Using ensemble sensitivity analysis to identify storm-scale characteristics associated with tornadogenesis in high resolution simulated supercells

Date

2021-08

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Abstract

Understanding tornadoes in supercell thunderstorms has been a persistent endeavor by meteorologists. Our physical understanding of supercell thunderstorms has improved drastically in the past 50 years, and meteorologists have been able to accurately forecast supercell outbreaks days in advance. While some environments are assuredly more conducive to tornado formation than others, storm-scale features within the parent supercells can compliment and enhance environmental support for tornadogenesis, or mitigate environmental aspects that are otherwise unsupportive of tornadogenesis. Due to their dangerous and chaotic nature, sampling tornadoes within supercell thunderstorms is difficult. By simulating supercells in high-resolution numerical models, researchers can attempt to dissect the features responsible for the formation of strong vertical vorticity near the surface. However, it has been shown that a single simulation can be misleading, and even minuscule changes to a model’s base state can drastically change the outcome of a supercell. To remedy these setbacks, this study aims to objectively identify storm-scale characteristics associated with tornado-like vortex (TLV) formation in an ensemble of high-resolution supercells. An ensemble of 51 supercells is created using Cloud Model Version 1 (CM1). The first member is initialized using a base state populated by the Rapid Update Cycle (RUC) proximity sounding near El Reno, Oklahoma on May 24, 2011. The other 50 ensemble members are created by randomly perturbing the base state after a supercell has formed. Despite only using small, random perturbations to the vertical sounding, there is considerable spread between ensemble members, with some supercells producing strong, long lived TLVs, while others do not produce a TLV at all. The ensemble is then analyzed using the Ensemble Sensitivity Analysis (ESA) technique. ESA uncovers persistent storm-scale characteristics that are dynamically relevant to TLV formation in both the forward flank and rear flank of the supercell. In the rear flank, outflow temperatures are not strongly related to TLV production, but divergence at the surface southeast of the TLV helps converge and contract existing vertical vorticity. In the forward flank, warm temperatures within the forward-flank cold pool are important to TLV production and magnitude, as is the longitudinal positioning of the cold pool, relative to the location of the mesocyclone. The positioning of strong streamwise vorticity, both in the horizontal and vertical, is also a clear indicator of TLV formation and strength, especially within 5 minutes of when the TLV is measured.

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Keywords

Supercell, Tornado, Ensemble Sensitivity Analysis

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