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Browsing by Author "Schumacher, Russ Stanley, author"

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    Quasi-stationary, extreme-rain-producing convective systems associated with midlevel cyclonic circulations
    (Colorado State University. Libraries, 2008) Schumacher, Russ Stanley, author; Johnson, Richard H., advisor
    Observations and numerical simulations are used to investigate the atmospheric processes responsible for initiating, organizing, and maintaining quasi-stationary mesoscale convective systems (MCSs) that form in association with midlevel mesoscale convective vortices or cutoff lows. Six events were identified in which an MCS remained nearly stationary for 6-12 hours and produced excessive rainfall that led to significant flash flooding. Examination of individual events and composite analyses reveals that the MCSs formed in thermodynamic environments characterized by very high relative humidity at low levels, moderate convective available potential energy, and very little convective inhibition. In each case, the presence of a strong low-level jet (LLJ) led to a pronounced reversal of the wind shear vector with height. Convection was initiated by lifting associated with the interaction between the LLJ and the midlevel circulation. One of these events was examined in detail using numerical simulations. This MCS, which occurred on 6-7 May 2000 in eastern Missouri, produced in excess of 300 mm of rain in 9 hours and led to destructive flash flooding. Simulations indicate that the MCS was long-lived despite the lack of a cold pool at the surface. Instead, a nearly stationary low-level gravity wave helped to organize the convection into a quasi-linear system that was conducive to extreme local rainfall amounts. Additionally, the convective system acted to reintensify the midlevel MCV and also caused a distinct surface low pressure center to develop in both the observed and simulated system. To further understand the important processes in these MCSs, idealized simulations using a low-level lifting mechanism and a composite thermodynamic profile are employed. These simulations successfully replicate many of the features of the observed systems. The low-level environment is nearly saturated, which is not conducive to the production of a strong surface cold pool; yet the convection quickly organizes into a quasi-linear system that produces very heavy local rainfall. As in the May 2000 case, a low-level gravity wave was responsible for this organization. The upstream development of new convective cells is shown to result from the interaction of the reverse-shear flow with these waves.