Relationship of Convection Initiation and Subsequent Storm Strength to Ensemble Simulated Environmental Conditions during IOP3b of VORTEX Southeast 2017

Abstract

A fifty-member convection allowing ensemble was used to examine environmental factors influencing afternoon convection initiation (CI) and subsequent severe weather on 5 April 2017 during Intensive Observing Period (IOP) 3b of the Verification of Rotation in Tornadoes Experiment in the Southeast (VORTEX-SE). This case produced several weak tornadoes (rated EF1 or less), and numerous reports of significant hail (diameter ≥ 2 inches), ahead of an eastward-moving surface cold front over eastern Alabama and southern Tennessee. Both observed and simulated CI was facilitated by mesoscale lower-tropospheric ascent maximized several tens of km ahead of the cold-frontal position, and the simulated mesoscale ascent was linked to surface frontogenesis in the ensemble mean. Simulated maximum 2-5-km AGL updraft helicity (UHmax) was used as a proxy for severe-weather producing mesocyclones, and considerable variability in UHmax occurred among the ensemble members. Ensemble members with UHmax > 100 m2 s-2 had stronger mesoscale ascent than in members with UHmax < 75 m2 s-2, which facilitated more timely CI by producing greater adiabatic cooling and moisture increases above the PBL. After CI, storms in the larger UHmax members moved northeastward toward a mesoscale region with larger convective available potential energy (CAPE) than in smaller UHmax members. The CAPE differences among members was influenced by differences in location of an antecedent mesoscale convective system, which had a thermodynamically stabilizing influence on the environment toward which storms were moving. Despite providing good overall guidance, the model ensemble overpredicted severe weather likelihoods in northeastern Alabama, where comparisons with VORTEX-SE soundings revealed a positive CAPE bias.

Reconstruction of Violent Tornado Environments in Europe: High‐Resolution Dynamical Downscaling of ERA5

“…A technique combining UH tracks from high‐resolution simulations with environmental values of STP (or other convective parameters) should be further explored. While in the United States, similar techniques have been applied operationally with ensemble‐based approaches (Clark et al., 2012; Gallo et al., 2016, 2018, 2019; Loken et al., 2017; Trier et al., 2021), very little is known about tornado forecasting performance in Europe. As evidenced in our study, despite a variety of synoptic patterns and types of convective environments, in the majority of cases, UH track near the location of the tornado report was associated with climatologically enhanced values of STP.…”