Orographic Influences on Coastal Refractivity

Analyses and forecasts of mesoscale refractive conditions, particularly radar trapping and anomalous propagation, are of critical importance to Naval communications and operations. The Navy's Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) is used in this investigation of the variability of refractive condition in the coastal zone. COAMPS skill in producing microwave refractivity forecasts has been evaluated using the VOCAR (Variability of Coastal Atmospheric Refractivity) dataset for which 425 radiosonde at eight observing stations in the California bight were released during August/September 1993. Statistics of model forecast duct base, strength, and thickness have been compared to verifying VOCAR radiosondes.

Utilizing COAMPS first as a research tool with homogeneous, idealized initial conditions, we examine the effect on the refractivity field of channeling the marine layer along several different simplified coastal configurations in which bend angles, orientations and positions are altered. Additionally, COAMPS real-data forecasts are validated with observations from the ONR sponsored field experiment COAST. This field study was conducted along the U.S. west coast during June/July 1996 to characterize the marine layer in the summertime California littoral. Comparisons of aircraft data with model fields reveal good agreement in the vertical profiles of the meteorological quantities as well as in the modified refractivity.

The observations suggest that the marine atmospheric boundary layer (MABL), when bounded above by a strong temperature inversion and laterally by coastal orography, may behave as supercritical, channeled flow. Reduced MABL heights and accelerated velocities are characteristic of supercritical flow leeward of coastal capes and points while blocked flow upwind of these topographic features often becomes subcritical, favoring stratus-capped, elevated MABL depths and reduced wind speeds. Here we explore the extent to which electromagnetic trapping layers, which tend to occur at the base of the MABL inversion are influenced by changes in the coastline and correlate to flow criticality. High-resolution model fields indicate that although these supercritical marine layer responses are tied to the topographic forcing, they exhibit substantial diurnal variability in shape and magnitude. Our results further show that locations of abrupt transition from elevated to surface based ducting tend to occur within supercritical coastal flow regimes often extending hundreds of kilometers downwind and well offshore.