NOUS41 KWBC 301620
PNSWSH
Technical
Implementation Notice 12-24
National
Weather Service Headquarters Washington DC
1220
PM EDT Mon Apr 30 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 Surface Wind Gust Speed Calculation in
the NAM and DGEX: Effective May 30, 2012
Effective
Wednesday, May 30, 2012, beginning with the 1200 Coordinated Universal Time
(UTC) run, the National Centers for Environmental Prediction (NCEP) will make a
modification to the surface wind gust speed algorithm in the North American
Mesoscale (NAM) Analysis and Forecast System and the Downscaled GFS by NAM
Extension (DGEX).
To
compute wind gust speed, the NAM/DGEX post-processing code determines the
height of the top of the planetary boundary layer (PBL). It then determines the
wind speed at the top of the PBL and computes the difference between this wind
speed and the speed at the surface. A fraction of the wind speed difference is
mixed down to the surface and is added to the surface wind speed to give the
wind gust speed.
In the
NAM/DGEX, the wind gust algorithm uses a PBL height based on a critical value
of turbulent kinetic energy (TKE). This form of PBL height has been generated
in all NAM versions going back through WRF-NMM and the Eta Model before
it. Recent comparisons to observations
have found this TKE-based PBL height is generally too high. Examination of the wind gust fields have
shown that the wind speed gusts are generally too high as well because wind
speeds generally increase with height and the higher PBL height is causing stronger wind speeds to be mixed down. To correct
this bias, the NAM/DGEX algorithm is being changed to use PBL height based on a
critical value of the bulk Richardson number (added to NAM output in October
2011). This method produces generally lower PBL heights and hence generally lower
wind gust speeds.
This
change will affect all NAM/DGEX products which contain the surface wind gust
speed.
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 X 7221
or
Eric Rogers
NCEP/Mesoscale Modeling Branch
Camp Springs , Maryland
301-763-8000 X 7227
NWS
National Technical Implementation Notices are online at:
http://www.nws.noaa.gov/os/notif.htm
$$
|