Kinematics and Thermodynamics of Nocturnal Tornadogenesis in the Severe 5-6 July 2015 South Dakota MCS During PECAN

This study documents the morphology and evolution of the severe 5-6 July 2015 South Dakota nocturnal mesoscale convective system (MCS) using observations from the Plains Elevated Convection At Night (PECAN) project and a diabatic Lagrangian analysis (DLA) of thermal and hydrometeor fields. PECAN observed the later stages of the merger of an externally-driven (Type 1) quasi-linear MCS and an internally-driven (Type 2) bowing MCS. The bowing segment of the 5-6 July MCS produced several severe wind reports (including one over 80 mph), several low-level mesocyclones, and an EF-0 tornado near Dolton, SD in the classical left bookend-vortex location. Observations from a unique pre-deployed array of 7 mobile Doppler radars are synthesized to generate wind fields in a regular Cartesian domain covering 12,600 sq. km at a uniform 500 m grid spacing; while in situ and MCS-environmental observations of thermal variables and airflow from mobile mesonets, pre-deployed lidars and AERI profilers, mobile and pre-deployed soundings, and research aircraft augment the DLA retrievals.

The radar wind syntheses and DLA are used to explore the mesoscale and storm-scale 3-D airflow, cold pool, cloud and precipitation structure associated with mesocyclogenesis in the tornado-producing storm cell and its parent 5-6 July MCS. A PECAN hypothesis asserts that severe straight-line surface winds may be achievable given favorable environmental convective available potential energy and bulk shear ingredients (which were present in the 5-6 July case), provided that the mesoscale nocturnal cold pool is surface-based and contains a strong thermal solenoid via precipitation-forced diabatic cooling to assist a vigorous descending rear-to-front (RTF) flow (e.g., see attached radar analysis image from the 6 July MCS). Previous studies have explored the potential contributions of both supercell and non-supercell mechanisms to tornadogenesis associated with bowing MCSs. Our radar and DLA results indicate that the tornadic storm cell is shown to share some characteristics with supercells, although it forms and develops an intense low-level mesocyclone at the EF-0 tornado location along the bowing MCS cold mesoscale outflow boundary ahead of a descending mesoscale rear-inflow jet. The Lagrangian air trajectories will help indicate whether the nocturnal convective and mesoscale updrafts and downdrafts and the mesoscale cold pool are surface-based or elevated, while also helping to define the thermodynamic evolution along trajectories that feed the tornadic low-level mesocyclone; as well as the deep convection, the mesoscale cold pool, and the trailing stratiform region.