Cloud Response to Short-term Sulfate Aerosol Forcing over the Tropics

Regional climate response to anthropogenic forcing exhibits considerable sensitivity to aerosol-cloud interactions. Clouds that exert strong control over a region’s climate are simultaneously sensitive to their large-scale environment and changes in cloud microphysics and aerosols. Difficulty in accurately observing and modelling such complex interactions, therefore, limits the fidelity of the simulated climate response. Specifically, the effect of sulfate aerosols (that act as cloud condensation and ice nuclei) on tropical clouds is not well understood and is addressed here.

We examine cloud response to short-term sulfate aerosols forcing by carrying out a fully coupled global model experiment using CESM (version-1.1.2, with CAM5 physics). The experiment consists of an ensemble of two-year long integrations of control (preindustrial sulfate aerosols) and perturbed (present day sulfate aerosols) cases with reduced intrinsic noise due to weather and longer term climate variability. Both subgrid scale parameterizations and their interactions are more physically realistic in CAM5 than its predecessor, improving instantaneous adjustment of clouds due to increased cloud condensation nuclei, as well as, finite time adjustments comprising radiative and dynamical effects on clouds (together known as aerosol-cloud interactions).

We find contrasting behavior in convective versus ice clouds (model simulated high level stratus) over tropical Indian Ocean and west Pacific sector in short-term climate response to sulfate aerosols. While convective activity (clouds) is significantly reduced - a response driven by cooling of underlying SST and large scale dynamics, there is significant enhancement of ice cloud amounts - perhaps an indirect response of high clouds mediated through aerosol-cloud interactions. Such interactions, for example, in liquid clouds can increase its amount through changes in microphysical parameters (increase in cloud droplet concentration, decrease in cloud droplet effective radii) and suppression or delay of rainfall.

This response of high clouds to sulfate aerosols, which are distributed throughout the atmospheric column is examined across microphysical, macrophysical and environmental features and evaluated against previous studies and observational evidence. Additionally, its effect on the local radiative budget is analyzed along with comments on the relevant regional climate response.