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Discussion papers
https://doi.org/10.5194/acp-2018-718
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2018-718
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 24 Aug 2018

Research article | 24 Aug 2018

Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Atmospheric Chemistry and Physics (ACP).

Cloud impacts on photochemistry: a new climatology of photolysis rates from the Atmospheric Tomography mission

Samuel R. Hall1, Kirk Ullmann1, Michael J. Prather2, Clare M. Flynn2, Lee T. Murray3, Arlene M. Fiore4,5, Gustavo Correa4, Sarah A. Strode6,7, Stephen D. Steenrod6,7, Jean-Francois Lamarque1, Jonathon Guth8, Béatrice Josse8, Johannes Flemming9, Vincent Huijnen10, N. Luke Abraham11,12, and Alex T. Archibald11,12 Samuel R. Hall et al.
  • 1Atmospheric Chemistry, Observations and Modeling Laboratory, National Center for Atmospheric Research, Boulder, CO 80301, USA
  • 2Department of Earth System Science, University of California, Irvine, CA, USA
  • 3Department of Earth and Environmental Sciences, University of Rochester, Rochester, NY, USA
  • 4Department of Earth and Environmental Sciences, Columbia University, NY, NY, USA
  • 5Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY, USA
  • 6NASA Goddard Space Flight Center, Greenbelt, MD, USA
  • 7Universities Space Research Association (USRA), GESTAR, Columbia, MD, USA
  • 8Centre National de Recherches Météorologiques, CNRS-Météo-France, UMR 3589, Toulouse, France
  • 9European Centre for Medium-Range Weather Forecasts, Reading, UK
  • 10Royal Netherlands Meteorological Institute, De Bilt, the Netherlands
  • 11Department of Chemistry, University of Cambridge, Cambridge, UK
  • 12National Centre for Atmospheric Science, UK

Abstract. Measurements from actinic flux spectroradiometers on board the NASA DC-8 during the Atmospheric Tomography (ATom) mission provide an extensive set of statistics on how clouds alter photolysis rates (J-values) throughout the remote Pacific and Atlantic Ocean basins. ATom made profiling circumnavigations of the troposphere over four seasons during 2016–2018. J-values are a primary chemical control over tropospheric ozone and methane abundances and their greenhouse effects. Clouds have been recognized for more than three decades as being an important factor in tropospheric chemistry. The ATom climatology of J-values is a unique test of how the chemistry models treat clouds. This work focuses on measurements over the Pacific during the first deployment (ATom-1) in August 2016. Nine global chemistry–climate or –transport models provide J-values for the domains measured in ATom-1. We compare mean profiles over a range of cloudy and clear conditions; but, more importantly, we build a statistical picture of the impact of clouds on J-values through the distribution of the ratio of J-cloudy to J-clear. In detail, the models show largely disparate patterns. When compared with measurements, there is some limited, broad agreement. Models here have resolutions of 50–200km and thus reduce the occurrence of clear sky when averaging over grid cells. In situ measurements also average the scattered sunlight, but only out to scales of 10s of km. A primary uncertainty remains in the role of clouds in chemistry, in particular, how models average over cloud fields, and how such averages can simulate measurements.

Samuel R. Hall et al.
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Status: final response (author comments only)
Status: final response (author comments only)
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
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Short summary
Photolysis (J rates) initiates and drives atmospheric chemistry, and J's are perturbed by factors of 2+ by clouds. The NASA Atmospheric Tomography (ATom) Mission provides the first comprehensive observations on how clouds perturb J's through the remote Pacific and Atlantic basins. We compare these cloud-perturbation J statistics with those from 9 global chemistry models. While basic patterns agree, there is a large spread across models, and all lack some basic features of the observations.
Photolysis (J rates) initiates and drives atmospheric chemistry, and J's are perturbed by...
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