Articles | Volume 8, issue 8
Geosci. Model Dev., 8, 2587–2595, 2015
https://doi.org/10.5194/gmd-8-2587-2015
Geosci. Model Dev., 8, 2587–2595, 2015
https://doi.org/10.5194/gmd-8-2587-2015

Model description paper 14 Aug 2015

Model description paper | 14 Aug 2015

Photolysis rates in correlated overlapping cloud fields: Cloud-J 7.3c

M. J. Prather M. J. Prather
  • Earth System Science Department, University of California, Irvine, California, USA

Abstract. A new approach for modeling photolysis rates (J values) in atmospheres with fractional cloud cover has been developed and is implemented as Cloud-J – a multi-scattering eight-stream radiative transfer model for solar radiation based on Fast-J. Using observations of the vertical correlation of cloud layers, Cloud-J 7.3c provides a practical and accurate method for modeling atmospheric chemistry. The combination of the new maximum-correlated cloud groups with the integration over all cloud combinations by four quadrature atmospheres produces mean J values in an atmospheric column with root mean square (rms) errors of 4 % or less compared with 10–20 % errors using simpler approximations. Cloud-J is practical for chemistry–climate models, requiring only an average of 2.8 Fast-J calls per atmosphere vs. hundreds of calls with the correlated cloud groups, or 1 call with the simplest cloud approximations. Another improvement in modeling J values, the treatment of volatile organic compounds with pressure-dependent cross sections, is also incorporated into Cloud-J.

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Short summary
A new approach for modeling photolysis rates (J values) in atmospheres with fractional cloud cover has been developed and is implemented as Cloud-J – a multi-scattering eight-stream radiative transfer model for solar radiation based on Fast-J. Using observations of the vertical correlation of cloud layers, Cloud-J provides a practical and accurate method for modeling atmospheric chemistry, which can be extended to solar heating rates.