[gradsusr] grib2map and skipping fields (time steps) inside of single record

[Sergey Varlamov]

Dear GrADS developers and experts,

My problem is that with the new version of gridmap for
grib2 files I can not skip record.fields in grib2 file that do not
fit to requested time stepping.

Working with old versions of grads (1.x) I often used
grads description files (ctl) to skip some time steps
from analysis by making custom description file
where was specified larger time step value (and smaller total
number of time steps) compared with these in the grib file(s).
Example: select only 0,3,6 etc. hours forecasts from the file
that includes 0,1,2,3,4,.. etc. hours forecasts.
Gribmap utility for grib1 files perfectly scanned such files (grib+ctl) and
generated index file where only matched records were
included and other skipped, as I expected.

But trying to do the same with the Japan Meteorological Agency (JMA)
grib2 files and gribmap for grads v 2.0.8, I met the problem that new
gribmap utility
for grib2 files stops scanning JMA files after the first non-matching
field is found:

1.11: lev1=101 var=0,3,1 valid time 2010110113:00 (t=1.33333) has
non-integer grid index

I was expecting that this field must be skipped as  NON-MATCHING
and next fields analyzed, but really all following field of analyzed record
were skipped.

As I found, this resulted from both specifics of JMA grib2 files
and design of gribmap utility:

1) JMA grib2 file has only one record where all subsequent
forecasts for all parameters are sub-records (fields in terms of gribmap).

2) Design of gribmap (gagmap.c file) is such that:
g2time_check functions return -99 (an error) and gribmap
function uses this error information to BREAK sub-records (fields) analysis
cycle, leaving all other VALID fields inside of the same grib2
record do not tested/indexed.

I would like to change BREAK statement to CONTINUE
in the fields analysis cycles of gribmap function.

Is it possible or I do not see some hidden problems of
such modification?
Could it be done in the official gribmap
utility distributed with grads (possibly with some new
option key added as it could increase analysis time and
verbose output size)?

Sincerely yours,
Sergey Varlamov

Senior Scientist
Research Institute for Global Change
JAMSTEC, JAPAN
E-mail: vsm at jamstec.go.jp