Indirect Aerosol Effects on Tropical Convection

Colorado State University, Fort Collins CO

Investigators

Abstract

The impact of aerosols--both naturally occurring and anthropogenic--upon clouds has been long recognized. In the case of more widespread stratiform type clouds (e.g., subtropical stratocumulus) and isolated convective clouds, these impacts have been rather extensively examined. Such is not the case for systems of deep clouds over broad equatorial regions, which are subject to incursions of wind-driven dust from emanating from continental deserts and urbanized areas. Work supported by this grant will be unique in examining groups of deep tropical convective cloud systems in considerable detail in order to better understand their sensitivity to incursions of microphysically active aerosol including cloud condensation nuclei (CCN) and ice nuclei (IN), which in turn influence the precipitation development and cloud-radiation interactions. Model runs will be conducted over large horizontal regions (~9600 km x 180 km at 38 vertical levels) integrated over long periods (~100 days), which will allow the simulated atmosphere to achieve a state of radiative-convective equilibrium and subsequent assessment of responses to variable amounts and vertical distributions of aerosols. Specific goals include analysis of the impacts that variable CCN and IN concentrations have upon: (1) the tropical water budget, including precipitation processes and distribution of latent heat release; (2) differentiation between the so-called "first" and "second" indirect aerosol effect, which relate to droplet sizes and number concentrations influencing radiative transfer through cloud layers; (3) cloud dynamics, such as those that may favor a previously noted tendency toward tri-modal distribution of convective cloud depth and including updrafts, downdrafts, development of low-level cold pools and other factors influencing cloud organization; (4) radiative-convective equilibrium across the tropics; (5) partitioning of water substance between liquid and ice; and (6) the anvil-cirrus properties of deep tropical convection. Simulated cloud statistics will also be compared with observed cloud properties emerging from NASA's CloudSat satellite to better assess model performance and ultimately identify those physical processes implicit in the observations. The intellectual merit of this study rests in developing an improved description of physics governing the behavior of tropical cloud systems and their role in the global climate system via a combination of cloud microphysical and radiative feedbacks. It will also advance understanding of the "radiative-convective equilibrium" paradigm thought to characterize much of the tropical atmosphere, and will facilitate extracting maximum value from newly emerging datastreams such as provided by CloudSat. Broader impacts of this work include improvement of techniques needed for medium-range forecasts of tropical cloud systems (such as those that occasionally engender tropical cyclones) as well as longer-term prediction of the earth's climate, and include graduate student education and increased involvement by underrepresented groups (viz. women in science).