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

O'Connor, F. M. and Johnson, C. E. and Morgenstern, O. and Abraham, N. L. and Braesicke, P. and Dalvi, M. and Folberth, G. A. and Sanderson, M. G. and Telford, P. J. and Voulgarakis, A. and Young, Paul J. and Zeng, G. and Collins, W. J. and Pyle, J. A. (2014) Evaluation of the new UKCA climate-composition model – Part 2: The Troposphere. Geoscientific Model Development, 7 (1). pp. 41-91. ISSN 1991-9603

PDF (gmd-7-41-2014)
gmd_7_41_2014.pdf - Published Version
Available under License Creative Commons Attribution.

Download (8MB)


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.

Item Type:
Journal Article
Journal or Publication Title:
Geoscientific Model Development
Additional Information:
© Author(s) 2014. This work is distributed under the Creative Commons Attribution 3.0 License.
Uncontrolled Keywords:
ID Code:
Deposited By:
Deposited On:
16 Apr 2014 09:06
Last Modified:
22 Nov 2022 00:48