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#! /bin/sh
set -efu
prefix=/tmp/CWAT
grib_path=$prefix.grib
json_path=$prefix.json
pgm_path=$prefix.pgm
png_path=$prefix.png
date=20191210
# Possible values for hour: 00 06 12 18
hour=12
url="https://nomads.ncep.noaa.gov/cgi-bin/filter_gfs_0p25.pl?file=gfs.t12z.pgrb2.0p25.anl&lev_entire_atmosphere_%5C%28considered_as_a_single_layer%5C%29=on&var_CWAT=on&leftlon=0&rightlon=360&toplat=90&bottomlat=-90&dir=%2Fgfs.$date%2F$hour"
curl -fsS "$url" > "$grib_path"
grib2json --data "$grib_path" > "$json_path"
width=$(jq '.[0].header.nx' < "$json_path")
height=$(jq '.[0].header.ny' < "$json_path")
# The maximum gray value. Must be bigger than 0 and less than 65536.
maxval=256
# pgm - Netpbm grayscale image format
# http://netpbm.sourceforge.net/doc/pgm.html
{
echo P2
echo "$width $height"
echo "$maxval"
cat "$json_path" |
jq --argjson maxval "$maxval" -c '
.[0].data[] * $maxval | round
' |
xargs -n "$width"
} > "$pgm_path"
convert "$pgm_path" "$png_path"
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