Summertime marine refractivity conditions along coastal California

Large vertical gradients of temperature and moisture, which are not uncommo n at the top of the marine atmospheric boundary layer (MABL), yield strong gradients in microwave refractivity that can result in anomalous electromag netic (EM) propagation, including ducting wherein energy is strongly channe led horizontally. Of particular importance to surface radars and other micr owave transmitters are surface-based ducts in which energy is ducted throug hout the entire depth of the MABL. The Naval Research Laboratory's Coupled Ocean-Atmosphere Mesoscale Prediction System (COAMPS) is used to define bou ndary layer structure during two coastal field experiments, and this model' s ability to forecast refractivity, including surface-based ducting, is ass essed. At three marine sites, COAMPS shows considerable skill in MABL forec asts during the Variability of Coastal Atmospheric Refractivity experiment, although it contains biases for the MABL to be somewhat shallow and for th e forecast duct strength, measured by the refractivity jump at MABL top, to be too weak. Nevertheless, for a total of 95 forecasts at these sites, COA MPS correctly forecasts surface-based ducting/no surface-based ducting even ts 82% of the time, with a false alarm rate of only 0.15. Abrupt variations in coastal MABL depth and wind speed have been observed a nd modeled when supercritical flow (Froude number. 1) interacts with coasta l terrain that is uniformly higher than the MABL depth. COAMPS and data fro m the Coastal Waves 1996 (CW96) experiment conducted along the northern Cal ifornia coast are used to address the implications of MABL variability to t he coastal EM propagation environment. Comparison of CW96 research aircraft cross sections and soundings with COAMPS forecast fields indicate that the mesoscale model captures much of the observed MABL vertical structure and horizontal variability, including the temperature inversion and moisture la pse capping the MABL, the along- and cross-shore MABL slopes, and the prese nce and intensity of a coastal low-level jet. Supercritical expansion fans form in the lee of Cape Blanco, Oregon, and Cape Mendocino, California, on 1 July 1996 during CW96, and COAMPS indicates the presence of a compression jump where the flow is blocked on the upwind (north) side of Cape Mendocin o. In conjunction with these MABL features, the EM propagation environment also exhibits substantial alongshore variation. Stronger, near-surface-base d ducting occurs in the expansion fans as the marine layer thins and accele rates; weaker, elevated ducting occurs in the blocked flow.