README: add ERA5 section

README.md

@@ -53,3 +53,14 @@ Please cite the Zenodo DOIs above and the CICE Consortium. For the atmospheric f
 
 - **JRA55**: Kobayashi, S., et al. (2015). The JRA-55 Reanalysis: General specifications and basic characteristics. *J. Meteor. Soc. Japan*, 93(1), 5–48. https://doi.org/10.2151/jmsj.2015-001
 - **JRA55-do**: Tsujino, H., et al. (2018). JRA-55 based surface dataset for driving ocean–sea-ice models (JRA55-do). *Ocean Modelling*, 130, 79–139. https://doi.org/10.1016/j.ocemod.2018.07.002
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+## ERA5 surface forcing (`ERA5/`)
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+Hourly ERA5 single-level reanalysis fields, added alongside the CICE data for building custom atmospheric forcing.
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+- **Source:** Copernicus Climate Change Service (C3S), ERA5 hourly data on single levels from 1940 to present (`reanalysis-era5-single-levels`), downloaded from the [Climate Data Store](https://cds.climate.copernicus.eu/datasets/reanalysis-era5-single-levels). Files are unmodified CDS output.
+- **Period:** 2005-01 – 2009-12, all hours, global 0.25° grid, one NetCDF file per month: `ERA5/<year>/era5_sl_<YYYYMM>.nc` (kept as `.zip` if CDS delivered a zip).
+- **Variables:** 10m_u_component_of_wind, 10m_v_component_of_wind, 2m_temperature, 2m_dewpoint_temperature, total_precipitation, snowfall, surface_solar_radiation_downwards, surface_thermal_radiation_downwards, mean_sea_level_pressure, surface_pressure.
+- **Manifest:** `ERA5/manifest.csv` (path, size, md5, time steps).
+- **License:** CC-BY 4.0. Contains modified Copernicus Climate Change Service information [2026]. Neither the European Commission nor ECMWF is responsible for any use of this information.
+- **Citation:** Hersbach, H., et al. (2020). The ERA5 global reanalysis. *Q. J. R. Meteorol. Soc.*, 146, 1999–2049. https://doi.org/10.1002/qj.3803