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

• Main Author: M. J. Prather
• Format: Article
• Language: English
• Published: Copernicus Publications 2015-08-01
• Series: Geoscientific Model Development
• Online Access: http://www.geosci-model-dev.net/8/2587/2015/gmd-8-2587-2015.pdf

A new approach for modeling photolysis rates (<i>J</i> 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 <i>J</i> 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 <i>J</i> values, the treatment of volatile organic compounds with pressure-dependent cross sections, is also incorporated into Cloud-J.