Last modified on Aug 05, 2026 08:22

Table of Contents

Introduction

ERA5 is the fifth generation ECMWF reanalysis for the global climate and weather and currently covers the past 80 years. It covers the period from January 1940 to the present with hourly sampling and continues to be extended forward with 5 days behind real time.

The ERA5 Analysis Ready Cloud Optimised (ARCO) data on single levels presented here is a subset of some parameters of the full Climate Data Store (CDS) ERA5 single levels hourly dataset on 0.25 degrees resolution and is stored in a ARCO format. It allows direct access to a subset of the surface and wave variables (see below) without downloading individual files, enabling efficient and scalable data access and retrieval. The ARCO format has a dual-chunking approach which means that data access can be optimised for retrieving long-time periods for a limited area (geo-chunked) or short-time period for large areas (time-chunked). The ARCO ERA5 geo-chunked is the source of ERA5 data that is used by the ERA-Explorer to ensure response times required for the interactive web-application and by the ERA5 hourly time-series data on single levels from 1940 to present catalogue entry to provide fast access to long time series ERA5 data.

Methodology

The process to generate this ERA5 ARCO data involves:

Fetching ERA5 data from in GRIB format.
Apply homogenization conventions, as applicable
- Rename spatial coordinates to latitude, longitude
- Ensure latitude, longitude ranges are between [-90, +90], [-180, +180]
- In the accumulation variables (e.g. Total precipitation), time is expressed in one single dimension called time which in ERA5 netCDFs is usually called valid_time.
- In the accumulation variables, if the number of hourly samples is incomplete (less than 24) the date is skipped (e.g. 1940-01-01).
- Drop "number", "step", "surface" and "valid_time".
Written to an ARCO Zarr archive (which is similar in structure to netCDF)

Data organization and access

The ARCO data is stored as Zarr datacubes to provide efficient access to time-chunked and geo-chunked ERA5 single level hourly data:

Variable set	Chunking strategy	URL for Zarr datacube
Surface and single level (Table 1)	Geo-chunked	https://arco.datastores.ecmwf.int/cadl-arco-geo-002/arco/reanalysis_era5_single_levels/sfc/geoChunked.zarr
Surface and single level (Table 1)	Time-chunked	https://arco.datastores.ecmwf.int/cadl-arco-time-002/arco/reanalysis_era5_single_levels/sfc/timeChunked.zarr
Wave (Table 2)	Geo-chunked	https://arco.datastores.ecmwf.int/cadl-arco-geo-003/arco/reanalysis_era5_single_levels/wav/geoChunked.zarr
Wave (Table 2)	Time-chunked	https://arco.datastores.ecmwf.int/cadl-arco-time-003/arco/reanalysis_era5_single_levels/wav/timeChunked.zarr

The variables available in the Single levels atmospheric and Wave Zarr files are listed in Table 1 and Table 2, respectively.

Target audience

Access to ARCO data is programmatic; therefore, users of these resources are expected to have some relevant programming experience.

The Zarr datacubes can be accessed using your CDS API key. Below is a simple plug-and-play code snippet and a more detailed Jupyter Notebook example. For more detailed and advanced access examples, please refer to the main Analysis Ready Cloud Optimised (ARCO) Data.

Plug and play xarray access

import xarray as xr

# Geo-chunked data for access optimised along the time dimension (e.g. for time-series at a single point)
# Surface
surface_geo_url = "https://arco.datastores.ecmwf.int/cadl-arco-geo-002/arco/reanalysis_era5_single_levels/sfc/geoChunked.zarr"
# Wave
wave_geo_url = "https://arco.datastores.ecmwf.int/cadl-arco-geo-003/arco/reanalysis_era5_single_levels/wav/geoChunked.zarr"

# Time-chunked data for access optimised along the space dimension (e.g. for short-time period for large areas)
# Surface
surface_time_url = "https://arco.datastores.ecmwf.int/cadl-arco-time-002/arco/reanalysis_era5_single_levels/sfc/timeChunked.zarr"
# Wave
wave_time_url = "https://arco.datastores.ecmwf.int/cadl-arco-time-003/arco/reanalysis_era5_single_levels/wav/timeChunked.zarr"

# Open one of the Zarr objects with xarray, the default example opens the geo-chunked surface variables
ds = xr.open_zarr(
    surface_geo_url,
    consolidated=True,
    storage_options={
        "headers": {"Authorization": f"Bearer <CDS-API-KEY>"}
    }
)

# Inspect the variables
print(ds)

Time-series access via the CDS

The ERA5 hourly time-series data on single levels from 1940 to present catalogue entry allows users to download long time series for a single-point or limited area via the CDS download webform or a CDS API request that users can build from the webform. The data download via the time-series entry is as described below with additional aspects:

Please be aware when you select a "Location" on the form, the latitude and longitude values are rounded to the closest neighborhood point of the 0.25 degrees grid. Similarly, selecting an “Area” returns all points within the region defined by the nearest neighborhood corner points to the user-specified S, W, N, and E latitude/longitude values (maximum 5° x 5°).
The download form allows users to select a choice of netCDF or CSV. The conversion is done directly with xarray with no additional processing from the Zarr description above.
- Given the computational cost of writing to csv, users are limited to smaller downloads with this option.
The surface and single level parameters (Table 1) and the wave parameters (Table 2) are in different Zarr stores, therefore they will be served in different output files.

