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HOME > Monitoring_and_Data > Oceanic and Atmospheric Data > Reanalysis: Atmospheric Data > wgrib2-merge_fcst

wgrib2: -merge_fcst

Introduction

The -merge_fcst option will try to combine adjacent messages by increasing the time period for forecast average, accumulation, minimum and maximum. For example,

$ wgrib2 prate.l.gdas.201404.ts -for 1:4
1:0:d=2014040100:PRATE:surface:0-1 hour ave fcst:
2:13083:d=2014040100:PRATE:surface:1-2 hour ave fcst:
3:26587:d=2014040100:PRATE:surface:2-3 hour ave fcst:
4:40437:d=2014040100:PRATE:surface:3-4 hour ave fcst:

$ wgrib2 preas.201404.ts -for 1:4 -merge_fcst 4 /tmp/all.grb
(listing of input)

$ wgrib2 /tmp/all.grb 
1:0:d=2014040100:PRATE:surface:0-4 hour ave fcst:

$wgrib2 prate.l.gdas.201404.ts -for 1:4 -merge_fcst 2 /tmp/all2.grb
(iisting of input)

$ wgrib2 /tmp/all2.grb 
1:0:d=2014040100:PRATE:surface:0-2 hour ave fcst:
2:29531:d=2014040100:PRATE:surface:2-4 hour ave fcst:

The -merge_fcst option needs the fields to be processed in order. Fields that are not fcst averages/accumlations are ignored. In the previous example, the input data was a time series of one varianble, and the fields were in order. The more common case is that you have the fhr06 (0-6 hour forecast), fhr12 (6-12 hour forecast), fhr18 (12-18 hour forecast) and fhr24 (18-24 hour forecast) files and you want the 0-24 hour mean. Each of the forecast files contain hundreds of fields.

$ cat fhr06 fhr12 fhr18 fhr24 | \
   wgrib2 - -set_grib_type c1 -set_grib_max_bits 20 -set_bin_prec 20 \
   -if ":PRATE:" -merge_fcst 4 OUT \
   -if ":APCP:" -merge_fcst 4 OUT \
   -if ":ACPCP:" -merge_fcst 4 OUT \
   -if ":BGRUN:" -merge_fcst 4 OUT

The above command works by

The first line writes the grib to stdout in chrnological order
The second line has wgrib2 read from stdin and sets the packing and precision
The third line process the PRATE, the -merge_fcst sees the data in the proper order
The 4-6 lines process the APCP, ACPCP and BGRUN fields

You can use the old and slow way which is

$ cat fhr06 fhr12 fhr18 fhr24 > IN.grb
$ wgrib2 IN.grb | sort -t: -k4,4 -k5,5 -k6,6n | wgrib2 -i IN.grb -grib OUT.grb
$ wgrib2 OUT.grb -merge_fcst 4 OUT

Usage

-merge_fcst (N=number fields to combine) (output grib file)

When N > 0, then N fields are merged together and then written to the output grib file
   (requires wgrib2 v2.0.1)
When N == 0, then there is no limit to the number of fields to be merged
             and intermediate steps are written out
              ex. 0-1 + 1-2 + 2-3 -> 0-1, 0-2, 0-3

The most common use would be get the 12-hour, daily, N-day average precipation forecasts from the N hour to N+6 hour precipitation forecasts.

Restrictions

Only 1 time range specificaion is allowed in the PDS (no nested time ranges)
Forecast time units must be the same between message to be merged.
The various metadata in the grib messages to be merged must be identical except for the averaging/accumulation interval.