Session 14.2 Precipitation, the Rear Flank Downdraft, and Tornadoes

Thursday, 7 October 2004: 1:45 PM
Mark Askelson, University of North Dakota, Grand Forks, ND; and J. M. Straka and E. N. Rasmussen

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Given their infrequent occurrence, supercell thunderstorms produce an inordinate amount of death and damage. As a result, they have been studied intensely for the past forty years. Even so, numerous important questions concerning supercells remain. One such question regards tornadoes and observations of supercells that appear to be similar but exhibit very different tornadic behavior. Recently, analyses of data from the VORTEX project revealed that the thermodynamic properties of the rear flank downdraft (RFD) may dictate whether or not a supercell becomes tornadic. Since hydrometeors are thought to be an important driving force for the RFD, it has been postulated that they may be important to its thermodynamic properties and, possibly, to tornadogenesis.

Potential kinematic and thermodynamic properties of RFDs are investigated using a 1.5 dimension downdraft model that is driven by hydrometeor fields. This downdraft model uses conservation equations for vertical momentum, temperature, mixing ratios of water vapor and cloud water, number concentration densities of raindrops and graupel/hail, and ice-water fraction of graupel/hail. Microphysical processes include saturation adjustment, evaporation/condensation of raindrops, evaporation/condensation of melting graupel/hailstones, melting of graupel/hailstones, and shedding of melting graupel/hailstones. Environmental conditions are set using prescribed soundings that span a wide range of conditions to illustrate the range of simulated-RFD outcomes. The initial model state is hydrostatically balanced to ensure that subsequent evolution results only from hydrometeors, which are inserted with the same properties at the same altitude at each time step.

Results indicate that downdraft strength is strongly dependent on environmental lapse rates of temperature. Moreover, the vertical distribution of these lapse rates is a strong determinant of the maximum originating altitude of air parcels that descend to the surface. In addition to being sensitive to environmental conditions, modeled downdrafts are also sensitive to the properties of the hydrometeor fields that drive them. From these effects and the expected vertical temperature profiles of RFD environments, potential explanations of VORTEX observations will be provided.

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