how to plot a radar picture

Dear,
   I have a radar data file, whose format is netcdf. I want to plot a radar
echo(unit:dbz) picture using NCL.
    The head file of this radar data file is as following:
   netcdf \1306250900 {
   dimensions:
 longtitude = 1019 ;
 latitude = 881 ;
 layer = 20 ;
variables:
 float longtitude(longtitude) ;
  longtitude:units = "degrees_east" ;
 float latitude(latitude) ;
  latitude:units = "degrees_north" ;
 int layer(layer) ;
  layer:units = "meter" ;
 byte reflectivity(layer, latitude, longtitude) ;
  reflectivity:name = "Reflectivity" ;
  reflectivity:units = "dBZ" ;
  reflectivity:scale_factor = 0.5f ;
  reflectivity:add_offset = -33s ;
  reflectivity:data_type = "unsigned byte" ;
  reflectivity:formula = "dBZ=-33+0.5*N" ;
  reflectivity:valid_min = 0s ;
  reflectivity:valid_max = 255s ;
  reflectivity:no_data = 0s ;
  reflectivity:bad_data = 1s ;
  reflectivity:no_observation = 2s ;
  reflectivity:censored_data = 3s ;
// global attributes:
   ...}
  
   The script file that I wrote to read this data file and plot the radar echo
at 5th level is as following:
   load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_code.ncl"
   load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_csm.ncl"
   load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/contributed.ncl"
begin
dbz=new((/881,1019/),"float")
 in=addfile("1306250900.nc","r")
 dbz=byte2flt(in->reflectivity(4,:,:))
  lon=in->longtitude
 lat=in->latitude
  
res=True
 res@gsnMaximize=True
 res@cnFillOn=True
 res@cnLinesOn=False
 res@sfXCStartV = 115 ; Define where contour plot
  res@sfXCEndV = 118 ; should lie on the map plot.
  res@sfYCStartV = 38
  res@sfYCEndV = 42
  res@mpDataSetName = "Earth..4"
  res@mpDataBaseVersion = "MediumRes" ; Medium resolution database
  res@mpOutlineOn = True ; Turn on map outlines
  res@mpOutlineSpecifiers = (/"China:states","Taiwan"/)
  res@mpLimitMode = "LatLon" ; Limit the map view.
  res@mpMinLonF = 115
  res@mpMaxLonF = 118
  res@mpMinLatF = 38
  res@mpMaxLatF = 42
  res@tmXTOn=False
  res@tmYROn=False
  res@cnLevelSelectionMode="ExplicitLevels"
 res@cnLevels=ispan(-5,65,5)
  wks=gsn_open_wks("x11","radarecho")
  gsn_define_colormap(wks,"WhiteBlueGreenYellowRed")
  plot=gsn_csm_contour_map_ce(wks,dbz({latitude|38:42},{longtitude|115:118}),
res)
end

         The distribution of the output plot radar echo is same as the reality.
However the value of the output plot radar echo is significantly larger than
the reality, even the value of some reality strong radar echo region is
negative.
How can I solve this problem?
 Thank you!
 Yours,
Hui He

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Received on Tue Jul 2 03:27:00 2013