## wind diraction

### wind diraction

I try to compare the results of me wind direction (fro U and V) and i read in the forum that i need to convert the U and V to the relative WRF model grid.
Can anyone explain to me what it means and how to do this?

Thanks,

I copy the post of samhawkins that explain the issue

"Report this postReply with quoteRe: Wind direction?
by samhawkins on Tue Nov 03, 2009 3:07 pm

You can calculate wind direction from the component vectors U (x direction) and V (y direction).

Be aware though, U and V are relative to the WRF model grid, so they may need to be rotated to be relative to the earth. This can be achieved by

Uearth = U*cosalpha + V*sinalpha, and
Vearth = V*cosalpha - U*sinalpha.

Also, be aware that if you want to compare this to observations, these typically record the direction the wind is blowing from, in degrees from North, so you need to make sure you compute this from the component vectors."
kozokg

Posts: 13
Joined: Mon Jul 12, 2010 4:50 am

### Re: wind diraction

The above formula is correct for converting from earth-relative to grid-relative.

The correct equations to rotate grid-relative to earth-relative wind are

Uearth = U*cosalpha - V*sinalpha
Vearth = V*cosalpha + U*sinalpha

(This is a quote from wrfhelp.)

See, for example, the code in share/wrf_fddaobs_in.F, which does the reverse (from earth to grid-relative).
BartBrashers

Posts: 1
Joined: Fri Mar 07, 2014 12:50 pm

### Re: wind diraction

i use this formula but i did not catch "alpha". what is alpha?
is it the tangent of (Vwind / Uwind)

Uearth = U*cosalpha + V*sinalpha,
Vearth = V*cosalpha - U*sinalpha.

Best Regards
zayfin

Posts: 8
Joined: Wed Nov 03, 2010 10:48 am