Synoptic and mesoscale influences on refractivity structure during offshore flow over the Persian Gulf

During synoptically driven offshore flow of warm dry air over a colder sea, stable internal boundary layers (SIBL) develop and can extend for hundreds of kilometers in the downwind direction. Depending on wind speed and direction, land /sea temperature difference, and to a lesser degree, land/sea humidity difference these layers tend to grow in height in the downwind direction. SIBLs create strong vertical thermodynamic gradients that lead to negative vertical profiles of refractivity and strong radio frequency (RF) trapping layers. These trapping layers or RF ducts tend to extend radar horizons far beyond those expected in a US Standard Atmosphere. The engineering requirements for radar designs to ameliorate the impact of such harsh refractivity environments requires a four dimensional understanding of SIBL development and the resulting refractivity structure. Mesoscale numerical weather prediction has evolved as significant tool for this purpose. On May 14, 2009, the US Navy Coupled Ocean Atmosphere Mesoscale Prediction System resolved SIBL growth with distance in northwesterly flow over the Persian Gulf. The resulting refractivity field down the middle of the Persian Gulf was characterized by strong RF trapping and increased radar horizons out to 600km downwind. Mesoscale circulations due to the 20K land/sea temperature difference modified the SIBL structure, resulting refractivity fields and radar horizon within 100km of the north and south coastlines of the Persian Gulf.

This paper will describe the diurnal three dimensional structures of the thermodynamic profiles in the SIBLs formed in the Persian Gulf due to synoptic and mesoscale forcing. The resulting four dimensional refractivity regimes and the influence on spatio-temporal radar horizon will also be explored.