Analysis of three years of boundary layer observations over the Gulf of Mexico and its shores

Boundary layer observations were made over the Gulf of Mexico, including its shore, over a three-year period in order to develop and test methods for estimating three-dimensional time-dependent wind, turbulence, and temperature fields for use in calculating transport and dispersion of pollutants released from offshore oil platforms. In addition to routinely available buoy and C-MAN shoreline surface data, the boundary layer observation network included five 915 MHz radar wind profilers (RWP) and Radio Acoustic Sounding Systems (RASS) mounted on oil platforms and one on the coast, and collocated surface stations at each RWP/RASS site. Four of the profilers operated for only one year. Because of sea clutter, the RASS and RWP seldom provided observations at heights less than about 200 m. Estimates of surface momentum, sensible heat, and latent heat fluxes were made from the surface observations using the COARE software, and diurnal, seasonal, and geographic variability were analyzed. Predictions of NCEP’s Eta model were compared with the observations of surface fluxes and boundary layer wind and temperature profiles. Eta predictions of wind speeds tended to exceed the RWP observations by 1-2 m/s near shore and by 2-6 m/s at distances of 100 to 200 km offshore. Eta predictions of surface fluxes were fairly close to COARE estimates based on surface observations, and the magnitudes of the fluxes were consistent with those found in studies at other over water sites. The boundary layer was found to be unstable over 90 % of the time, with sea-air temperature differences of about 1 to 3 C. Occasionally, very large stability or instability was observed near-shore with advection of warm air over cold water or cold air over warm water. Most of the time, COARE-derived fluxes at the several offshore stations were within a factor of two of each other at any given hour. The “open-ocean” COARE algorithm was not able to simulate the slight diurnal variations in fluxes at the C-MAN coastal stations, which have surface characteristics in-between those of water and land (some are in tidal marshes, some are on barrier islands, and some are on beaches). COARE should be modified so it can be more confidently applied to shallow water and coastal sites. Based on the overall results, some recommendations for future enhanced field studies can be made, involving shoreline surface towers with flux observations, minisodars (to provide data below 200 m) at the RWP stations, and a focused study with several surface stations and profilers along a cross-section from 10 km offshore to 10 km onshore at a marshy location.