The Influence of Mesoscale Humidity and Evapotranspiration Fields on a Model Forecast of a Cold-Frontal Squall Line

Abstract

Satellite imagery and rain gauge data are combined to create mesoscale detail in the initial states of relative humidity (RH) and surface moisture availability (M) for a mesoscale model simulation. The most profound impact of inserting the mesoscale initial fields was the development of a strong vertical circulation transverse to an intensifying cold front that triggered an intense frontal rainband similar to a severe squall line that was observed to develop explosively. This paper explores the causative factors leading to the formation of this intense circulation and the sensitivity of the model to the mesoscale initial fields. A substantial gradient in the initialized RH and M fields occurred across the cold front in the region where the observed frontal squall line formed. In contrast to the control run, the model simulations that incorporated the mesoscale initial analysis displayed considerable daytime warming just ahead of the front. This warming was due principally to a reduction in the RH (and, hence, low-level cloud cover) east of the front, although an increase in the cross-frontal M gradient did contribute about 25% of the warming. Increased sensible heat fluxes at the expense of decreased latent heat fluxes led to a much deeper and well-mixed prefrontal boundary layer, a more erect frontal surface, and an updraft jet just ahead of the front. A density current?like flow developed in the cold air immediately behind the front only in the presence of this cross-frontal gradient in sensible heating. Much improved forecasts of the location and timing of the frontal squall line and other precipitation systems resulted from the mesoscale initial analysis. The initial RH and M fields possessed sufficient resolution and consistency with the model dynamics to have a positive influence on the forecasts for a period of at least 12 h. This study provides evidence that differential cloud cover and evapotranspiration fields can have important impacts on frontal behavior when strong synoptic dynamics are present. Future research should attempt to improve the modeling of evapotranspiration processes, develop more objective satellite-based humidity analysis techniques, and obtain in situ mesoscale data for verification of the retrieved atmospheric and soil moisture fields.