The Coastal Environment

Ducting conditions are associated with strong, localised, vertical gradients in meteorological variables[Skolink, 1980]. For example, ducts are frequently observed over the oceans, due to a strong decrease in water vapour pressure immediately above the sea surface. Such evaporation ducts are typically up to 30m deep. Advection ducts may be produced by the movement of relatively warm and dry continental air over a cooler sea. A stable, marine internal boundary layer (MIBL) is formed, capped by a temperature inversion that separates the modified moist air beneath from the unmodified air above. A surface duct, with a height approximately equal to that of the MIBL, is generated by the accumulation of moisture within the stable layer which gives rise to a strong gradient of water vapour pressure across the inversion. Stable MIBL development results in a horizontally inhomogeneous refractivity environment over distances of several hundred kilometres.

Assessment of propagation within coastal environments is a particularly difficult problem since the meteorological conditions can vary quite significantly over both space and time. The refractive index of the atmosphere for radar frequencies can be determined from its temperature, pressure and moisture content, using a well-established semi-empirical formula[Skolink, 1980]. Previously, assessments were made directly from soundings taken at 12 hourly intervals, and assuming horizontal homogeneity across distances of $\sim 100$km. However, the refractivity can also be obtained as a diagnostic from numerical meteorological models, and thereby estimated at arbitrary times and places. We have used a high-resolution mesoscale model[Golding, 1987] ($\Delta x=1$ to $6$km, $\Delta z \sim 10$m in the boundary layer) to study the propagation environment in cases of warm air advection from the Arabian deserts over the cooler waters of the Persian Gulf[Atkinson et al., 2001]. Model results compared favourably with detailed aircraft observations[Brooks et al., 1999] and have been used as input to radar propagation models[Levy and Craig, 1996]. To illustrate the value of our approach, some examples are shown below. Coastal mesoscale processes are seen to have strong influences on the propagation. Work is continuing in order to study such effects more generically.