[nws-changes] 20120904: chg in land sfs model in global fcst sys lFwd: ADMIN NOTICE NOUS41 KWBC

• To: nws-changes@xxxxxxxxxxxxxxxx
• Subject: [nws-changes] 20120904: chg in land sfs model in global fcst sys lFwd: ADMIN NOTICE NOUS41 KWBC
• From: Linda Miller <lmiller@xxxxxxxxxxxxxxxx>
• Date: Tue, 04 Sep 2012 14:29:53 -0600

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

125
NOUS41 KWBC 041930
PNSWSH

National Weather Service Headquarters Washington DC
330 PM EDT Tue Sep 4 2012

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

          Science Plans Branch Chief
          Office of Science and Technology

          System and Associated Cool and Moist Bias in Near
          Surface Temperature and Moisture Fields.

Beginning with the 1200 Coordinated Universal Time (UTC) model
run on Wednesday, September 5, 2012, the National Centers for
Environmental Prediction (NCEP) Central Operations will implement
a fix to the Land Surface Model used to drive the Global Forecast
System (GFS).  This correction is expected to improve the cool
and moist bias in the near surface air temperature and moisture
fields during the warm season.

Starting in mid-June 2012, NCEP confirmed a problem with the GFS
near-surface temperature and moisture simulations. The model was
not verifying in the late afternoon over the central United
States when drought conditions existed. Specifically, users noted
a significant 2m cold and wet bias in both the MOS and GFS
gridded products. The NCEP Environmental Modeling Center (EMC)
traced the problem to a look-up table used in the land surface
scheme that modulates evapotranspiration based on vegetation type
and root zone depth. Current settings allowed for excessive
transpiration and plant-extraction of soil moisture from deeper
soil layers, which caused the lower atmospheric boundary layer to
become too moist and cool.

This summer, EMC conducted a parallel test of the GFS with
corrected land surface parameters. EMC found the change to the
land surface model significantly reduced the cold/wet bias over
and improved the 0-3 day precipitation forecasts.  An analysis of
the parallel test results can be viewed here:

www.emc.ncep.noaa.gov/GFS/ppt/gfs_lsm_analysis_090412_final.pptx

In addition, the Meteorological Development Lab (MDL) evaluated
the GFS MOS guidance generated from the GFS parallel output. MDL
concluded the corrections in the GFS parallel output had the
desired effect of reducing the cold/wet bias resulting in
improved bias and error scores in the GFS MOS temperature and
dewpoint guidance. Slight improvement was also noted in the GFS
MOS probability of precipitation guidance generated from the GFS
parallel output. A summary of MDLs evaluation and comparisons of
GFS MOS text bulletins generated from the GFS parallel output can
be viewed at:

http://www.mdl.nws.noaa.gov/~mos/mos/gfs_coolfix/comp_mavmex.php

NCEP and MDL also conducted a 1.5 month GFS parallel evaluation
from Jan-Feb 2012 to test the land surface model correction
during the U.S. cold season. As expected, analysis of results
indicated the change in the land surface model had little impact
on the simulated 2m temperature and dewpoint given the low solar
forcing during the winter months. MDLs analysis of the MOS
guidance from the GFS parallel during the cool season sample also
indicated minimal impact on the 2m temperature and dewpoint.

Based on the positive results of the GFS parallel tests and the
severity of the cold and wet bias in the near surface air
temperature, NCEP will implement the correction to the land
surface model beginning with the 1200 UTC cycle on Sep. 5, 2012.
This change requires no action by users to continue receiving
data.

NCEP thanks the users who alerted us to this problem. For
questions regarding the correction to the GFS Land Surface Model
and resulting improvement to the near surface cold and moist bias
please contact:

  NCEP EMC/College Park, Maryland
  301-683-3700
  Bill.Lapenta@xxxxxxxx

For questions regarding the impacts on the GFS MOS text and
gridded products please contact:

  MDL/Silver Spring, Maryland
  301-713-0023, Ext. 130
  Kathryn.Gilbert@xxxxxxxx

For questions regarding the impacts to the GFS LAMP text and
gridded products please contact:

  MDL/Silver Spring, Maryland
  301-713-0056, Ext. 194
  Judy.Ghirardelli@xxxxxxxx

Links to the MOS products and descriptions are online at:

Descriptions of previous and upcoming GFS implementations can be
found at:

  http://www.emc.ncep.noaa.gov/GFS/impl.php

NWS national TINs are online at:

$$