NUMERICAL-SIMULATION OF AIRMASS TRANSFORMATION OVER THE GULF OF MEXICO

A mesoscale numerical simulation (35 km) of a return-flow event over the Gulf of Mexico that occurred during the Gulf of Mexico Experiment (GUFMEX) is presented in order to examine the structure and the transformation of the polar air mass and to assess the model's skill in simulating the event. The study deals with the phase of cold-air outbreak over the Gulf of Mexico and the subsequent rapid modification of the cold air mass by the underlying warm ocean, prior to the onset of return flow. The investigation focuses on the physical processes operating during the airmass transformation, notably the air-sea fluxes and the vertical destabilization of the airmass. The results are compared with various data gathered during GUFMEX and suggest that a realistic simulation of airmass transformation can be obtained. The results indicate a strong interplay between 1) large-scale subsidence above the planetary boundary layer behind the front and 2) destabilization near the sea surface and in the boundary layer. In particular, advective processes play a central role in the airmass modification above the boundary layer and in the maintenance of a strong capping inversion. However, very large surface energy fluxes and vigorous turbulent vertical mixing appear as dominant mechanisms within the boundary layer itself. A sensitivity experiment where surface energy fluxes are turned off supports these conclusions and clearly demonstrates their impact on the advance of the cold air mass over the Gulf and on the changes in moisture and stability of the return flow.