Surface thermodynamic characteristics of RFDs as measured by a mobile mesonet

Although rear-flank downdrafts (RFDs) have been believed to be critical for tornado genesis and maintenance for nearly 30 years, direct observations within them have been scarce. This presentation will summarize observations made by a mobile mesonet of the surface thermodynamic characteristics (e.g., buoyancy, potential buoyancy, equivalent potential temperature, and pressure) of RFDs associated with tornadic and non-tornadic supercells.

Evidence will be presented in support of the following conclusions: (1) RFDs associated with non-tornadic supercells are driven largely by evaporation and entrainment of potentially cold midlevel air; (2) evaporative cooling and entrainment of midlevel potentially cold air play a smaller or negligible role in the forcing of RFDs associated with tornadic supercells compared to non-tornadic supercells; (3) tornadogenesis is more likely as the surface buoyancy and potential buoyancy in the RFD increase, and as the convective inhibition associated with RFD parcels at the surface decreases; (4) tornadogenesis is more likely as the equivalent potential temperature deficit in RFD at the surface decreases; (5) baroclinity at the surface within the hook echo is not a necessary condition for tornadogenesis.

The final two conclusions are more tentative: (6) the presence of surface-based convective available potential energy in the RFD is a necessary condition for tornadogenesis; (7) tornado longevity and intensity increase as CAPE increases and CIN decreases in the surface RFD parcels.