly influence the radiation, which subsequently alters the surface heating and cooling that potentially lead to newly–formed clouds. Likewise, rain falling from convective clouds creates downdrafts that can greatly alter location, strength, and amount of subsequent updrafts and thunderstorms due to assumptions of rain drop or hail size and amount.
Additionally, there are a host of uncertainties within the microphysical schemes in general, some of which are amplified when applied to areas with complex terrain.
NCAR–RAL numerical model developers are working on all of these challenging areas in an attempt to find the best optimal solution for each application of the model.
Model Development and Enhancement