Session 15.3 The effect of variations in low level thermodynamic structure on the rear flank downdraft of simulated supercells

Thursday, 30 October 2008: 11:00 AM
North & Center Ballroom (Hilton DeSoto)
Jason A. Naylor, University of North Dakota, Grand Forks, ND ; and M. A. Askelson

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Tornadoes, although the subject of numerous studies, remain somewhat of a mystery.  While there have been improvements in the prediction of tornadoes and tornado outbreaks, it is still unknown how tornadoes actually form and why one particular storm will produce a tornado while a similar storm will not.  Recent studies have suggested that the rear flank downdraft (RFD) of a supercell is crucial to the development of a tornado.  Markowski et al. (2002) analyzed 30 supercell storms (18 tornadic and 12 non-tornadic) and found that, on average, the tornadic supercell storms had RFDs with pseudoequivalent potential temperature (θep) values closer to those observed at the surface in the storm's environment than did the nontornadic supercells.  This was particularly true in the case of strong tornadoes [defined by Markowski et al. (2002) to be tornadoes of ≥ F2 intensity or tornadoes that persisted longer than 5 minutes].

Previous numerical simulations of RFDs only produced ‘cold' downdrafts [e.g., Schlesinger (1975), Wilhelmson and Klemp (1978), Klemp et al. (1981)].  This could be due to the absence of a layer of reduced lapse rate, or cap, in the storm environment which can deter air aloft from descending to the surface.  Askelson et al. (2004) found that the θep deficit at the surface was minimized in a storm environment that had a cap just above the boundary layer.  This cap prevented upper level air with low θep values from descending to the surface, thus minimizing surface θep deficits and creating an environment that, according to Markowski et al. (2002), is favorable for tornadogenesis.

Askelson et al. (2004) used a simplified downdraft model that did not consider pressure perturbation forcing.  The model used in this study is the Weather Research and Forecasting (WRF) model.  The WRF model is a three dimensional model that allows both hydrometeor and pressure-perturbation forcing of the RFD.  This study expands on the work of Markowski et al. (2002) and Askelson et al. (2004) by investigating (a) the characteristics of the pre-storm environment that will produce RFDs with relatively high θep and (b) if the results obtained in Askelson et al. (2004) remain valid in a full scale cloud model that considers pressure perturbation forcing.  In addition, the effects of microphysical parameterizations on the thermodynamic structure of the RFD are examined.  

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