A study of aerosol indirect effects for cumulus clouds on a global scale

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Yuan, Tianle
Li, Zhanqing
Using case study approach my investigation on aerosol's effects on fair weather cumulus clouds shows that not only can aerosol reduce cloud droplet sizes like in the case of stratiform clouds, but also they can increase droplet sizes. Atmospheric water vapor loading explains nearly 70% of variations in the dependence of droplet size on aerosol loading for cases over Eastern United States. This finding withstands serious scrutinizing under different scenarios of artificial correlations. A further study on a global scale indicates that only two areas, Eastern US and coastal region of Southeast China, show increasing trend of droplet size with aerosol loading. Results from other regions agree well with findings from past studies further ruling out artificial correlation. Relationship between aerosol loading and cloud liquid water path differs significantly for marine stratocumulus clouds and continental cumuli. Two possible explanations for our findings are confirmed by state-of-the-art cloud resolving model simulations. Deep convective clouds properties are shown to obey a few universally observable relationships. Their cloud top ice particle sizes are positively correlated with their vertical height and they are significantly affected by topography; their optical depth distributions have signature shapes associated with individual regions; their brightness temperature distributions show agreement with the fixed anvil temperature hypothesis. A conceptual model is proposed to understand cloud hydrometeor evolution and is used to study aerosol's influence. Anthropogenic pollution and smoke are shown to decrease ice particle sizes by delaying coalescence process and prolonging condensational growth. As a result cloud glaciation height is increased that possibly leads to invigoration of cloud development. Dust particles are demonstrated to increase ice particle sizes probably by acting as giant condensation nuclei or ice nuclei. Ice particle size vertical structure is shown to have a significant latitudinal variation. Far reaching implications of our results are envisioned for climate studies.