Water vapor, clouds and precipitation are critically important to the global energy budget and climate system through radiation and the hydrologic cycle. Cumulus clouds can occupy up to a quarter of the global cloud fraction and act to moisten the free troposphere through detrainment, which is influenced by both microphysics and environmental thermodynamics. Through both direct and indirect forcings, aerosols act to alter both environmental thermodynamics and cloud microphysics, respectively, in turn affecting detrainment. As the world increases its aerosol producing activities, the effects of these aerosols on the global climate must be understood, especially on water processes.
The primary goal of this research is to better understand the impacts of aerosols on cumulus detrainment and the resulting vertical distribution of moisture through investigations of both environmental thermodynamics and cloud processes. This will be achieved through the use of large eddy simulations (LES) employing aerosol dependent microphysics and radiation. A series of experiments in which the number concentration of sulfate aerosols (CCN) are varied from "pristine" to "polluted" for each of the three cumulus cloud regimes (shallow, congestus, and deep) will be performed. Observed soundings from the Dynamics of the Madden-Julian Oscillation (DYNAMO) experiment will be used to represent each of the three cumulus regimes in the LES model. This approach will allow explicit calculations and budgets of both water processes and radiation to assess the impacts of aerosols on the vertical distribution of water vapor due to cumulus detrainment. As water vapor is the strongest greenhouse gas, its vertical distribution is highly important for the global radiation budget.
Intellectual Merit This research will provide new insights into the role aerosols play in modifying tropospheric moistening due to cumulus detrainment through the following measures: (1) assessing the changes in the vertical distribution of water vapor, direct aerosol forcing impacting detrainment, and indirect aerosol forcing due to microphysical processing of aerosol - all of which will assist future radiation parameterizations in regional and climate models; (2) quantifying changes in cloud characteristics that impact detrainment such as: cloud top height, drop size distributions, and precipitation efficiency - these will be beneficial to the development of future cumulus parameterizations; and (3) showing the importance of using data from a field campaign (DYNAMO) designed to understand large-scale phenomena for the investigation of small-scale processes. The described intellectual merits will also assist in bridging the gap in our community between scientists operating on small and large scales.
Broader Impacts The broader impacts of the research ultimately aid in our understanding of two highly influential aspects of the atmosphere: water and aerosols. More specifically, the research will help provide: (1) a better understanding of aerosol forcing on the global climate; (2) increased knowledge of cumulus convection, which occupies up to a quarter of the global cloud cover, and its role in the vertical transport of water vapor (the strongest greenhouse gas); (3) awareness of the value in field campaigns, such as DYNAMO, in not only collecting in-situ data but coupling those data with models to better assess physical processes occurring in the atmosphere; and (4) co-mentoring of graduate students and education in numerical modeling through the development of a cloud systems modeling course.
