Mesoscale Disturbances and Orographic Precipitation Distribution: Three Special Case Scenarios
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2007-04-05
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Abstract
Flow patterns and precipitation distribution in the vicinity of mountains are often attributed to whether the flow is in a blocked or unblocked regime, which, in turn, is dictated by the wind speed and static stability far upstream of the mountain and the mountain height. Herein, the notion that mesoscale disturbances immediately upstream of mountains and/or terrain inhomogeneities could alter the flow patterns significantly, resulting in a precipitation distribution that does not fit typical patterns for blocked vs. unblocked flows, was studied via consideration of specific case studies and idealized numerical experiments. The first scenario considered that of an orographically trapped stable layer upstream of a mountain. A series of quasi-idealized simulations suggests that flow patterns and precipitation distribution in the vicinity of a mountain are affected not only by the presence of such a layer, but by the strength of the capping inversion of this layer. Part two of this research investigated the effects of a pre-existing convective system upstream of a mountain for conditionally unstable flow, where a pre-existing convective system is defined as one that is triggered by some mechanism other than lifting by the mountain in question. Idealized numerical experiments with differing upstream wind speeds reveal that if the flow was in a blocked regime, the introduction of a pre-existing precipitation system had little effect on the precipitation distribution and flow patterns in the vicinity of the mountain. If the flow was in an unblocked regime, the introduction of a pre-existing precipitation system was associated with flow reversal upstream of the mountain and an upstream shift of the locus of maximum precipitation. The last part of the thesis considered the effects of terrain inhomogeneities on precipitation distribution via full-physics simulations of two heavy precipitation events along the western face of the Sierra Nevada mountains. Of the terrain features that were tested, the greatest sensitivity in the precipitation distribution was to directional changes in slope orientation along the upstream face of the range. In sensitivity tests where these undulations were removed, isolated maxima along the upslope that were present in the corresponding reference simulations were either reduced or non-existant. The overriding conclusion from this thesis is that although the wind speed and static stability far upstream of a mountain are important, mesoscale flow phenomena immediately upstream of a mountain can significantly change the flow and precipitation patterns.
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precipitation, mountain
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Degree
PhD
Discipline
Marine, Earth and Atmospheric Sciences
