Photolysis Rates in Correlated Overlapping Cloud Fields


A new approach for modeling photolysis rates (J values) in atmospheres with fractional cloud cover has been developed and implemented as Cloud-J, a multiscattering 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 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% to 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 versus 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, also is incorporated into Cloud-J.


Prather, M. J. 2015. “Photolysis Rates in Correlated Overlapping Cloud Fields: Cloud-J 7.3c,” Geoscientific Model Development 8, 2587–95. DOI: 10.5194/gmd-8-2587-2015.