Bulk Microphysical Sensitivities within the MM5 for Orographic Precipitation. Part II: Impact of Barrier Width and Freezing Level

Abstract

This paper investigates the impact of barrier width and freezing level on the microphysical processes and pathways within the Reisner2 bulk microphysical parameterization (BMP) using a two-dimensional version of the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5). As the barrier half-width is decreased incrementally from 50 km (relatively wide mountain) to 10 km (narrow mountain) for a deep orographic cloud and a 750-mb freezing level, the percentage of water vapor loss (WVL) rate over the windward slope leading to snow deposition decreases from 23% to 7%, while condensation increases from 74% to 93% of WVL rate. A narrow (10 km) barrier has less snow aloft, twice as much cloud water over the windward slope, and a shallow region of intense riming over the crest that results in twice as much graupel as the wide (50 km) barrier. It is found that a relatively wide barrier (≥30 km half-width) allows more time for snow growth aloft; therefore, it is more sensitive to snow parameters over the windward slope such as the slope intercept for number concentration and fall speeds. In contrast, a narrower barrier is more sensitive to rain and graupel processes, such as the cloud water autoconversion and graupel fall speeds. The wide barrier has a larger sensitivity to cloud water processes when the freezing level is elevated to 500 mb, while the narrow barrier is more sensitive to snow processes when the freezing level is lowered to 1000 mb. For a 1000-mb freezing level, the lack of riming and accretion reduces the rapid increase in maximum precipitation that is shown to occur when the barrier half-width is reduced from 20 and 10 km for a higher freezing level.