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

Abstract

In some prominent extreme precipitation and flash flood events, radar and rain gauge observations have suggested that the heaviest short-term rainfall accumulations (up to 177 mm h−1) were associated with supercells or mesovortices embedded within larger convective systems. In this research, we aim to identify the influence that rotation has on the storm-scale processes associated with heavy precipitation. Numerical model simulations conducted herein were inspired by a rainfall event that occurred in central Texas in October 2015 where the most extreme rainfall accumulations were collocated with meso-β-scale vortices. Five total simulations were performed to test the sensitivity of precipitation processes to rotation. A control simulation, based on a wind profile from the aforementioned event, was compared with two experiments with successively weaker low-level shear. With greater environmental low-level shear, more precipitation fell, in both a point-maximum and an area-averaged sense. Intense, rotationally induced low-level vertical accelerations associated with the dynamic nonlinear perturbation vertical pressure gradient force were found to enhance the low- to midlevel updraft strength and total vertical mass flux and allowed access to otherwise inhibited sources of moisture and CAPE in the higher-shear simulations. The dynamical accelerations, which increased with the intensity of the low-level shear, dominated over buoyant accelerations in the low levels and were responsible for inducing more intense low-level updrafts that were sustained despite a stable boundary layer.

Budget Analyses of a Record‐Breaking Rainfall Event in the Coastal Metropolitan City of Guangzhou, China

“…The storm was a back‐building quasi‐stationary MCS, favoring extreme local rainfall. The characteristics of this case are different from the case of Nielsen and Schumacher (), in which the dynamical acceleration dominated over buoyant acceleration in the low levels during an extreme rainfall associated with meso‐β‐scale vortices that occurred in central Texas in October 2015. There may be many factors causing these differences, such as the supply of low‐level moisture in different regions (coastal in this study and inland in their study), the static stability of the environments, the vertical structure of the storms' heating profiles, and the low‐level wind shear (direction and strength).…”

Strange Floods: The Upper Tail of Flood Peaks in the United States

“…Cotton et al (2010) note that "storms producing the largest hailstones occur in strongly sheared environments; thus, in general, we should not expect that the storm systems producing the largest hailstones are also heavy rain producing storms" (see also Hamada et al, 2015;Zipser et al, 2006). Doswell et al (1996) and Smith et al (2001) take a different perspective, pointing to the most intense thunderstorms as underappreciated agents of extreme flooding (see also Bunkers & Doswell, 2016;Hitchens & Brooks, 2013;Llasat et al, 2016;Nielsen et al, 2015;Nielsen & Schumacher, 2018;Rogash & Racy, 2002). The 1903The , 1956The , and 1965 floods in eastern Oregon seem strange partly because we have relatively little insight to the storms that produced them.…”

Quantifying Precipitation Efficiency and Drivers of Excessive Precipitation in Post-Landfall Hurricane Harvey

“…In addition, supercell thunderstorms are known to locally enhance rainfall rates due to their strong updrafts in the presence of rotation (Nielsen and Schumacher 2018). Traditional supercells that develop in continental air and are influenced by baroclinic instability are known to have a PE< 100% due to the influence of hail, deeplayer shear resulting in limited drop residence time, and additional cold rain processes (eg., Marwitz 1972;Foote and Fankhauser 1973;Browning 1977).…”