Physical influences on east coast cyclogenesis

Three examples of cyclogenesis off the east toast of North America are studied using an 8-level primitive equations model. The model includes input of sensible and latent heat from the sea surface, both parameterized convective and large-scale precipitation and release of latent heat, surface frictional drag and orography. The grid size is 190 km. Twenty-four-hour prognoses were made for four time periods with similar results: orography and fluxes of sensible and latent heat from the ocean were of little consequence, while the effect of the land-water roughness contrast was significant. The lack of appreciable orographic influences is attributable to the fact that the Lows studied crossed the Appalachians south of the highest terrain. The cyclones formed along well-developed frontal systems where the difference between air or dew-point temperature and sea temperature in the warm sector was small or negative. Consequently fluxes of both sensible and latent heat near the Low centres during cyclogenesis were small. They were large, as expected, in the cold air flowing out over the water to the rear of the cyclones. However, the Laplacian of the flures did not favour intensification at the Low centres. Moreover, heating in the cold air destroys available potential energy. It is true that the warm sector air probably had an initial vertical temperature and moisture distribution conducive to precipitation and release of latent heat. However, this arose due to fluxes prior to deepening. The dramatic effect of frictiona drag was associated with a decrease in low-level mass convergence as the Lows moved from rough land to smooth sea.