Simulation of climate response to aerosol direct and indirect effects with aerosol transport‐radiation model

TLDR

A global aerosol transport‑radiation model coupled to a general circulation model, augmented with a Köhler‑theory based microphysical parameterization, simulates aerosol‑induced changes in cloud, precipitation, temperature, and radiative forcing. The model reproduces satellite‑observed cloud droplet effective radius, radiative forcing, and precipitation, showing a global reduction in effective radius from anthropogenic aerosols, with cloud water and precipitation changes driven mainly by dynamical hydrological cycle and temperature shifts, while the second indirect effect can locally increase cloud water and reduce precipitation; global mean radiative forcings are −0.1 and −0.9 W m⁻², and aerosols are estimated to offset roughly 40 % of anthropogenic greenhouse‑gas–induced surface warming.

Abstract

With a global aerosol transport‐radiation model coupled to a general circulation model, changes in the meteorological parameters of clouds, precipitation, and temperature caused by the direct and indirect effects of aerosols are simulated, and its radiative forcing are calculated. A microphysical parameterization diagnosing the cloud droplet number concentration based on the Köhler theory is introduced into the model, which depends not only on the aerosol particle number concentration but also on the updraft velocity, size distributions, and chemical properties of each aerosol species and saturation condition of the water vapor. The simulated cloud droplet effective radius, cloud radiative forcing, and precipitation rate, which relate to the aerosol indirect effect, are in reasonable agreement with satellite observations. The model results indicate that a decrease in the cloud droplet effective radius by anthropogenic aerosols occurs globally, while changes in the cloud water and precipitation are strongly affected by a variation of the dynamical hydrological cycle with a temperature change by the aerosol direct and first indirect effects rather than the second indirect effect itself. However, the cloud water can increase and the precipitation can simultaneously decrease in regions where a large amount of anthropogenic aerosols and cloud water exist, which is a strong signal of the second indirect effect. The global mean radiative forcings of the direct and indirect effects at the tropopause by anthropogenic aerosols are calculated to be −0.1 and −0.9 W m −2 , respectively. It is suggested that aerosol particles approximately reduce 40% of the increase in the surface air temperature by anthropogenic greenhouse gases on the global mean.