Fronts and frontogenesis as revealed by high time resolution data Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/8k71nm17g

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  • Upper air soundings taken every three hours are used to examine a cold front of average intensity over a period of 24 hours. Vertical cross sections of potential temperature and wind and horizontal analyses are compared and adjusted until they are consistent with one another. These analyses are then used to study the evolution of the front. The front is found to consist of a complex system of fronts occurring at all levels of the troposphere. Low level fronts are strongest at the surface and rapidly weaken with height. Fronts in the middle and upper troposphere are much more intense. The warm air ahead of the fronts is nearly barotropic while the cold air behind the fronts is baroclinic through deep layers. A deep mixed layer is observed to grow in this cold air. Examination of cross sections of potential temperature and potential vorticity indicates that the air in at least the upper portions of the upper level fronts originates in the stratosphere. No evidence is found, however, of an extrusion of stratospheric air to very low levels. Diabatic processes seem to destroy the extrusion as it descends. The structure of the upper level fronts is complex. These fronts are observed to split apart, recombine, and descend to low elevations. This descent is due to the incorporation into the front of pre-existing stable/baroclinic layers. An equation for parcel-following frontogenesis in isentropic coordinates is developed and applied. No single process was found to be dominant in changing frontal intensity. Frontogenesis occurs on the leading edge of the fronts and frontolysis on the trailing edge. The magnitudes of the computed frontogenesis decrease downstream from the axis of the upper level trough. Isentropic trajectories are constructed in order to verify the computed values of parcel-following frontogenesis. Poor correlations are found between the computed and trajectory-following values of frontogenesis. This is believed to be due to nonlinearities in the field of frontogenesis and to errors in the trajectories. Such nonlinearities cast doubt on the usefulness of conventional data for the study of frontogenesis. Vertical velocities are computed using a kinematic technique. Reasonable fields of vertical velocity are obtained in the vicinity of the fronts and jet streaks. Good correlations are found between the vertical displacement between endpoints of the trajectories and the value of computed vertical velocity integrated over the path of the trajectory. The field of vertical velocity is also found to be highly nonlinear.
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