Evaluating the Snow Crystal Size Distribution and Density Assumptions within a Single-Moment Microphysics Scheme

The Canadian CloudSat/Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) Validation Project (C3VP) was a field campaign designed to obtain aircraft, surface, and radar observations of clouds and precipitation in support of improving the simulation of snowfall and cold season precipitation, their microphysical processes represented within forecast models, and radiative properties relevant to remotely sensed retrievals. During the campaign, a midlatitude cyclone tracked along the U.S.–Canadian border on 22 January 2007, producing an extensive area of snowfall. Observations of ice crystals from this event are used to evaluate the assumptions and physical relationships for the snow category within the Goddard six-class, single-moment microphysics scheme, as implemented within the Weather Research and Forecasting (WRF) model. The WRF model forecast generally reproduced the precipitation and cloud structures sampled by radars and aircraft, permitting a comparison between C3VP observations and model snowfall characteristics. Key snowfall assumptions in the Goddard scheme are an exponential size distribution with fixed intercept and effective bulk density, and the relationship between crystal diameter and terminal velocity. Fixed values for the size distribution intercept and density did not represent the vertical variability of naturally occurring populations of aggregates, and the current diameter and fall speed relationship underestimated terminal velocities for all sizes of crystals.