NOUS41 KWBC 281900



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


           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



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


      Eric Rogers

      NCEP/Mesoscale Modeling Branch

      Camp Springs, Maryland

      301-763-8000 x7227


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