Cloud Condensation and Ice Nuclei Impacts on Tropical Convection in Cloud Resolving Model Simulations in Radiative Convective Equilibrium

Evaluation of the impacts of aerosols on the initiation and development of convection is necessary in order to enhance our understanding of precipitation and radiative effects of convection on climate. As found in the recent IPCC report, whether anthropogenic- or naturally-produced, aerosols are considered one of the most uncertain aspects in climate studies. A region important in maintaining the Earth's climate, the Tropics, represents a balanced radiative-convective environment. Previous model studies of cloud condensation nuclei (CCN) impacts on tropical convection have found varying responses in different cloud types and cloud modes. Yet, large-scale organization remains dynamically influenced. This present study attempts to present an evaluation of the impacts of ice nuclei (IN) in addition to CCN on tropical convection development. IN are globally present aerosols and have different microphysical characteristics than CCN. The cloud dynamical and microphysical influences of each of these aerosol species are the goal of this study.

This study investigates a pristine and polluted tropical environment. The two-dimensional idealized simulations performed for this study were conducted using the Regional Atmospheric Modeling System (RAMS). The model was initiated with a Tropical Ocean and Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA-COARE) sounding, and utilized fixed sea surface temperatures. The model was run using a domain of 10,000 km with grid spacing of 1 km for a time period of 100 days. Such a model setup allowed for the development of multiple convective events. Sensitivity tests were conducted once the CONTROL simulations reached radiative convective equilibrium, a process that takes about 60 simulation days. In these sensitivity tests, the concentrations of aerosols that can serve as CCN or IN were independently and then simultaneously varied. Aerosol indirect effects were then evaluated by varying aerosols that can serve as CCN and IN separately and together, with concentrations ranging from a clean to polluted environment.

Results of this study show varying IN concentrations affect aerosol indirect effects on tropical convection differently than CCN. When both species are present, the effect is not dominated by either aerosol species alone. Investigation of these separate and simultaneous effects on cloud fraction, precipitation, updraft strength, liquid water path, ice water path, and the vertical distribution of liquid water and ice will be presented.