Airmass Modification over the Gulf of Mexico: Mesoscale Model and Airmass Transformation Model Forecasts

Abstract

Several numerical models are used to examine strong air-sea fluxes and resultant airmass modification following a cold-frontal passage over the Gulf of Mexico. Data from the Gulf of Mexico Experiment (GUFMEX), which was conducted in February-March 1988, are used for model validation. To provide a benchmark by which to evaluate the role of diabatic processes in airmass modification, the mesoscale model was initially run with surface fluxes deleted. Subsequent full physics runs show profound alterations to the boundary layer due to the diabatic processes. A one-dimensional airmass transformation (AMT) boundary-layer model is also tested and compared with the mesoscale model and GUFMEX data. The Lagrangian character of the AMT model is a useful compliment to the mesoscale model output. Further, at least in one forecast, the AMT model yields a better forecast of boundary-layer depth. Strong sensible and latent heat fluxes in the vicinity of the cold front act frontolytically, while a subsidence-induced local maximum in latent heat flux appears in the return flow that is established in the western Gulf. The precipitable-water field shows a tongue of moist air returning to the Louisiana coast and indicates that substantial mesoscale horizontal gradients in the moisture field are to be expected in the return flow.