Analysis of rear-flank downdrafts and their evolution during project WIRL



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Texas Tech University


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.