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Geoscientific Model Development An interactive open-access journal of the European Geosciences Union
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Volume 2, issue 1
Geosci. Model Dev., 2, 59–72, 2009
© Author(s) 2009. This work is distributed under
the Creative Commons Attribution 3.0 License.
Geosci. Model Dev., 2, 59–72, 2009
© Author(s) 2009. This work is distributed under
the Creative Commons Attribution 3.0 License.

  20 May 2009

20 May 2009

Upgrading photolysis in the p-TOMCAT CTM: model evaluation and assessment of the role of clouds

A. Voulgarakis1,*, N. H. Savage2, O. Wild3, G. D. Carver1, K. C. Clemitshaw4, and J. A. Pyle1 A. Voulgarakis et al.
  • 1Centre for Atmospheric Science, University of Cambridge, Cambridge, UK
  • 2Met Office, Exeter, UK
  • 3Lancaster Environment Centre, University of Lancaster, Lancaster, UK
  • 4Department of Earth Sciences, Royal Holloway, University of London, London, UK
  • *now at: Columbia University, Center for Climate Systems Research, NASA Goddard Institute for Space Studies, New York, USA

Abstract. A new version of the p-TOMCAT Chemical Transport Model (CTM) which includes an improved photolysis code, Fast-JX, is validated. Through offline testing we show that Fast-JX captures well the observed J(NO2) and J(O1D) values obtained at Weybourne and during a flight above the Atlantic, though with some overestimation of J(O1D) when comparing to the aircraft data. By comparing p-TOMCAT output of CO and ozone with measurements, we find that the inclusion of Fast-JX in the CTM strongly improves the latter's ability to capture the seasonality and levels of tracers' concentrations. A probability distribution analysis demonstrates that photolysis rates and oxidant (OH, ozone) concentrations cover a broader range of values when using Fast-JX instead of the standard photolysis scheme. This is not only driven by improvements in the seasonality of cloudiness but also even more by the better representation of cloud spatial variability. We use three different cloud treatments to study the radiative effect of clouds on the abundances of a range of tracers and find only modest effects on a global scale. This is consistent with the most relevant recent study. The new version of the validated CTM will be used for a variety of future studies examining the variability of tropospheric composition and its drivers.

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