Getting Meteorological Data from ERA5

For the current pycequeau meteorological workflow, the recommended starting point is ERA5 from the Copernicus Climate Data Store (CDS). ERA5 is a global atmospheric reanalysis distributed by the Copernicus Climate Change Service and accessed through the public CDS web portal and API.

If you are starting a new project, ERA5 is usually the simplest source to use because it is global, well documented, and already distributed in NetCDF-ready formats that fit naturally with pycequeau workflow.

Why ERA5?

ERA5 is a practical choice for pycequeau because it provides:

• global spatial coverage

• long historical records

• hourly products that can be aggregated to daily values

• easy access through the CDS web interface and API

The pycequeau meteorological workflow is built around gridded NetCDF datasets. In practice, that means ERA5 can be downloaded, preprocessed into daily fields, and then ingested into pycequeau.meteo.meteo_netcdf.NetCDFMeteo.

Where to get the data

ERA5 data can be downloaded from the official Copernicus Climate Data Store:

• Climate Data Store documentation: https://confluence.ecmwf.int/display/CKB/Climate+Data+Store+%28CDS%29+documentation

• ERA5 hourly data on single levels: https://cds.climate.copernicus.eu/datasets/reanalysis-era5-single-levels?tab=download

• ERA5 hourly data on pressure levels: https://cds.climate.copernicus.eu/datasets/reanalysis-era5-pressure-levels?tab=download

For most pycequeau applications, the single-level ERA5 product is the one to start with. Pressure-level products are usually only needed for more specialized atmospheric analyses and are not the usual entry point for the standard pycequeau workflow.

How to access ERA5 with the CDS API

The recommended way to automate downloads is the official cdsapi client. This documentation intentionally keeps that part brief, because account setup, API credentials, licence acceptance, and request syntax are maintained by the Copernicus team and may evolve over time.

Please follow the official CDS guidance for:

• creating an account

• accepting dataset licences

• installing cdsapi

• configuring your API key

• writing download requests

Official references:

• CDS documentation: https://confluence.ecmwf.int/display/CKB/Climate+Data+Store+%28CDS%29+documentation

• CDS migration and quick-guide notice: https://confluence.ecmwf.int/display/CKB/Please+read%3A+CDS+and+ADS+migrating+to+new+infrastructure%3A+Common+Data+Store+%28CDS%29+Engine

In other words, pycequeau does not replace the Copernicus download step. You should first retrieve the raw ERA5 files from CDS, then prepare those files for the pycequeau meteorological workflow.

What pycequeau expects

The pycequeau meteorological workflow works with daily gridded NetCDF variables. The ingestion layer normalizes supported variables through the meteorological schema, and the NetCDF workflow then interpolates those fields to the CE grid.

Some of the main variable names currently recognized by pycequeau include:

• tasmax or tMax for daily maximum temperature

• tasmin or tMin for daily minimum temperature

• tp, pr or pTot for daily precipitation

• ssrd, ssr or rsds for shortwave radiation

• strd or msdwlwrf for longwave radiation

• d2m for dewpoint temperature

• tcc or clt for cloud cover

• sp for surface pressure

• wind or sfcWind for scalar wind speed

At the moment, the workflow is centered on daily inputs, not raw hourly ERA5 time steps. That means the CDS download is only the first part of the workflow. The ERA5 fields still need to be aggregated or converted into the daily variables expected by pycequeau.

Units

The meteorological inputs must include a valid units attribute in the NetCDF variables. pycequeau can normalize several common ERA5 and meteorology unit conventions during preprocessing, but the metadata still needs to be present and correct.

For the full list of supported unit strings and the canonical units expected by the workflow, see Meteorological Units.

Recommended ERA5 variables to download first

A practical starting set is:

• 2 m temperature

• total precipitation

• surface solar radiation downwards

• surface thermal radiation downwards

• 2 m dewpoint temperature

• total cloud cover

• surface pressure

• 10 m u-component of wind

• 10 m v-component of wind

This combination gives you enough information to prepare most of the variables used in the current gridded workflow.

A simple preparation strategy

In practice, a typical workflow looks like this:

1. Download ERA5 data from CDS for the basin extent and study period.

2. Store the downloaded fields as NetCDF files.

3. Aggregate hourly variables into daily products where needed.

4. Convert derived variables before ingestion when appropriate. For example:

   • build daily wind speed from u10 and v10 with pycequeau.meteo.calculators.wind_speed.WindSpeedCalculator

   • derive vapor pressure from dewpoint temperature, if that variable is needed, with pycequeau.meteo.calculators.vapor_pressure.VaporPressureCalculator

5. Load the prepared files into pycequeau.meteo.meteo_netcdf.NetCDFMeteo.

6. Interpolate the meteorological fields to the CE grid and export them to the CEQUEAU-ready format.

The preprocessing utilities in pycequeau.meteo.calculators can help with some of these derivations once the raw NetCDF files have already been downloaded. A separate page will document that workflow in more detail.

What this page does not cover

This page does not try to reproduce the official CDS API manual or provide a full ERA5 request cookbook. Those details are better maintained in the official Copernicus documentation.

The goal here is simply to document the recommended data source and show how it connects to the pycequeau meteorological workflow.