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 resolution of 5 km, is used with measurements taken during intensive observation 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 precipitation bands, with embedded convective elements, are present along the coastal front in the vicinity of the Gulf Stream. As the frontogenesis evolves, the simulated surface frontal zone becomes fractured, and discontinuous lines of confluence and mesoscale ascent become apparent. A collapse of the cross-frontal thermal gradient is driven by intense gradients of the surface fluxes in the vicinity of the Gulf Stream. A mesoscale wave train, consisting of a series of shallow, weak vortices with horizontal scales between 50 and 100 km, forms along the front in agreement with the Doppler radar data and surface observations. Diagnostic 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 vortices along the coastal front. The formation mechanisms of these vortices may have important implications for the genesis of coastal cyclones and polar lows along shallow baroclinic zones.