NONHYDROSTATIC SIMULATIONS OF COASTAL MESOBETA-SCALE VORTICES AND FRONTOGENESIS

A nonhydrostatic version of the Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model, with a horizontal re solution of 5 km, is used with measurements taken during intensive obs ervation period 2 of the Genesis of Atlantic Lows Experiment to study the offshore mesobeta-scale coastal front structure. Results from the 24-h model simulation and Doppler radar data indicate that precipitati on bands, with embedded convective elements, are present along the coa stal front in the vicinity of the Gulf Stream. As the frontogenesis ev olves, the simulated surface frontal zone becomes fractured, and disco ntinuous lines of confluence and mesoscale ascent become apparent. A c ollapse of the cross-frontal thermal gradient is driven by intense gra dients of the surface fluxes in the vicinity of the Gulf Stream. A mes oscale wave train, consisting of a series of shallow, weak vortices wi th horizontal scales between 50 and 100 km, forms along the front in a greement with the Doppler radar data and surface observations. Diagnos tic analysis of the model simulation and a series of model sensitivity experiments indicate that shearing instability along the frontal zone focuses the lower-tropospheric convergence. Subsequently, stretching of cyclonic vorticity, modulated by latent heating associated with the banded precipitation, leads to the generation of the mesobeta-scale v ortices along the coastal front. The formation mechanisms of these vor tices may have important implications for the genesis of coastal cyclo nes and polar lows along shallow baroclinic zones.