The Sensitivity of a Numerically Simulated Idealized Squall Line to the Vertical Distribution of Aerosols

Abstract

hanges in the aerosol number concentration are reflected by changes in raindrop size and number concentration that ultimately affect the strength of cold pools via evaporation. Therefore, aerosol perturbations can potentially alter the balance between cold pool?induced and low-level wind shear?induced circulations. In the present work, simulations with increased aerosol loadings below approximately 3 km, between approximately 3 and 10 km, and at all vertical levels are performed to specifically address both the overall sensitivity of a squall line to the vertical distribution of aerosols and the extent to which low-level aerosols can affect the convective strength of the system. The results suggest that low-level aerosol perturbations have a negligible effect on the overall storm strength even though they act to enhance low-level latent heating rates. A tracer analysis shows that the low-level aerosols are either predominantly detrained at or below the freezing level or are rapidly lifted to the top of the troposphere or the lower stratosphere within the strongest convective cores. Moreover, it is shown that midlevel aerosol perturbations have nearly the same effect as perturbing the entire domain, increasing the convective updraft mass flux by more than 10%. These changes in strength are driven by a complex chain of events caused by smaller supercooled droplets, larger graupel, and larger raindrops. Combined, these changes tend to reduce the low-level bulk evaporation rate, thus weakening the cold pool and enhancing updraft strength. The results presented herein suggest that midlevel aerosol perturbations may exhibit a much larger effect on squall lines, at least in the context of this idealized framework.