Are U & V wind components grid relative?

[kagelos]

I know that the U10 & V10 wind components are grid relative and by using this formula http://forum.mmm.ucar.edu/phpBB3/viewtopic.php?f=78&t=179&p=338&hilit=sinalpha, one can convert to earth relative ones.
My question though is about the U & V components on the various eta levels. There doesn't seem to be a SINALPHA / COSALPHA pair for these values. Do they need converting somehow or are they earth relative already?

 

[weiwang]

The 3D U and V are grid-relative too. To convert them, one first needs to compute U and V at the center of the grid by averaging U in x-direction and V in y-direction. Then, same SINALPHA and COSALPHA can be used to rotate them to earth-relative.

 

[kagelos]

Indeed, U & V have to be averaged to the destaggered grid first, I figured out later on. Thank you very much! For anyone interested in a simple way to achieve this, the WRF python package by NCAR does this and a lot more.