Journal cover Journal topic
Geoscientific Model Development An interactive open-access journal of the European Geosciences Union
Geosci. Model Dev., 9, 3447-3460, 2016
© Author(s) 2016. This work is distributed
under the Creative Commons Attribution 3.0 License.
Model experiment description paper
27 Sep 2016
The Radiative Forcing Model Intercomparison Project (RFMIP): experimental protocol for CMIP6
Robert Pincus1,2, Piers M. Forster3, and Bjorn Stevens4 1Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309, USA
2NOAA Earth System Research Lab, Physical Sciences Division, Boulder, CO 80305, USA
3Institute for Climate and Atmospheric Science, School of Earth and Environment, University of Leeds, Leeds, UK
4Max Planck Institute for Meteorology, Hamburg 20146, Germany
Abstract. The phrasing of the first of three questions motivating CMIP6 – “How does the Earth system respond to forcing?” – suggests that forcing is always well-known, yet the radiative forcing to which this question refers has historically been uncertain in coordinated experiments even as understanding of how best to infer radiative forcing has evolved. The Radiative Forcing Model Intercomparison Project (RFMIP) endorsed by CMIP6 seeks to provide a foundation for answering the question through three related activities: (i) accurate characterization of the effective radiative forcing relative to a near-preindustrial baseline and careful diagnosis of the components of this forcing; (ii) assessment of the absolute accuracy of clear-sky radiative transfer parameterizations against reference models on the global scales relevant for climate modeling; and (iii) identification of robust model responses to tightly specified aerosol radiative forcing from 1850 to present.

Complete characterization of effective radiative forcing can be accomplished with 180 years (Tier 1) of atmosphere-only simulation using a sea-surface temperature and sea ice concentration climatology derived from the host model's preindustrial control simulation. Assessment of parameterization error requires trivial amounts of computation but the development of small amounts of infrastructure: new, spectrally detailed diagnostic output requested as two snapshots at present-day and preindustrial conditions, and results from the model's radiation code applied to specified atmospheric conditions. The search for robust responses to aerosol changes relies on the CMIP6 specification of anthropogenic aerosol properties; models using this specification can contribute to RFMIP with no additional simulation, while those using a full aerosol model are requested to perform at least one and up to four 165-year coupled ocean–atmosphere simulations at Tier 1.

Citation: Pincus, R., Forster, P. M., and Stevens, B.: The Radiative Forcing Model Intercomparison Project (RFMIP): experimental protocol for CMIP6, Geosci. Model Dev., 9, 3447-3460, doi:10.5194/gmd-9-3447-2016, 2016.
Publications Copernicus
Short summary
This paper describes an experimental protocol to understand the changes in energy balance (the "radiative forcing") that arise due to changes in atmospheric composition and why this value is not the same across climate models. The protocol includes a way to determine the total forcing to which each model is subjected, experiments designed at teasing out why certain errors occur, and experiments to identify any robust signals caused by atmospheric particles from human activities.
This paper describes an experimental protocol to understand the changes in energy balance (the...