Re: Question about netCDF info as provided by ncdump and ncl

AJ,

"f" is one of the normal attribute suffixes in ncdump output. It is
just a data type code meaning "float". This is probably documented
somewhere with ncdump or on the Netcdf website.

Also, the ncdump output shows that this SST data is in packed
format, compressed from 32 bit floats to 16 bit integers by a simple
linear formula. This cuts the file size in half.

After the unpacking formula is applied, you will probably see that
the SST values are within the expected range. You can apply the
formula explicitly. This is an array formula:

   sst_unpacked = f->sst * sst_at_scale_factor + sst_at_add_offset

But it is better practice to use the NCL unpacking function made for
this purpose. Also it preserves the coordinates and attributes with
the data variable:

   sst_unpacked = short2flt (f->sst)

http://www.ncl.ucar.edu/Document/Functions/Contributed/short2flt.shtml

Packing is discussed here in the CF convention standards:

   http://cf-pcmdi.llnl.gov/documents/cf-conventions/1.4/ch08.html

Dave Allured
CU/CIRES Climate Diagnostics Center (CDC)
http://cires.colorado.edu/science/centers/cdc/
NOAA/ESRL/PSD, Climate Analysis Branch (CAB)
http://www.cdc.noaa.gov/psd1/

AJ Smit wrote:
> Dear all,
>
> I am puzzled by the behaviour of ncl, specifically when comparing the
> outputs of ncdump vs that of ncl. In short, this is what's worrying me:
>
> From ncdump for the sst variable:
> sst:valid_range = -5.f, 40.f ;
> sst:actual_range = -1.7999996f, 35.310001f ;
> From ncl for the sst variable:
> valid_range : ( -5, 40 )
> actual_range : ( -1.8, 35.31 )
>
> Despite the correct ranges indicated above, in ncl the sst values in
> this variable are bogus - the values do not make sense at all.
> Conversely, ndump retains sensible sst values even though the ranges
> indicated above contain an "f". What does this mean?
>
> I am sure there is an obvious solution, but it escapes me for now. How
> do I correctly read in the data using ncl?
>
> The full header info provided by netCDF and ncl is at the end of the
> message.
>
> With kind regards,
> AJ
>
> ncl 7> in = addfile("sst.mnmean.nc","r")
> ncl 8> print(in)
> Variable: in (file variable)
>
> filename: sst.mnmean
> path: sst.mnmean.nc
> file global attributes:
> title : NOAA Optimum Interpolation (OI) SST V2
> Conventions : CF-1.0
> history : Wed Apr 6 13:47:45 2005: ncks -d time,0,278
> SAVEs/sst.mnmean.nc sst.mnmean.nc
> Created 10/2002 by RHS
> comments : Data described in Reynolds, R.W., N.A. Rayner, T.M.
> Smith, D.C. Stokes, and W. Wang, 2002: An Improved In Situ and Satellite
> SST Analysis for Climate, J. Climate
> platform : Model
> source : NCEP Climate Modeling Branch
> institution : National Centers for Environmental Prediction
> references : http://www.emc.ncep.noaa.gov/research/cmb/sst_analysis/
> http://www.cdc.noaa.gov/cdc/data.noaa.oisst.v2.html
> dimensions:
> lon = 360
> lat = 180
> nbnds = 2
> time = 333 // unlimited
> variables:
> float lat ( lat )
> units : degrees_north
> long_name : Latitude
> actual_range : ( 89.5, -89.5 )
> standard_name : latitude_north
> axis : y
> coordinate_defines : center
>
> float lon ( lon )
> units : degrees_east
> long_name : Longitude
> actual_range : ( 0.5, 359.5 )
> standard_name : longitude_east
