Thermodynamic characterization of supercell rear flank downdrafts in Project ANSWERS 2003

With the introduction of the mobile mesonet (MM) during the 1990s (Straka et al. 1996), direct surface thermodynamic observations within rear-flank downdrafts (RFD) became possible. Research results from the Verifications of the Origins of Rotation in Tornadoes Experiment (VORTEX, Rasmussen et al. 1994) and subsequent projects showed that tornadogenesis is more likely and tornado intensity and longevity increase as the equivalent potential temperature (θ_e), virtual potential temperature (θ_v), and convective available potential energy (CAPE) near the surface within the RFD increase and the convective inhibition (CIN) within the RFD decreases (Markowski et al. 2002).

Project ANSWERS 2003 (Analysis of the Near-Surface Wind and Environment Along the Rear Flank of Supercells) was conducted on the Plains during May and June 2003. The data collection objective of Project ANSWERS 2003 was to gather high-resolution thermodynamic and kinematic near-surface data along the RFD boundary and within the RFD in both tornadic and nontornadic supercells. Given the limited nature of the RFD observational database to date and the variety of RFD evolutionary scenarios (e.g., cyclic supercells), an objective of this research project involved a thermodynamic analysis of RFDs in both tornadic and nontornadic supercells. In particular, near-surface θ_e, θ_v, CAPE, and CIN are compared for both tornadic and nontornadic supercell RFDs.

During this project, 12 RFDs were sampled in several cyclic supercells. Tornadic supercell RFD θ_e values were approximately 2.5 – 4 K warmer than nontornadic supercell RFD values and tornadic supercell RFD θ_v values were approximately 2.5 – 3 K warmer than nontornadic supercell RFD values. Surface-based CAPE present in tornadic supercell RFDs was approximately 600 – 1200 J/kg greater than the surface-based CAPE found in nontornadic supercell RFDs. The tornadic supercell RFD CIN ranged from 35 J/kg less to 20 J/kg more than nontornadic supercell RFDs. RFD thermodynamic characteristics in the current study are similar to those found in Markowski et al. (2002) for θ_e, θ_v, and CAPE. Trends in CIN differ from Markowski et al. (2002), which could be an artifact of several thermodynamically warm nontornadic RFDs present in the current data set. Three of the nontornadic RFDs developed in an environment where tornadic RFDs were produced, which confirms that a thermodynamically warm RFD is not sufficient for tornadogenesis and other factors certainly are important.