Changes in the Sensitivity of Convective Storms and Tornadoes to the Microphysics Parameterization in Environments with Different Lifting Condensation Levels PDF Download

Author: Shawn Murdzek
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Languages : en
Pages : 0

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Book Description
Several studies have documented how simulations of convective storms are sensitive to the microphysics parameterization. Very few studies, however, have examined how this sensitivity changes with different environmental conditions. This project explores how changing the environmental lifting condensation level (LCL), a proxy for near-ground relative humidity, impacts the sensitivity of both ordinary and supercellular convection to the microphysics. Additionally, how the sensitivity of supercellular tornadogenesis to the microphysics changes with the LCL is also examined. To explore these sensitivities, several sets of perturbed-microphysics ensembles are run, where each ensemble member uses a different variation of the microphysics scheme and each ensemble uses an environment with a different LCL. Sensitivity to the microphysics is evaluated using the ensemble spread of various cold pool metrics. In the supercell simulations, processes contributing to cold pool strength are examined using a new technique, where buoyancy budgets are computed along parcel trajectories. For ordinary convection, cold pools in environments with higher LCLs are more sensitive to the microphysics owing to the drier conditions associated with the higher LCLs, which magnify differences in evaporation rates that already exist between ensemble members owing to the microphysics scheme variations. The same increase in sensitivity with higher LCLs appears in supercellular convection, but the primary reason in these supercell simulations is the more rapid increase in base-state potential temperature with height in the prescribed environment for the low-LCL simulations. This relatively high-potential temperature air is brought to the surface in downdrafts in the low-LCL simulations, which partially counters the cooling from rain evaporation and limits the strength of the cold pool in the coldest low-LCL simulations. This reduces the spread of cold pool metrics in the low-LCL ensembles, but has little impact on the high-LCL ensembles because the potential temperature does not increase as rapidly with height. Unlike the cold pool properties, the sensitivity of tornadogenesis to the microphysics when there is a marginal wind profile does not change much with LCL, with all the ensembles producing a mix of tornadic and nontornadic simulations except for the low-LCL, high-level of free convection (LFC) ensemble, which produced no tornado-like vortices. This low-LCL, high-LFC environment is found to be the most unfavorable for tornadogenesis owing in part to weak low-level updrafts and a poor positioning of the near-surface circulation far away from the mesocyclone in several of the members. Altogether, these results suggest that convective storms are generally more predictable in low-LCL environments, but tornadogenesis still has poor predictability in environments that couple a favorable LCL with a marginal wind profile.