Numerical simulations of cyclogenesis over the western United States

Jiun-Dar Chern, Purdue University

Abstract

In this study the Purdue Mesoscale Model (PMM) was used to study a winter cyclone that developed in the Great Basin region on 1-3 March 1985. The primary goal of this study is to improve the understanding of the process of explosive development and the evolution of the cyclone over complex terrain. To achieve this goal, a real case simulation was combined with sensitivity tests and diagnostic analyses of the model outputs. The control simulation was able to replicate the rapid development and the movement of the cyclone. The central pressures of the simulated low centers were within 3 mb of the observed values and the predicted cyclone paths were within 135 km of the ECMWF analyses. The predicted precipitation amounts and patterns were in general agreement with observed fields. To identify the importance of various physics processes in the cyclogenesis, six sensitivity experiments were conducted. It was found that the development of the cyclone was mainly controlled by adiabatic dynamics in this case study. The effect of topography was also important to the location and movement of the cyclone. Without the mountains, the surface low center over Nevada disappeared and the low center over Idaho was located farther north. Over the entire domain, the effects of surface friction and latent heating were significant. However, these physical processes only had a minor effect on the cyclone development in the Great Basin region. Diagnostic analyses were performed to help pinpoint the dynamical essence of cyclogenesis. It was found that upper-level forcing was essential to the rapid development of the surface cyclones. Based on potential vorticity analysis, we have been able to describe the development in terms of the advection of a positive PV anomaly. It was found that the rapid cyclogenesis occurred when the upper-level high PV spread over the low-level baroclinic zone. The time evolution of the tropopause height suggested that the positive PV anomaly coincided with a tropopause folding. It was also found that the surface cyclone over Nevada was situated inside the major ascending motion induced by the upper-level PV anomaly during the rapid deepening period. From analyses of the divergence fields and ageostrophic flows, it was shown that the observed patterns were very complicated and could not be fully described in terms of idealized two-dimensional jet streak dynamics. On the other hand, vertical cross sections of ageostrophic circulations indicated that the development of the cyclone over Nevada coincided with the ascending branch of an indirect circulation in the left quadrant exit region of the anticyclonic jet streaks upstream of the developing trough. The heavy snowfall over South Dakota also can be understood as the interaction of an upper-leveljet-frontal system and a strong surface warm front in the vicinity of the entrance region of an upper-level jet streak.

Degree

Ph.D.

Advisors

Sun, Purdue University.

Subject Area

Atmosphere

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