Modeling Studies of Frontogenesis in the Upper and Middle Troposphere.
Abstract
This research addresses fundamental questions associated with upper-level frontogenesis during the evolution of a mid-latitude disturbance in the framework of three -dimensional primitive-equation model simulations on a beta-plane. Simulations are performed by using two different models based on pressure and isentropic vertical coordinates starting with similar initial conditions in a nearly adiabatic atmosphere. The performance of the models is evaluated by using different spatial resolutions for various initial conditions to Select the most realistic upper-level frontogenesis simulation. Dynamical processes involved in upper-level frontogenesis are analyzed separately in selected pressure and isentropic coordinate model simulations. Analyses reveal many similarities as well as differences in the process of simulated frontogenesis and tropopause folding with these models. The isentropic coordinate model produces stronger and very narrow upper -level fronts and deep tropopause foldings compared to the pressure coordinate model. Analyses suggest that this difference in the isentropic model simulation is a result of the existence of a positive-feedback mechanism suggested by Mudrick (1974) acting during the upper-level frontogenesis. With this feedback, as isentropic surfaces are tilted vertically in the upper-level fronts to enhance the horizontal temperature gradients, stratospheric air with high potential vorticity penetrates into the troposphere. Local increases in the static stability enhances vertical tilting further which leads to very local and narrow frontal developments. Difficulties resulting from intersection of isentropic surfaces with lower boundary with isentropic coordinate model remain unsolved. Consequently, near the lower boundary, potential vorticity and temperature fields are unrealistic. Therefore, combining isentropic coordinate with a sigma -coordinate, which follows the terrain, appears very attractive. However, there are many technical difficulties associated with such a general coordinate system. In this study, a method to design such a hybrid coordinate system is discussed in detail.
- Publication:
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Ph.D. Thesis
- Pub Date:
- 1992
- Bibcode:
- 1992PhDT.......189K
- Keywords:
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- Physics: Atmospheric Science