High resolution mobile mesonet observations of RFD surges in the June 9 Basset, Nebraska supercell during Project ANSWERS 2003

There are many unresolved issues regarding tornadogenesis in supercells. One question that has received recent attention in the research community is the role of the rear flank downdraft (RFD) and rear flank downdraft boundary (RFDB) in the tornadogenesis process. Project ANSWERS (Analysis of the Near-Surface Wind and Environment Along the Rear Flank of Supercells) 2003 was a field experiment designed to collect near-surface data in order to document the kinematic and thermodynamic environment in the vicinity of the RFDB, and to try to determine the RFD/RFDB’s contribution to low-level mesocyclogenesis, tornadogenesis and tornado maintenance.

Approximately 12 different RFDs (both tornadic and nontornadic) were sampled by Project ANSWERS using an array of four mobile mesonets during the late spring and early summer of 2003. One case of particular interest occurred on June 9th near Basset, Nebraska in which a weak tornado developed in very close proximity to the project’s mobile mesonet array. During the 10 minute period prior to tornadogenesis, the team was deployed such that high spatial resolution data could be collected in the RFD region as the storm approached the mesonet array from the northwest. Atmospheric variables were sampled every 2 seconds.

A preliminary analysis of the high resolution data using 12 s time averages revealed several interesting features as the storm slowly passed to the north and east of the team. Two smaller RFD ‘surges’ embedded within a broader RFD were detected in the mesonet array in the 10 minutes preceding tornadogenesis. The passage of the first RFD surge revealed several small vortices along the RFD surge boundary, one of which passed almost directly over one of the mobile mesonets. The second RFD surge passed through the array approximately 1-2 minutes prior to tornadogenesis. As the second RFD surge passed, a distinct small scale (~1-2 km wide) convergence/divergence couplet was detected in the mesonet array. Tornadogenesis occurred a few hundred meters to the northeast of one of the mesonets following the passage of the second RFD surge. Results and additional analysis from this case will be presented, and the possible role of RFD surges in tornadogenesis will be discussed.