NOUS41 KWBC 271115


Public Information Statement, Comment Request

National Weather Service Headquarters Washington, DC

715 AM EDT Thu Sep 27 2012

To: Subscribers:

-Family of Services

-NOAA Weather Wire Service

-Emergency Managers Weather Information Network


Other NWS Partners, Users and Employees

From: John Derber

Acting Chief, Global Climate and Weather Modeling


NCEP/Environmental Modeling Center

Subject: Soliciting Public Comments through November 2, 2012,

on the Removal of NCEPs Legacy Global Data

Assimilation System


The National Centers for Environmental Prediction (NCEP) is

proposing to stop producing the legacy Global Data Assimilation

System (GDAS2) and discontinue all associated output products.

This proposal does not affect any output from the current Global

Data Assimilation System (GDAS1). The NWS is seeking comments on

this proposed change through November 2, 2012.


The GDAS2 are legacy products with a horizontal resolution of

triangular truncation T62 (about 208 km on 28 sigma layers).

The current operational GFS runs at a much higher horizontal and

vertical resolution (T574, about 23 km on 64 hybrid sigma-

pressure layers). GDAS2 files were originally created for

downstream applications and customers unable to handle GFS

resolutions greater than 208 km x 28 vertical levels. The GFS

moved beyond T62 in August 1987 when GFS horizontal resolution

went from R40 to T80. Horizontal resolution increased to T126

four years later (March 1991). Vertical resolution increased from

18 layers to 28 layers 2 years after this (August 1993). Since

then there has been a steady increase in GFS horizontal and

vertical resolution. The operational GFS resolution of T574L64

has been in place since July 2010. EMC is currently working on a

GFS resolution increase package for a future implementation.


The near order magnitude reduction in horizontal resolution of

GDAS2 files represents a significant loss of information in the

analysis and forecast fields with respect to GDAS1 files. Not

only do GDAS2 files have less vertical resolution (28 layers),

the vertical coordinate used in GDAS2 files (pure sigma =

terrain following) differs from that used in current operations

(hybrid sigma-pressure = terrain following near surface

transitioning to isobaric surfaces in the upper atmosphere).

Higher resolution GDAS1 products are available for all GDAS2

products and should be used by downstream applications and

customers in lieu of obsolete GDAS2 files.

The obsolete GDAS2 products and the current GDAS1 operational

products are both available at:


where YYYYMMDD is the 4-digit year, 2-digit month and 2-digit



The following files will be removed if the GDAS2 is discontinued.

The replacement GDAS1 files have the same filename, but with

gdas1 as the prefix in place of gdas2. All of these

corresponding GDAS1 files are currently available at the links

listed above.



gdas2.txxz.bfyy where yy=00,03,06,09

gdas2.txxz.pgrbanl.grib2 and gdas2.txxz.pgrbanl.grib2.idx

gdas2.txxz.pgrbfyy.grib2 where yy=00,03,06,09

gdas2.txxz.pgrbfyy.grib2.idx where yy=00,03,06,09


gdas2.txxz.sfyy where yy=00,03,06,09

gdas2.txxz.sfluxgrbfyy.grib2 where yy=00,03,06,09

gdas2.txxz.sfluxgrbfyy.grib2.idx where yy=00,03,06,09


In the filenames above, xx equals 00, 06, 12, or 18 depending on

the model cycle.


NWS will evaluate all comments to determine whether to proceed

with this change. If approved, a TIN will be issued giving 30

days notice of the implementation date.

Send comments on this proposal to:

John Derber

NWS/NCEP Environmental Modeling Center

College Park, MD


Rebecca Cosgrove

NWS/NCEP Central Operations

College Park, MD

National Public Information Statements are online at: