I reviewed the article titled “Aerosol Effects on Clouds, Precipitation, and the Organization of Shallow Cumulus Convection” from the Journal of The Atmospheric Sciences, Volume 65, February 2008. The authors had a desire to study the aerosol effects on clouds and climate. Using large eddy simulations they investigated the effects of aerosol on clouds, precipitation, and organization of trade wind cumuli. The conceptual structure for this study follows the idea of greater aerosol loading suppresses precipitation formation, increases cloud liquid water, and leads to a long lived cloud with larger cloud fractions. Recent field studies have had a theme in focusing on the interaction between precipitation, boundary layer dynamics, and cloud organization with illustrative findings that precipitation is often associated with mesoscale variations. This article focuses mainly on using simulations to help understand ways in which precipitation effects the organization and structure of trade wind cumuli. Data collection during the ATEX (Atlantic Trade Wind Experiment) is used as reference and comparison data.
This simulation uses a 3D dynamical resolving microphysical scheme. Warm cloud microphysical processes are simulated by having aerosol particles larger than dry critical size that activate based on equilibrium theory at a local temperature and supersaturation. In comparison to the ATEX study, a Beer’s law radiation scheme is used to represent a net export of radiant energy.
In a regime where surface precipitation rates are substantial there is an expected tendency of cloudiness to increase with increasing aerosol concentrations. A heavy precipitation regime gives expected tendency of cloud water to increase with decreasing precipitation. This would indicate a significant liquid water amount and that it is not efficiently removed from the area of interest. This would mean that the tendency for water to remain due to longer evaporation time scales overshadows the tendency for gravity to return it to the surface. The precipitation rate decreases as aerosol concentrations increase. The main difference between the aerosol mixing ratio with and/or without precipitation shows that precipitation may be important in determining cloud and boundary layer evolution.
This study using the large eddy simulation study delivered a clear representation of suppression of precipitation with increasing aerosol. Aerosol tends to increase cloud fraction when the aerosol mixing ratio increases. Also further increases in the aerosol mixing ratio leads to a decrease in cloud fraction most likely due to a fast evaporation of smaller cloud droplets. Two different regimes for aerosol effects on the clouds life cycle are considered. The first one, precipitation regime, shows that increases in aerosol increase the dwelling time of cloud condensate. A second one, weakly precipitation regime, shows increases in aerosol reduce the dwelling time of condensate through enhanced evaporation. Their study found no effect of aerosols on the lifetime of individual cells. One might resolve that changes in convective frequency, or even changes in the size of individual clouds would be a materialization of the changes in cloud fractions.