The Development of Severe Vortices within Simulated High-Shear, Low-CAPE Convection

Abstract

Environments characterized by large values of vertical wind shear and modest convective available potential energy (CAPE) are colloquially referred to as high-shear, low-CAPE (HSLC) environments. Convection within these environments represents a considerable operational forecasting challenge. Generally, it has been determined that large low-level wind shear and steep low-level lapse rates—along with synoptic-scale forcing for ascent—are common ingredients supporting severe HSLC convection. This work studies the specific processes that lead to the development of strong surface vortices in HSLC convection, particularly associated with supercells embedded within a quasi-linear convective system (QLCS), and how these processes are affected by varying low-level shear vector magnitudes and lapse rates. Analysis of a control simulation, conducted with a base state similar to a typical HSLC severe environment, reveals that the key factors in the development of a strong surface vortex in HSLC embedded supercells are (i) a strong low- to midlevel mesocyclone, and (ii) a subsequent strong low-level updraft that results from the intense, upward-pointing dynamic perturbation pressure gradient acceleration. Through a matrix of high-resolution, idealized simulations, it is determined that sufficient low-level shear vector magnitudes are necessary for the development of low- to midlevel vertical vorticity [factor (i)], while steeper low-level lapse rates provide stronger initial low-level updrafts [factor (ii)]. This work shows why increased low-level lapse rates and low-level shear vector magnitudes are important to HSLC convection on the storm scale, while also revealing similarities between surface vortexgenesis in HSLC embedded supercells and higher-CAPE supercells.

Creating physically coherent and spatially correlated perturbations to initialize high‐resolution ensembles of simulated convection

“…The Sherburn and Parker (2019) high‐shear low‐CAPE base‐state sounding (Figure 1a, generated via the MetPy software: May et al ., 2008) initializes the horizontally homogeneous environment, though small modifications are made to the lowest 100 mb of the sounding (e.g., increased near‐surface lapse rate) to maintain boundary‐layer turbulence in the simulation. Following Sherburn and Parker (2019), a $$$ - $$$10‐K $$$ \theta $$$ perturbation is inserted along the western edge of the domain to simulate a frontal boundary that provides the low‐level forcing necessary to initiate convection. The perturbation decreases as a cosine function of the height above ground level (AGL) and distance from the western boundary edge, and extends 260 km east of the domain boundary and 6 km above the surface.…”

“…Subgrid‐scale turbulence is parameterized using a variant of the Deardorff (1980) turbulent kinetic energy scheme. Coriolis acceleration only acts on perturbation winds, which is equivalent to assuming the base‐state wind field is in geostrophic balance (Roberts et al ., 2016; Coffer et al ., 2017; Sherburn and Parker, 2019). The National Severe Storms Laboratory (NSSL) double‐moment microphysics scheme (Mansell et al ., 2010) parameterizes precipitation processes.…”

A method for generating a quasi-linear convective system suitable for observing system simulation experiments

“…The initial sounding (Fig. 1a), introduced by Sherburn and Parker (2019), is modified in the lower troposphere to support the development of robust boundary layer turbulence. High-shear, low-CAPE environments, which support approximately half of the Contiguous United States (CONUS) significant tornadoes (EF2+) (Schneider et al, 2006), are the primary target of Verifications of the Origins of Rotation in Tornadoes Experiment-Southeast (VORTEX-SE) field experiments.…”

The Transient Responses of an Axisymmetric Tropical Cyclone to Instantaneous Surface Roughening and Drying

“…Numerical simulation experiments in axisymmetric geometry are performed using the Bryan Cloud Model (CM1), version 19.7 (v19.7). CM1 is suitable for use in a broad range of atmospheric science applications across scales, including hurricanes (Chavas and Emanuel 2014;Peng et al 2018) and severe thunderstorms (Sherburn and Parker 2019;Trapp et al 2018). CM1 satisfies nearexact conservation of both mass and energy in a reversible saturated environment (Bryan and Fritsch 2002;Rotunno and Bryan 2012).…”