Aerosol processes and, in particular, aerosol-cloud interactions cut across the traditional physical-Earth system boundary of coupled Earth system models and remain one of the key uncertainties in estimating anthropogenic radiative forcing of climate. Here we calculate the historical aerosol effective radiative forcing (ERF) in the HadGEM3-GA7 climate model in order to assess the suitability of this model for inclusion in the UK Earth system model, UKESM1. The aerosol ERF, calculated for the year 2000 relative to 1850, is large and negative in the standard GA7 model leading to an unrealistic negative total anthropogenic forcing over the twentieth century. We show how underlying assumptions and missing processes in both the physical model and aerosol parameterizations lead to this large aerosol ERF. A number of model improvements are investigated to assess their impact on the aerosol ERF. These include an improved representation of cloud droplet spectral dispersion, updates to the aerosol activation scheme, and black carbon optical properties. One of the largest contributors to the aerosol forcing uncertainty is insufficient knowledge of the preindustrial aerosol climate. We evaluate the contribution of uncertainties in the natural marine emissions of dimethyl sulfide and organic aerosol to the ERF. The combination of model improvements derived from these studies weakens the aerosol ERF by up to 50% of the original value and leads to a total anthropogenic historical forcing more in line with assessed values.

Vertical profiles of aerosol size, composition, and hygroscopic behavior from Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) Twin Otter and National Oceanic and Atmospheric Administration R/V Ronald H. Brown observations are used to construct a generic optical model of the Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia) aerosol. The model accounts for sulfate, black carbon, organic carbon, sea salt, and mineral dust. The effects of relative humidity and mixing assumptions (internal versus external, coating of dust by pollutants) are explicitly accounted for. The aerosol model is integrated with a Monte Carlo radiative transfer model to compute direct radiative forcing in the solar spectrum. The predicted regional average surface aerosol forcing efficiency (change in clear-sky radiative flux per unit aerosol optical depth at 500 nm) during the ACE-Asia intensive period is -65 Wm(-2) for pure dust and -60 Wm(-2) for pure pollution aerosol (clear skies). A three-dimensional atmospheric chemical transport model (Chemical Weather Forecast System (CFORS)) is used with the radiative transfer model to derive regional radiative forcing during ACE-Asia in clear and cloudy skies. Net regional solar direct radiative forcing during the 5-15 April 2001 dust storm period is -3 Wm(-2) at the top of the atmosphere and -17 Wm(-2) at the surface for the region from 20degreesN to 50degreesN and 100degreesE to 150degreesE when the effects of clouds on the direct forcing are included. The model fluxes and forcing efficiencies are found to be in good agreement with surface radiometric observations made aboard the R. H. Brown. Mean cloud conditions are found to moderate the top of atmosphere (TOA) radiative forcing by a factor of similar to3 compared to clear-sky calculations, but atmospheric absorption by aerosol is not strongly affected by clouds in this study. The regional aerosol effect at the TOA (climate forcing) of -3 Wm(-2) is comparable in magnitude, but of opposite sign, to present-day anthropogenic greenhouse gas forcing. The forcing observed during ACE-Asia is similar in character to that seen during other major field experiments downwind of industrial and biomass black carbon sources (e.g., the Indian Ocean Experiment (INDOEX)), insofar as the primary effect of aerosol is to redistribute solar heating from the surface to the atmosphere.