Aircraft observations of the mean and turbulent structure of a shallow boundary layer over the Persian Gulf

Stable internal boundary layers form when warm air is advected over a cooler surface, a common occurrence in coastal areas. The internal boundary layer deepens with distance along-wind, eventually reaching equilibrium with the surface and becoming a fully developed marine boundary layer. We present observations of the late stages of internal boundary-layer evolution made by the U.K. Meteorological Office's C-130 Hercules research aircraft over the Persian Gulf in April 1996. Northwesterly winds brought warm dry air from the surrounding desert landmass across the cooler waters of the Gulf. Loss of heat to the surface resulted in the formation of a shallow, stable internal boundary layer downwind of the coast. The aircraft measurements were made several hundred kilometres downwind, by which point the original deep convective boundary layer had been eroded away and the internal boundary layer was well developed, effectively a new marine atmospheric boundary layer. Throughout most of its depth the boundary layer was statically stable and a downward heat flux of approximately 15 W m(-2) was observed; however, an exceptionally strong latent heat flux, in excess of 250 W m(-2) near the surface, was sufficient to overcome the downwards heat flux and maintain weak buoyant convection in the lower 30-50% of the boundary layer. Scaling of boundary-layer turbulence statistics using local similarity theory produces results in agreement with previous studies. Because of the strong humidity contribution to the buoyancy flux, however, care is required with the definition of the similarity scales. It is usual for either the sensible heat or buoyancy flux to be used in the definitions of both the temperature and length scales; the latter being used over water where humidity plays a significant role in determining stability. In the present case we find that while the buoyancy flux is appropriate in the definition of the length scale, the temperature scale must be defined in terms of the sensible heat flux.