EASE Ice Motion Vectors

Hi Graham

The NSIDC EASE-grid ice motion u and v vectors are parallel to the x and
y coordinates of the grid, not lat and lon (e.g. not zonal and
meridional components). This is in the documentation somewhere but I
couldn't find it immediately when I looked just now. We'll have to deal
with that.

You have to rotate the u and v vectors based on longitude. A script to
do this for the Arctic is below. It should work for the Antarctic.

Andy Barrett

NSIDC
University of Colorado at Boulder

;;**********************************************************************
;; coord_rotate - performs a coordinate rotation for vector data.

undef ("coord_rotate")
procedure coord_rotate ( x1:numeric, y1:numeric, theta:numeric, \
                          x2:numeric, y2:numeric )
local rtheta, dtor
begin

   dtor = 0.0174533 ;; converts degrees to radians

   rtheta = theta * dtor

   x2 = ( x1 * cos( rtheta ) ) + ( y1 * sin( rtheta ) )
   y2 = ( y1 * cos( rtheta ) ) - ( x1 * sin( rtheta ) )

end
;;**********************************************************************

<snip>

    ; icef is ice motion file
    umotion = icef->umotion ; u vector of ice motion
    vmotion = icef->vmotion ; v vector of ice motion
    lat2d = icef->lat2d ; 2d lat array
    lon2d = icef->lon2d ; 2d lon array

;;----------------------------------------------------------------------
;; Ice motion vectors are relative to EASE grid coordinate. These
;; must be rotated according to longitude. Western hemisphere vectors
;; must be rotated clockwise and eastern hemisphere vectors anti-
;; clockwise. Longtitudes are multiplied by -1.0 to achieve this.

    urot = umotion
    vrot = vmotion
    coord_rotate( umotion, vmotion, lon2d, urot, vrot )

<snip>
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Received on Thu Oct 4 14:26:19 2012