Ashley, Walker S.
M.S. (Master of Science)
Department of Geographic and Atmospheric Sciences
Baroclinic instability is one of the primary drivers of the extratropical cyclogenesis and maturation. During the cool-season months in the continental mid- and high-latitudes, changes in lower-tropospheric thermal gradients and, thus, baroclinity can occur due to alterations in land-surface snow coverage. As the climate system warms, reduced cool-season snow pack may occur across these continental regions. An assessment of how this possible change in seasonal snow pack affects extratropical cyclone tracks and intensity is important since these storms produce a large proportion of the hazards and sensible weather in these regions. Using a numerical modeling framework, this thesis analyzes the effects of enhanced, as well as reduced, snow pack on a North American cyclone case that was driven primarily by lower-tropospheric thermal gradients and related baroclinic instability. Several ensemble families of the Weather Research and Forecasting model (WRF) using various schemes of land surface, microphysics, and radiation are generated to study the effects of changing snow pack character on the selected storm. Diagnostic images such as surface charts, 850--700 hPa frontogenesis, and precipitation fields are analyzed to determine the influence, if any, snow pack had on the simulated extratropical cyclone's intensity, track, and precipitation. It was hypothesized that under an enhanced snow pack, the lower-tropospheric thermal gradient near the storm would be more intense, resulting in larger baroclinic instability. This enhancement of a fundamental ingredient for extratropical system formation and intensification was expected to promote a stronger storm characterized by a deeper central sea level pressure, as well as a possible shift in track and resulting precipitation fields. Conversely, the reduced snow pack simulations were expected to result in reduced baroclinity and storm intensity. Results from the experimental modeling framework demonstrate that low-level baroclinity acted to modify the frontogenesis of the cyclone, changing the coverage and intensity of precipitation, while the cyclone's minimum pressure and track remained largely unchanged. This case study suggests that future climate scenarios in which reduced snow cover is present may result in cyclones with precipitation covering a smaller area, but with at a higher intensity.
Fritzen, Robert Clayton, "The influence of a pre-existing snow pack on an extratropical cyclone" (2017). Graduate Research Theses & Dissertations. 5904.
Northern Illinois University
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