Modification of the Thermodynamic Structure of the Lower Troposphere by the Evaporation of Precipitation ahead of a Cold Front

Abstract

The evolution of an oceanic prefrontal subcloud layer of continental origin was examined by analyzing data gathered on the subsynoptic and mesoscale during Phase III of the Australian Cold Fronts Research Program. Characteristics of the atmosphere ahead of the surface cold front included deep ascent through the troposphere and horizontal low-level warm air advection. Temporal and spatial variation in the thermodynamic structure of the 3.5-km thick prefrontal subcloud layer confirmed that significant local cooling was occurring despite the presence of horizontal warm air advection. Over a 24-hour period this cooling created a new low-level baroclinic zone several hundred kilometers ahead of the front. Observations from surface stations, rawinsodes, radars, satellites, and research aircraft were combined to demonstrate that the observed local cooling in the prefrontal air mass was most plausibly due to evaporation of precipitation. The region in question contained mesocale bands of precipitation, including convective showers that generated strong downdrafts. The observed mesoscale features are consistent with those obtained in a numerical modelling study of the generation of similar prefrontal rainbands, and appear to account for the observed modification of the baroclinic structure of the frontal zone.