Please be aware that the generation of this dataset is using an alternative source for the ERA5 data and may be subject to changes over time (e.g. file format, data file structure, deprecation etc). This dataset should therefore be regarded as “experimental” and is not recommended for use in a production environment.

Notification of changes via the catalogue entry warning banner and/or in the Forum will be provided on best efforts.

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.

This dataset presented here is a subset of some parameters of the full CDS ERA5 single levels hourly dataset on 0.25 degrees resolution and is stored in Analysis Ready Cloud Optimised (ARCO) format, which has been implemented for retrieving long time-series for a single point in a efficient way. It is this source of ERA5 data that is used by the ERA-Explorer to ensure response times required for the interactive web-application.

Methodology

The process to generate this dataset involves:

Fetching ERA5 data 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)
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.

The data is now available from the Climate Data Store (CDS), either interactively through its download web form or programmatically using the CDS API service:

ERA5 hourly time-series data on single levels from 1940 to present

Spatial grid

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

Temporal frequency

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

Data format

In the CDS, there are the options of retrieving the data in netCDF format or CSV format.

Note that the surface and single level parameters (Table 1) and the wave parameters (Table 2) are on different spatial grids, therefore they will be served in different output files to avoid any conflict in coordinate definition.

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)

Count	Name	Units	Variable name	Notes
1	10 metre U wind component	m s^-1	10m_u_component_of_wind
2	10 metre V wind component	m s^-1	10m_v_component_of_wind
3	2 metre dewpoint temperature	K	2m_dewpoint_temperature
4	2 metre temperature	K	2m_temperature
5	Mean sea level pressure	Pa	mean_sea_level_pressure
6	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.
7	Surface pressure	Pa	surface_pressure
8	Total precipitation	m	total_precipitation

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.}