Model Experiments: Specifications

The Basic Experiment: Fixed season ±2K SST perturbation (Cess et al., 1990)

Slab Ocean 1XCO2 and 2XCO2 Experiment

Appendices

Model Experiments: Specifications

It is well recognised that it is relatively easy to suggest a model intercomparison exercise; however designing an experiment in a manner that maximises the chances of answering specific questions can be much more difficult. Since the magnitude of cloud feedback in an individual model can depend critically on the precise configuration of that model it is crucial that all model experiments that are designed to intercompare cloud feedbacks are conducted with the consistent experimental protocols and with consistent model versions.

In order to maximise overlap with existing successful model intercomparison exercises it is recommended that the Atmospheric Model Intercomparison Project (AMIP) guidelines be adopted wherever possible and moreover it is strongly recommended that the model version used for any of the experiments outlined below be as close as possible to a version submitted for inclusion in the AMIP and CMIP data base, and that the data management protocols of AMIP be followed.

The model experiments considered follow the outlines of the CLIVAR implementation plan and include SST perturbation experiments and slab ocean experiments. In the case of SST perturbation experiments there is a very wide range in choices that could be made from fixed season experiments to experiments with a full seasonal cycle and from SST perturbations that are uniform over the globe or specific regions or where the SST perturbation varies geographically in some prescribed manner either analytically or from some previous experiment. Choices with slab ocean experiments have less possible variation in experimental protocol but nevertheless constraints need to be imposed.

The Basic Experiment: Fixed season ±2K SST perturbation (Cess et al., 1990)

It is fully recognised that there are a number of problems associated with the experimental protocol and the interpretation of results from this experiment and also that the modelling community has gone a long beyond such a perturbation experiment but it is precisely the fact that there is a long and honourable tradition of such experiments that make it so valuable to set current models in an unique historical context.

The basic analysis output of the experiment will be the globally averaged clear and cloudy sky sensitivity (defined as in Cess et al., 1990) separated into long wave and short wave contributions. In addition it is strongly advised that direct output of data from the "in line" version of the ISCCP Simulator be provided. If this is not possible then the data necessary for input to an "off line" version of the ISCCP simulator should be saved and the ISCCP information (see the ISCCP simulator appendix) be provided separately

The Experimental Conditions.

The experiment should be conducted with the perpetual month of July. It is recommended that the SST data set chosen be the AMIP 2 SST climatology for July (earlier experiments were conducted with Alexander and Mobley SST data set – experiments have shown that the experimental results for the perturbation are not very sensitive the actual SST used). Both snow and soil moisture are to be held fixed at their initial conditions, so as to remove a potential albedo feedback (the snow and soil moisture fields used may be chosen by the modelling centre). The SST perturbation of ±2K is applied uniformly. Although much of the analysis can be accomplished using data saved from the ± 2K experiments alone it is recommended at a 0K control experiment also be performed (expressively for the purpose of obtaining information on the clouds in the control climate). Data should be collected for at least 120 days after the initial spin up (participants to check on spin-up).

Slab Ocean 1XCO2 and 2XCO2 Experiment

This type of experiment has generally formed part of the “normal” development path for any model used for climate change research and hence runs should be relatively easy to do. The computational cost is higher than a SST perturbation experiment because of the longer time needed to reach equilibrium.
It is expected that, provided sufficient model data is stored, these experiments will be analysed in many more ways other than those aspects specifically related to cloud feedback.

For the purpose of this proposal the basic analysis will consist of; (i) the determination of change in surface air temperature and precipitation and (ii) the change in cloud forcing (shortwave and long wave) components at the top of atmosphere and also the change in the components of the surface energy budget. This analysis will be augmented wherever possible by an investigation of the simulated clouds as determined by the ISCCP simulator.

Experimental Conditions

The atmospheric model (and associated astronomical and radiative forcing) should conform to all AMIP 2 requirements (except of course for the SST). A “q-flux” correction should be applied to the slab ocean to ensure that SST errors in the control (1*CO2) simulation are less than 0.5K and that errors in the sea-ice area in the control case are less than 5%. The SST and sea ice concentration data set to be used in construction of the q flux should be the AMIP2 climatological mean. It is suggested that the specially constructed sea ice thickness data set available at: http://www-pcmdi.llnl.gov/amip/RESOURCES/synice.html could be used when constructing the q-flux under ice following the method discussed by McFarlane et al,1992. The 2*CO2 run should be started from the same initial conditions as the control and annual mean data for the full spin-up be saved for several of the sub-projects (see the Data requirements appendix)

Appendices

1. Data Requirements
2. Data Formats
3. The ISCCP simulator