Numerical simulations of cold air advection over the Appalachian Mountains and the Gulf Stream

Cold air advection over the Gulf Stream off the Carolinas and the Appalachian Mountains was studied using idealized two-dimensional cases for the Genesis of Atlantic Lows Experiment (GALE) IOP 2 conditions. An anelastic hydrostatic mesoscale model was used. Turbulent transfer in the planetary boundary layer, diurnal heating, cloud dynamics, atmospheric longwave and shortwave radiation and subgrid cumulus parameterization were included in the model. Model results show that the geometry of the oceanic and coastal rain bands depends on the direction of the ambient flow (onshore or offshore) . For onshore flows, the rain bands remain in the vicinity of the oceanic baraclinic zone. The rain bands become transient and migrate down wind of the Gulf Stream front for offshore flows. Depths of the marine boundary layer (MBL) and the cloud (or rain) bands depend more on the ambient flow speed than its direction. The rain bands develop primarily in response to the strong low level convergence. As expected, southward winds are produced at the eastern side of the Appalachian Mountains for onshore conditions. A significant amount of the turning, however, results from the baroclinic zone over the ocean. Upstream influence of the mountain intensifies the updraughts in the MBL and moves the oceanic rain bands further offshore. The effects of the atmospheric longwave and shortwave radiation, subgrid cloud heating and diurnal ground heating are of secondary importance in influencing the structure of the MBL as compared to the surface turbulent heat fluxes. Diurnal effects can change the coastal inland flow regime considerably, resulting in a local breeze and the formation of another cloud (or rain) band. (A)