AbstractsPhysics

Considerable progress has been made in the past decades on the life cycle of rapidly deepening winter cyclones. However, little attention has been paid to the roles of mesoscale convective systems (MCSs) in extratropical cyclogenesis that occurs within weak baroclinic environments. In this thesis, the impact of an MCS on the subsequent surface cyclogenesis is investigated, using a 36-h three-dimensional, high-resolution simulations of the famous 10-12 June squall line that occurred during PRE-STORM. The model simulates remarkably well the initiation of the squall line, numerous mesoscale surface pressure perturbations and midlevel circulation structures during the mature stage, and the subsequent surface cyclogenesis after the dissipation of the system. It is found that the squall line is initiated ahead of a weak surface front with the aid of baroclinic forcing. Once initiated, the squall system is more or less driven by the interaction of convectively generated circulations with the potential unstable environment ahead. The baroclinic forcing only provides a favorable environment for the evolution of the squall system. As the squall system rapidly intensifies and accelerates eastward, it enhances the larger-scale baroclinicity and produces a phase-lag between the pressure and thermal waves so that the baroclinic environment is more favorable for surface cyclogenesis. To isolate the roles of moist convection in the surface cyclogenesis, a "moist" and a "dry" simulation are compared. It is found that in the absence of moist convection the model could also produce a surface cyclone, but with much weaker intensity, much smaller extent and slower displacement. The effects of moist convection are shown not only to increase the upper-level and decrease the lower-level height of isobaric surfaces, but also to condition the baroclinic environment by increasing the phase lag between the pressure and thermal waves and enhancing the large-scale baroclinicity.