The Dependence of the Convective Instability Available to Squall Lines on the Environmental Low-tropospheric Shear: A Layer-lifting Perspective

The morphology of squall lines is known to be highly dependent on the strength of the environmental low-tropospheric vertical wind shear, with previous studies emphasizing the vorticity balance between the cold pool and the environmental shear. This investigation suggests an additional mechanism through which low-tropospheric shear affects the intensity of squall lines, namely by modulating the system-relative inflow of relevant parameters throughout the troposphere, e.g. CAPE and the water vapor mixing ratio. The layer-lifting model of convection (LLMC) is proposed for measuring such effects, wherein system-relative fluxes are computed from the storm’s propagation speed and the kinematic and thermodynamic characteristics of the environment.

The skill of LLMC diagnostics is analyzed in the case of mature idealized squall lines with trailing-stratiform precipitation, simulated with a cloud resolving model under a variety of kinematic and thermodynamic environments. Latent heating under the LLMC accounts for most of the inter-case variability in updraft strength and convective mode as revealed by radar reflectivity plots (e.g. slabular or cellular), while the precipitation rate is strongly constrained by the water vapor inflow rate. Results suggest that previous investigations of squall line dependence on the environmental shear overestimate the impacts of cold pool-shear balance on precipitation rates and updraft strength, as system-relative fluxes under the LLMC explain much of the inter-case variance of such features, after controlling for environmental thermodynamics. This study concludes that, in addition to well-known vorticity balance effects, shear fundamentally affects the intensity of squall lines by modulating the mean convective instability of system-relative inflowing air.