Low-Level Vorticity Budget Analysis of a Simulated Long-Lived Violent Tornado

A recent study by Orf et al. (2016) presented an overview of a very high-resolution numerical simulation of a long-lived EF5 tornado embedded within a parent supercell.  A forward trajectory analysis during the genesis and mature phases of the tornado revealed several source regions for air entering the low-levels of the tornadic circulation:  air originating along the forward-flank downdraft boundary (FFDB), air originating at low levels deep within the cold pool well behind the FFDB, and air originating in the rear-flank downdraft (RFD).  Of particular note were smaller-scale vortices that developed along the FFDB and then moved rearward along the boundary (in a storm-relative sense) and merged with the tornado.  A cursory look at the possible vertical and streamwise horizontal vorticity sources for the tornado indicated that the air originating in the RFD appeared to make the smallest potential contribution to the low-level vorticity in the tornado, especially during the genesis phase.  This is in contrast to recent theories of tornadogenesis which argue that baroclinic generation of streamwise vorticity in the RFD is a key process in tornadogenesis (Davis-Jones and Brooks, 1993). 

As first step in quantitatively assessing possible low-level vorticity sources for the simulated tornado, we will present preliminary results from a vorticity budget analysis (including both horizontal and vertical vorticity components) in the lowest few model layers of the simulated tornado.  A sector analysis approach will be utilized so that contributions from the various source regions (FFDB, FFD, RFD, etc.) can be calculated and compared to assess the relative contributions to the low-level tornado rotation.  An analysis of the vorticity budget during the genesis, mature and dissipation phases of the tornado will also be presented.

References:

Orf, L., R. Wilhelmson, B. Lee, C. Finley, and A. Houston, 2016: Evolution of a Long-Track Violent Tornado within a Simulated Supercell.  Bull. Am. Meteorol. Soc., online early release, doi: 10.1175/BAMS-D-15-00073.1.

Davis-Jomes, R.P. and H. E. Brooks, 1993: Mesocyclogenesis from a theoretical perspective. The Tornado: Its Structure, Dynamics, Prediction, and Hazards, Geophys. Monogr., Vol. 79, Amer. Geophys. Union, 105–114.