CDO

Hello,

I'm trying to determine the optimal approach for remapping rotated grid data (latlon WGS84) to normal (non-rotated) latlon.

The method adopted is based on the post at https://code.zmaw.de/boards/1/topics/2632?r=2634#message-2634 and lots of online searching including the CDO manual.

I've used 'cdo sinfo' to identify the real (ie., non-rotated) spatial boundaries of the data, and 'gdalinfo' to find the files spatial resolution (see i and ii below).

Then the ouput target grid (seasia.nc) is used for remapping the rotated nc file to normal latlon coordinates using cdo's remapbil (see iii below)

My queries:

1. I couldn't find any information related to the '-topo' flag used for creating the seasia.nc target grid. Can anyone point me to any info on this flag. The flag is needed or the command fails otherwise.

2. I've used bilinear interpolation - would remapcon be better in this case with daily precipitation values?

3. 'cdo sinfo' outputs the long and lat of the grid boundaries - this command appears to ignore the rotated pole parameters or is it remapping to normal (non-rotated) latlon WGS84 coordinates?

Thanks for your feedback.

Cheers
Richard

(i) Find spatial extent of nc file (in normal non-rotated latlon WGS84):

cdo sinfo cahpa.f1jan.05216.nc
File format : netCDF
-1 : Institut Source Ttype Levels Num Points Num Dtype : Parameter ID
1 : unknown unknown instant 1 1 36864 1 F32 : -1
Grid coordinates :
1 : curvilinear : points=36864 (192x192) * longitude : 90.3057 to 136.942 degrees_east
latitude : -13.0722 to 30.24 degrees_north*
Vertical coordinates :
1 : surface : levels=1
Time coordinate : 30 steps
RefTime = 1860-12-01 00:00:00 Units = days Calendar = 360_day Bounds = true
YYYY-MM-DD hh:mm:ss YYYY-MM-DD hh:mm:ss YYYY-MM-DD hh:mm:ss YYYY-MM-DD hh:mm:ss
1951-01-01 12:00:00 1951-01-02 12:00:00 1951-01-03 12:00:00 1951-01-04 12:00:00
1951-01-05 12:00:00 1951-01-06 12:00:00 1951-01-07 12:00:00 1951-01-08 12:00:00
1951-01-09 12:00:00 1951-01-10 12:00:00 1951-01-11 12:00:00 1951-01-12 12:00:00
1951-01-13 12:00:00 1951-01-14 12:00:00 1951-01-15 12:00:00 1951-01-16 12:00:00
1951-01-17 12:00:00 1951-01-18 12:00:00 1951-01-19 12:00:00 1951-01-20 12:00:00
1951-01-21 12:00:00 1951-01-22 12:00:00 1951-01-23 12:00:00 1951-01-24 12:00:00
1951-01-25 12:00:00 1951-01-26 12:00:00 1951-01-27 12:00:00 1951-01-28 12:00:00
1951-01-29 12:00:00 1951-01-30 12:00:00
cdo sinfo: Processed 1 variable over 30 timesteps ( 0.01s )

(ii) Use gdalinfo to find spatial resolution (0.22 degrees)

gdalinfo cahpa.f1jan.05216.nc 
Driver: netCDF/Network Common Data Format
Files: cahpa.f1jan.05216.nc
Size is 192, 192
Coordinate System is `'
Origin = (-16.909989696522658,15.349998811152593)
Pixel Size = (0.219999992410550,-0.219999987417491)
...
  precipitation_flux#units=kg m-2 s-1
  rotated_latitude_longitude#grid_mapping_name=rotated_latitude_longitude
  rotated_latitude_longitude#grid_north_pole_latitude=75
  rotated_latitude_longitude#grid_north_pole_longitude=289
...
Geolocation:
  LINE_OFFSET=0
  LINE_STEP=1
  PIXEL_OFFSET=0
  PIXEL_STEP=1
  SRS=GEOGCS["WGS 84",DATUM["WGS_1984",SPHEROID["WGS 84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],TOWGS84[0,0,0,0,0,0,0],AUTHORITY["EPSG","6326"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.0174532925199433,AUTHORITY["EPSG","9108"]],AUTHORITY["EPSG","4326"]]
  X_BAND=1
  X_DATASET=NETCDF:"cahpa.f1jan.05216.nc":longitude
  Y_BAND=1
  Y_DATASET=NETCDF:"cahpa.f1jan.05216.nc":latitude
Corner Coordinates:
Upper Left  ( -16.9099897,  15.3499988) 
Lower Left  ( -16.9099897, -26.8899988) 
Upper Right (  25.3300088,  15.3499988) 
Lower Right (  25.3300088, -26.8899988) 
Center      (   4.2100096,  -5.7700000)

(iii) Remap to normal (non-rotated) latlon WGS84 with 0.25 degree resolution:

*cdo -f nc -sellonlatbox,90,140,35,-15 -remapbil,r1440x720 -topo seasia.nc

cdo -f nc remapbil,seasia.nc cahpa.f1jan.05216.nc ofile.nc*