Re: [netcdf-java] Grib1 Gaussian Grid Relative Humidity Interpolation Issue

Hi Jochen:

Yes, please send us the single grib record, so we can test.

thanks!

John

On 11/15/2013 6:04 AM, Jochen Kähler wrote:
Hi all,

we’re implementing some service that reads ECMWF 0.125° Grib files in
gaussian grid. The NetCDF-Library automatically interpolates the
gaussian grid to a regular lat/lon-grid. For the relative humidity we
found some strange values in output after that interpolation.

Using wgrib I’ve extracted some isobaric relative humidity record to a
single grib file called „dpd_test.grib“ (which i could send as well if
needed). According to wgrib, it is an gaussian grid and min and max
values are 0% and 100% - as I would expect them for a relative humidity
(see attached output).

Next I’ve written a small program which extracts that relative humidity
parameter via netcdf-java-lib and step thru all values to find minimum
and maximum in it. Source is also attached - just in case I did
something wrong there.

The output of that states the following...
2013-11-14 13:45:15,378 INFO : App -
dpd_test.grib/Relative_humidity_isobaric/850.0hPa:
min=-4.462653160095215 max=108.53846740722656
…indicating the minimum relative humidity is around -4.46% and the
maximum at 108.54%.

Browsing your code on Github I think you’re using cubic interpolation to
convert gaussian to regular grid, which might cause this problem.
https://github.com/Unidata/thredds/blob/6b1052455e597797f3a5980165a23292172920ce/grib/src/main/java/ucar/nc2/grib/QuasiRegular.java
 Line
219 - 221:
outpt[oIdx] = (float) (a * inpt[iIdx + low] + b * inpt[iIdx + hi]
         + ((a * a * a - a) * y2d[low]
         + (b * b * b - b) * y2d[hi]) / 6.0);
Is the problem in the parametrization of this formula?

Is it possible to use Linear Interpolation here?

