Browsing by Author "Hirth, Brian D."
Now showing 1 - 3 of 3
Results Per Page
Sort Options
Item Analysis of rear-flank downdrafts and their evolution during project WIRL(Texas Tech University, 2005-08) Hirth, Brian D.; Schroeder, John L.; Peterson, Richard E.; Weiss, Christopher C.The existence of hook echoes and rear-flank downdrafts (RFDs) in supercell thunderstorms has been well documented over the past several decades (Stout and Huff 1953; van Tassell 1955; Fujita 1958, 1973, 1975; Browning and Donaldson 1963, Browning 1964, 1965; Lemon 1977; Burgess et al. 1977; Brandes 1977; Barnes 1978a,b). The identification of these features has played a vital role in severe thunderstorm research as supercell thunderstorms account for a large percentage of severe weather occurrences each year. Initial investigations have attempted to directly link hook echoes with tornado occurrences; however recent research has shown that a variety of supercell thunderstorms possessing hook echoes fail to produce low-level circulations (Markowski 2002). A recent study using WSR-88D radar data over a seven year period from 1992-1999 found that roughly 15% of mid-level mesocyclones and 40% of low-level mesocyclones actually produced tornadic circulations at the surface (Trapp and Stumpf 2002). Nonetheless hook echoes and their associated RFDs are still thought to play key roles in the development of rotation near the ground. The development of mobile mesonet instrumented vehicles by the National Severe Storms Laboratory propelled supercell thunderstorm research forward in the mid-1990s (Straka et al. 1996). These instrumented suites were produced as a response to the scientific demands of very high spatial and temporal resolution datasets. Since it would be unlikely for a given storm to pass over a fixed network, the practical alternative was to make the network mobile. This new ability to sample supercell characteristics directly at the surface allowed for the direct investigation of RFD surface thermodynamics and kinematics during the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX; Rasmussen et al. 1994). Markowski et al. (2002) encountered consistent relationships between RFD thermodynamics and tornadogenesis and tornadogenesis failure in a study using 18 tornadic and 12 nontornadic cases from VORTEX. Though an unprecedented study, the near instantaneous nature of these observations did not take into consideration the evolution of an entire RFD lifecycle. Embedded features within a broad RFD circulation could potentially play their own individually significant roles in the generation of tornadoes. The principle objective of this investigation is to document the progression of thermodynamic and kinematic features as they potentially move cyclonically through the RFD region. In doing so, the following questions will be examined: 1. Do RFD equivalent potential temperature and virtual potential temperature values trend colder with time compared to those values measured in the storm-relative inflow? This trend would suggest that RFD air is descending from progressively higher levels allowing for more ambient environmental air to be entrained into the RFD. This result may also suggest that the efficiency of evaporation is increasing as the drier environmental air mixes with the RFD parcels during this entrainment process. 2. Though the equivalent potential temperature and virtual potential temperature perturbations within a particular RFD may be relatively small compared to that of the storm-relative inflow, what effects may weak surface flow have on tornadogenesis and its resulting intensity and longevity? This will be examined to see if any relationships between strong inflow/RFD winds and more significant tornadic episodes can be distinguished. It is now well understood that the mid-level rotation comprising a supercell is obtained through the vertical titling of ambient environmental streamwise vorticity. This component of vorticity is maximized when environmental flow both increases in magnitude and veers with height (Davies-Jones 1984). However the production of low-level vorticity is not as well understood. Solenoidally generated vorticity likely exists along the baroclinic boundaries that define both the forward-flank and rear-flank downdrafts. A secondary aspect of this research will be to investigate the implications that thermodynamic gradients have on the generation of this vorticity.Item Analysis of rear-flank downdrafts and their evolution during project WIRL(2005-08) Hirth, Brian D.; Schroeder, John L.; Peterson, Richard E.; Weiss, Christopher C.The existence of hook echoes and rear-flank downdrafts (RFDs) in supercell thunderstorms has been well documented over the past several decades (Stout and Huff 1953; van Tassell 1955; Fujita 1958, 1973, 1975; Browning and Donaldson 1963, Browning 1964, 1965; Lemon 1977; Burgess et al. 1977; Brandes 1977; Barnes 1978a,b). The identification of these features has played a vital role in severe thunderstorm research as supercell thunderstorms account for a large percentage of severe weather occurrences each year. Initial investigations have attempted to directly link hook echoes with tornado occurrences; however recent research has shown that a variety of supercell thunderstorms possessing hook echoes fail to produce low-level circulations (Markowski 2002). A recent study using WSR-88D radar data over a seven year period from 1992-1999 found that roughly 15% of mid-level mesocyclones and 40% of low-level mesocyclones actually produced