Zheng-yi Peng

Thesis Approved June 1993

Mesoscale Potential Vorticity Analysis of a Warm Front in a Marine Cyclone

Extratropical cyclones and their associated fronts have been a matter for vigorous scientific debate for many years. Because of the explosive deepening of these cyclones on temporal and spatial scales of a few hours and a few hundred kilometers, with deepening rates of ten millibars or more per six hours, such development requires a unique interaction of synoptic, mesoscale, and boundary-layer proceses. Current observations and technologies make it feasible to observe cyclones and the associated fronts over the ocean and critically evaluate the conceptual and theoretical models.

The mesoscale properties of a surface warm front within a moderate marine cyclone over the ocean is studied. Especially, the potential vorticity structure within the warm front is analyzed. It is found that the air on the warm side of the front and above the front is subject to conditional symmetric instability (CSI). The precipitation on the warm side of the front could be the result of the release of this instability. This warm front is characterized by the high potential vorticity in the frontal zone. This result shows that potential vorticity can be viewed as a valuable indicator of the surface frontal position.

The effects of diabatic processes, such as air-sea interaction, turbulent mixing and latent heating as well as friction, on the potential vorticity structure are estimated. These results indicate that turbulent mixing processes play a dominant role in the formation of the high potential vorticity area. A simple mixing model is constructed to illustrate the turbulent mixing effects. These results confirm the results obtained from numerical modelling studies by other scientists (Keysey and Anthes, 1982; Baldwin et al., 1984; Knight, 1987).

Zheng-yi is currently studying for his Ph.D. at York University in Toronto.