Failed to get index of longitude array

ncl-talk,

My job was to obtain wind value of a specified grid point from a grib file.
Before executing, I extracted the lat and lon array from the grib so that I
know in advance (23.84,146.4) is a valid grid point.

Here is my script

begin
gribfile="GRMC_GZLM.2014050100000.grb2"
in = addfile (gribfile,"r")
raw_lat = in->lat_0
raw_lon = in->lon_0
centre_lat = 23.84
centre_lon = 146.4
centre_lat_id = ind(raw_lat.eq.centre_lat)
centre_lon_id = ind(raw_lon.eq.centre_lon)
print("lat id of " + centre_lat + ": " + centre_lat_id + " | lon id of " +
centre_lon + ": " + centre_lon_id)
u_wind = in->UGRD_P0_L103_GLL0(centre_lat_id, centre_lon_id)
v_wind = in->VGRD_P0_L103_GLL0(centre_lat_id, centre_lon_id)
print("Wind speed at (" + centre_lat + "," + centre_lon + "): " +
sqrt(u_wind^2+v_wind^2));
end

This script got the correct index of lat_0 but not that of lon_0. It is
always -999 and I had no idea why. Is there anything I overlooked?

I am using NCL v5.1.1, and the grib file to be read can be downloaded at
http://www.grapes-trams.org.cn/Data/Download/gzlm/GRMC_GZLM_2014050100/GRMC_GZLM.2014050100000.grb2

Thanks a lot.

Regards,
Samuel Leung

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Received on Thu May 01 20:07:42 2014