Thanks so much for the information and code Andy! It worked perfectly, with the ice motion vectors plotted correctly.
Thanks again,
Graham
On 05/10/2012, at 6:26 AM, Andy Barrett wrote:
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 17:08:43 2012
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