[Wrf-users] Obs_nudging : U and V are assumed to be the wind components rotated to the model map-projection

[Craig Mattocks]

On Mon, 18 Feb 2013 at 16:04:56 +0800 Elena Maria Pison San Pedro <emp at dhi.com.sg> wrote:

> I'm trying to run WRF with obs_nudging option, transforming my own
> observation data into little_r format for input. 
> in this link (http://www.mmm.ucar.edu/wrf/users/wrfv3 ... _fdda.html
> <http://www.mmm.ucar.edu/wrf/users/wrfv3.1/How_to_run_obs_fdda.html> ) I
> found that 
> 
> -  U and V are assumed to be the wind components rotated to the model
> map-projection
> -  SPD, DIR and Td fields are ignored.
> 
> my wind data is in Wspeed and Wdir form, so I can change this to u and v
> but how can I rotate the wind components to Mercator projection? 


Hi Elena,

There is no need to rotate the winds on a Mercator projection because "grid north" = "earth/true north" -- the y-axis is parallel to (aligned with) meridians of longitude.

Craig