Low-Tropospheric Shear in the Structure of Squall Lines: Impacts on Latent Heating under Layer-Lifting Ascent

Abstract

Abstract This study analyzes the dependence of the intensity of simulated midlatitude squall lines (SLs) at maturity on the strength of the environmental low-tropospheric shear, focusing on the amplitude of the latent heating produced within the deep convective region. The hypothesis motivating this investigation is that shear fundamentally affects system strength by modulating the mean convective instability of the storm-relative inflowing air, which is justified by the layer-lifting nature of convection in SLs. The layer-lifting model of convection (LLMC) is proposed for measuring convective instability in the context of SLs, wherein latent heating is estimated by contemplating the storm-relative inflow of CAPE. This framework is used for defining LLMC indices for the precipitation rate and the updraft’s strength and verticality. Idealized SLs at maturity, simulated in a variety of kinematic and thermodynamic environments, encompass wide-ranging values of LLMC indices and degrees of cold pool–shear balance within the spectrum of cold pool–dominated storms. LLMC indices account for much of the intercase variability in the updraft’s strength and verticality, the precipitation rate, and the convective mode apparent in radar reflectivity plots. It is found that the low-tropospheric shear fundamentally affects the intensity of SLs through its effects on latent heating, with stronger shear leading to larger inflowing convectively unstable air as a fraction of the total storm-relative inflow, favoring system intensity. This behavior could largely explain the dependence of storm intensity on the strength of shear documented in previous investigations, as cold pool–shear balance appears to be less restrictive on the intensity of mature SLs than the strength of the shear alone.