THE SENSITIVITY OF SIMULATED STORM STRUCTURE AND INTENSITY TO THE LIFTED CONDENSATION LEVEL AND THE LEVEL OF FREE CONVECTION

Using a full-physics numerical cloud model, convective storm overturning efficiency and structure are shown to be highly sensitive to the altitudes of cloud base and the level of free convection. For a moderately unstable atmosphere with CAPE=2000 J/kg, a fixed buoyancy profile shape, a fixed semicircularly curved hodograph, and a constant equivalent potential temperature in the subcloud layer, a series of storms was simulated in horizontally homogeneous environments designed so that the LFC, specified to coincide with cloud base, occurred at eight distinct levels ranging from 0.45 to 2.64 km above the surface. Statistics were then compiled on storm behavior during the second hour of the simulations.

The simulated storms showed a well-defined tendency to have strongest updrafts when the LFC was located near 1.6-2.0 km above the surface. For LFCs lower than this optimum altitude, peak updraft strengths were systematically weaker, with the decrease in updraft strength being directly related to lowering of the specified LFC. Meanwhile, for LFC's higher than the optimum, peak updraft strengths declined slowly. Storms having optimum LFC altitudes produced peak updrafts near 55 m/s, while storms having their LFC at the lowest tested level, near 0.45 km, had peak updrafts of only 32 m/s. Surface outflows became systematically colder as the LFC and cloud base were raised higher, and the associated storms showed signs of weakening due to outflow domination late in the second hour.

Use of a special initial ambient thermodynamic profile having a near-optimum LFC at 1.63 km, but a cloud base at 0.45 km, and a nearly saturated pseudoadiabatic ambient thermal profile in between, produced a storm rather similar in overturning efficiency to that found when both the LFC and cloud base were set at 1.63 km. This demonstrates that the LFC height, not cloud base, largely determines peak updraft strength, all other things being held constant. However, in this special case the enhanced relative humidity in the subcloud layer restrained the production of cold surface outflow, and the storm maintained its intensity throughout the second hour.

The sensitivity of the storm updrafts to LFC height has significant implications for the ability of storms to grow large hail. The optimum LFC height found in the simulations also appears to coincide well with the altitudes of LFC heights observed in climatologies of Great Plains severe storms.