3M.1A The effects of orography on the propagation and precipitation distribution of pre-existing mesoscale convective systems under different Froude number flow regimes

Wednesday, 26 October 2005: 10:30 AM
Alvarado GH (Hotel Albuquerque at Old Town)
Heather Dawn Reeves, North Carolina State University, Raleigh, NC; and Y. L. Lin

Observations from field programs such as MAP, TAMEX, and HaRP reveal that pre-existing mesoscale convective systems (i. e. convective systems not generated by orographic lifting) moving toward an orographic barrier sometimes stagnate or stall on the upstream side of the barrier leading to high precipitation accumulations and the potential for flash flooding. Some previous researchers have noted that stagnation occurs when the Froude number of the basic state flow (F) is low. In this study we explore whether or not stagnation is related to F through a series of two-dimensional, idealized simulations of a squall line impinging on a mountain. In these simulations, the uniform basic state wind speed was varied so that there were cases of blocked and unblocked flow. For the blocked flow cases (i. e. low F cases), the convective system stagnated far upstream of the mountain with large accumulations of precipitation at the location of stagnation, consistent with previous observations. For cases of unblocked, or high F flow, there were moderate precipitation accumulations over the mountain peak or upslope and low accumulations elsewhere. This result is not consistent with linear theory. Further analysis from the unblocked cases reveals that evaporative cooling associated with the pre-existing convective system acted to stabilize the low-level airstreams immediately upstream of the mountain, leading to a marked decrease in F. This decrease in F, which was sufficient to shift the flow into a non-linear or blocked regime for ambient wind speeds less than a critical wind speed (18 m/s for the simulations herein), appears to be responsible for the lower precipitation accumulations over the mountain peak. In order to determine whether the mechanisms uncovered in the idealized simulations are active in the real atmosphere, full physics simulations of stagnated vs. nonstagnated cases were conducted. These simulations show there is a dependence on evaporative cooling and mountain height (i. e. F) for squall line stagnation and, hence, precipitation accumulation.
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