Description and evaluation of aerosol in UKESM1 and HadGEM3-GC3.1 CMIP6 historical simulations

Abstract

Abstract. We document and evaluate the aerosol schemes as implemented in the physical and Earth system models, HadGEM3-GC3.1 (GC3.1) and UKESM1, which are contributing to the 6th Coupled Model Intercomparison Project (CMIP6). The simulation of aerosols in the present-day period of the historical ensemble of these models is evaluated against a range of observations. Updates to the aerosol microphysics scheme are documented as well as differences in the aerosol representation between the physical and Earth system configurations. The additional Earth-system interactions included in UKESM1 leads to differences in the emissions of natural aerosol sources such as dimethyl sulfide, mineral dust and organic aerosol and subsequent evolution of these species in the model. UKESM1 also includes a stratospheric-tropospheric chemistry scheme which is fully coupled to the aerosol scheme, while GC3.1 employs a simplified aerosol chemistry mechanism driven by prescribed monthly climatologies of the relevant oxidants. Overall, the simulated speciated aerosol mass concentrations compare reasonably well with observations. Both models capture the negative trend in sulfate aerosol concentrations over Europe and the eastern United States of America (US) although the models tend to underestimate the sulfate concentrations in both regions. Interactive emissions of biogenic volatile organic compounds in UKESM1 lead to an improved agreement of organic aerosol over the US. Simulated dust burdens are similar in both models despite a 2-fold difference in dust emissions. Aerosol optical depth is biased low in dust source and outflow regions but performs well in other regions compared to a number of satellite and ground-based retrievals of aerosol optical depth. Simulated aerosol number concentrations are generally within a factor of 2 of the observations with both models tending to overestimate number concentrations over remote ocean regions, apart from at high latitudes, and underestimate over Northern Hemisphere continents. Finally, UKESM1 includes for the first time a representation of a primary marine organic aerosol source. The impact of this new aerosol source is evaluated. Over the pristine Southern Ocean, it is found to improve the seasonal cycle of organic aerosol mass and cloud droplet number concentrations relative to GC3.1 although underestimations in cloud droplet number concentrations remain. This paper provides a useful characterization of the aerosol climatology in both models facilitating the understanding of the numerous aerosol-climate interaction studies that will be conducted as part of CMIP6 and beyond.

Description and evaluation of the UKCA stratosphere-troposphere chemistry scheme (StratTrop vn 1.0) implemented in UKESM1

“…CC BY 4.0 License. Mulcahy et al, 2019), in which sulphate and secondary organic aerosol (SOA) formation is driven by prescribed oxidant fields. In the UKCA-StratTrop configuration described here, the oxidants driving secondary aerosol formation are fully interactive; this coupling between UKCA chemistry and GLOMAP-mode is fully described in Mulcahy et al (2019).…”

“…Mulcahy et al, 2019), in which sulphate and secondary organic aerosol (SOA) formation is driven by prescribed oxidant fields. In the UKCA-StratTrop configuration described here, the oxidants driving secondary aerosol formation are fully interactive; this coupling between UKCA chemistry and GLOMAP-mode is fully described in Mulcahy et al (2019). Together with dynamic vegetation and a terrestrial carbon/nitrogen scheme (Sellar et al, 2019a), GA7.1/GL7.0 and UKCA StratTrop make up the atmospheric and land components of the UK Earth System Model, UKESM1 (Sellar et al, 2019a) which will be used as part of the UK contribution to the 6th Coupled Model Intercomparison Project (CMIP6; Eyring et al, 2016).…”

“…When UKCA StratTrop is coupled to the UKCA aerosol scheme, GLOMAP-mode (Mann et al, 2010) as here, there are additional trace gas aerosol-precursor emissions for dimethyl sulphide (DMS), sulphur dioxide (SO2), and monoterpenes (C10H16). These emissions will be discussed in the context of the UKESM1 aerosol performance in Mulcahy et al (2019); the focus here will solely be on the tropospheric ozone precursor emissions. Table 11 and Figures 5-8 summarise the mean global annual emissions totals for the time period considered here (2005-2014) and their global and seasonal distributions.…”

“…(Sellar et al, 2019a) is determined by oxidants (OH, O3, H2O2, NO3) modelled interactively by the UKCA StratTrop chemistry scheme. For further details on the oxidation of sulphate and SOA precursors, chemistry-aerosol coupling, and the scientific performance of the aerosol scheme (GLOMAPmode; Mann et al, 2010) in UKCA and UKESM1, the reader is referred to Mulcahy et al (2019).…”

U.K. Community Earth System Modeling for CMIP6

“…For computational reasons, a significant fraction of the development and tuning of the coupled biogeochemical cycles in UKESM1 was carried out using a model configuration with prescribed atmospheric chemistry (referred to as UKESM1‐CN). In parallel to this, coupling and tuning of the interactive trace gas chemistry scheme (UKCA, Archibald et al., 2019) with the GLOMAP‐mode aerosol scheme (Mulcahy et al., 2019) and relevant atmosphere and land physical parameterizations was carried out using the atmospheric component of HadGEM3‐GC3.1, GA7.1, with interactive chemistry enabled. Once the coupled biogeochemical cycles (in UKESM1‐CN) and the atmospheric chemistry‐aerosol processes (in GA7.1) were performing acceptably, this updated interactive chemistry scheme was activated in UKESM1‐CN, leading to the full configuration of UKESM1.…”