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

Hi Jochen,
I took a quick look, and you are correct about the interpolation. While the cubic fit gives smoother results, there is the possibility of exceeding physical maxima and minima using this method, especially if the data points jump significantly in value from one to the next. I thought about introducing a limiter, but this bit of code does not have access to min/max values. 

I will have to send this issue into JIRA, our bug tracking system.

Regards,
  Lansing Madry
  Unidata
  Boulder, Colorado

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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