Experimental design for Transpose-AMIP II
 

• 64 global hindcasts are to be produced with the centre’s AGCM, each hindcast being 5 days in length.
• 4 sets of 16 hindcasts are to be run, the first in each set starting at 00Z on the 15th of the following months and then subsequently at 30 hour intervals: October 2008, January 2009, April 2009 and July 2009. This ensures sampling throughout the annual and diurnal cycles for each grid-point for a given lead times. These periods have been chosen to tie in with the Year of Tropical Convection (YOTC) and various IOPs (see below).
• Model state variables are to be initialised from ECMWF YOTC analyses available from http://data-portal.ecmwf.int/data/d/yotc. Guidance on carrying out the interpolation can be found in the ECMWF IFS documentation: http://www.ecmwf.int/research/ifsdocs_old/TECHNICAL/.)
• Atmospheric composition, solar forcing and land use should be as the final year of the CMIP5 AMIP experiment (2008). SSTs from ECMWF YOTC should be used in the hindcasts.
• Land surface models should be initialised using one of the following methods: 

• Initialise from fields produced by a land surface assimilation system (e.g. ECMWF or GLDAS)
• Initialise using a suitable climatology: e.g. from GSWP2 or derived from the model’s AMIP simulation.
• Initialised with a nudging method as described by Boyle et al. (2005).

• Aerosols concentrations should either be initialised using the final year of the model’s AMIP simulation, or initialised using the nudging method of Boyle et al. (2005).
• Non-state variable prognostics which spin-up quickly (such as cloud fraction for models with a prognostic scheme) can either be initialised from zero, or  initialised using the nudging method of Boyle et al. (2005).
• Details of the initialisation methods used should be given in the ‘comment’ attribute of the netCDF headers of submitted data.
• The AGCMs submitted should be the same (both in terms of physics and resolution) as those used for the CMIP5 AMIP experiment in order to compare model biases across timescales. 
• Whilst maximum benefit from the project is expected for climate centres, the Transpose-AMIP II experiment is also open to NWP centres to participate. NWP centres planning to run an AMIP simulation for CMIP5 are encouraged to participate in Transpose-AMIP II, submitting the forecasts at the same resolution as the CMIP5 submissions.
• In order to test the sensitivity of the results to the choice of analysis, we are requesting centres to optionally also submit a second set of experiments. Centres with their own assimilation system can initialise these from their own analyses. Centres without their own assimilation system should initialise this second set of experiments from GMAO/MERRA re-analyses.
  

 Benefits and Project Linkages
 
Seamless predication is high on the agenda of both the WWRP and WCRP. Centres which run both climate and NWP models in a unified system frequently find that model errors are common across timescales and that analysis and evaluation of NWP simulations for particular meteorological events can yield significant insight into the cause of the error. Analysis in regions where extra observations are deployed can be particularly useful and the extra model diagnostics being proposed over the ARM and CloudNet sites, as well as along the GCSS Pacific Cross-section Intercomparison for Transpose-AMIP II should prove beneficial in this regard. In addition, the period over which the hindcasts are to be run has been chosen to tie in with the Year of Tropical Convection project, and the hindcast periods are aligned with one or more of the IOPs for VOCALS (VAMOS Ocean-Cloud-Atmosphere-Land Study), AMY (Asian Monsoon Years) and T-PARC (THORPEX Pacific Asian Regional Campaign). 
 
As well as providing benefits for the individual centre in terms of providing insight into the potential cause of model biases, comparison with other CMIP5 experiments will enable investigation of whether model differences seen on longer timescales can also be seen on short timescales and potentially gain knowledge of the underlying processes. This should allow a more thorough assessment of confidence in the controlling processes operating within the CMIP5/AR5 models. It is envisaged that there will be close links with the process-based understanding and evaluation work on clouds which will be conducted in CFMIP. Specifically, the use of ‘case studies’ in Transpose-AMIP II will complement the high temporal resolution climate data being collected through CFMIP. 

Project timeline
 
It is hoped that modelling groups will perform these runs alongside or immediately after their CMIP5 experiments in order to enable analysis of model errors across timescales to be related, and to benefit from the data collection/distribution facilities which will be available for CMIP5. Whilst it is recognised that the core CMIP5 experiments are a significant use of modelling groups' resources and additional experiments may not be welcome, adding the Transpose-AMIP II experiment is lightweight in terms of computer time as it totals less than 1 year of model simulation.