[nws-changes] 20120501: chgs to surface wind gust speed calc Fwd: ADMIN NOTICE NOUS41 KWBC

• To: nws-changes@xxxxxxxxxxxxxxxx
• Subject: [nws-changes] 20120501: chgs to surface wind gust speed calc Fwd: ADMIN NOTICE NOUS41 KWBC
• From: Linda Miller <lmiller@xxxxxxxxxxxxxxxx>
• Date: Tue, 01 May 2012 11:51:36 -0600

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

342
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
      Geoff.DiMego@xxxxxxxx
or
      Eric Rogers
      NCEP/Mesoscale Modeling Branch
      Camp Springs , Maryland
      301-763-8000 X 7227
      Eric.Rogers@xxxxxxxx


NWS National Technical Implementation Notices are online at:

   http://www.nws.noaa.gov/os/notif.htm

$$