Evaluation of a bulk cloud microphysics model in simulating low-level arctic mixed-phase clouds using a new single column model

The MM5 NCAR/Penn State mixed-phase bulk microphysical parameterization is evaluated using a new single column model (ARCSCM) in the context of arctic low-level mixed-phase clouds. ARCSCM is developed from the Arctic Regional Climate System Model (ARCSyM). Three mixed-phase clouds over SHEBA in May 1998 are simulated using the MM5 parameterization. Liquid water path (LWP) is underpredicted by ~ 75% compared to observations for the two cases that have a significant ice content, while LWP is accurately predicted (within 5% of observations) for the predominately liquid case. Precipitation, primarily snowfall, is substantially over-predicted in all three cases. It is suggested that the errors in LWP and precipitation are due to the parameterization of ice processes and water/ice phase transitions, which are dependent upon the specified ice crystal number concentration and the autoconversion threshold for snow. The simulated LWP is shown to be highly sensitive to the ice crystal number concentration and autoconversion threshold, while the precipitation is relatively insensitive to the ice crystal number concentration and autoconversion theshold. Thus, the large errors in precipitation cannot be resolved by simple parameter adjustment within the microphysical process parameterizations.