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 form 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 westerly for half of the month and easterly for the other half. The average U wind (and V wind) component is zero but the average (mean) wind speed is 10ms-1.
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