The Sensitivity of Simulated Storm Structure, Intensity, and Precipitation Efficiency to Environmental Temperature

Abstract

Prior parameter space studies of simulated deep convection are extended to embrace shifts in the environmental temperature. Within the context of the parameter space study design, shifts in this environmental temperature are roughly equivalent to changes in the ambient precipitable water (PW). Two series of simulations are conducted: one in a warm environmental regime that is associated with approximately 60 mm of precipitable water, and another with temperatures 8°C cooler, so that PW is reduced to roughly 30 mm. The sets of simulations include tests of the impact of changes in the buoyancy and shear profile shapes and of changes in mixed- and moist layer depths, all of which have been shown to be important in prior work. Simulations discussed here also feature values of surface-based pseudoadiabatic convective available potential energy (CAPE) of 800, 2000, or 3200 J kg?1, and a single semicircular hodograph having a radius of 12 m s?1, but with variable vertical shear. The simulations reveal a consistent trend toward stronger peak updraft speeds for the cooler temperature (reduced PW) cases, when the other environmental parameters are held constant. Roughly comparable increases in updraft speeds are noted for all combinations of mixed- and moist layer depths. These increases in updraft strength evidently result from both the reduction of condensate loading aloft and the lower altitudes at which the latent heat release by freezing and deposition commences in the cooler, low-PW environments. As expected, maximum storm precipitation rates tend to diminish as PW is decreased, but only slightly, and by amounts not proportionate to the decrease in PW. The low-PW cases thus actually feature larger environment-relative precipitation efficiency than do the high-PW cases. In addition, more hail reaches the surface in the low-PW cases because of reduced melting in the cooler environments. Although these experiments were designed to feature specified amounts of pseudoadiabatic CAPE, it appears that reversible CAPE provides a more accurate prediction of updraft strength, at least for the storms discussed here.