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A Study of the Development of Extratropical Cyclones with an Analytic Model. Part I: The Effects of Stratospheric Structure

Paul A. HirschbergDepartment of Meteorology, Naval Postgraduate School, Monterey, California

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J. Michael FritschDepartment of Meteorology, The Pennsylvania State University, University Park, Pennsylvania

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Abstract

A new five-layer, quasigeostrophic model of baroclinic development is utilized to examine the initial-value problem sensitivity of extratropical cyclogenesis to the variation of stratospheric thermal and geopotential configurations associated with tropopause undulations. Previous studies have suggested that such undulations or potential vorticity anomalies can influence both the structure and evolution of lower-tropospheric cyclones. A series of experiments with the five-layer model are performed to evaluate the sensitivity of the model height, vertical motion, and height tendency patterns to various stratospheric temperature and geopotential distributions. It is found that idealized tropospheric baroclinic systems do not show typically observed characteristics unless certain stratospheric temperature, geopotential, and wind anomaly configurations associated with tropopause undulations are present. Furthermore, for given tropospheric patterns, there are particular lower-stratospheric configurations that optimize the development of model lower-tropospheric cyclones. These stratospheric configurations are functions of 1) the value of the lower-stratospheric temperature anomaly, 2) the amplitude of the tropopause undulation, and 3) the horizontal location of the undulation relative to the tropospheric temperature anomalies. Finally, both the rate of cyclogenesis and the amplification of the tropopause undulation increase if tropospheric static stability is reduced.

Abstract

A new five-layer, quasigeostrophic model of baroclinic development is utilized to examine the initial-value problem sensitivity of extratropical cyclogenesis to the variation of stratospheric thermal and geopotential configurations associated with tropopause undulations. Previous studies have suggested that such undulations or potential vorticity anomalies can influence both the structure and evolution of lower-tropospheric cyclones. A series of experiments with the five-layer model are performed to evaluate the sensitivity of the model height, vertical motion, and height tendency patterns to various stratospheric temperature and geopotential distributions. It is found that idealized tropospheric baroclinic systems do not show typically observed characteristics unless certain stratospheric temperature, geopotential, and wind anomaly configurations associated with tropopause undulations are present. Furthermore, for given tropospheric patterns, there are particular lower-stratospheric configurations that optimize the development of model lower-tropospheric cyclones. These stratospheric configurations are functions of 1) the value of the lower-stratospheric temperature anomaly, 2) the amplitude of the tropopause undulation, and 3) the horizontal location of the undulation relative to the tropospheric temperature anomalies. Finally, both the rate of cyclogenesis and the amplification of the tropopause undulation increase if tropospheric static stability is reduced.

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