A STUDY OF THE DEVELOPMENT OF EXTRATROPICAL CYCLONES WITH AN ANALYTICMODEL .2. SENSITIVITY TO TROPOSPHERIC STRUCTURE AND ANALYSIS OF HEIGHT TENDENCY DYNAMICS

An analytic quasigeostrophic model is used to examine the sensitivity of type B cyclogenesis to the vertical structure of the troposphere gi ven a particular stratospheric temperature configuration. it is found that there is an optimal tropospheric configuration that produces the largest negative height tendency at the center of the 1000-mb model cy clone. Based on the response of the 1000-mb height tendencies, alterin g the baroclinicity in the model planetary boundary layer (PBL) does n ot significantly affect the instantaneous quasigeostrophic dynamics of the deep atmosphere. Rather, the PBL temperature anomalies affect the development of lower-tropospheric model lows by hydrostatically shift ing or steering the cyclone centers to locations beneath more (or less ) favorable deep atmospheric quasigeostrophic conditions for developme nt. Diagnostic analyses of three individual stratospheric-tropospheric model configurations are also performed to examine the dynamics that drive the height (pressure) tendency field. Generally, the analytic mo del findings confirm previous observational and numerical investigatio ns of height tendency mechanisms and support the notion of a stratosph eric level of insignificant dynamics. In the optimal development case, the 1000-mb low is located almost directly underneath the region of s trongest 200-mb temperature advection associated with a tropopause und ulation (potential vorticity anomaly). This strong lower-stratospheric warm advection instantaneously overwhelms adiabatic cooling in the st ratosphere and troposphere so that there are height falls over and dow nstream of the 1000-mb low. When the static stability is lowered in th e troposphere and raised in the stratosphere to realistic ''warm-secto r'' values, the vertical motion increases, and the local warming in th e stratosphere and cooling in the troposphere decrease. The reduced tr opospheric cooling results in larger net local column warming that int ensifies the 1000-mb height falls. The intensified vertical circulatio n also acts to amplify the tropopause undulation. As the amplitude of the undulation increases, characteristics of the occlusion process can be identified.