Evaluation of the new UKCA climate-composition model – Part 2: The Troposphere

Abstract

Abstract. In this paper, we present a description of the tropospheric chemistry component of the UK Chemistry and Aerosols (UKCA) model which has been coupled to the Met Office Hadley Centre's HadGEM family of climate models. We assess the model's transport and scavenging processes, in particular focussing on convective transport, boundary layer mixing, wet scavenging and inter-hemispheric exchange. Simulations with UKCA of the short-lived radon tracer suggest that modelled distributions are comparable to those of other models and the comparison with observations indicate that apart from a few locations, boundary layer mixing and convective transport are effective in the model as a means of vertically re-distributing surface emissions of radon. Comparisons of modelled lead tracer concentrations with observations suggest that UKCA captures surface concentrations in both hemispheres very well although there is a tendency to underestimate the observed geographical and interannual variability in the Northern Hemisphere. In particular, UKCA replicates the shape and absolute concentrations of observed lead profiles, a key test in the evaluation of a model's wet scavenging scheme. The timescale for inter-hemispheric transport, calculated in the model using a simple krypton tracer experiment, does appear to be long relative to other models and could indicate deficiencies in tropical deep convection and/or insufficient boundary layer mixing. We also describe the main components of the tropospheric chemistry and evaluate it against observations and other tropospheric chemistry models. In particular, from a climate forcing perspective, present-day observed surface methane concentrations and tropospheric ozone concentrations are reproduced very well by the model, thereby making it suitable for long centennial integrations as well as studies of biogeochemical feedbacks. Results from both historical and future simulations with UKCA tropospheric chemistry are presented. Future projections of tropospheric ozone vary with the Representative Concentration Pathway (RCP). In RCP2.6, for example, tropospheric ozone increases up to 2010 and then declines by 13% of its year-2000 global mean by the end of the century. In RCP8.5, tropospheric ozone continues to steadily rise throughout the 21st century, with methane being the main driving factor. Finally, we highlight aspects of the UKCA model which are undergoing and/or have undergone recent developments and are suitable for inclusion in a next-generation Earth System Model.

Earth System Chemistry integrated Modelling (ESCiMo) with the Modular Earth Submodel System (MESSy) version 2.51

“…Prather et al, 2012;Prinn et al, 2005, with 11.2 ± 1.3 and 10.2 (+0.9/−0.7) years, respectively). The wide range of lifetime estimates is mainly caused by different methods of calculation and applied weighting (Lawrence et al, 2001), whereas varying included vertical layers due to different tropopause heights have a minor impact (see also O'Connor et al, 2014). The global mean temperature nudged RC1SD-base-08 simulation (royalblue dashed) predicts the shortest lifetime.…”

Tropospheric ozone and its precursors from the urban to the global scale from air quality to short-lived climate forcer

“…The O 3 mixing ratios presented in Fig. 4 are generated from photochemical modelling output generated using the UKCA model (O'Connor et al, 2014;Archibald et al, 2011). Regions in Fig.…”

Reduced cooling following future volcanic eruptions

“…Sulphate precursor emissions are partly prescribed (anthropogenic, land dimethyl sulphide and volcanic degassing) and partly interactive (ocean dimethyl sulphide). Sulphate oxidation rates are calculated with the coupled tropospheric chemistry scheme (O'Connor et al 2014). All other aerosols rely on prescribed monthly emissions.…”

“…From the surface to 3 model levels above the dynamically diagnosed tropopause, ozone is simulated dynamically with a tropospheric chemistry scheme (O'Connor et al 2014). Stratospheric halogen chemistry is not included, and so from the 4th level above the tropopause, stratospheric ozone is prescribed from an observationally-based zonally averaged climatology.…”