tornadic circulations at the surface (Trapp and Stumpf 2002). Nonetheless hook echoes and their associated RFDs are still thought to play key roles in the development of rotation near the ground. The development of mobile mesonet instrumented vehicles by the National Severe Storms Laboratory propelled supercell thunderstorm research forward in the mid-1990s (Straka et al. 1996). These instrumented suites were produced as a response to the scientific demands of very high spatial and temporal resolution datasets. Since it would be unlikely for a given storm to pass over a fixed network, the practical alternative was to make the network mobile. This new ability to sample supercell characteristics directly at the surface allowed for the direct investigation of RFD surface thermodynamics and kinematics during the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX; Rasmussen et al. 1994). Markowski et al. (2002) encountered consistent relationships between RFD thermodynamics and tornadogenesis and tornadogenesis failure in a study using 18 tornadic and 12 nontornadic cases from VORTEX. Though an unprecedented study, the near instantaneous nature of these observations did not take into consideration the evolution of an entire RFD lifecycle. Embedded features within a broad RFD circulation could potentially play their own individually significant roles in the generation of tornadoes. The principle objective of this investigation is to document the progression of thermodynamic and kinematic features as they potentially move cyclonically through the RFD region. In doing so, the following questions will be examined: 1. Do RFD equivalent potential temperature and virtual potential temperature values trend colder with time compared to those values measured in the storm-relative inflow? This trend would suggest that RFD air is descending from progressively higher levels allowing for more ambient environmental air to be entrained into the RFD. This result may also suggest that the efficiency of evaporation is increasing as the drier environmental air mixes with the RFD parcels during this entrainment process. 2. Though the equivalent potential temperature and virtual potential temperature perturbations within a particular RFD may be relatively small compared to that of the storm-relative inflow, what effects may weak surface flow have on tornadogenesis and its resulting intensity and longevity? This will be examined to see if any relationships between strong inflow/RFD winds and more significant tornadic episodes can be distinguished. It is now well understood that the mid-level rotation comprising a supercell is obtained through the vertical titling of ambient environmental streamwise vorticity. This component of vorticity is maximized when environmental flow both increases in magnitude and veers with height (Davies-Jones 1984). However the production of low-level vorticity is not as well understood. Solenoidally generated vorticity likely exists along the baroclinic boundaries that define both the forward-flank and rear-flank downdrafts. A secondary aspect of this research will be to investigate the implications that thermodynamic gradients have on the generation of this vorticity.Item Examination of the Coastal Transition Zone in Hurricane Frances (2004)(2011-05) Hirth, Brian D.; Schroeder, John L.; Weiss, Christopher C.; Smith, Douglas A.Understanding the structure of the coastal internal boundary layer (IBL) during the landfall of a tropical cyclone has important ramifications on operational forecasting, structural design, and post-storm damage assessment. Despite these important issues, it is unclear how the structure of the IBL evolves at the coastline on micro- and meso-scales during a landfalling hurricane. Knowledge of the vertical kinematic structure within tropical cyclones over water has improved greatly through aircraft reconnaissance missions and the advent of GPS dropsondes and the Stepped Frequency Microwave Radiometers. Unfortunately, reconnaissance and research aircraft are limited to over-water missions resulting in a poor understanding of vertical kinematic structure near the coastal interface where changes in IBL structure are expected due to changes in coastal geometry and surface roughness. Additionally, IBL structure may evolve due to the passage of convective precipitation and associated downdrafts. A unique observational dataset was collected from the coastal transition zone in the onshore flow region of Hurricane Frances (2004) over Cape Canaveral, FL. Single- and dual-Doppler radar data collected by the Shared Mobile Atmospheric Research and Teaching radars provide the ability to discern horizontal and vertical mean IBL structure over a complex coastal interface while assessing the influence of a variable underlying surface and the passage of transient convective wind gusts. Additional wind speed data were collected by a meso-network of surface towers operated by the Cape Canaveral Air Force Station and Kennedy Space Center along with a portable surface tower deployed by Texas Tech University. Radar and tower data analyses reveal that IBL mean structure over the Cape Canaveral remains quite consistent during the landfall of Hurricane Frances, though IBL growth is suppressed when compared to empirical growth models. Additionally, transient convective gusts commonly perturb the mean structure at the top of the IBL, though the higher momentum associated with these gusts is typically not able to descend to the surface within an established IBL.