Evaluation of Warm-rain Microphysical Parameterizations in Mesoscale Simulations of the Cloudy Marine Boundary Layer

Several warm-rain microphysical parameterizations are evaluated in a regional forecast model setting (using the Naval Research Laboratory's Coupled Ocean-Atmosphere Mesoscale Prediction System) by evaluating how accurately the model is able to represent the marine boundary layer (MBL). Cloud properties from a large suite of simulations using different parameterizations and concentrations of cloud condensation nuclei are compared to ship-based observations from the VOCALS-REx field campaign conducted over the southeastern Pacific (SEP). As in previous studies, the simulations systematically underestimate liquid water path and marine boundary layer cloud depth. On the other hand, the simulations overestimate precipitation rates relative to those derived from the scanning C-band radar aboard the ship. Most of the simulations exhibit a diurnal cycle, although details differ somewhat from a recent observational study of SEP cloud variability. In addition to direct comparisons with the observations, the internal microphysical consistency of simulated MBL cloud properties is assessed by comparing simulation output to a number of observationally and theoretically derived scalings for precipitation and coalescence scavenging. Simulation results are broadly consistent with these scalings, suggesting COAMPS is behaving in a microphysically consistent fashion. However, microphysical consistency as defined in the analysis is highly dependent upon the horizontal resolution of the model.