Observations of strong surface radar ducts over the Persian Gulf

Ducting of microwave radiation is a common phenomenon over the oceans. The height and strength of the duct are controlling factors for radar propagati on and must be determined accurately to assess propagation ranges. A surfac e evaporation duct commonly forms due to the large gradient in specific hum idity just above the sea surface; a deeper surface-based or elevated duct f requently is associated with the sudden change in temperature and humidity across the boundary layer inversion. In April 1996 the U.K. Meteorological Office C-130 Hercules research aircra ft took part in the U.S. Navy Ship Antisubmarine Warfare Readiness/Effectiv eness Measuring exercise (SHAREM-115) in the Persian Gulf by providing mete orological support and making measurements for the study of electromagnetic and electrooptical propagation. The boundary layer structure over the Gulf is influenced strongly by the surrounding desert landmass. Warm dry air fl ows from the desert over the cooler waters of the Gulf. Heat loss to the su rface results in the formation of a stable internal boundary layer. The lay er evolves continuously along wind, eventually forming a new marine atmosph eric boundary layer. The stable stratification suppresses vertical mixing, trapping moisture within the layer and leading to an increase in refractive index and the formation of a strong boundary layer duct. A surface evapora tion duct coexists with the boundary layer duct. In this paper the authors present aircraft- and ship-based observations of both the surface evaporation and boundary layer ducts. A series of sawtooth aircraft profiles map the boundary layer structure and provide spatially d istributed estimates of the duct depth. The boundary layer duct is found to have considerable spatial variability in both depth and strength, and to e volve along wind over distances significant to naval operations (similar to 100 km). The depth of the evaporation duct is derived from a bulk paramete rization based on Monin-Obukhov similarity theory using near surface data t aken by the C-130 during low-level (30 m) flight legs and by ship-based ins trumentation. Good agreement is found between the two datasets. The estimat ed evaporation ducts are found to be generally uniform in depth; however, l ocalized regions of greatly increased depth are observed on one day, and a marked change in boundary layer structure resulting in merging of the surfa ce evaporation duct with the deeper boundary layer duct was observed on ano ther, Both of these cases occurred within exceptionally shallow boundary la yers (less than or equal to 100 m), where the mean evaporation duct depths were estimated to be between 12 and 17 m. On the remaining three days the b oundary layer depth was between 200 and 300 m, and evaporation duct depths were estimated to be between 20 and 35 m, varying by just a few meters over ranges of up to 200 km. The one-way radar propagation factor is modeled for a case with a pronounce d change in duct depth. The case is modeled first with a series of measured profiles to define as accurately as possible the refractivity structure of the boundary layer, then with a single profile collocated with the radar a ntenna and assuming homogeneity. The results reveal large errors in the pro pagation factor when derived from a single profile.