NOUS41 KWBC 041930
PNSWSH
Technical Implementation
Notice 12-42
National Weather Service
Headquarters Washington DC
330 PM EDT Tue Sep 4 2012
To: Subscribers:
-Family
of Services
-NOAA
Weather Wire Service
-Emergency
Managers Weather Information Network
-NOAAPORT
-Other
NWS Partners, Users and Employees
From: Tim
McClung
Science
Plans Branch Chief
Office
of Science and Technology
Subject: Change in Land Surface Model in Global Forecast
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:
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 MDL’s 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. MDL’s 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:
Bill Lapenta
NCEP EMC/College
Park, Maryland
301-683-3700
For questions regarding the
impacts on the GFS MOS text and gridded products please contact:
Kathryn Gilbert
MDL/Silver
Spring, Maryland
301-713-0023,
Ext. 130
For questions regarding the
impacts to the GFS LAMP text and gridded products please contact:
Judy Ghirardelli
MDL/Silver
Spring, Maryland
301-713-0056,
Ext. 194
Links to the MOS products and descriptions are online at:
http://www.nws.noaa.gov/mdl/synop
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:
http://www.weather.gov/os/notif.htm
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