Evaluating and Improving Multi-Moment Cloud Microphysical Schemes. Part II: Sedimentation Section

A new moment order group, 0-3-4 moments, are selected to establish a more appropriate parameter-solving equation group, based on which a raindrop sedimentation (one-column) model has been developed. Measured raindrop spectra on three different heights are employed to motivate and evaluate simulated results to distinguish questionable source/sink terms. A continuous precipitation event impacting all three measuring sites is simulated by WRF model and the profiles of meteorological elements, with a temporal resolution as high as five seconds, are put out to serve as forcing data to the one-column model. It is found that a future modification to Morrison Scheme misses the real physical nature so that the simulated number mixing ratio does not reflect correct magnitude orders. Moreover, the combination between Kain-Fritsch Cumulus Scheme and Morrison Scheme overestimates the diagnosed raindrop number mixing ratio of sub-grid cumulus clouds. After turning these two improper terms off, a more acceptable simulation result has been achieved by which profiles of diverse raindrop moments are captured. Source/sink terms show different sensitivities to such a triple-moment conversion. Largest uncertainty is detected for evaporation, because the substitution of triple-moment gamma size distribution function changes the magnitude of its rate which implies the significance of physical experiments determining raindrop evaporating strength. All of these conclusions are limited in a condition where sedimentation processes dominate the source/sink term group such as below the heavily precipitating cloud system bases. Further efforts will extent such a sedimentation model into a whole warm cloud model to examine accretion and auto-conversion parameterizations.