Estimates of rear-flank downdraft buoyancy as a predictor of tornadogenesis

Supercell thunderstorms frequently occur without tornadogenesis (~80% of the time; Burgess 2000, personal communication). Parameters often used in forecasting tornado potential, such as 0-6km vertical shear and storm-relative helicity, are summary measures of processes that cause storm-scale rotation, or supercells, and may be more useful at predicting storm rotation rather than tornadogenesis.

Based on VORTEX findings of differences in the equivalent potential temperature of air within rear-flank downdrafts (RFDs) of tornadic versus nontornadic supercells, Rasmussen (1999, personal communication) suggests that RFD buoyancy might determine whether a supercell produces a tornado and, possibly, the intensity of the tornado. We present a method for estimating RFD buoyancy from proximity soundings of ambient conditions.

Our method lifts a surface parcel to a predetermined level, allowing for adjustments to the parcel's thermal properties by rain water and entrainment. Then, the parcel descends to the surface. RFD buoyancy is the difference between the modified parcel temperature and the ambient surface temperature.

Because conflicting theories for the source region of RFD air exist, we have examined different scenarios. In the simplest circumstance, we have raised a surface parcel to the lifted condensation level, and lowered it to the surface moist adiabatically. Thus, RFD air has originated in the updraft. A more complicated situation is modeled by lifting a surface parcel to 700 mb, adjusting for rain water loss and entrainment, and descending to the surface (moist adiabatically until the rain water evaporates). Thus, RFD air is considered to be some combination of updraft and ambient air.

Our results for these two scenarios suggest that either model is useful for discriminating between ambient conditions that support weak (F0-F1) and strong (F2-F5) tornados, although the LCL method produces greater separation. Including a parameter to estimate downdraft formation due to evaporative cooling of ambient mid-level air enhances discrimination in both cases.