NOUS41 KWBC 281900
PNSWSH
Technical
Implementation Notice 12-32
National Weather
Service Headquarters Washington DC
255 PM EDT Thu
Jun 28 2012
To: Subscribers:
-Family of Services
-NOAA Weather Wire Service
-Emergency Managers Weather
Information Network
-NOAAPORT
Other NWS Partners, Users and
Employees
From: Timothy McClung
Chief, Science Plans Branch
Office of Science and Technology
Subject: Changes to the Calculation of Snow Cover
Fraction
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.
For questions
regarding this change, please contact:
Geoff DiMego
NCEP/Mesoscale Modeling Branch
Camp Springs, Maryland
301-763-8000 x7221
Geoff.DiMego@noaa.gov
or
Eric Rogers
NCEP/Mesoscale Modeling Branch
Camp Springs, Maryland
301-763-8000 x7227
Eric.Rogers@noaa.gov
NWS National
Technical Implementation Notices are online at:
http://www.nws.noaa.gov/os/notif.htm
$$