THE INFLUENCE OF MESOSCALE HUMIDITY AND EVAPOTRANSPIRATION FIELDS ON A MODEL FORECAST OF A COLD-FRONTAL SQUALL LINE

Satellite imagery and rain gauge data are combined to create mesoscale detail in the initial states of relative humidity (RH) and surface mo isture availability (M) for a mesoscale model simulation. The most pro found impact of inserting the mesoscale initial fields was the develop ment of a strong vertical circulation transverse to an intensifying co ld front that triggered an intense frontal rainband similar to a sever e squall line that was observed to develop explosively. This paper exp lores the causative factors leading to the formation of this intense c irculation and the sensitivity of the model to the mesoscale initial f ields. A substantial gradient in the initialized RH and M fields occur red across the cold front in the region where the observed frontal squ all line formed. In contrast to the control run, the model simulations that incorporated the mesoscale initial analysis displayed considerab le daytime warming just ahead of the front. This warming was due princ ipally to a reduction in the RH (and, hence, low-level cloud cover) ea st of the front, although an increase in the cross-frontal M gradient did contribute about 25% of the warming. Increased sensible heat fluxe s 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 f low developed in the cold air immediately behind the front only in the presence of this cross-frontal gradient in sensible heating. Much imp roved forecasts of the location and timing of the frontal squall line and other precipitation systems resulted from the mesoscale initial an alysis. The initial RH and M fields possessed sufficient resolution an d consistency with the model dynamics to have a positive influence on the forecasts for a period of at least 12 h. This study provides evide nce that differential cloud cover and evapotranspiration fields can ha ve important impacts on frontal behavior when strong synoptic dynamics are present. Future research should attempt to improve the modeling o f evapotranspiration processes, develop more objective satellite-based humidity analysis techniques, and obtain in situ mesoscale data for v erification of the retrieved atmospheric and soil moisture fields.