Mesoscale Modeling of Summertime Refractive Conditions in the Southern California Bight

Abstract

Large vertical gradients of temperature and moisture, often present at the top of the atmospheric boundary layer, can result in anomalous electromagnetic propagation. Layers in which the modified refractive index M decreases with height can act to trap microwave energy depending on the frequency and angle of incidence of the signal. Here the authors examine the ability of a mesoscale model to forecast the topography of such a trapping layer and to predict temporal trends in trapping-layer structure and depth. Data from the Variability of Coastal Atmospheric Refractivity (VOCAR) experiment are used to examine the fidelity of model forecasts. The intensive observing period of VOCAR occurred from 23 August to 3 September 1993 in the Southern California bight. The mesoscale numerical weather prediction model used has a sophisticated physics package that includes a second-order closure turbulence scheme, detailed radiative flux computations, and explicit cloud physics. The impact of several specific mesoscale and synoptic events (e.g., sea?land breezes, a migrating low) upon the refractivity field is examined along with the model?s capacity to forecast these features. The model exhibits significant promise in its ability to forecast trends in the height of the microwave trapping layer. Furthermore, these trends in trapping-layer depth are found to correlate rather well with the temporal behavior of the measured propagation factor.