The entry may be temporarily disabled or completely deprecated at any point and it does not come with the same level of operational support as the parent entry. Additionally, the formats and structure of the data files are subject to change.

Spatial grid

The atmospheric parameters have a grid of 0.25 degrees as the wave parameters have a grid of 0.5 degrees.

Temporal frequency

Temporal frequency of the data is hourly and every day has 24 hourly steps.

Data update frequency

ERA5 is updated on a daily basis. New daily data in ERA5 becomes available in ERA5 Time Series in a few hours.

Variations in delivery times may occur due to the non-operational nature of this CDS service, as issues may arise which cause delays.

Parameters listings

Table 1 below lists the surface and single level parameters (levtype=sfc) and Table 2 lists the wave parameters.

Table 1: surface and single level parameters (instantaneous and accumulations)

	Name	Units	Variable name	Notes
1	10 metre U wind component	m s^-1	10m_u_component_of_wind
2	100 metre V wind component	m s^-1	10m_v_component_of_wind
3	10 metre U wind component	m s^-1	100m_u_component_of_wind
4	100 metre V wind component	m s^-1	100m_v_component_of_wind
5	10m wind gust since previous post-processing	m s^-1	10m_wind_gust_since_previous_post_processing
6	2 metre dewpoint temperature	K	2m_dewpoint_temperature
7	2 metre temperature	K	2m_temperature
8	Mean sea level pressure	Pa	mean_sea_level_pressure
9	Sea surface temperature	K	sea_surface_temperature	Sea surface temperature is available at the usual times, eg hourly, but its content is only updated once daily.
10	Skin temperature	K	skin_temperature
11	Surface pressure	Pa	surface_pressure
12	Surface latent heat flux	J m^-2	surface_latent_heat_flux
13	Surface sensible heat flux	J m^-2	surface_sensible_heat_flux
14	Surface solar radiation downwards	J m^-2	surface_solar_radiation_downwards
15	Surface thermal radiation downwards	J m^-2	surface_thermal_radiation_downwards
16	Total precipitation	m	total_precipitation
17	Total cloud cover	(0 - 1)	total_cloud_cover
18	Total sky direct solar radiation at surface	J m^-2	total_sky_direct_solar_radiation_at_surface
19	Cloud base height	m	cloud_base_height
20	Boundary layer height	m	boundary_layer_height

Table 2: wave parameters: instantaneous

Count	Name	Units	Variable name
1	Mean wave direction	Degrees	mean_wave_direction
2	Mean wave period	s	mean_wave_period
3	Significant height of combined wind waves and swell	m	significant_height_of_combined_wind_waves_and_swell

Known issues

Please refer to the ERA5 documentation.

References

Further ERA5 references are available from the ECMWF website.

_{This document has been produced in the context of the Copernicus Climate Change Service (C3S).}

_{The activities leading to these results have been contracted by the European Centre for Medium-Range Weather Forecasts, operator of C3S on behalf of the European Union (Delegation Agreement signed on 11/11/2014 and Contribution Agreement signed on 22/07/2021). All information in this document is provided "as is" and no guarantee or warranty is given that the information is fit for any particular purpose.}

_{The users thereof use the information at their sole risk and liability. For the avoidance of all doubt , the European Commission and the European Centre for Medium - Range Weather Forecasts have no liability in respect of this document, which is merely representing the author's view.}

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Introduction

Methodology

Data organization and access

Time-series access via the CDS

Spatial grid

Temporal frequency

Data update frequency

Parameters listings

Table 1: surface and single level parameters (instantaneous and accumulations)

Table 2: wave parameters: instantaneous

Known issues

References

Related articles

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ERA5 hourly Analysis Ready Cloud Optimised (ARCO) data on single levels from 1940 to present: Product User Guide (PUG)

Introduction

Methodology

Data organization and access

Time-series access via the CDS

Spatial grid

Temporal frequency

Data update frequency

Parameters listings

Table 1: surface and single level parameters (instantaneous and accumulations)

Table 2: wave parameters: instantaneous

Known issues

References

Related articles