Tropopause Undulations and the Development of Extratropical Cyclones.

This study evaluates the hypothesis that the development of extratropical cyclones is influenced by the evolution of temperature anomalies associated with meso-alpha scale tropopause undulations. A diagnostic analysis of an observed cyclone is used to show that the three-dimensional distribution of height change is sensitive to the distribution of temperature change in the lower stratosphere, especially near 200 mb. Quantitative analysis of the storm with a geopotential height tendency equation indicates that a synergistic process developed between the upper and lower levels whereby the vertical motion pattern maintained and intensified the lower-stratospheric temperature anomalies while the subsequent upper-level warm advection led to an enhanced vertical circulation. Using the results of this diagnostic study, a new physically appealing conceptual model is constructed. The conceptual model is based on the hydrostatic and vertical motion adjustments that occur as tropopause undulations propagate over favored regions of tropospheric warm advection and less stable air. In the second part of the study, an analytic five -layer model is developed to examine the governing hypothesis and the conceptual model. The analytic model yields nonlinear diagnostic solutions to simple initial-value problems. Results of series of model experiments substantiate the results of the case study and many aspects of the new conceptual model. Specifically, it is found that tropospheric cold core baroclinic systems do not show their traditionally observed characteristics unless certain lower-stratospheric temperature configurations associated with tropopause undulations are present. Furthermore, for given tropospheric temperature structures, there are particular lower-stratospheric temperature configurations which optimize the development of the 1000 mb model low. These stratospheric configurations are functions of: (1) the magnitude of the lower-stratospheric temperature perturbation; (2) the elevation of the tropopause undulation; (3) the amplitude of the tropopause undulation; and (4) the horizontal location of the undulation relative to the tropospheric temperature anomalies. Another important finding from the model study is that altering the baroclinicity of the PBL does not significantly affect the instantaneous quasi-geostrophic dynamics. Rather, low-level temperature perturbations affect cyclonic development indirectly by acting to hydrostatically shift cyclone centers to locations with more favorable conditions for development.