A family of frontal cyclones over the western Atlantic Ocean. Part I: A 60-h simulation

Despite marked improvements in the predictability of rapidly deepening extr atropical cyclones, many operational models still have great difficulties i n predicting frontal cyclogenesis that often begins as a mesoscale vortex e mbedded in a large-scale (parent) cyclone system. In this paper, a 60-h sim ulation and analysis of a family of frontal cyclones that were generated ov er the western Atlantic Ocean during 13-15 March 1992 are performed using t he Pennsylvania State University-National Center for Atmospheric Research m esoscale model with a fine-mesh grid size of 30 km. Although it is initiali zed with conventional observations, the model reproduces well the genesis, track and intensity of the frontal cyclones, their associated thermal struc ture and precipitation pattern, as well as their surface circulations, as v erified against the Canadian Meteorological Centre analysis and other avail able observations. It is shown that each frontal cyclone is initiated successively to the sout hwest of its predecessor in the cold sector, first appearing as a pressure trough superposed on a baroclinically unstable basic state in the lowest 15 0-300 hPa. Then, it derives kinetic energy from the low-level available pot ential energy as it moves over an underlying warm ocean surface (with weak static stability) toward a leading large-scale frontal zone and deepens rap idly by release of latent heat occurring in its own circulations. One of th e frontal cyclones, originating in the cold air mass, deepens 44 hPa in 42 h and overwhelms the parent cyclone after passing over the warm Gulf Stream water into the leading frontal zone. These cyclones have diameters ranging from 500 to 1100 km (as denoted by the last closed isobar) and are spaced 1000-1400 km apart (between their circulation centers) during the mature st age. They begin to establish their own cold/warm frontal circulations once their first closed isobars appear, thus distorting the leading large-scale frontal structures and altering the distribution and type (convective versu s stratiform) of precipitation. It is found that the frontal cyclones accelerate and experience their centr al pressure drops as they move from high to low pressure regions toward the parent cyclone center, and then they decelerate and fill as they travel aw ay from the parent cyclone. Their spatial and temporal scales, vertical str uctures, as well as deepening mechanisms, are shown to differ significantly from those typical extratropical cyclones as previously studied.