Aerosol-cloud impacts on aerosol detrainment and rainout in shallow maritime tropical clouds

This study investigates how aerosol-induced changes tocloud properties subsequently influence the overall aerosol budget throughchanges to detrainment and rainout. We simulated an idealized field ofshallow maritime tropical clouds using the Regional Atmospheric ModelingSystem (RAMS) and varied the aerosol loading and type between 16simulations. The full aerosol budget was tracked over the course of the48 h simulation, showing that increasing the aerosol loading leads to anincrease in aerosol regeneration and detrainment aloft at the expense ofaerosol removal via rainout. Under increased aerosol loadings, clouddroplets are smaller and more likely to evaporate before they formprecipitation-sized hydrometeors. As a result, the aerosol particlescontained inside these droplets are released into the environment ratherthan being removed to the surface via rainout. However, the few raindropswhich do happen to form under increased aerosol loadings tend to be larger,since the cloud water available for collection is divided among fewerraindrops, and thus raindrops experience less evaporation. Thus, in contrastto previous work, we find that increases in aerosol loading lead to decreases inaerosol rainout efficiency, even without a decrease in the overallprecipitation efficiency. We further used tobac, a package for tracking andidentifying cloud objects, to identify shifts in the overall cloudpopulation as a function of aerosol loading and type, and we found contrastingaerosol effects in shallow cumulus and congestus clouds. Shallow cumulusclouds are more sensitive to the increase in cloud edge and/or top evaporation withincreased aerosol loading and thereby tend to rain less and remove feweraerosols via rainout. On the other hand, larger congestus clouds are moreprotected from evaporation and are thereby able to benefit from warm-phaseinvigoration. This leads to an increase in rain rates but not in domain-wideaerosol rainout, as the domain total rainfall becomes concentrated over asmaller horizontal area. Trends as a function of aerosol loading wereremarkably consistent between the different aerosol types tested. Theseresults represent a pathway by which a polluted environment not only hashigher aerosol loadings than a pristine one but is also less able toregulate those loadings by removal processes, instead transporting aerosolsto the free troposphere where they remain available for reactivation andfurther aerosol-cloud interactions.