The Dependence of QPF on the Choice of Microphysical Parameterization for Lake-Effect Snowstorms

Abstract

everal lake-effect-snow forecasts are compared to assess how the choice of microphysical parameterization affects quantitative precipitation forecasting (QPF). Eight different schemes, with different numbers of moments and categories of hydrometeors, are considered. Half of the schemes are in the steady regime (so named because the precipitation rates are nearly constant with time), and the remaining experiments are in the unsteady regime, which has a high temporal variation in precipitation. The steady-regime members have broader precipitation shields and 24-h accumulations that range from 43 to 50 mm. In the unsteady regime, the precipitation shields are narrower, leading to higher accumulations (ranging from 55 to 94 mm). These differences are the result of lower terminal velocities ?t in the steady regime, which allows for relofting or suspension of hydrometeors (assuming the vertical velocity is sufficiently large) and, hence, a longer in-cloud residence time and stronger downstream transport. In the six-category experiments, low ?t values in the steady regime occur in conjunction with a lower production of graupel, which is primarily due to less accretion of rain by snow. In the five-category experiments, differences are due to the way ?t is functionally dependent on environmental temperature and the degree of riming, with the steady regime having a more conservative relation. The steady regime compares better to available observations, although both have notable forecast errors.