Numerical modeling of the propagation environment in the atmospheric boundary layer over the Persian Gulf

Strong vertical gradients at the top of the atmospheric boundary layer affe ct the propagation of electromagnetic waves and can produce radar ducts. A three-dimensional, time-dependent, nonhydrostatic numerical model was used to simulate the propagation environment in the atmosphere over the Persian Gulf when aircraft observations of ducting had been made. A division of the observations into high-and low-wind cases was used as a framework for the simulations. Three sets of simulations were conducted with initial conditio ns of varying degrees of idealization and were compared with the observatio ns taken in the Ship Antisubmarine Warfare Readiness/ Effectiveness Measuri ng (SHAREM-115) program. The best results occurred with the initialization based on a sounding taken over the coast modified by the inclusion of data on low-level atmospheric conditions over the Gulf waters. The development o f moist, cool, stable marine internal boundary layers (MIBL) in air flowing from land over the waters of the Gulf was simulated. The MIBLs were capped by temperature inversions and associated lapses of humidity and refractivi ty. The low-wind MIBL was shallower and the gradients at its top were sharp er than in the high-wind case, in agreement with the observations. Because it is also forced by land-sea contrasts, a sea-breeze circulation frequentl y occurs in association with the MIBL. The size, location, and internal str ucture of the sea-breeze circulation were realistically simulated. The grad ients of temperature and humidity that bound the MIBL cause perturbations i n the refractivity distribution that, in turn, lead to trapping layers and ducts. The existence, location, and surface character of the ducts were wel l captured. Horizontal variations in duct characteristics due to the sea-br eeze circulation were also evident. The simulations successfully distinguis hed between high-and low-wind occasions, a notable feature of the SHAREM-11 5 observations. The modeled magnitudes of duct depth and strength, although leaving scope for improvement, were most encouraging.