How to convert GRIB to CSV

Last modified on Jun 10, 2021 13:41

You might find it easier to handle data in a CSV file instead of a GRIB file. This article demonstrates how to extract data from a GRIB file using ECMWF's ecCodes and save the output as a CSV file. You are expected to have ecCodes installed on a Linux machine before you continue. You are also recommended to add ecCodes' bin directory to your PATH. Work below was tested with ecCodes 2.5.0.

Step-by-step guide

Download 2m temperature from the Copernicus Climate Change Service (C3S)'s Climate Data Store (CDS) and save the data as file t2m_20000801.grib. In this example, selections are set as below:
1. Dataset: ERA5 hourly data on single levels from 2000 to 2017 (you may see different years as new data is released)
2. Parameter : 2m temperature
3. Product type: Reanalysis
4. Year : 2000
5. Month : August
6. Day : 01
7. Time : 06:00, 12:00 and 18:00
8. Format : GRIB

Check the GRIB file by issuing command grib_ls -P time t2m_20000801.grib, you should see three GRIB messages in the file at three different times: 600, 1200, 1800 as shown below. Remember three time values were selected in step 1. Option -P time shows key "time", typing grib_ls for help.

t2m_20000801.grib
time         edition      centre       typeOfLevel  level        dataDate     stepRange    dataType     shortName    packingType  gridType     
600          1            ecmf         surface      0            20000801     0            an           2t           grid_simple  regular_ll  
1200         1            ecmf         surface      0            20000801     0            an           2t           grid_simple  regular_ll  
1800         1            ecmf         surface      0            20000801     0            an           2t           grid_simple  regular_ll  
3 of 3 messages in t2m_20000801.grib

3 of 3 total messages in 1 files

Say I want to extract lat, lon, 2t (2m temperature) at time = 12:00 from the GRIB file. Run command grib_get_data -w time=1200 t2m_20000801.grib > temp.csv and you will get a file containing data like below. Option -w time=1200 filters data for time = 12:00 only, typing grib_get_data for help.
```
Latitude, Longitude, Value
   90.000    0.000 2.7346786499e+02
   90.000    0.250 2.7346786499e+02
   90.000    0.500 2.7346786499e+02
   90.000    0.750 2.7346786499e+02
...
```

Format the CSV file. You may use script below.

#!/usr/bin/env python
"""
Save as format.py, then run "python format.py".
 
Input file : temp.csv
Output file: t2m_20000801.csv
"""
with open('temp.csv', 'r') as f_in, open('t2m_20000801.csv', 'w') as f_out:
    f_out.write(next(f_in))
    [f_out.write(','.join(line.split()) + '\n') for line in f_in]

You have a CSV file t2m_20000801.csv, which is ready to be imported in Excel. Notice there are over 1 million records in the file! You may now want to extract data for time=600 and 1800.
```
Latitude, Longitude, Value
90.000,0.000,2.7346786499e+02
90.000,0.250,2.7346786499e+02
90.000,0.500,2.7346786499e+02
90.000,0.750,2.7346786499e+02
...
```

If you have CDS API and ecCodes (with its Python interface) installed, you will be able to retrieve (in GRIB), extract and export data to CSV by writing up a Python script.

This document has been produced in the context of the Copernicus Atmosphere Monitoring Service (CAMS) and 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 CAMS and C3S on behalf of the European Union (Delegation Agreement signed on 11/11/2014). 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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This document has been produced in the context of the Copernicus Atmosphere Monitoring Service (CAMS) and the Copernicus Climate Change Service (C3S).

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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