Extreme Short-term Precipitation in Supercells and Mesovortices: Insights from Numerical Simulations

Building on evidence suggesting that extreme short-term precipitation accumulations (e.g., > 75 mm/h) are often associated with supercells or other meso-beta-scale vortices, this research uses numerical simulations to investigate the storm-scale processes responsible for the production of heavy precipitation.  First, simulations are conducted for several observed cases at convection-allowing (dx=3-4 km) and higher (dx=400-500 m) resolutions.  These cases include flash floods near Houston, TX on 18 April 2009 and 18 April 2016; Mobile, AL/Pensacola, FL on 29-30 April 2014, and Austin/San Antonio, TX on 30 October 2015.  The environments of all of these events featured moist conditions with a statically stable layer near the surface, and very strong low-level wind shear.  The simulations reveal that within the heavily raining convective systems, the highest short-term rain rates are indeed found almost exclusively near strong, rotating low-level updrafts that are identifiable either as high-precipitation supercells, or mesovortices embedded within a convective line.

The conditions found in observations and case-study simulations are used to design idealized numerical experiments aimed at investigating the influence of low-level shear on the convective structures and rainfall production.  These simulations suggest that, similar to idealized simulations of individual supercells, increased low-level shear leads to stronger near-surface updrafts and stronger low-level rotation within the convective system.  Furthermore, the high-shear simulations produce more precipitation, both locally and in an area-integrated sense.  Further work will aim to more fully explore the parameter space associated with, and the dynamical processes supporting extreme precipitation and mesovortices.