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|Title: ||Mesoscale Precursors to the Hurricane Gaston Flooding Event as Diagnosed from Observations and Numerical Simulations|
|Authors: ||Brown, Zachary Gordon|
|Advisors: ||Gary Lackmann, Committee Member|
Michael Kaplan, Committee Co-Chair
Yuh-Lang Lin, Committee Co-Chair
|Keywords: ||convective-symmetric instability|
|Issue Date: ||3-Aug-2007|
|Discipline: ||Marine, Earth and Atmospheric Sciences|
|Abstract: ||The causes of severe flooding in Richmond, Virginia during the passage of Hurricane Gaston on 30 August, 2004 are explored using an ingredients based methodology and numerical simulations. Gaston's precipitation was unusual as the worst flooding occurred more than a day after landfall and was focused over a very small area. The convection was observed to produce tornados and was especially intense between the hours of 1500 UTC on 30 August and 0000 UTC 31 August.
The first part of this study uses the ingredients method to focus on the key factors that lead to the heavy rainfall. High convective available potential energy (CAPE) due to shortwave radiational heating and a steady supply of moisture advecting off the Gulf Stream were the keys to explosive convective growth and maintenance. The convective line appeared to organize along a convergence band rotating around Gaston that later became phased with a baroclinic zone formed by differential solar heating between the cloudy and clear skies. This low level organization occurred under an area of low inertial stability in the upper troposphere as indicated by the RUC20 analysis. It was theorized that the low inertial stability in the upper level mesoscale ridge north of Gaston could account for the longevity of the convective system, but the observational datasets available were of insufficient resolution to make a determination.
The Non-Hydrostatic Mesoscale Atmospheric Simulation System (NHMASS) was then run to increase the resolution of our dataset and to explore the ability of a numerical model to simulate a complicated tropical convective system. Experiments on the initial datasets resulted in the Global Forecast System (GFS) analysis being chosen and a moisture synthesis scheme was implemented to improve the moisture and cloud representation in the model. An outer grid with spacing of 18 km was run with inner grids nested at 6 km and 2 km. The 6 km and 2 km grids produced a similar rainfall pattern to observations and were used for further dynamical analysis. This revealed the presence of a tropospheric-deep mesoscale convective circulation in conjunction with the precipitation system. It is shown to have similarities to mesoscale convective complex (MCC) circulations that exploit weak inertial stability in the upper troposphere for maintenance. The inertial stability turns to instability once the convective updrafts perturb the upper troposphere creating a convective symmetric instability that drives the circulation and maintains the heavy precipitation rates for the duration observed.|
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