Bulk microphysical sensitivities within the MM5 for orographic precipitation. Part I: The Sierra 1986 event

This paper investigates the microphysical pathways and sensitivities within the Reisner2 bulk microphysical parameterization (BMP) of the fifth-generation Pennsylvania State University-National Center for Atmospheric Research (PSU-NCAR) Mesoscale Model (MM5) for a precipitation event over the central Sierra Nevada on 12 February 1986. Using a single sounding initialization, the MM5 was run two-dimensionally at 2-km horizontal grid spacing, which was needed to realistically simulate the embedded convective cells within the orographic cloud. Unlike previous modeling studies of this event, a microphysical budget over the windward slope was calculated for each experiment, in which the importance of each microphysical process was quantified relative to the water vapor loss (WVL) rate. For the control MM5, the largest microphysical processes that contribute to surface precipitation over the Sierra windward slope are condensation (63% of WVL), snow deposition (33%), riming to form graupel (35%), and melting of graupel (28%). The amount of supercooled water aloft is larger than observed and in previous modeling studies of this event using the Regional Atmospheric Modeling System ( RAMS). The surface precipitation and microphysical processes over the Sierra Nevada are most sensitive to those parameters associated with the snow distribution, cloud condensation nuclei (CCN) concentrations, and snow/graupel fall speeds, while there is less sensitivity to ice initiation and autoconversions; however, all experiments overpredict the surface precipitation over the windward slope. If ice production is turned off in the cloud-ice region (above 7 km or <250 K), deposition acting on the small amount of cloud ice nucleated at warmer temperatures can still generate a similar snow cloud below 4 km and surface precipitation. The precipitation differences between the BMPs in the MM5 are greater than any single process experiment within Reisner2. The process experiments do help reveal some of the fundamental differences between BMP schemes.