Postfrontal Boundary-Layer Modification over the Western Gulf of Mexico during GUFMEX

Abstract

Cold-frontal passages over the Gulf of Mexico in late winter or early spring are frequently followed by return-flow episodes in which modified polar air and warm, moist tropical air move toward the Gulf coast. While both advection and airmass modification due to boundary-layer physics 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 weather prediction model to determine the source regions of air arriving on the coat at several different levels. A one-dimensional airmass transformation model is also used in order to delineate boundary-layer physical processes. Simulations were conducted at two sites along the Gulf coast to investigate geographic variability in this return-flow episode, including the effect on boundary-layer structure of sea surface temperature variations in shelf waters. By careful examination of temporal variations in surface flux and advective forcing and by examining changes due both to surface heat flux and differential advection in the forecast vertical profiles of potential temperature and specific humidity, the authors demonstrate that surface fluxes are important in heating and moistening the boundary layer as the air moves south across the Gulf. In the return flow, the complex vertical structure of differential advective heating and drying from multiple source regions plays an important role as well.