Comparing Aerosol and Low-Level Moisture Influences on Supercell Tornadogenesis: Three-Dimensional Idealized Simulations

Abstract

our three-dimensional, nested-grid numerical simulations were performed using the Regional Atmospheric Modeling System (RAMS) to compare the effects of aerosols acting as cloud condensation nuclei (CCN) to those of low-level moisture [and thus convective available potential energy (CAPE)] on cold-pool evolution and tornadogenesis within an idealized supercell storm. The innermost grid possessed horizontal grid spacing of 111 m. The initial background profiles of CCN concentration and water vapor mixing ratio varied among the simulations (clean versus dusty and higher-moisture versus lower-moisture simulations). A fifth simulation was performed to factor out the impact of CAPE. The higher-moisture simulations produced spatially larger storms with stronger peak updrafts and low-level downdrafts, heavier precipitation, greater evaporative cooling, and stronger cold pools within the forward and rear flank downdrafts. Each simulated supercell produced a tornado-like vortex. However, the lower-moisture simulations produced stronger, longer-lived vortices, as they were associated with weaker cold pools and less negative buoyancy within the rear flank downdraft. Raindrop and hailstone concentrations (sizes) were reduced (increased) in the dusty simulations, resulting in less evaporative cooling and weaker cold pools compared to the clean simulations. With greater terminal fall speeds, the larger hydrometeors in the dusty simulations fell nearer to the storm?s core, positioning the cold pool closer to the main updraft. Tornadogenesis was related to the size, strength, and location of the cold pools produced by the forward and rear flank downdrafts. Not surprisingly, while the aerosol effect was evident, the influences of low-level moisture and CAPE had markedly larger impacts on tornadogenesis.