ERA-Interim production stopped on 31st August 2019
For the time being and until further notice, ERA-Interim (1st January 1979 to 31st August 2019) shall continue to be accessible through the ECMWF Web API. Keeping in mind that ERA-Interim is published with an offset of about three months from the dataset's reference date, ERA-Interim August 2019 data will be made available towards the end of October 2019.
ERA5 is now 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 migrate from ECMWF Web API to 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.
