Technical Note: Estimating aerosol effects on cloud radiative forcing

S J Ghan

doi:10.5194/acp-13-9971-2013

Abstract

Abstract. Estimating anthropogenic aerosol effects on the planetary energy balance through the aerosol influence on clouds using the difference in cloud radiative forcing from simulations with and without anthropogenic emissions produces estimates that are positively biased. A more representative method is suggested using the difference in cloud radiative forcing calculated as a diagnostic with aerosol scattering and absorption neglected. The method also yields an aerosol radiative forcing decomposition that includes a term quantifying the impact of changes in surface albedo. The method requires only two additional diagnostic calculations: the whole-sky and clear-sky top-of-atmosphere radiative flux with aerosol scattering and absorption neglected.

Highlights

The Intergovernmental Panel on Climate Change Fifth Assessment Report recommends that anthropogenic aerosol effects on the planetary energy balance be expressed as an effective radiative forcing (ERF) that allows clouds to respond to the aerosol while surface temperature is prescribed
Lohmann et al (2010) showed that the radiative flux perturbation (RFP) estimate of aerosol radiative forcing agrees well with estimates using diagnostic radiation calls with presentday and preindustrial aerosol and the same meteorology. This suggests the RFP method can be used to estimate aerosol effects involving all of the “fast physics” of climate, including precipitation
Further decomposition of the aerosol effect on cloud radiative forcing into contributions from indirect effects and semi-direct effects would require another pair of simulations in which aerosol absorption is neglected (G12)

Summary

Introduction

The Intergovernmental Panel on Climate Change Fifth Assessment Report recommends that anthropogenic aerosol effects on the planetary energy balance be expressed as an effective radiative forcing (ERF) that allows clouds to respond to the aerosol while surface temperature is prescribed. One recommended way to calculate the ERF is using the radiative flux perturbation (RFP) method from the top-of-atmosphere energy balance difference between simulations with and without anthropogenic emissions but the same ocean surface conditions (Hanson et al, 2005; Haywood et al, 2009; Lohmann et al, 2010).

Results

Conclusion