Browsing by Author "Michael Kaplan, Committee Co-Chair"

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Examining Planetary, Synoptic and Mesoscale Features that Enhance Precipitation Associated with Landfalling Tropical Cyclones in North Carolina.
(2005-10-31) Croke, Meredith Suzanne; Michael Kaplan, Committee Co-Chair; Gary Lackmann, Committee Member; Lian Xie, Committee Co-Chair
Landfalling tropical cyclones (TCs) over North Carolina often produce excessive rain well inland from the location of landfall. A three-part study consisting of a precipitation analysis, climatological analysis and numerical model simulations was performed to determine the dominant mechanisms that influence precipitation associated with TCs that move over North Carolina. The goal was to determine the intrinsic or environmental features that enhance precipitation associated with the TCs influencing North Carolina and create a conceptual model to enable forecasters to better assess the likelihood of enhanced precipitation during TCs. In the precipitation analysis of 28 TCs that made landfall or tracked along North Carolina's immediate coastline from 1953-2003, the spread of precipitation and track of each storm across North Carolina was considered. The potential correlation between several intrinsic features (i.e. maximum storm intensity, landfall intensity and translation speed) and the 3-day storm average precipitation from 52 rain gauge stations across North Carolina were examined. The results indicated no statistically significant correlation between precipitation and any of the above features. Due to this lack of correlation, the preexisting synoptic/dynamic environment the TC was entering at landfall was examined to determine if the environmental features could be the dominant precipitation enhancing mechanism. In the climatological analysis the 28 TCs were divided into relatively heavy and relatively light rainfall groups so that composite analyses of several environmental features from 72 hours prior to landfall could be examined. The features included 250 hPa geopotential heights, 850-700 hPa and 500-250 hPa potential vorticity, 925-850 hPa moisture flux, 1000 hPa frontogenesis, temperatures, winds and mean sea level pressure. The results indicated that there are several significant planetary, synoptic and mesoscale climatological differences from 72 to 6 hours prior to landfall between the heavy and light rainfall groups. The numerical simulations served to test the agreement between the climatological analysis and two case studies. The two case studies used were Hurricane Floyd (1999) and Tropical Storm Arthur (1996) which represented heavy and light rain events, respectively. All numerical simulations were performed using the Non-Hydrostatic Mesoscale Atmospheric Simulation System (NHMASS) model. The model was run at four different scales to allow features from the synoptic to meso- β scales to be examined. The results were consistent with the climatological study, showing that preexisting environmental features influence the mesoscale environment, leading to enhanced precipitation well inland from the location of landfall. A conceptual model from 72-hours prior to landfall is now available for forecasters to incorporate into the ir tropical forecasting routine.
Mesoscale Precursors to the Hurricane Gaston Flooding Event as Diagnosed from Observations and Numerical Simulations
(2007-08-03) Brown, Zachary Gordon; Gary Lackmann, Committee Member; Michael Kaplan, Committee Co-Chair; Yuh-Lang Lin, Committee Co-Chair
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.