# [ncl-talk] how to plot divergent winds, divergence and velocity potential in one single plot?

Vyankatesh Manoj Mundhada vyankatesh at iiserb.ac.in
Sat Feb 24 02:33:57 MST 2018

```Hello,

I am new to NCL and I am trying to plot annual mean climatology for
divergent winds (vectors), divergence (contours (shading)) and velocity
potential (contours (lines)) in a single plot using velocities u and v
only. I am using two input .nc files f1 and f2, where f1 contains u and v
and f2 contains velocity potential.
I am trying to plot divergent velocities over velocity potential using the
following script:

begin
;*************************************************
; open file and read in data: data are on a gaussian grid
;*************************************************
u    = f1->u
v    = f1->v
vp    = f2->velopot
;*************************************************
; calculate divergence on a gaussian grid
;*************************************************
div = uv2dvG_Wrap(u,v)                                        ; u,v ==>
divergence
div at long_name  = "Divergence"
div at units      = "s-1"
;*************************************************
; calculate velocity potential
;*************************************************
;chi = ilapsG_Wrap ( div , 0)
chi  = new ( dimsizes(vp), double, vp at _FillValue )
chi = (/vp/1e4/)                                                 ;
arbitrary scale
copy_VarCoords(vp, chi )
;chi at long_name = "velocity potential"
;chi at units     = "m^2/s"
;*************************************************
; calculate divergent wind component
; --
; note: the calculation uses a procedure, so memory
; must be preallocated.
;*************************************************
ud    = new ( dimsizes(u), double, u at _FillValue )
vd    = new ( dimsizes(v), double, v at _FillValue )

dv2uvg(div,ud,vd) ; div  ==> divergent  wind components

copy_VarCoords(u, ud )
copy_VarCoords(v, vd )
ud at long_name  = "Zonal Divergent Wind"
ud at units      = u at units
vd at long_name  = "Meridional Divergent Wind"
vd at units      = v at units

;udAve = avg(ud(:,:,:,:))
;vdAve = avg(vd(:,:,:,:))
;scale    =(/vdAve/udAve/)
;ud    =    (/scale*ud/)

ud = (/ud/1e3/)
vd = (/vd/1e3/)
div = (/div*1e3/)

;UD = month_to_annual(ud, 1)
;VD = month_to_annual(vd, 1)
;DIV = month_to_annual(div, 1)
;CHI = month_to_annual(chi, 1)

UDavg    =    dim_avg_n_Wrap(ud, (/0,1/))
VDavg    =    dim_avg_n_Wrap(vd, (/0,1/))
DIVavg    =    dim_avg_n_Wrap(div, (/0,1/))
CHIavg    =    dim_avg_n_Wrap(chi, (/0,1/))
CHIavg    =    abs(CHIavg)

;*************************************************
; plot results
;*************************************************
wks  = gsn_open_wks("png","vp_dv_d")             ; send graphics to PNG
file
nc   = dimsizes(cmap(:,0))

res                          = True
res at gsnDraw                                 = False
res at gsnFrame                             = False
cnres1                                        = res
cnres2                                        = res
vcres                                            = res

cnres1 at cnFillOn           = True               ; color on
cnres1 at cnLinesOn          = False              ; turn off contour lines
cnres1 at cnFillPalette      = cmap(:nc-4,:)
cnres1 at tiMainString       = "Divergent Winds, Divergence and Velocity
Potential"
cnres1 at cnLevelSelectionMode     = "ManualLevels"    ; manually set the
contour levels with the following 3 resources
cnres1 at cnMinLevelValF              = -5.                                ;
set the minimum contour level
cnres1 at cnMaxLevelValF              = 5.                                ;
set the maximum contour level
cnres1 at cnLevelSpacingF             = 0.5                                ;
set the interval between contours
cnres1 at lbOrientation           = "vertical"
