A numerical investigation of a family of frontal cyclogenesis events during CASP II.

摘要

In this thesis, a series of (48-60 h) numerical simulations of a family of frontal cyclogenesis events that occurred over western Atlantic Ocean during 13-15 March 1992 are conducted using a nested-grid version of the PSU/NCAR mesoscale model (MM4) with a fine-mesh grid size of 30 km. It is shown that MM4 captures very well the genesis, track and intensity of three secondary cyclones, their associated thermal structure and precipitation pattern as well as their surface circulations.;It is found that an upper-level potential vorticity (PV) ring plays an important role in determining the initiation and track of the frontal cyclones. The cyclones appear to form as a consequence of the superposition of upper-level PV anomalies on the low-level intense baroclinicity in the cold sector behind the slow moving primary cold front, and then they propagate into colder air towards the parent cyclone's center. It is also found that as the MFC intensifies, a mesoscale trough is induced in the low-to-middle troposphere, creating a favorable phase lag between the new pressure trough and a slow moving thermal wave. This phase lag provides a baroclinic conversion mechanism by which the system's kinetic energy could increase rapidly at the expense of available potential energy.;Diagnosis of sensitivity experiments reveals (i) dry dynamics determines the initiation and track of the frontal cyclones, accounting for about 59% of the final intensity of the MFC; (ii) the low-level baroclinicity and the upper-level PV anomalies are near-equally important in the genesis of the dry systems; (iii) the Ekman spin-down tends to slow substantially the development of the frontal cyclones; and (iv) surface heat and moisture fluxes could produce a significant impact (i.e., 59%) on the final intensity of the cyclones in the presence of latent heat release, but its impact is small in the dry dynamical framework. (Abstract shortened by UMI.)