ERA-Interim production stopped on 31st August 2019
For ERA-Interim (1st January 1979 to 31st August 2019) access through the ECMWF Web API stopped on 01 June 2023
Its successor ERA5 is available from the Climate Data Store (CDS) (What are the changes from ERA-Interim to ERA5?) and users are strongly advised to migrate to ERA5 (How to download ERA5).
For those users who still need access to ERA-Interim after 01 June 2023 (subject to further notice), they can do so via the Climate Data Store (CDS) API.
Problem
I use ERA Interim Monthly Means of Daily Means data for:
- 10 metre U wind component (short name '10u', ID 165)
- 10 metre V wind component (short name '10v', ID 166)
- 10 metre wind speed (short name 10si, ID 207)
I then calculate sqrt(10u*10u+10v*10v), ie, calculate the wind speed from the mean U and V wind components.
However, the result is different from the 10 metre wind speed (10si). Why is this?
Answer
The mean wind speed (10si) is not equal to the speed of the mean wind components.
The mean wind speed for the month is produced by calculating the wind speed at each individual time within the month and then averaging over the month.
Example
Consider a wind of 10ms-1 that is westward for half of the month and eastward for the other half. The mean wind speed is 10ms-1, but the mean of the U wind (and V wind) component is zero. The speed of the mean components is also zero.
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). 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.
