An observational and numerical study of boundary layer structure in a Gulf Coast Environment (Formerly P5.3 and 1.9A)

Although simple in topography, the boundary layer structure and winds in the Gulf regions of Southeastern Texas can be highly variable due to frequent presence of sea-land breeze circulations, strong low-level jets, and a larger scale diurnal wind oscillation with a period close to one diurnal cycle as a result of the region's proximity to the critical latitude. The interaction of these local and regional meteorological phenomena pose a great challenge for mesoscale numerical weather forecasting and air quality prediction for the region. This paper presents results from an observational study of boundary layer structure and evolution and the use of the data to help improve mesoscale predictions using the MM5 model. The observations, which took place during a 10-day period in July 2004 at a site in Galveston, TX, consisted of frequent PBL soundings using a tethered balloon and measurements of turbulent fluxes from a 10-m tower over native Gulf Coast prairie. These measurements were complemented by measurements made farther inland, which included hourly wind observations by a radar wind profiler, daily radiosonde soundings, and hourly surface meteorology from an extensive surface network. The tetheredsonde observations captured the vertical structure and time variation of the boundary layer wind, temperature, and moisture associated with sea breeze circulations. They also revealed, in detail, the structure of the nocturnal low level jets. The combination of the soundings and the measurements of turbulent sensible and latent heat fluxes allowed an examination of the energy budget in the boundary layer. The comparison of MM5 simulations with the detailed observations of the near-surface and PBL energy budget revealed large deficiencies in the model's prediction of the downward radiative fluxes. The comparisons also identified the errors in the predicted longwave radiation and turbulent mixing as the primary reason for the discrepancy between the observed and predicted low-level jet structure.