MESOSCALE MODELING OF SUMMERTIME REFRACTIVE CONDITIONS IN THE SOUTHERN CALIFORNIA BIGHT

Large vertical gradients of temperature and moisture, often present at the top of the atmospheric boundary layer, can result in anomalous el ectromagnetic propagation. Layers in which the modified refractive ind ex M decreases with height can act to trap microwave energy depending on the frequency and angle of incidence of the signal. Here the author s examine the ability of a mesoscale model to forecast the topography of such a trapping layer and to predict temporal trends in trapping-la yer structure and depth. Data from the Variability of Coastal Atmosphe ric Refractivity (VOCAR) experiment are used to examine the fidelity o f model forecasts. The intensive observing period of VOCAR occurred fr om 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 schem e, detailed radiative flux computations, and explicit cloud physics. T he impact of several specific mesoscale and synoptic events (e.g., sea -land breezes, a migrating low) upon the refractivity field is examine d along with the model's capacity to forecast these features. The mode l exhibits significant promise in its ability to forecast trends in th e height of the microwave trapping layer. Furthermore, these trends in trapping-layer depth are found to correlate rather well with the temp oral behavior of the measured propagation factor.