Dynamical Insights into Extreme Short-Term Precipitation Associated with Supercells and Mesovortices

In several prominent extreme precipitation and flash flood events, radar and rain-gauge observations have suggested that the heaviest short-term rainfall accumulations (exceeding 75 mm/h) were associated with supercells or mesovortices embedded within larger convective systems (e.g., multicell clusters or MCSs). Previous observationally based research has show that between 40% and 50% of these extreme short-term rainfall accumulations from 2013-2015 were associated with rotation. In this research, we aim to identify the influence that rotation has on the storm-scale processes associated with heavy precipitation.

Convection-allowing, semi-idealized numerical model simulations were conducted in CM1 based up an extreme short-term rainfall event that occurred between Austin and San Antonio, TX in October 2015 where the extreme rainfall accumulations were collocated with meso-beta-scale vortices. Three total simulations were preformed to test the sensitivity of precipitation processes to rotation. Each simulation was initialized with the extreme rainfall composite thermodynamic profile from Schumacher and Johnson (2009, WAF). Three different kinematic profiles, one for each simulation, were created based upon the profile observed in the above event where the 0—1km shear was decreased from observed values in two increments (i.e., from 15.2 m/s to 10.7m/s to 7.6m/s); however, care was taken so the predicted motion for the right mover for each profile was approximately equal. In order to access the dynamical effects of rotation, a full decomposition of the 3D vertical perturbation pressure gradient accelerations was undertaken for each simulation to decompose the forcing into the buoyant and dynamic accelerations.

The simulations suggest that increased low-level shear leads to stronger low-level rotation, similar to results from supercell simulations. The higher shear simulations produce larger precipitation accumulations both in a point maximum and area-averaged sense. Intense low-level vertical accelerations associated with the dynamic non-linear perturbation vertical pressure gradient force were found in the high shear case, and the area average was found to be positively related with the amount of low-level shear/rotation. These results suggest the potential for an analogy between extreme rainfall in high shear and tornadogenesis (e.g., Markowski and Richardson 2014, JAS). Furthermore, these results relate to potential mechanisms behind concurrent, collocated tornado and flash flood events (e.g., Nielsen et al. 2015, WAF).