Usage of NCL to Access remote HDF-EOS5 MLS Files
1.
Introduction
In this document we describe how to use NCL[1]
to visualize a remote HDF-EOS5 MLS files. For the general usages of NCL, please refer to the NCL user's
guides[2], or websites.
2.
Sample MLS File
The sample MLS file name is MLS-Aura_L2GP-CO_v02-23-c02_2008d277.he5.
You can download it from
here.
3.
How to Open and Read OPeNDAP data wtih NCL
The following code shows a typical way to use NCL to open and read a variable through OPeNDAP.
This code needs to be typed at the prompt that NCL shows.
In the above code, the
url points to the remote MLS file being served by OPeNDAP. Use
print command to check if the
url file is opened successfully and to see what variables are available inside the file.
4.
Setting up key MLS Variables
The next code snippet shows a typical way to handle MLS variables. MLS file holds the key data under L2gpValue and related geo-location information under Latitude, Longitude and Pressure variable. You need to adjust the numeric variable index inside the parentheses to match the right variables.
Figure 2 Handle MLS specific variables; From the above, we know that the 12th variable is L2gpValue.
T = f->$vNames(12)$
; Re-scale the values.
T = T*1e7
T@units= T@units + "*1e7"
lat = f->$vNames(8)$
lon = f->$vNames(5)$
; The below mapping assumes that the OPeNDAP hdf5_handler maps nTimes to time
; and Pressure to lev variable.
; Otherwise, you need to use numeric index.
lev = f->lev
time = f->time
ntime = dimsizes(time)
; This is useful to set manual contour levels. See "ManualLevels" below.
printVarSummary(T)
In the above code, we re-scaled the
L2gpValue va since they are too small. You need to adjust the scale factor depending on the MLS product.
5.
Visualizing MLS Data
The final code snippet is almost identical to the official NCL MLS example code in NCL HDF Application Page.
Figure 3 MLS Data Visualization; This is useful to set manual contour levels. See "ManualLevels" below.
printVarSummary(T)
; The rest of the code is almost identical to hdf5eos_3.ncl
printMinMax(T, True)
print("==============")
; time units: NCL does not *yet* have a function analogous
; to ut_calendar for TAI93 (International Atomic Time).
; Create an "elapsed time" variable.
telapse = (time - time(0))/60
telapse@long_name = "Elapsed Time (minutes)"
telapse@units = "minutes since "+time(0)
;************************************************
; plot
;************************************************
pltType= "ps"
pltName= fName+".L2gpValue.ncl"
wks = gsn_open_wks (pltType,pltName); open workstation
gsn_define_colormap(wks,"amwg") ; choose colormap
dumb= NhlNewColor(wks,0.7,0.7,0.7); add gray to colormap
res = True
res@gsnMaximize= True ; make ps/eps/pdf
res@gsnSpreadColors= True
res@gsnSpreadColorEnd= -2 ; do not use gray for contours
;res@gsnPaperOrientation= "portrait" ; force portrait
res@tiMainString = fName
res@cnFillOn = True
res@cnLinesOn= False
res@cnLineLabelsOn = False
res@cnFillMode = "RasterFill" ; faster
res@cnRasterSmoothingOn= True
res@lbLabelAutoStride= True
res@lbOrientation= "Vertical"
; Change these values based on min/max values of L2gpValue dataset.
res@cnLevelSelectionMode = "ManualLevels" ; set manual contour levels
res@cnMinLevelValF = -6. ; set min contour level
res@cnMaxLevelValF = 6. ; set max contour level
res@cnLevelSpacingF=1. ; set contour spacing
res@trYReverse = True ; reverse y-axis
T&lev= lev; assign "pressure" coordinates
T&time = telapse; assign temporal coordinates
res@gsnYAxisIrregular2Linear = True
res@tmYLFormat = "f"; force minimal precision
res@tiYAxisString= "Pressure (hPa)"
res@gsnLeftString= T@title
plot= gsn_csm_contour (wks,T(lev|:,time|:) , res)
;***************************************
; create trajectory plot
;***************************************
mpres= True; Plot options desired.
mpres@gsnFrame = False ; Don't advance the frame
mpres@gsnMaximize= True
mpres@mpLandFillColor= "gray70"; color of land
;mpres@gsnPaperOrientation= "portrait"; force portrait
plot = gsn_csm_map_ce(wks,mpres) ; Draw map
gsres= True; "Graphic Style" resources
gsres@gsMarkerSizeF= 10.0; Marker size
gsres@gsMarkerThicknessF = 1.0 ; Marker thickness
gsres@gsMarkerColor= "Blue"; Marker color
gsres@gsMarkerIndex= 1 ; Marker style
mpres@tiMainString = fName
gsn_polymarker(wks,plot,lon,lat,gsres) ; plot trajectory
frame(wks)
end
In the above code, we adjusted the
res@cnMinLevelValF and
res@cnMaxLevelValF to get a nice graph since the values in that range are the most meaningful values based on the pressure level.
See the complete code
here.
Run the code as follows:
Then, it will create a
MLS-Aura_L2GP-CO_v02-23-c02_2008d277.he5.L2gpValue.ncl.ps file as a result. The PostScript file will have two images like
Figure 5 and
Figure 6.
6.
References
Last modified: 08/09/2011
