[Wrf-users] Grid Magnetic Angle

[Bart Brashers]

Here's the subroutine I use to convert to TRUE North (not magnetic North, but maybe you can tweak this for your needs):

  subroutine uv2sd(u,v,lon,speed,dir)
!
! Converts U and V (vector components) to speed and direction.
! If lon >= -90., rotate the winds from the local projection to E-N.
!
    USE met_fields
!-----Vars needed from met_fields:
!     cenlat
!     conefact
    implicit none
!
!-----Variable declaration
!
    real :: u,v,lon           ! Vector components, Longitude
    real :: speed,dir         ! Output speed and direction
    real :: lon_diff, sign
    real, parameter :: pi = 3.14159265
    real, parameter :: d2r = pi/180.
!
!-----Entry point
!
    speed = sqrt(u**2 + v**2)
    dir   = atan2(u,v) / d2r ! in degrees, not radians

    if (lon >= -90. .and. lon <= 90.) then

       lon_diff = lon - clon
       if ( lon_diff >  180. ) lon_diff = lon_diff - 360.
       if ( lon_diff < -180. ) lon_diff = lon_diff + 360.

       sign = 1                   ! northern hemisphere
       if (cenlat < 0.) sign = -1 ! southern hemisphere

       dir = dir + lon_diff * conefact * sign

    end if

    if (dir <   0.) dir = dir + 360.
    if (dir > 360.) dir = dir - 360.

  end subroutine uv2sd

Bart Brashers

From: wrf-users-bounces at ucar.edu [mailto:wrf-users-bounces at ucar.edu] On Behalf Of Jason Padovani Ginies
Sent: Wednesday, January 25, 2012 11:49 PM
To: wrf-users
Subject: [Wrf-users] Grid Magnetic Angle

Dear all,

I am to understand that u- and v- wind components in WRF are given with respect to the x and y coordinates respectively according to Grid North. How would I go about converting from Grid north to magnetic North (Grid Magnetic angle) on a Lambert Conformal map?

Kind regards,

Jason

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