[conduit] Difference Tables for NAM/NDAS versus GFS/GDAS

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  • Subject: [conduit] Difference Tables for NAM/NDAS versus GFS/GDAS
  • From: Geoff DiMego <Geoff.Dimego@xxxxxxxx>
  • Date: Wed, 02 Jan 2008 15:48:19 -0500
Folks; [apologies to repeat recipients - let me know if you wish to be
taken off my distribution lists for occasional updates related to NAM]

We seem to be at that point again when many of our users have forgotten
what are the differences between NAM and GFS and between their
respective data assimilation systems, NDAS and GDAS.  This comes up
every time there is a big difference between the two solutions (I hear
some of you asking "... like this doesn't happen EVERY day?")  People
then want to know why and I'm sure most everyone has an opinion and I've
heard quite a few.  Sorry to say, many of them are just not accurate or
are based on an unwarranted assumption.  So here, in all their glory,
are ALL the differences laid out in table form on the web at
http://wwwt.emc.ncep.noaa.gov/mmb/mmbpll/misc/NAM+GSI-vs-GFS+GSI.html .

Table 1 has a column labeled "Priority" in which I've listed a year when
we hope to have brought the two systems together in the particular
attribute or when we have ruled it out as a contributor.

Table 2 lists the model differences and this table has no Priority
column.  This is due to the fact most of our modeling efforts in the
next couple of years will be devoted to two major projects: 1) tuning
the 'explicit' 4 km nested version of the NMM for inclusion in NAM in
the 2009-2010 time frame and b) converting over to the NCEP
Environmental Modeling System (NEMS) that Steve Lord detailed at the
last two NCEP Production Suite Reviews (see

As I mentioned in my presentation at the NCEP Production Suite Review,
we continue to see situations of poor NAM forecasts being improved
when we rerun them using GFS initial conditions.  We have examined Table
1 or ones like it for years trying to isolate the critical differences
between NDAS and GDAS.  Even now, though both use the same analysis code
in the Gridpoint Statistical Interpolation (GSI), we are still seeing
cases where rerunning with GFS initial conditions improves the NAM
forecast.  Please remember it is not possible to run NAM off GFS initial
conditions in the Production Suite at this time.  It is, however, our
intention to evolve towards a Production Suite where NAM and GFS are run
concurrently as Steve stated.

This Winter we have decided to run a near-real-time parallel where we
return to a partial-cycling strategy in the parallel NDAS.  In this we
use GFS as the source of atmospheric fields at the T-12 starting point
of each NDAS but continue to use the cycled land-surface states of the
previous NDAS.  This is running now as NAMY and can be seen at Eric
Rogers' page http://www.emc.ncep.noaa.gov/mmb/mmbpll/nampll_namy/ . We
are using the NAMEXP as the NAMY's control.  The NAMEXP is the new
NAM/NDAS upgrade bundle being tested for a planned February 2008
implementation.  See my remarks at
or check out the parallel change logs for information on the NAMEXP.
TIN 07-96 has already been issued on this upgrade bundle and can be
found at http://www.nws.noaa.gov/om/notif.htm .  This partial cycling
approach could be run in current production suite and we will consider
it for the next NAM/NDAS bundle in late Fall 2008 if we see consistently
higher scores.  However, we have not given up on the idea of independent
NAM cycling and we have GSI development resources working on several of
the items in Table 1.  Please don't give up on us yet.

As usual, if you have any questions or comments - I'm all ears.

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