The importance of riming for orographic precipitation as revealed by the development of a new bulk microphysical parameterization

Several previous studies have noted snow overprediction aloft over terrain within many bulk microphysical parameterizations (BMPs), such as during the Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE-2) over the central Oregon Cascades. This can lead to excessive surface precipitation over the windward slope and/or too much lee side spillover. It was hypothesized that much of this overprediction results from how the various schemes treat riming within the orographic cloud, since this controls the fallout and time scale for snow growth. Most BMPs treat snow and graupel as two separate categories, so there is no partial riming within the cloud. A new bulk microphysical parameterization (BMP) scheme developed at Stony Brook (SBU-YLIN) is presented that includes a diagnosed riming intensity and temperature dependent ice characteristics. As a result, the new scheme represents a continuous spectrum from pristine ice particles to heavily-rimed particles and graupel using one prognostic variable (precipitating ice or PI) rather than two separate variables (snow and graupel). In contrast to most existing parameterization schemes that use fixed empirical relationships to describe ice particles, general formulations are proposed to consider the influences of riming intensity and temperature on the projected area, mass, and fall velocity of PI particles.

The new SBU-YLIN scheme has been implemented in Weather Research and Forecasting (WRFv3.1) model and compared with three other WRF schemes for two events during IMPROVE-2 over the central Oregon Cascades. The new scheme reduces the snow amounts aloft by 30-40% as compared to other WRF schemes and thus compares better with the NOAA-P3 and Convair research aircraft observations, especially for the 13-14 December 2001 event with moderate riming aloft. The cloud water amounts in the new scheme also compare better with observations, and the surface precipitation is comparable to observed over the windward slope. Sensitivity tests suggest both reduced snow depositional growth rate and more efficient fallout due to riming contribute to the reduction of ice water content aloft in the new scheme, with the larger impact from the partially rimed snow and fallout.