Presenter: Heather D. Reeves
Advisor(s): Yuh-Lang Lin
Author(s): Heather D. Reeves
Graduate Program: Marine, Earth, and Atmospheric Sciences

Title: The effects of orography on the propagation and precipitation distribution of pre-existing convective systems

Abstract: Moist flow impinging on steep mountains can cause heavy precipitation and flash flooding. This, in turn, can lead to property damage and loss of life. One type of orographically modified precipitation system that has received relatively little attention in the scientific community is that of a pre-existing convective (or squall) line impinging on a mountain range. Observations of squall lines impinging on various mountain ranges world-wide show that the squall lines occasionally stagnate over the foothills of the mountain range leading to copious accumulations of precipitation in areas typically not affected by heavy orographic precipitation. It is unclear what processes are responsible for stagnation of a pre-existing convective system. Furthermore, the ability of forecast modes to adequately capture squall line stagnation has been shown to be rather poor.

Previous researchers have argued that the distribution and magnitude of precipitation maxima associated with a pre-existing convective system impinging on a mountain may be related to the Froude number (F) of the basic state flow, with stagnation occurring when F is low. This hypothesis is tested herein through a series of idealized, two-dimensional simulations. Contrary to previous arguments, these simulations reveal that stagnation occurs for flows with moderate to high values of F. Additionally, these simulations suggest that squall line stagnation is due to evaporative cooling upstream of the mountain range. This finding has important consequences for numerical weather prediction as forecast models typically have a very poor skill at resolving evaporative cooling for heavy precipitation events.