Sensitivity of aerosol-induced effects on numerically simulated squall lines to the vertical distribution of aerosols

The sensitivity of aerosol-induced enhancement of convective strength and precipitation to the vertical distribution is analyzed in the context of numerically simulated squall lines. Recent investigations have hypothesized and demonstrated that an increase in an aerosol loading may lead to enhanced vertical updrafts and potentially more precipitation in a variety of deep convective systems (e.g., supercells, mid-latitude squall lines, and tropical squall lines). One of the generally accepted hypotheses for such an enhancement in convective strength suggests that the predominant effect of an increase in aerosol loading is related to enhanced latent heat release in the mid- to upper-levels of the convective cores. This enhancement has been attributed to an increase in supercooled liquid water that tends to exist in clouds formed in more polluted environments and it is suggested that this water is lofted from below the freezing level to the mixed-phase region of the cloud where the latent heating effects are maximized. However, deep convective cores are quite strong (updrafts of several meters per second to tens of meters per second) and so a reduction in cloud droplet size due to enhanced aerosol number concentration (which reduces the terminal fall speed) ought to have a negligible effect on the trajectory of the droplets (since the updraft velocity is much larger than the terminal fall speed). Thus, it should be expected that low-level aerosol pollution would have little to no effect on latent heating rates aloft since the droplets will end up in the mixed-phase region regardless of size. Moreover, more recent investigations have shown that aerosol perturbations, especially in squall lines, can lead to less intense cold pools and thus a more optimal state according to RKW theory.

Numerical simulations of idealized squall lines are performed to specifically analyze the sensitivity of the aforementioned effects to the vertical distribution of aerosols. The simulations suggest that low-level air tends to either be detrained at the bottom of the convective cores or remains in the convective cores throughout the troposphere and is then detrained in the anvil region of the cloud. This should come as no surprise since the maximum updraft velocity occurs between 8 and 10 km above ground level, thus suggesting that air parcels are accelerating through the mid levels, entraining mid-level environmental air in and around the mixed-phase region of the cloud. As a result, not only is it shown that an increase in low-level aerosols has little to no effect on latent heating rates, but also on the bulk cloud properties as well. In other words, the effects on supercooled liquid water (and the subsequent effects on graupel formation and rain drop production) are minimal. Thus additionally, no effects are observed on cold pool dynamics. This does however elude to the potential for mid-level aerosols to be entrained into the convective cores and, in the case of an increase in these aerosols, promote enhanced convection, increased supercooled liquid water and graupel formation, and ultimately a reduction in rain water but an increase in size that acts to reduce cold pool intensity. These effects are analyzed in the context of RKW theory to demonstrate that the effect on the storm as a whole is to produce a more optimal line. This key result suggests that low-level local pollution sources have little effect on squall line dynamics and instead, mid-level distant pollution sources can have an appreciable effect on the storm strength and precipitation.