cnres1 at lbBoxSeparatorLinesOn = False
cnres1 at lbTitleString             = div at long_name
cnres1 at lbTitlePosition        = "Left"
cnres1 at lbAutoManage             = False
cnres1 at lbTitleFontHeightF    = 0.012
;cnres1 at lbLabelStrings =  (/"-5.0","-2.5","0","2.5","5.0"/)
cnres1 at mpMinLonF          =   0
cnres1 at mpMaxLonF          = 360
cnres1 at mpCenterLonF       = 180     ; This is necessary to get the
correct map
cnres1 at mpMinLatF          = -80
cnres1 at mpMaxLatF          = 80

vcres at vcRefMagnitudeF     = 10.                 ; make vectors larger
vcres at vcRefLengthF        = 0.05              ; ref vector length
vcres at vcGlyphStyle        = "CurlyVector"      ; turn on curly vectors
vcres at vcMinDistanceF      = 0.012              ; thin the vectors
vcres at vcRefAnnoOrthogonalPosF  = 0        ; Move ref anno into plot
vcres at vcRefAnnoBackgroundColor = "Background"
vcres at vcRefAnnoString2On    = False
;vcres at vcRefAnnoSide            =    "Left"

;cnres2 at gsnCenterString    = "Chi scaled by 1e8"
cnres2 at cnFillOn                        = False
cnres2 at cnLinesOn                    = True
;cnres2 at mpMinLonF          =   0
;cnres2 at mpMaxLonF          = 360
;cnres2 at mpCenterLonF       = 180     ; This is necessary to get the
correct map
cnres2 at trGridType                     = "TriangularMesh"

plot1 = gsn_csm_vector(wks,UDavg(:,:),VDavg(:,:),vcres)
plot2    =
gsn_csm_contour_map_overlay(wks,DIVavg(:,:),CHIavg(:,:),cnres1,cnres2)

overlay(plot2,plot1)

draw(plot2)
frame(wks)

end

I am getting the following warning messages:

(0)    check_for_y_lat_coord: Warning: Data either does not contain a valid
latitude coordinate array or doesn't contain one at all.
(0)    A valid latitude coordinate array should have a 'units' attribute
equal to one of the following values:
(0)        'degrees_north' 'degrees-north' 'degree_north' 'degrees north'
'degrees_N' 'Degrees_north' 'degree_N' 'degreeN' 'degreesN' 'deg north'
(0)    check_for_lon_coord: Warning: Data either does not contain a valid
longitude coordinate array or doesn't contain one at all.
(0)    A valid longitude coordinate array should have a 'units' attribute
equal to one of the following values:
(0)        'degrees_east' 'degrees-east' 'degree_east' 'degrees east'
'degrees_E' 'Degrees_east' 'degree_E' 'degreeE' 'degreesE' 'deg east'

and the plot which is attached to this mail. I think I am getting proper
divergence but velocity potential lines are not getting plotted at all and
proper plot without error. I am expecting humble reply from your side.

Thanking you,

On Wed, Feb 21, 2018 at 5:40 PM, Vyankatesh Manoj Mundhada <
vyankatesh at iiserb.ac.in> wrote:

> Thank you very much. That was really helpful.
>
> Regards,
> Vyankatesh
>
> On Wed, Feb 21, 2018 at 4:03 PM, Guido Cioni <guidocioni at gmail.com> wrote:
>
>> Hi,
>> This error usually pops up when you're doing assignment that delete the
>> original attributes of the variable, including the reference to the
>> coordinate variables.
>>
>> I'm just speculating but...could it be this line?
>>
>>     plot1 = gsn_csm_vector_map(wks,0.001*ud(0,0,:,:),0.001*vd(0,0,:,:),v
>> cres)
>>
>> You are doing 0.001*ud(0,0,:,:), so merely assigning a new variable
>> without copying the attributes.
>> You should instead do the scaling first
>>
>> ud=0.001*ud(:,:,:,:) ; here the attributes are preserved
>>
>> and then plot ud(0,0,:,:).
>> Or just a define a new variable based un ud first.
>>
>> Let us know whether that works, that's the first thing that comes to my
>> mind.
>>
>>
>>
>> On 21. Feb 2018, at 10:20, Vyankatesh Manoj Mundhada <
>> vyankatesh at iiserb.ac.in> wrote:
>>
>> 0.001*vd(0,0,:,:)
>>
>>
>>
>> Guido Cioni
>> http://guidocioni.altervista.org
>>
>>
>
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