Support for the Radiation-Intercomparison Project as part of the Chemistry Climate Model Validation exercise (CCMVal)

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Uncertainties in radiation schemes are a large source of error in climate studies. The PI is leading an international effort to validate the radiative transfer models used in Chemistry Climate Models (CCMs). This project is part of the SPARC CCM Validation (CCMVal) exercise. SPARC/WCRP recognise that CCMs require extensive validation and are providing the framework but not the funding to carry out this project. Phase I of this project is nearing completion and this NERC Small Grant proposal is to support Phases II and III of the project. Phase I. For this the PI collected and/or created (by stripping radiation subroutines out of main programs) offline versions of five GCM and CCM radiative transfer codes. Single profile clear sky comparisons of radiative heating rates and forcings were made and these were compared to those calculated with a sophisticated radiation model). This approach required substantial coding and debugging to get all radiation schemes performing on a single computer platform. Phase II will widen the comparison to the international CCM community and involve many more radiation schemes (12+), compared to the 5 analysed in Phase I. Groups themselves will need to perform the requested offline calculations. Heating rates, radiative temperature changes and radiative relaxation times will be compared in offline radiative transfer schemes for multiple clear-sky atmospheric profiles, simulating different months and different times of day. To explain the differences found between schemes it is also important to document the various parameterisations and approximations used in each code. Therefore it is necessary for the PI to visit modelling groups for a few days, impress on them the need for action and then to enlist willing scientists to help produce and/or document an offline radiation code. Following on from this contact groups will be required to submit calculations in a timely fashion and help with interpretation of the results. A suite of requested documentation and calculations has already been designed. Discovering causes of radiation scheme error will be an iterative process with the modelling groups performing additional calculations when requested. An independent set of reference calculations are also being performed by Dr Robert Portmann (NOAA-Bolder). Phase III will involve the CCM groups themselves with our input, employing the results from Phase II to improve their radiative transfer schemes and look at how improved schemes affect CCM integrations.