Microphysics Contrasts between Stratus and Cumulus Clouds

点击翻译按钮获取中文摘要

This award supports research into connections between cloud types and the tiny particles that cause water to condense to form cloud droplets. Those tiny particles are also called cloud condensation nuclei (CCN). Since many CCN come from air pollution, their effects on clouds, which is commonly considered as the aerosol indirect effect (AIE), cause the largest climate uncertainty. Effects of CCN determine the amount of solar radiation that reaches the Earth’s surface. This largely controls global climate. But clouds also change CCN by chemical reactions within cloud droplets and coalescence among cloud droplets. These cloud process effects on CCN in turn further impact the cloud properties that control global climate. In one analyzed data set cloud processing seemed to enhance AIE but in another dataset AIE seemed to be reduced by cloud processing. The proposed research will extend this analysis to many other existing data sets collected in different parts of the world and at various time periods in various types of clouds. This research could help determine whether AIE is appreciably offsetting or enhancing greenhouse warming of the global climate. Thus the result could provide the scientific basis for national and international energy policies. This fundamental research would increase knowledge of cloud processing and lead to improved weather and climate forecasts as well as evaluate cloud brightening geoengineering schemes proposed to offset global warming. In addition, a graduate student will be trained and exposed to a wide array of cloud field campaign data to start his/her scientific career. Bimodal aerosol size distributions are due to increased dissolved material within cloud droplets after they evaporate as they usually do. The chemical reactions by trace gases diffused into cloud droplets and coalescence among cloud droplets from cloud processing form the larger particle. The required larger droplets for coalescence are more abundant in thicker cumulus clouds whereas the longer lifetimes of stratus clouds favor chemical cloud processing. The opposite effects of bimodal aerosol on cumulus and stratus clouds may be due to these different types of cloud processing or to inherent differences in cloud dynamics between these cloud types. These opposite cloud effects are manifested in differences in cloud brightness and cloud lifetime both of which impact climate. The proposed analysis method is based on bimodality/unimodality of detailed high resolution CCN spectral measurements compared with cloud characteristics such as droplet concentration, mean diameter, spectral width, and drizzle amounts. These cloud microphysics characteristics will be examined over various cloud liquid water contents to determine the competing role of entrainment on cloud microphysics and CCN modality. Results of different hierarchies of cloud characteristics according to associated CCN spectra grouped according to modality should reveal how cloud and drizzle characteristics respond to CCN modality in various environments and cloud types. This could confirm or refute opposite impacts of bimodal CCN on AIE in stratus and cumulus clouds and could untangle effects of chemical or coalescence processing from differences in vertical winds between these cloud types.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.