Numerical study of an idealized cyclone evolution and its sub-synoptic features

Ahmet Yildirim, Purdue University

Abstract

The Purdue mesoscale model is applied to study the evolution of cyclones within a baroclinic zone in a periodic channel domain. Initially, the basic state is stably stratified with a reduced static stability in the lower atmosphere ($<$700 mb) and increased static stability above that level as compared to a standard atmosphere. A hyperbolic tangential variation of temperature is imposed in the y-direction. Consistent with the thermal wind relation, a jet also exists near the tropopause. At the outset, small amplitude perturbations are superimposed on the mean temperature and geopotential fields, and then integration is run for twelve days or longer in order to study the life cycle of the cyclone. Results from both an inviscid atmosphere and a viscous atmosphere with condensation are presented. In the inviscid case, the perturbation grows rapidly mainly due to the baroclinic instability, and reaches its mature stage in about a week. Subsequently, the cyclone starts to fill in and completes its life cycle. The model simulation produces a cold front, warm front, occluded front, and warm air seclusion. In the viscous case that includes condensation, the central pressure of the cyclone drops slightly more than it does in the inviscid case simulation, mainly due to the latent heat of condensation. In this case, a well defined frontal band forms in the cold side of the occluded front and produces precipitation. This band is shallow (below 700 mb) and short lived. In order to gain better insight about the evolution of the cyclone, fronts and small-scale frontal waves, a simple energy conversion and relative vorticity budget analysis are applied. It is shown that in the early stages of the cyclone development, the perturbations grow mainly by baroclinic instability while the effect of condensation becomes important in the mature stage. After the mature stage, cyclone starts to decay due to the decrease of the horizontal temperature gradient as the available potential energy is consumed by the perturbations. Comparison with other numerical simulations and observations are also discussed in this study.

Degree

Ph.D.

Advisors

Sun, Purdue University.

Subject Area

Atmosphere

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