Summertime Marine Refractivity Conditions along Coastal California

Abstract

Large vertical gradients of temperature and moisture, which are not uncommon at the top of the marine atmospheric boundary layer (MABL), yield strong gradients in microwave refractivity that can result in anomalous electromagnetic (EM) propagation, including ducting wherein energy is strongly channeled horizontally. Of particular importance to surface radars and other microwave transmitters are surface-based ducts in which energy is ducted throughout the entire depth of the MABL. The Naval Research Laboratory’s Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) is used to define boundary layer structure during two coastal field experiments, and this model’s ability to forecast refractivity, including surface-based ducting, is assessed. At three marine sites, COAMPS shows considerable skill in MABL forecasts during the Variability of Coastal Atmospheric Refractivity experiment, although it contains biases for the MABL to be somewhat shallow and for the 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, COAMPS correctly forecasts surface-based ducting/no surface-based ducting events 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 and modeled when supercritical flow (Froude number > 1) interacts with coastal terrain that is uniformly higher than the MABL depth. COAMPS and data from the Coastal Waves 1996 (CW96) experiment conducted along the northern California coast are used to address the implications of MABL variability to the 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 lapse capping the MABL, the along- and cross-shore MABL slopes, and the presence 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 Mendocino. In conjunction with these MABL features, the EM propagation environment also exhibits substantial alongshore variation. Stronger, near-surface-based ducting occurs in the expansion fans as the marine layer thins and accelerates; weaker, elevated ducting occurs in the blocked flow.