Near-Term Effects of the Lower Atmosphere in Simulated Northwest Flow Snowfall Forced over the Southern Appalachians

Abstract

orthwest flow snowfall (NWFS) impacts the southern Appalachian Mountains after the upper-level trough has departed from the region, when moist northwesterly flow near the ground is lifted after encountering the mountains. Snowfall associated with these events is highly localized and challenging to predict as the clouds generating the accumulation are mesoscale in structure and depend on rapidly varying structures of moisture, instability, and wind in the planetary boundary layer (PBL) and on the relief of the local topography. The purpose of this study is to investigate the near-term impact of heat and moisture fluxes at the ground on the evolution of an NWFS event using several simulations of the Advanced Research core of the Weather Research and Forecasting mesoscale model. Model simulations indicate that convective banding is responsible for the snowfall accumulations in the southern Appalachians during the event and the structure of the banding is sensitive to the vertical positions of maximum wind shear and minimal stability within the PBL. Sensible heat fluxes at the ground upstream of the mountains in the daytime tend to deepen the PBL, reduce cloud water content, and reduce snowfall accumulations. At the same time, however, the daytime sensible heating also increases the overall vapor of the PBL through increased turbulent mixing and the transport of vapor made available to the atmosphere through upward latent heat fluxes at the ground. Latent heat fluxes at the ground upstream of the southern Appalachian Mountains provide a source of moisture that contributes a significant fraction of the overall simulated snowfall accumulations.