if ((field1 eq 'T') and (winds(0) eq 'U') and (winds(1) eq 'V')) $
	or ((field1 eq 'tk') and (winds(0) eq 'U') and (winds(1) eq 'V')) $
	or ((field1 eq 'HGT') and (winds(0) eq 'U') and (winds(1) eq 'V')) then begin

;print, 'computing Ertel potential vorticity'
;;---------------------------------------------------
;;** ERTEL POTENTIAL VORTICITY
;;---------------------------------------------------
;; vort scalaire grad_theta / rho
;;               and rho equal p / gH (because H ~ RT/g)
;H=10.
;g=3.72
;for i=0, west_east-1 do begin
;for j=0, south_north-1 do begin
;     theta=220.+var[i,j,*,nt]
;     dtheta_dz=DERIV(z*1000., theta)
;     fac=dtheta_dz*H*g/z     ;;fac est EPV/(abs. vort)
;     what_I_plot(i,j)=fac(theloop)
;endfor
;endfor
;; alternative method : ksi * g * dtheta_dp
;
;
;;--------------------------
;; coriolis parameter 'f'
;;--------------------------
;omeg=7.0721E-5
;coriolis=2*omeg*sin(lat*!pi/180)
;ny=n_elements(coriolis)
;nx=n_elements(what_I_plot(*,1))
;f=fltarr(nx,ny)
;for i=0,nx-1 do begin
;for j=0,ny-1 do begin
;	f(i,j)=coriolis(j)
;endfor
;endfor
;print, 'coriolis'
;print, mean(coriolis)
;
;;----------------------------------
;; horizontal coordinates (meters)
;;----------------------------------
rad=3397000.
deg_m=2*rad*!pi/360.
dlon=deg_m*cos(y*!pi/180)
dlat=deg_m
xcal=(x-min(x))*dlon
ycal=(y-min(y))*dlat

;---------------------------------
; wind gradients (s-1)
;---------------------------------
grad, dv_dx, overvector_y, xcal, ycal, 1
grad, dv_dy, overvector_y, xcal, ycal, 2
grad, du_dx, overvector_x, xcal, ycal, 1
grad, du_dy, overvector_x, xcal, ycal, 2

;;-----------------------------------------
;; absolute and relative vorticity (s-1)
;;-----------------------------------------
;;what_I_plot=(f+dv_dx-du_dy)*what_I_plot*1.e6 
;what_I_plot=(dv_dx-du_dy)*1.e6
what_I_plot=(dv_dx-du_dy)*1.e4 
			;now vort. is in PVU !!
			;which is 1e-6 K.kg-1.m2.s-1
;w=where(abs(what_I_plot) gt 1.e10)
;if (w(0) ne -1) then what_I_plot[w]=1.e38

;overcontour = (dv_dx-du_dy)*1.e4


;overcontour = sqrt((overvector_x + overvector_y)^2)
;overvector_x=0
;overvector_y=0
minfield_init=0.
maxfield_init=0.
title='Vorticity (1e-4 s!U-1!N)'

endif