Evaluation of a Two-Dimensional Kinematic Cloud Model Using Data from a Central Sierra Nevada Orographic Cloud System

Abstract

A two-dimensional kinematic cloud model was developed to include snowflake growth by aggregation as well as riming and vapor deposition. The model includes new parameterizations of ice crystals and aggregates of ice crystals. The model was run and evaluated using a dataset gathered by the Sierra Cooperative Pilot Project (SCPP). The model was tested for its sensitivity to two model parameters, a constant relating the mean mass of collected ice crystals to the mean mass of all ice crystals in the cloud, and an ice nucleation rate constant. Varying the collection constant by a factor of 2.5 had only minor effects on the model behavior, but varying the ice nucleation rate by a factor of 50 had significant effects on the relative importance of riming and vapor deposition growth. Two problems that did develop in the model were the result of the model's bulk microphysical assumptions relating graupel and raindrop size to the magnitude of the mixing ratio, so that small concentrations behaved as cloud particles and would not fall out as precipitation. In general, the model worked well in representing the distribution of ice crystals and aggregates in the cloud as well as the relative importance of the riming, aggregation, and vapor deposition processes. The location of ice crystals and snow aggregates compared well with aircraft observations in the real cloud including the location of a convergence zone with higher ice-crystal concentrations noted by the aircraft near the crest.