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 redistributing 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 rise steadily 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.

The influence of model spatial resolution on simulated ozone and fine particulate matter for Europe: implications for health impact assessments

“…In particular, the deeper boundary layer in summer simulated at the finer resolution may lead to greater vertical lofting from the surface, producing lower PM 2.5 levels compared to that simulated at the coarse resolution. In addition, differences in simulated precipitation (especially smaller-scale convective precipitation) between the two resolutions may be important, through its influence as the dominant mechanism in UKCA for removal of aerosols through wet deposition (O'Connor et al, 2014). Spatial patterns of convective precipitation differences between the two resolutions are shown in Sect.…”

“…Both model configurations are driven by decadal mean present-day emissions from Lamarque et al (2010), representative of the decade centred on 2000. Biogenic emission of isoprene and monoterpenes are calculated interactively following Pacifico et al (2011) and the biogenic emissions of methanol and acetone are prescribed, taken from Guenther et al (1995). A full description of other biogenic emissions and the coarse and finer configurations can be found in Neal et al (2017).…”

“…In both configurations, a 63 level hybrid height vertical co-ordinate system is used with 50 levels below 18 km and a surface level at 40 m. Gas phase chemistry is simulated within HadGEM3 by a tropospheric configuration of the United Kingdom Chemistry and Aerosol (UKCA) model (Morgenstern et al, 2009;O'Connor et al, 2014). The chemistry scheme used for both configurations is the UKCA Extended Tropospheric Chemistry (UKCA-ExtTC) scheme (Folberth et al, 2018) which is an extension to the TropIsop standard chemistry scheme (O'Connor et al, 2014) and includes 89 chemical species. Boundary layer mixing for both configurations is based on Lock et al (2000) and includes an explicit entrainment parameterisation and non-local mixing in unstable layers.…”

On the emissions and transport of bromoform: sensitivity to model resolution and emission location

“…Isoprene emissions are taken from the POET database (Granier et al, 2005;Olivier et al, 2003). The general circulation for the periods under analysis is forced by prescribing monthly mean sea surface temperatures and sea ice cover from the AMIP data set (http://www-pcmdi.llnl.gov/ projects/amip).…”

Influence of Sea Ice‐Derived Halogens on Atmospheric HOx as Observed in Springtime Coastal Antarctica

“…We chose to investigate the role that halogens play by using a zero-dimensional model employing a simple representation of bromine release to the boundary layer. We used the BOXMOX modeling framework of Knote et al (Knote et al, 2015), employing the UKCA "CHeST" whole atmosphere chemistry mechanism (Morgenstern et al, 2009;O'Connor et al, 2014) and photolysis rates derived from the Tropospheric Ultraviolet and Visible (TUV) radiation model (Madronich, 2002) scaled to match the observed JO 1 D and JNO 2 . We initialized the model with measured temperature and pressure, from which water vapor concentrations were derived using the Goff-Gratch parameterization, as well as observed levels of O 3 , NO x , and VOCs (Read et al, 2007), at midnight local time.…”