Thermodynamic Constraints on the Morphology of Simulated Midlatitude Squall Lines

Abstract

Abstract This study examines how environmental thermodynamics constrain the morphology of simulated idealized midlatitude squall lines (SLs). The thermodynamic soundings used for simulating various SLs are specified primarily by prescribed vertical profiles of the convective available potential energy (CAPE) and the level of free convection. This framework, which contemplates the latent instability properties of both low- and midtropospheric air, is considered to be convenient for investigating layer-lifting convective phenomena. Results show that frequently used CAPE indices are unsuitable for diagnosing SL characteristics, while integrated CAPE (ICAPE) discriminates the amplitude of the storm-induced heating for a given value of environmental shear. The skill of ICAPE follows from its relation to the buoyancy attained by low- and midtropospheric parcels as they ascend over the cold pool under layer-lifting convection. Environmental kinematics also affect the storm-induced heating, with stronger low-level shear leading to a greater proportion of inflowing latent unstable air among total storm-relative inflow, thus producing higher temperatures aloft. The precipitable water accounts for much of the precipitation-rate variation for a given value of shear. The precipitation efficiency is lower in environments with weaker shear and dryer midtropospheric conditions. Cold pool temperatures are slightly affected by environmental variations beneath the layer of minimum moist static energy, with drier midtropospheric conditions and weaker shear leading to warmer cold pools. SLs with a small vertical gradient of cold pool buoyancy propagate less rapidly and produce small surface wind speeds. Cold pool properties could be affected by a descending branch of the front-to-rear flow, which crosses over with the rear inflow jet.