Improved Aerosol Processes and Effective Radiative Forcing in HadGEM3 and UKESM1

Abstract

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.

UKESM1: Description and Evaluation of the U.K. Earth System Model

“…In the introduction to this special issue, Senior et al (2020) outline the development strategy for UKESM1 and its physical core HadGEM3-GC3.1, which included a step (before model finalization) of estimating effective radiative forcing (ERF) due to key anthropogenic forcing agents (GHGs, aerosols, and land use change), in order to avoid unrealistic historical performance at a global mean scale. See Mulcahy et al (2018) in this special issue for action which was taken on aerosol forcing as a result of this strategy. These ERF experiments assessed only the 1850-2000 forcing, but as Figure 29 demonstrates, it is possible to have good agreement in global mean surface temperature toward the end of the historical period (implying that the integrated historical forcing is accurate, perhaps through a compensation of errors) and still have significant deviations prior to this.…”

“…• Ocean biogeochemistry: MEDUSA (Model of Ecosystem Dynamics, nutrient Utilisation, Sequestration and Acidification; Yool et al, 2013), with the addition of a diagnostic submodel for DMS surface seawater concentration (Anderson et al, 2001) to support interactive DMS emissions to the atmosphere. • Atmospheric composition: UKCA (United Kingdom Chemistry and Aerosol model; Archibald et al, 2019;Mulcahy et al, 2018) with unified stratospheric-tropospheric chemistry and close coupling between chemistry and aerosols. UKESM1 is a successor to HadGEM2-ES (Collins et al, 2011), and many of its components, namely, the land surface, atmospheric chemistry, and the core physical model, have been developed from that model's components.…”

“…In this section we summarize all of the parameters tuned in the development of the fully coupled UKESM1 and the rationale for that tuning. Note that prior to coupled testing, the component models were developed and tuned in uncoupled mode, and this tuning is described in the documentation papers for those components (Harper et al, 2016(Harper et al, , 2018Morgenstern et al, 2009;Mulcahy et al, 2018;O'Connor et al, 2014;Walters et al, 2019;Williams et al, 2018;Yool et al, 2013). The parameter values from that component model tuning acted as the starting point for the tuning of UKESM1; in the case of physical model parameters, the starting point was the values used in GC3.1 (the physical core of UKESM1).…”

Strong Dependence of Atmospheric Feedbacks on Mixed‐Phase Microphysics and Aerosol‐Cloud Interactions in HadGEM3

“…All the tickets are GA tickets, except for the GL tickets in the Lnd package. The Erf package is documented in Mulcahy et al ().…”

“…GA6.0 is documented in Walters et al (2017), and GA7.0 in Walters et al (2019), and Williams et al (2018). GA7.1 is an incremental evolution targeting a better representation of the aerosol radiative forcing (Mulcahy et al, 2018). More than 40 model changes were introduced between GA6.0 and GA7.1, ranging from simple bug fixes to the implementation of an entirely new aerosol scheme.…”