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The wind direction D clockwise from North can be calculated as
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$$ D = tan_{2}^{-1} \fracatan2 ({u/U},{v/V}) + 180^{\circ} + \alpha, (4) $$ |
where 𝛼 where 𝛼 is the local rotation of the model grid relative to North, and tan2-1 atan2 is the very specific 2-argument arcus tangens function atan2, which is included in most programming languages. With atan2 both the sign in the numerator and the denominator are independently important for the resulting angle. Take For definition of the atan2 function, see for instance at https://en.wikipedia.org/wiki/Atan2. Take care to check if the atan2 result is in radians, in which case it should be converted to degrees with the factor 180°/π. Also take care to check if the resulting direction is between 0° and 360°. Note that the wind direction is the direction from which the wind comes! The grid rotation angle 𝛼 can be computed with this script: https://github.com/metno/NWPdocs/wiki/FromExamples/#wind-direction-obtained-from-x-y-wind-to-wind-direction.
10-metre u and v wind gust components are also output. These are computed from the diagnosed 10-metre winds and the turbulent kinetic energy (pers. comm. Gwenaëlle Hello, Meteo France, 2007).
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