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1. Forecast system version
Identifier code: Météo-France System 9
First operational forecast run: April 2025
2. Configuration of the forecast model
Is the model coupled to an ocean model? Yes
Coupling frequency: 1 hour
2.1 Atmosphere and land surface
| Model | ARPEGE v6.45 SURFEX v8 |
|---|---|
| Horizontal resolution and grid | TL359; 0.5° reduced Gauss grid |
| Atmosphere vertical resolution | 137 layers |
| Top of atmosphere | top layer 1 Pa |
| Soil levels (layers) | 14 Layer 1 : 0 - 0.01 m |
| Time step | 10 min |
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SURFEX: http://www.umr-cnrm.fr/surfex//IMG/pdf/surfex_scidoc_v8.1.pdf
2.2 Ocean and cryosphere
| Ocean model | NEMO v4.2.0 |
|---|---|
| Horizontal resolution | 0.25° ORCA grid |
| Vertical resolution | 75 levels |
| Time step | 15 min |
| Sea ice model | SI3 |
| Sea ice model resolution | 0.25° ORCA grid |
| Sea ice model levels | 5 |
| Wave model | None |
| Wave model resolution | NA |
Detailed documentation:
NEMO:NEMO documentationGELATO: GELATO documentation Madec et al. (2022). NEMO ocean engine reference manual. Zenodo. https://doi.org/10.5281/zenodo.6334656
SI3: Vancoppenolle et al. (2023). NEMO Sea Ice Engine (SI3). Zenodo. https://doi.org/10.5281/zenodo.7534900
3. Boundary conditions - climate forcings
Most forcing data comes from the CMIP6 protocol.
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Voldoire, A., Saint-Martn, D., Sénési, S., Decharme, B., Alias, A., Chevallier, M., et al (2019). Evaluation of CMIP6 DECK experiments with CNRM-CM6-1. Journal of Advances in Modeling Earth Systems, 11. https://doi.org/10.1029/2019MS001683
4. Initialization and initial condition (IC) perturbations
4.1 Atmosphere and land
| Hindcast | Forecast | |
|---|---|---|
| Atmosphere initialization | ERA5 | ERA5T |
| Atmosphere IC perturbations | None | None |
Land Initialization | ERA5 | ERA5T |
| Land IC perturbations | None | None |
| Soil moisture initialization | ERA5 | ERA5T |
| Snow initialization | ERA5 | ERA5T |
| Unperturbed control forecast? | NA | NA |
Detailed documentation: see ECMWF page
4.2 Ocean and cryosphere
| Hindcast | Forecast | |
|---|---|---|
| Ocean initialization | GLORYS12V1 MERCATOR-OCEAN | PSY4V3R1 MERCATOR-OCEAN |
| Ocean IC perturbations | None | None |
| Unperturbed control forecast? | NA | NA |
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GLORYS12V1: https://data.marine.copernicus.eu/product/GLOBAL_MULTIYEAR_PHY_001_030/description
PSY4V3R1: https://www.mercator-ocean.eu/wp-content/uploads/2017/02/SYSTEM-sheet-_PSY4V3R1_2017.pdf
5. Model Uncertainties perturbations:
| Model dynamics perturbations | Yes |
|---|---|
| Model physics perturbations | No |
If there is a control forecast, is it perturbed? | NA |
Detailed documentation: Batté, L. and Déqué, M., 2016: Randomly correcting model errors in the ARPEGE-Climate v6. 1 component of CNRM-CM: applications for seasonal forecasts, Geoscientific Model Development,9, 2055-2076, doi:10.5194/gmd-9-2055-2016
6. Forecast system and hindcasts
| Forecast frequency | month |
|---|---|
| Forecast ensemble size | 51 |
| Hindcast years | 1993-2018 |
| Hindcast ensemble size | 25 |
| On-the-fly or static hindcast set? | static |
| Calibration (bias correction) period | NA |
7. Other relevant information
Ensemble start dates
The forecast uses three start dates:
- The penultimate Thursday of the previous month (25 members in the forecasts, 12 members in the hindcasts)
- The last Thursday of the previous month (25 members in the forecasts, 12 members in the hindcasts)
- the 1st of the month (1 member in the forecast/hindcast)
Data assimilation method for analysis
Coupled initialization run nudged towards ERA5T (ERA5 in the hindcast) in the atmosphere and Mercator Ocean International analyses in the ocean. Sea ice initial conditions are provided by a separate NEMO-GELATO ORCA 0.25 forced run nudged towards the same ocean analyses.
Interpolation details
Météo-France System 8 is built on the CNRM-CM coupled climate model which embeds an output manager called the XIOS server (Meurdesoif et al, 2018). XIOS is an input/output parallel server software enabling to perform online field operations in the course of the model integration and writing the final result on NetCDF files. All operations such as time sampling, time averaging, horizontal regridding or vertical interpolation are specified through xml files and performed by XIOS such that no or little post-processing is needed: the only output files written on disk are the files featuring the requested C3S 1°x1° horizontal grid and pressure levels.
Horizontal Interpolation
The ARPEGE atmospheric model is a spectral model, and horizontal fields such as temperature or geopotential are originally computed as spherical harmonic coefficients, but can be converted into grid point data on the corresponding reduced Gaussian grid (e.g 0.5° reduced Gaussian grid for Tl359 spectral resolution) using an internal function of the model. The XIOS server can perform online interpolation of this grid point data on any user specified grid such as the rectilinear C3S 1°x1° grid. As for the oceanic fields from NEMO, they are internally defined as grid point data and can directly be interpolated by XIOS onto the 1°x1° grid. For both atmospheric and oceanic data, the interpolation method is a first order conservative remapping.
Vertical interpolation
ARPEGE uses sigma coordinates in the vertical, meaning that near-surface model levels follow the orography. Transformation into data on pressure levels is done directly on request by ARPEGE through a linear interpolation, or linear extrapolation for data below the surface.
Meurdesoif, Y. (2018) XIOS fortran reference guide. IPSL http://forge.ipsl.jussieu.fr/ioserver/svn/XIOS/trunk/doc/XIOS_reference_guide.pdf
8. Where to find more information
Technical implementation details can be found in http://www.umr-cnrm.fr/IMG/pdf/system8-technical.pdf
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