Island Wake Dynamics and Wake Influence on the Evaporation Duct and Radar Propagation

Abstract

The conditions under which atmospheric island wakes form leeward of Kauai, Hawaii, are investigated using idealized numerical simulations and real data forecasts from the U.S. Navy's Coupled Ocean?Atmosphere Mesoscale Prediction System (COAMPS). Nondimensional mountain height ? is varied in a series of idealized simulations by altering the island's terrain height; with increasing ?, the wake configuration varies from two small counterrotating vortices to a straight wake to a meandering wake to a von Kármán vortex street. In both the idealized and real data forecasts, stability changes across the wake alter the surface layer temperature and moisture profiles, thereby modifying the refractivity and evaporation duct height (EDH) fields. An electromagnetic (EM) propagation model and a radar clutter model are used to demonstrate that the alterations to the refractivity field created by the wake are capable of strongly affecting near-surface EM propagation. Substantial azimuthal variability in radar sea clutter was observed during radar performance tests conducted by the USS O'Kane leeward of Kauai in December of 1999; these anomalies were postulated to result from an island wake. Results from the linkage of COAMPS output with the two EM codes are compared with the radar returns collected aboard the O'Kane, and metrics are developed for comparing COAMPS forecast EDH values with those calculated directly from the shipboard observations.