Browsing by Subject "mesoscale"
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Item Environmental control of cloud-to-ground lightning polarity in severe storms(2009-05-15) Buffalo, Kurt MatthewIn this study, it is hypothesized that the mesoscale environment can indirectly control the cloud-to-ground (CG) lightning polarity of severe storms by directly affecting their structural, dynamical, and microphysical properties, which in turn directly control cloud electrification and CG flash polarity. A more specific hypothesis, which has been supported by past observational and laboratory charging studies, suggests that broad, strong updrafts and associated large liquid water contents in severe storms lead to enhanced positive charging of graupel and hail via the noninductive charging mechanism, the generation of an inverted charge structure, and increased positive CG lightning production. The corollary is that environmental conditions favoring these kinematic and microphysical characteristics should support severe storms generating an anomalously high (> 25%) percentage of positive CG lightning (i.e., positive storms), while environmental conditions relatively less favorable should sustain storms characterized by a typical (? 25%) percentage of positive CG lightning (i.e., negative storms). Forty-eight inflow proximity soundings were analyzed to characterize the environments of nine distinct mesoscale regions of severe storms (four positive and five negative) on six days during May ? June 2002 over the central United States. This analysis clearly demonstrated significant and systematic differences in the mesoscale environments of positive and negative storms, which were consistent with the stated hypothesis. When compared to negative storms, positive storms occurred in environments associated with a drier low to midtroposphere, higher cloud base height, smaller warm cloud depth, stronger conditional instability, larger 0-3 km AGL wind shear, stronger 0-2 km AGL storm-relative wind speed, and larger buoyancy in the mixed-phase zone, at a statistically significant level. Differences in the warm cloud depth of positive and negative storms were by far the most dramatic, suggesting an important role for this parameter in controlling CG lightning polarity. Subjective visual inspection of radar imagery revealed no strong relationship between convective mode and CG lightning polarity, and also illustrated that positive and negative severe storms can be equally intense.Item Modeling Frameworks for Representing the Mechanical Behavior of Tissues with a Specific Look at Vasculature(2013-08-27) Andersohn, AlexanderMany mechanicstic models aimed at predicting tissue behavior attempt to connect constitutive factors (such as effects due to collagen or fibrin concentrations) with the overall tissue behavior. Such a link between constitutive and material behaviors would allow for a better understanding of the mechanobiology of diseased states and how one might return the tissue to a healthy state. Therefore, a literature search into present mechanistic models was performed and yielded a variety of models that were analyzed in order to determine their uniqueness, a requisite characteristic for this aim. It was found that many of these models did not make uniqueness a defining characteristic in their development and thus cannot be used for multiscale modeling (connecting constitutive behavior to material behavior).The literature search was then extended and narrowed to specifically analyze mechanical models describing vascular wall behavior. Once again, it was found that uniqueness was lacking in these models. To develop a unique model for inflation strains, an inflation experiment utilizing a bladder, syringe, and a pressure sensor was conducted to provide pressure vs. volume data for a sheep aorta. The data was then used to develop a unique model for inflation strains in an aorta utilizing a constitutive framework developed by Dr. John Criscione.Item Observations, dynamics and predictability of the mesoscale convective vortex event of 10-13 June 2003(Texas A&M University, 2006-08-16) Hawblitzel, Daniel PatrickThis study examines the dynamics and predictability of the mesoscale convective vortex (MCV) event of 10-13 June 2003 which occurred during the Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX). The MCV formed from a preexisting upper-level disturbance over the southwest United States on 10 June and matured as it traveled northeastward. The BAMEX field campaign provided a relatively dense collection of upper air observations through dropsondes on 11 June during the mature stage of the vortex. While several previous studies have focused on analysis of the dynamics and thermodynamics of observed and simulated vortices, few have addressed the ability to predict MCVs using numerical models. This event is of particular interest to the study of MCV dynamics and predictability given the anomalously strong and long-lived nature of the circulation and the dense data set. The first part of this study explores the dynamics of this MCV through an in-depth analysis of data from the profiler network and BAMEX dropsonde observations, in addition to the conventional surface and sounding observations as well as radar and satellite images. Next, issues relating to model performance are addressed through anevaluation of two state-of-the-art mesoscale models with varying resolutions. It is determined that the ability of a forecast model to accurately predict this MCV event is directly related to its ability to simulate convection. It is also shown that the convective-resolving Weather Research and Forecast (WRF) model with horizontal grid increments of 4 km displays superior performance in its simulation of this MCV event. Finally, an ensemble of 20 forecasts using mesoscale model MM5 with horizontal grid increments of 10 km are employed to evaluate probabilistically the dynamics and predictability of the MCV through the examination of the ensemble spread as well as the correlations between different forecast variables among ensemble members. It is shown that after MCV development, the ensemble mean performs poorly while individual ensemble members with good forecasts of convection at all stages of the MCV also forecast the midlevel vortex well. Furthermore, correlations among ensemble members generally support the findings in the observational analysis and in previous literature.Item Total lightning observations of severe convection over North Texas(2009-05-15) McKinney, Christopher MichaelFive severe convective cells over North Texas from three separate dates were examined to determine what three dimensional, or ?total? lightning data can add to the understanding of a convective cell?s intensity, propagation, and severe weather potential. Total lightning data were obtained from Vaisala Inc.?s Dallas/Fort Worth (D/FW) Lightning Detection and Ranging (LDAR) network. Radar data from two Weather Surveillance Radar ? 1988 Doppler (WSR-88D) sites were used for position data and information regarding the intensity and kinematic properties of each cell. Total lightning products used by the National Weather Service Forecast Office in Fort Worth, Texas were compared to total lightning flash rate; a quantity that has been shown to be correlated to changes in cell intensity inferred from other sources, such as radar and satellite data. These products, specifically flash extent density (FED) were also compared to CG flash rate and radar derived measures from the WSR-88D sites. The results of this work show that FED and total flash rate are well correlated, with an average Pearson correlation value of 0.73, indicating that previous total flash rate results may also apply to FED. Lightning hooks, holes, and notches in FED displays indicated likely updraft regions, while appendages were observed to develop prior to deviant motion with two supercells. These results, combined with a greater update frequency provided a useful complement to radar data in the warning decision process. FED jumps were observed prior to several severe weather reports, indicating that total lightning activity may be related to updraft strength as found in past studies. However, FED jumps were sometimes observed without any associated severe event. More work is clearly needed to define what FED changes are of most importance in the short-term prediction of storm severity. The usefulness of the total lightning data on these dates was dependant upon LDAR network status and distance of the cell from the network center. The results of this study suggest that combining total flash rate trends with visual displays of FED provides the greatest added benefit to forecasters in maintaining situational awareness during warning operations.