> axis : x
> coordinate_defines : center
>
> <! Snip: time - display same as ncdump -h; see below >
>
> double time_bnds ( time, nbnds )
> long_name : Time Boundaries
>
> short sst ( time, lat, lon )
> long_name : Monthly Mean of Sea Surface Temperature
> valid_range : ( -5, 40 )
> actual_range : ( -1.8, 35.31 )
> units : degC
> add_offset : 0
> scale_factor : 0.01
> missing_value : 32767
> precision : 2
> least_significant_digit : 2
> var_desc : Sea Surface Temperature
> dataset : NOAA Optimum Interpolation (OI) SST V2
> level_desc : Surface
> statistic : Mean
> parent_stat : Weekly Mean
> standard_name : sea_surface_temperature
> cell_methods : time: mean (monthly from weekly values
> interpolated to daily)
> _FillValue : 32767
>
> Here is the corresponding ncdump output:
>
> Pelagia:OISSTv2 ajsmit$ ncdump -h sst.mnmean.nc
> netcdf sst.mnmean {
> dimensions:
> lon = 360 ;
> lat = 180 ;
> nbnds = 2 ;
> time = UNLIMITED ; // (333 currently)
>
> variables:
> float lat(lat) ;
> lat:units = "degrees_north" ;
> lat:long_name = "Latitude" ;
> lat:actual_range = 89.5f, -89.5f ;
> lat:standard_name = "latitude_north" ;
> lat:axis = "y" ;
> lat:coordinate_defines = "center" ;
> float lon(lon) ;
> lon:units = "degrees_east" ;
> lon:long_name = "Longitude" ;
> lon:actual_range = 0.5f, 359.5f ;
> lon:standard_name = "longitude_east" ;
> lon:axis = "x" ;
> lon:coordinate_defines = "center" ;
> double time(time) ;
> time:units = "days since 1800-1-1 00:00:00" ;
> time:long_name = "Time" ;
> time:actual_range = 66443., 76548. ;
> time:delta_t = "0000-01-00 00:00:00" ;
> time:avg_period = "0000-01-00 00:00:00" ;
> time:prev_avg_period = "0000-00-07 00:00:00" ;
> time:standard_name = "time" ;
> time:axis = "t" ;
> double time_bnds(time, nbnds) ;
> time_bnds:long_name = "Time Boundaries" ;
> short sst(time, lat, lon) ;
> sst:long_name = "Monthly Mean of Sea Surface Temperature" ;
> sst:valid_range = -5.f, 40.f ;
> sst:actual_range = -1.7999996f, 35.310001f ;
> sst:units = "degC" ;
> sst:add_offset = 0.f ;
> sst:scale_factor = 0.0099999998f ;
> sst:missing_value = 32767s ;
> sst:precision = 2s ;
> sst:least_significant_digit = 2s ;
> sst:var_desc = "Sea Surface Temperature" ;
> sst:dataset = "NOAA Optimum Interpolation (OI) SST V2" ;
> sst:level_desc = "Surface" ;
> sst:statistic = "Mean" ;
> sst:parent_stat = "Weekly Mean" ;
> sst:standard_name = "sea_surface_temperature" ;
> sst:cell_methods = "time: mean (monthly from weekly values interpolated
> to daily)" ;
>
> // global attributes:
> :title = "NOAA Optimum Interpolation (OI) SST V2" ;
> :Conventions = "CF-1.0" ;
> :history = "Wed Apr 6 13:47:45 2005: ncks -d time,0,278
> SAVEs/sst.mnmean.nc sst.mnmean.nc\n",
> "Created 10/2002 by RHS" ;
> :comments = "Data described in Reynolds, R.W., N.A. Rayner, T.M.\n",
> "Smith, D.C. Stokes, and W. Wang, 2002: An Improved In Situ and
> Satellite\n",
> "SST Analysis for Climate, J. Climate" ;
> :platform = "Model" ;
> :source = "NCEP Climate Modeling Branch" ;
> :institution = "National Centers for Environmental Prediction" ;
> :references = "http://www.emc.ncep.noaa.gov/research/cmb/sst_analysis/\n",
> "http://www.cdc.noaa.gov/cdc/data.noaa.oisst.v2.html" ;
> }
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Received on Sun Oct 11 2009 - 23:57:27 MDT