The Impact on Simulated Storm Structure and Intensity of Variations in the Mixed Layer and Moist Layer Depths

Abstract

The sensitivities of convective storm structure and intensity to variations in the depths of the prestorm mixed layer, represented here by the environmental lifted condensation level (LCL), and moist layer, represented by the level of free convection (LFC), are studied using a three-dimensional cloud model containing ice physics. Matrices of simulations are generated for idealized environments featuring both small and large LCL = LFC altitudes, using a single moderately sheared curved hodograph trace in conjunction with convective available potential energy (CAPE) values of either 800 or 2000 J kg?1, with the matrices consisting of all four combinations of two distinct choices of buoyancy and shear profile shape. For each value of CAPE, the LCL = LFC altitudes are also allowed to vary in a separate series of simulations based on the most highly compressed buoyancy and shear profiles used for that CAPE, with the environmental buoyancy profile shape, subcloud equivalent potential temperature, subcloud lapse rates of temperature and moisture, and wind profile held fixed. Two other special simulations, one for each CAPE, are conducted using the high LFC and the lowered LCL, with a neutrally buoyant environmental thermal profile specified in between, such that the equivalent potential temperature was similar to that at the LCL. These latter two cases correspond to situations where the moist layer depth exceeds that of the mixed layer, whereas in all the other cases the two depths were equal. Results show that for the CAPE-starved environments (CAPE = 800 J kg?1) the simulated storms are supercells that are generally largest and most intense when LCL = LFC altitudes lie in the approximate range 1.5?2.5 km above the surface. The simulations show similar trends for the shear-starved (CAPE = 2000 J kg?1) environments, except that a tendency toward outflow dominance and multicell morphology is more evident when the LCL = LFC is high. For choices of LCL = LFC height within the optimal 1.5?2.5-km range, peak storm updraft overturning efficiency may approach 100% relative to parcel theory, while for lower LCL = LFC heights, overturning efficiency is reduced significantly. The enhancements of overturning efficiency with increasing LFC height are shown to be associated with systematic increases in both updraft effective diameter and the mean equivalent potential temperature of the low-level updraft, which reaches a maximum near the LFC. For the shear-starved environments, the tendency for outflow dominance is eliminated, but a large overturning efficiency maintained, when a low LCL is used in conjunction with a high LFC. The result regarding outflow dominance at large LCL derives from enhanced evaporation of precipitation in the deeper and drier subcloud layer, but the beneficial effect of a high LFC on convective overturning efficiency, at first glance surprising, derives from the enhanced depth of the moist layer containing the maximum CAPE. The importance of the moist layer depth is highlighted in tests that show that high-LFC storms simulated in environments where the neutrally buoyant sub-LFC layer contains a layer of reduced equivalent potential temperature experience a corresponding decrease in updraft strength. The simulation findings presented here appear to be consistent with statistics from previous severe storm environment climatologies, but provide a new framework for interpreting those statistics.