The effect of aerosol concentration on the depth of deep convection during the CACTI campaign

Science

Increasing concentrations of atmospheric aerosols have been shown to shift the droplet size distribution in clouds robustly toward increasing numbers of smaller droplets. It has been hypothesized and found in some studies, but not in others, that this can yield more vigorous updrafts and taller cloud tops in mixed-phase deep convective clouds. Using a new, comprehensive data set (the CACTI campaign) for addressing this topic, we do not find any evidence of taller clouds with increasing aerosol concentration, and in fact find indications that the opposite may be true in our data set.

Impact

Our findings provide another data point in the open question of how aerosols affect mixed-phase deep convective clouds. Answering this question is important for future climate projections, as deep convective clouds are an integral part of the global circulation and radiation balance. We also showed that (1) aerosol concentrations correlate with key meteorological parameters that strongly affect convective depth, and (2) aerosol optical depth is not necessarily a good proxy for the aerosol concentration being ingested into cloud base. Both of those findings should improve the methods and outcome of future studies on the topic.

Summary

An aerosol indirect effect on deep convective cores (DCCs), by which increasing aerosol concentration increases cloud-top height via enhanced latent heating and updraft velocity, has been proposed in many studies. However, the magnitude of this effect remains uncertain due to aerosol measurement limitations, modulation of the effect by meteorological conditions, and difficulties untangling meteorological and aerosol effects on DCCs. The Cloud, Aerosol, and Complex Terrain Interactions (CACTI) campaign in 2018–19 produced concentrated aerosol and cloud observations in a location with frequent DCCs, providing an opportunity to examine the proposed aerosol indirect effect on DCC depth in a rigorous and robust manner. For periods throughout the campaign with well-mixed boundary layers, we analyze relationships that exist between aerosol variables (condensation nuclei concentration > 10 nm, 0.4% cloud condensation nuclei concentration, 55–1000-nm aerosol concentration, and aerosol optical depth) and meteorological variables (level of neutral buoyancy [LNB], convective available potential energy, midlevel relative humidity, and deep-layer vertical wind shear) with the maximum radar-echo-top height and cloud-top temperature (CTT) of DCCs. Meteorological variables such as LNB and deep-layer shear are strongly correlated with DCC depth. LNB is also highly correlated with three of the aerosol variables. After accounting for meteorological correlations, increasing values of the aerosol variables (with the exception of one formulation of aerosol optical depth [AOD]) are generally correlated at a statistically significant level with a warmer CTT of DCCs. Therefore, for the study region and period considered, increasing aerosol concentration is mostly associated with a decrease in DCC depth.

The CACTI campaign was funded by the Department of Energy's Atmospheric Radiation Measurement (ARM) user facility.