[nws-changes] 20120628: snow cover chgs-Fwd: ADMIN NOTICE NOUS41 KWBC

• To: nws-changes@xxxxxxxxxxxxxxxx
• Subject: [nws-changes] 20120628: snow cover chgs-Fwd: ADMIN NOTICE NOUS41 KWBC
• From: Linda Miller <lmiller@xxxxxxxxxxxxxxxx>
• Date: Thu, 28 Jun 2012 13:52:37 -0600

-------- Original Message --------

976
NOUS41 KWBC 281900
PNSWSH

Technical Implementation Notice 12-32
National Weather Service Headquarters Washington DC
255 PM EDT Thu Jun 28 2012

           -Family of Services
           -NOAA Weather Wire Service
           -Emergency Managers Weather Information Network
           -NOAAPORT
           Other NWS Partners, Users and Employees

           Chief, Science Plans Branch
           Office of Science and Technology

           and  GOES Simulated Brightness Temperature in
           the NAM Analysis and Forecast System: Effective
           August 7, 2012.

Effective on or about Tuesday, August 7, 2012, beginning with the
1200 Coordinated Universal Time (UTC) run, the National Centers
for Environmental Prediction (NCEP) will make fixes to the
calculation of snow cover fraction and the GOES Simulated
Brightness Temperatures in the North American Mesoscale (NAM)
Analysis and Forecast System.

When computing the simulated GOES brightness temperature, the
radiative transfer model in the NAM post-processor fails at land
points defined as "permanent snow/ice" (i.e., glaciers) which had
no snow cover in the NAM snow analysis. When this occurs, the
point in question has undefined brightness temperature and will
be bitmapped out in the output GRIB GOES brightness temperature
field. For example, on the 12 km CONUS grid #218, the number of
points with this problem would start at near zero at 00-h to
about 100-150 by 84-h (about 0.05% of the grid). The presence of
this bit map for brightness temperatures in this grid caused
failures in the GRIB2 unpacking utility degrib. The code has been
modified to force grid points defined as permanent snow/ice to
always have snow cover present so the radiative transfer model
will not fail and return a valid brightness temperature.

To compute snow cover fraction, the NAM post-processor uses a
snow depth threshold for snow cover of 100% at a grid point. This
threshold is different for each vegetation type. The current NAM
routine is using the old USGS vegetation type definitions, not
the new MODIS IGBP vegetation types implemented in the NAM in
October 2011. The code has been modified to use the IGBP
vegetation types so it is consistent with the forecast model.
This change will affect all NAM products which contain snow cover
fraction.

NCEP urges all users to ensure their decoders can handle changes
in content order, changes in the scaling factor component within
the product definition section (PDS) of the GRIB files, and
volume changes. These elements may change with future NCEP model
implementations. NCEP will make every attempt to alert users to
these changes before implementation.

      NCEP/Mesoscale Modeling Branch
      Camp Springs, Maryland
      301-763-8000 x7221
      Geoff.DiMego@xxxxxxxx
or
      Eric Rogers
      NCEP/Mesoscale Modeling Branch
      Camp Springs, Maryland
      301-763-8000 x7227
      Eric.Rogers@xxxxxxxx

$$