Kind Regards,
Jochen



— wgrib output ---------------
Losty-MacBook:Desktop jkaehler$ ~/wgrib/wgrib -V dpd_test.grib
rec 1:0:date 2013111300 R kpds5=157 kpds6=100 kpds7=850 levels=(3,82)
grid=255 850 mb 6hr fcst:
   R=Relative humidity [%]
   timerange 0 P1 6 P2 0 TimeU 1  nx -1 ny 800 GDS grid 4 num_in_ave 0
missing 0
   center 98 subcenter 0 process 143 Table 128 scan: WE:NS winds(N/S)
   thinned gaussian: lat  89.828000 to -89.828000
           long 0.000000 to 359.900000, 843490 grid pts   (-1 x 800)
scan 0 mode 0 bdsgrid 1
       18   25   32   40   45   50   60   60   72   72   75   81   90
96  100
      108  120  120  125  128  144  144  150  160  160  180  180  192
  192  200
      200  216  216  225  240  240  240  250  250  256  270  288  288
  288  300
      300  320  320  320  324  360  360  360  360  360  360  375  375
  384  400
      400  400  405  432  432  432  432  450  450  450  480  480  480
  480  480
      486  500  500  512  512  540  540  540  540  540  576  576  576
  576  576
      576  600  600  600  600  640  640  640  640  640  640  640  648
  675  675
      675  675  675  720  720  720  720  720  720  720  729  729  750
  750  750
      750  768  768  768  800  800  800  800  800  800  810  864  864
  864  864
      864  864  864  864  864  864  900  900  900  900  900  900  900
  960  960
      960  960  960  960  960  960  960  960  960  960  972  972 1000
1000 1000
     1000 1000 1000 1024 1024 1024 1024 1024 1080 1080 1080 1080 1080
1080 1080
     1080 1080 1080 1080 1125 1125 1125 1125 1125 1125 1125 1125 1125
1152 1152
     1152 1152 1152 1152 1200 1200 1200 1200 1200 1200 1200 1200 1200
1200 1200
     1215 1215 1215 1215 1280 1280 1280 1280 1280 1280 1280 1280 1280
1280 1280
     1280 1280 1280 1280 1280 1296 1296 1296 1296 1350 1350 1350 1350
1350 1350
     1350 1350 1350 1350 1350 1350 1350 1350 1350 1440 1440 1440 1440
1440 1440
     1440 1440 1440 1440 1440 1440 1440 1440 1440 1440 1440 1440 1440
1440 1440
     1440 1440 1440 1440 1440 1440 1440 1458 1458 1458 1458 1458 1458
1458 1500
     1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500 1500
1500 1500
     1500 1536 1536 1536 1536 1536 1536 1536 1536 1536 1536 1536 1536
1536 1536
     1536 1536 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600
1600 1600
     1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600
1600 1600
     1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600
1600 1600
     1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600
1600 1600
     1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600
1600 1600
     1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600
1600 1600
     1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600
1600 1600
     1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600
1600 1600
     1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600
1600 1600
     1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600
1600 1600
     1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600 1600
1600 1600
     1600 1600 1600 1536 1536 1536 1536 1536 1536 1536 1536 1536 1536
1536 1536
     1536 1536 1536 1536 1500 1500 1500 1500 1500 1500 1500 1500 1500
1500 1500
     1500 1500 1500 1500 1500 1500 1458 1458 1458 1458 1458 1458 1458
1440 1440
     1440 1440 1440 1440 1440 1440 1440 1440 1440 1440 1440 1440 1440
1440 1440
     1440 1440 1440 1440 1440 1440 1440 1440 1440 1440 1440 1350 1350
1350 1350
     1350 1350 1350 1350 1350 1350 1350 1350 1350 1350 1350 1296 1296
1296 1296
     1280 1280 1280 1280 1280 1280 1280 1280 1280 1280 1280 1280 1280
1280 1280
     1280 1215 1215 1215 1215 1200 1200 1200 1200 1200 1200 1200 1200
1200 1200
     1200 1152 1152 1152 1152 1152 1152 1125 1125 1125 1125 1125 1125
1125 1125
     1125 1080 1080 1080 1080 1080 1080 1080 1080 1080 1080 1080 1024
1024 1024
     1024 1024 1000 1000 1000 1000 1000 1000  972  972  960  960  960
  960  960
      960  960  960  960  960  960  960  900  900  900  900  900  900
  900  864
      864  864  864  864  864  864  864  864  864  810  800  800  800
  800  800
      800  768  768  768  750  750  750  750  729  729  720  720  720
  720  720
      720  720  675  675  675  675  675  648  640  640  640  640  640
  640  640
      600  600  600  600  576  576  576  576  576  576  540  540  540
  540  540
      512  512  500  500  486  480  480  480  480  480  450  450  450
  432  432
      432  432  405  400  400  400  384  375  375  360  360  360  360
  360  360
      324  320  320  320  300  300  288  288  288  270  256  250  250
  240  240
      240  225  216  216  200  200  192  192  180  180  160  160  150
  144  144
      128  125  120  120  108  100   96   90   81   75   72   72   60
60   50
       45   40   32   25   18
   min/max data 0 100  num bits 8  BDS_Ref 0  DecScale 0 BinScale -1
——


— Java-Source ——
GridDataset dataset = GridDataset.open(gribFile.getAbsolutePath());

try {
GridDatatype grid = dataset.findGridDatatype("Relative_humidity_isobaric");
if(grid != null) {
CoordinateAxis1D isbl = grid.getCoordinateSystem().getVerticalAxis();
double[] isblValues = isbl.getCoordValues();
for (int isblIdx = 0; isblIdx < isblValues.length; isblIdx++) {
double[] data = (double[]) grid.readDataSlice(0, isblIdx, -1,
-1).get1DJavaArray(double.class);
double min = Double.MAX_VALUE;
double max = Double.MIN_VALUE;
for (double d : data) {
min = Math.min(min, d);
max = Math.max(max, d);
        }
LOG.info("{}/{}/{}{}: min={} max={}", gribFile.getName(),
grid.getFullName(), isblValues[isblIdx], isbl.getUnitsString(), min, max);
}
}
} finally {
if (dataset != null) {
dataset.close();
}
}

--------


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