13.4
Precipitation-independent supercell tornadogenesis
Adam L. Houston, Univ. of Nebraska, Lincoln, NE; and M. D. Parker
Tornadogenesis ultimately requires the generation and amplification of surface vertical vorticity. In the absence of preexisting boundaries, the organization and amplification of available vorticity into surface vertical vorticity positioned underneath the principal updraft of the parent supercell is usually assumed to rely upon the storm's precipitation. However, it is unclear if precipitation has a causal or merely coincidental relationship to tornadogenesis. The objective of this work is to examine the possibility of precipitation-independent supercell tornadogenesis by conducting numerical experiments to test hypothetical mechanisms for generating surface vertical vorticity in the absence of precipitation. If precipitation-independent supercell tornadogenesis is possible, its existence would have implications for short-term forecasting and warning decisions.
Existing theories of tornadogenesis require the precipitation-driven rear-flank downdraft (RFD). However, we posit that in the absence of precipitation, a dynamically induced downdraft may still be possible and that this downdraft could act similarly to the RFD by transporting mesocyclonic angular momentum to the surface. Further amplification of surface vertical vorticity could occur as the surface vertical vorticity induces an occlusion downdraft. In this work we will test the following hypothetical mechanisms for dynamic downdraft generation: 1) The buoyancy gradient in place along the edges of the principal updraft will generate “compensating” downdrafts that could penetrate into the planetary boundary layer (PBL); 2) Pressure excesses (due to fluid extension/compression) could form at the base of the low-level inversion (generally located at cloud base) as negative buoyancy causes parcels rising slowly in the outer annuli of the principal low-level updraft to rapidly decelerate. These pressure excesses in concert with the pressure deficits generally found at the surface underneath the rotating updraft could yield a downward-directed pressure gradient force; 3) The stretching of planetary vorticity (often neglected in numerical simulations of tornadogenesis) could yield a larger pressure deficit underneath the updraft that, in concert with the hypothesized mechanisms above, could be necessary for a dynamic downdraft within the PBL.
Preliminary experiments without precipitation have yielded vertical vorticity in excess 0.09 s-1 at the lowest grid point underneath the right-rear flank of the principal updraft. Analysis is underway and results will be presented at the conference.
.Session 13, Tornado Vortex Dynamics and Genesis
Thursday, 9 November 2006, 10:30 AM-12:00 PM, St. Louis AB
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