Questions regarding gsn_csm_pres_hgt_vector

Hi Everyone,

I have a few questions regarding the gsn_csm_pres_hgt_vector function:
I created a small script (see below) to plot 3 examples (see attached
plot). The first vector plot has a constant vertical component and a
null horizontal component, the second is the reverse and the third has
constant horizontal and vertical components (both equal to 1). The first
two plots show constant vector field as defined while the third plot
shows vectors changing direction with height. I don't understand the
resulting third plot. My vector field is constant but the vector's
directions vary. Can yoanyone explain why? Did I miss something? Is it
related to the non-linear pressure axis? If so how come the first two
plots are ok?

Thank you,
Erwan Monier

***********************************************************
vect = new((/9,9/),float)
lat = (/0,1,2,3,4,5,6,7,8/)
lat_at_units = "degree_north"
lev = (/10,9,8,7,6,5,4,3,2/)
lev_at_units = "hPa"
vect!0="lev"
vect!1="lat"
vect&lat = lat
vect&lev = lev
vect_null = vect
do i=0,8
        do j=0,8
                vect(i,j) = 1
                vect_null(i,j) = 0
        end do
end do

 wks = gsn_open_wks ("ps", "vector-example" )
 plot = new(3,graphic)

 res = True
 res_at_gsnDraw = False
 res_at_gsnFrame = False
 res_at_vcMinDistanceF = 0.02
 res_at_vcRefLengthF = 0.04
 res_at_lbLabelBarOn = False

 plot(0) = gsn_csm_pres_hgt_vector(wks,vect,vect_null,vect,res )
 plot(1) = gsn_csm_pres_hgt_vector(wks,vect,vect,vect_null,res )
 plot(2) = gsn_csm_pres_hgt_vector(wks,vect,vect,vect,res )

 resP = True
 resP_at_gsnPanelRowSpec = True
 gsn_panel(wks,plot,(/2,1/),resP)

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