Numerical Forecasts of Explosive Winter Storms: Sensitivity Experiments with a Meso‐α scale Model

Abstract

Mesoscale numerical forecasts of cases of explosive cyclogenesis during the Canadian Atlantic Storms Program are presented in order to examine the evolution and structure of the simulated storms, and to assess the model's skill in forecasting significant weather elements. The investigation focuses on a series of sensitivity experiments with various horizontal resolutions, sea surface temperatures (SST), surface energy fluxes and condensation schemes in order to understand the physical mechanisms responsible for explosive deepening. The results confirm the conclusions of several other investigators that realistic simulations of explosive storms and many of their subsynoptic and mesoscale features can be obtained using high resolution models with a complete set of physical processes, even when starting with synoptic-scale analyses only. In particular, the formation and maintenance of an intense southerly low-level jet (LLJ) ahead of the surface cold front appears to be instrumental throughout the rapid deepening phase. Variations in physical parameterization schemes or SST analyses have an impact mostly in the lower levels. However, a critical factor for simulating marine explosive cyclogenesis is identified as evaporation from the ocean. The results indicate that a large fraction of the moisture available for condensation processes and further deepening originates from the air?sea interactions. The coupling provided by the LLJ then becomes a very efficient mechanism by which heat and moisture are carried from the source region into the developing storm. The overall skill of the model in forecasting quantitative precipitation is found to be quite good. Horizontal distributions of cloud and precipitation compare favorably with satellite imagery and the simulated splitting of precipitation into different types agrees quite well with surface reports.