The Influence of Lifting Condensation Level on Low-Level Outflow and Rotation in Simulated Supercell Thunderstorms

Abstract

This paper reports on results of idealized numerical simulations testing the influence of low-level humidity, and thus lifting condensation level (LCL), on the morphology and evolution of low-level rotation in supercell thunderstorms. Previous studies have shown that the LCL can influence outflow buoyancy, which can in turn affect generation and stretching of near-surface vertical vorticity. A less explored hypothesis is tested: that the LCL affects the relative positioning of near-surface circulation and the overlying mesocyclone, thus influencing the dynamic lifting and intensification of near-surface vertical vorticity. To test this hypothesis, a set of three base-state thermodynamic profiles with varying LCLs are implemented and compared over a variety of low-level wind profiles. The thermodynamic properties of the simulations are sensitive to variations in the LCL, with higher LCLs contributing to more negatively buoyant cold pools. These outflow characteristics allow for a more forward propagation of near-surface circulation relative to the midlevel mesocyclone. When the mid- and low-level mesocyclones become aligned with appreciable near-surface circulation, favorable dynamic updraft forcing is able to stretch and intensify this rotation. The strength of the vertical vorticity generated ultimately depends on other interrelated factors, including the amount of near-surface circulation generated within the cold pool and the buoyancy of storm outflow. However, these simulations suggest that mesocyclone alignment with near-surface circulation is modulated by the ambient LCL, and is a necessary condition for the strengthening of near-surface vertical vorticity. This alignment is also sensitive to the low-level wind profile, meaning that the LCL most favorable for the formation of intense vorticity may change based on ambient low-level shear properties.

Improving the Physical Basis for Updraft Dynamics in Deep Convection Parameterizations

“…We have refrained from a detailed analysis of updrafts in the PBL because the structure of such updrafts is likely to be strongly dependent on a range of processes, such as dry convection in the PBL, cold pools, synoptic scale forcing for ascent, dynamics within the lower parts of existing updrafts, and terrain features to name a few. It is possible that such a framework for extending updraft properties into the PBL could be developed in the future through an analysis of dynamic and buoyant pressure accelerations below cloud base (e.g., Brown & Nowotarski, 2018) and along the edge of cold pools (e.g., Bryan & Rotunno, 2014; Jeevanjee & Romps, 2015); however, we leave such analysis to future work.…”

Southeastern U.S. Tornado Outbreak Likelihood Using Daily Climate Indices

“…5, but for the DJF period. Blanchard 1998;Thompson et al 2003) that has been shown to impact the positioning and strengthening of nearsurface circulation in supercell environments (Brown and Nowotarski 2019). This positive trend in CAPE places SC day values beyond the bounds of HSLC CAPE criteria.…”

The Prestorm Environment and Prediction for Local‐ and Nonlocal‐Scale Precipitation: Insights Gained From High‐Resolution Radiosonde Measurements Across China

“…Static stability measures the gravitational resistance of the atmosphere to vertical displacements (Bluestein, 1992). The burst of KHI, which is the major triggering source for turbulence (Sharman et al, 2012), is characterized by the occurrence of Ri falling below a critical value that is frequently taken as 1/4 (Fritts & Alexander, 2003).…”

“…The probability distributions of convective available potential energy (CAPE) and 0–6 km bulk shear vary greatly by severe weather (Púčik et al., 2015). For instance, low‐level (0–1 km) vertical wind shear and lifting condensation levels (LCLs) can skillfully distinguish between nontornadic and tornadic supercells (Brown & Nowotarski, 2019). Therefore, attempts have been made to infer the environment of convective storms based on a variety of proximity sounding parameters.…”