Last modified on Aug 03, 2026 16:05

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 pressure levels presented here is a 6-hourly subset of some parameters and pressure levels of the full Climate Data Store (CDS) ERA5 pressure levels hourly dataset on 0.25 degrees resolution and is stored in a ARCO format. It allows direct access to a subset of the pressures 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).

Methodology

The process to generate this dataset involves:

Fetching ERA5 Pressure Levels data in GRIB format.
Applying homogenization conventions, as applicable:
- Ensure latitude, longitude ranges are between [-90, +90], [-180, +180].
Rename vertical dimension to pressureLevels, to homogenize all parameters.
Leave only essential coordinates latitude, longitude, time and pressureLevels.
Writing to an ARCO Zarr archive (which is similar in structure to netCDF).
The CDS requests the data from the Zarr archive using xarray and writes the data to netCDF or CSV (as requested).

Data organization and access

The ARCO data is stored as Zarr datacubes to provide efficient access to time-chunked and geo-chunked ERA5 pressure levels 6-hourly data:

Variable set	Chunking strategy	URL for Zarr datacube
Pressure levels (Table 1)	Geo-chunked	https://arco.datastores.ecmwf.int/cadl-arco-geo-048/arco/reanalysis_era5_pressure_levels/pl/geoChunked.zarr
Pressure levels (Table 1)	Time-chunked	https://arco.datastores.ecmwf.int/cadl-arco-time-048/arco/reanalysis_era5_pressure_levels/pl/timeChunked.zarr

Target audience

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

The Zarr datacubes can now 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)
# Pressure
pressure_geo_url = "https://arco.datastores.ecmwf.int/cadl-arco-geo-048/arco/reanalysis_era5_pressure_levels/pl/geoChunked.zarr"

# Time-chunked data for access optimised along the space dimension (e.g. for short-time period for large areas)
# Pressure
pressure_time_url = "https://arco.datastores.ecmwf.int/cadl-arco-time-048/arco/reanalysis_era5_pressure_levels/pl/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 time-series data on pressure 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 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 pressure levels parameters have a grid of 0.25 degrees.

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 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.

Temporal frequency

Temporal frequency of the data is 6-hourly at 00:00 , 06:00, 12:00, 18:00 UTC.

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.

Level list

The levels available in the files are: 50, 100, 150, 200, 250, 300, 400, 500, 600, 700, 850, 925, 1000 hPa.

Please be aware that in the timeseries entry users cannot chose levels and they will all be in the file.

Parameters listings

Table 1 below lists the pressure levels parameters.

Table 1: pressure levels parameters


geopotential
specific_humidity
temperature
u_component_of_wind
v_component_of_wind
vertical_velocity

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.}

	Name	Units	Variable name
1	Geopotential	m² s^-2	geopotential
2	Specific humidity	kg kg^-1	specific_humidity
3	Temperature	K	temperature
4	U component of wind	m s^-1	u_component_of_wind
5	V component of wind	m s^-1	v_component_of_wind
6	Vertical velocity	Pa s^-1	vertical_velocity

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Introduction

Methodology

Data organization and access

Time-series access via the CDS

Spatial grid

Temporal frequency

Data update frequency

Level list

Parameters listings

Table 1: pressure levels parameters

Known issues

References

Related articles

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ERA5 Analysis Ready Cloud Optimised (ARCO) data on pressure 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

Level list

Parameters listings

Table 1: pressure levels parameters

Known issues

References

Related articles