POSTFRONTAL BOUNDARY-LAYER MODIFICATION OVER THE WESTERN GULF-OF-MEXICO DURING GUFMEX

Cold-frontal passages over the Gulf of Mexico in late winter or early spring are frequently followed by return-flow episodes in which modifi ed polar air and warm, moist tropical air move toward the Gulf coast. While both advection and airmass modification due to boundary-layer ph ysics are important in this sequence of events, the relative roles of these processes are unclear. In the present study, the authors utilize data from the Gulf of Mexico Experiment and two distinctive numerical models in addressing this issue. In forecasts of a return-flow event, trajectory computations are performed using a mesoscale numerical wea ther prediction model to determine the source regions of air arriving on the coast at several different levels. A one-dimensional airmass tr ansformation model is also used in order to delineate boundary-layer p hysical processes. Simulations were conducted at two sites along the G ulf coast to investigate geographic variability in this return-flow ep isode, including the effect on boundary-layer structure of sea surface temperature variations in shelf waters. By careful examination of tem poral variations in surface flux and advective forcing and by examinin g changes due both to surface heat flux and differential advection in the forecast vertical profiles of potential temperature and specific h umidity, the authors demonstrate that surface fluxes are important in heating and moistening the boundary layer as the air moves south acros s the Gulf. In the return flow, the complex vertical structure of diff erential advective heating and drying from multiple source regions pla ys an important role as well.