Hi John, that an easy one: let’s go for option #4! ;-) ... Well, regarding to our current project, linear interpolation would be totally fine. If we could get some configuration switch or even some option to change interpolationmethod programmatically, that would be excellent. Wgrib provides access to the original numbers - that would be an interesting option as well. As my colleague Martijn already pointed out here: http://www.unidata.ucar.edu/mailing_lists/archives/netcdf-java/2013/msg00125.html , we also have some issues around the chosen resolution, which we could work around in a somewhat more elegant way if we had access to the raw data... Jochen Am 19.11.2013 um 18:37 schrieb John Caron <caron@xxxxxxxxxxxxxxxx>: > Hi Jochen: > > We call these "reduced grids" or "thin grids", their use is technically > independent of whether its a gaussian lat/lon (just means the lats are spaced > with a gaussian), although in practice they often go together (perhaps always > in the model - im just talking about the output files). We probably stole > the cubic spline code from GEMPAK/nawips which made us think it was a > reassonable thing to do. > > IFAIU, cubic splines allow the interpolated numbers to be outside the range > of existing ones, so at this point i dont suspect an error. > > You are right that we could use the actual min, max of the numbers to clip > the cubic spline results. > > Not sure what the right thing to do here is. we could clip it based on actual > min/max (easy). We could try to make an option to allow linear interpolation, > which would prevent the interpolated numbers from exceeding the min/max of > existing numbers (harder). We could try to allow user access to the original > numbers (harder, need lower level API). We could construct a quantity X > conserving algorithm, which reads the users' mind as to what X should be > (requires faster than light technology). > > What does wgrib do? > > Suggestions from anyone are welcome. > > 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(); >>>> } >>>> } >>>> >>>> -------- >>>> >>>> >>>> _______________________________________________ >>>> netcdf-java mailing list >>>> netcdf-java@xxxxxxxxxxxxxxxx >>>> For list information or to unsubscribe, visit: >>>> http://www.unidata.ucar.edu/mailing_lists/ >>>> >>> >>> _______________________________________________ >>> netcdf-java mailing list >>> netcdf-java@xxxxxxxxxxxxxxxx >>> For list information or to unsubscribe, visit: >>> http://www.unidata.ucar.edu/mailing_lists/ >> > > _______________________________________________ > netcdf-java mailing list > netcdf-java@xxxxxxxxxxxxxxxx > For list information or to unsubscribe, visit: > http://www.unidata.ucar.edu/mailing_lists/
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