Update : Changed this question to better reflect my current understanding.
I have a NetCDF version 4.5 Grib2Record object. Given a (x,y) grid point and a variable name I want to extract all the forecast data for that variable by forecast-time from the object (if the record contains the forecast for that variable). Because of the default behavior of writing a disk index file I do not want to use the higher level NetCDFFile interface.
I have tried looking at lower level code (Grib2Rectilyser, Grib2Customizer etc.) But the code is too dense and I am looking for help with where to start.
I would appreciate any pointers on how to take a Grib2Record and 1. check whether a particular forecast variable is contained in it, and 2. if it is, then extract the forecast data by forecast-valid-time for a given xy grid point and z-level.
I have worked with grib2 files for wind predictions, this is how I get the records and how to process it to get wind (VU components)
Grib2Input input = new Grib2Input(getRandomAccessFile());
if (!input.scan(false, false)) {
logger.error("Failed to successfully scan grib file");
return;
}
Grib2Data data = new Grib2Data(getRandomAccessFile());
List<Grib2Record> records = input.getRecords();
for (Grib2Record record : records) {
Grib2IndicatorSection is = record.getIs();
Grib2IdentificationSection id = record.getId();
Grib2Pds pdsv = record.getPDS().getPdsVars();
Grib2GDSVariables gdsv = record.getGDS().getGdsVars();
long time = id.getRefTime() + (record.getPDS().getForecastTime() * 3600000);
logger.debug("Record description at " + pdsv.getReferenceDate() + " forecast "
+ new Date(time) + ": " + ParameterTable.getParameterName(is.getDiscipline(), pdsv.getParameterCategory(), pdsv.getParameterNumber()));
float[] values = data.getData(record.getGdsOffset(), record.getPdsOffset(), 0);
if ((is.getDiscipline() == 0) && (pdsv.getParameterCategory() == 2) && (pdsv.getParameterNumber() == 2)) {
// U-component_of_wind
int c = 0;
for (double lat = gdsv.getLa1(); lat >= gdsv.getLa2(); lat = lat - gdsv.getDy()) {
for (double lon = gdsv.getLo1(); lon <= gdsv.getLo2(); lon = lon + gdsv.getDx()) {
logger.debug(lat + "\t" + lon + "\t" +
values[c]);
c++;
}
}
} else if ((is.getDiscipline() == 0) && (pdsv.getParameterCategory() == 2) && (pdsv.getParameterNumber() == 3)) {
// V-component_of_wind
int c = 0;
for (double lat = gdsv.getLa1(); lat >= gdsv.getLa2(); lat = lat - gdsv.getDy()) {
for (double lon = gdsv.getLo1(); lon <= gdsv.getLo2(); lon = lon + gdsv.getDx()) {
logger.debug(lat + "\t" + lon + "\t" +
values[c]);
c++;
}
}
}
}
private RandomAccessFile getRandomAccessFile() {
RandomAccessFile raf = null;
try {
raf = new RandomAccessFile(path, "r");
raf.order(RandomAccessFile.BIG_ENDIAN);
} catch (IOException e) {
logger.error("Error opening file " + path, e);
}
return raf;
}
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