Global Models corrections-NOUS (fwd)

602 
NOUS41 KWBC 271621
PNSNMC
 
NATIONAL TECHNICAL INFORMATION MESSAGE 96-18
NATIONAL WEATHER SERVICE HEADQUARTERS
100 PM EDT THU JUN 27 1996
 
 
TO:       USERS OF NCEP GLOBAL SPECTRAL MODEL (AVN AND MRF) 
 
FROM:     JOSEPH R. BOCCHIERI
          CHIEF... SCIENCE AND TRAINING CORE
 
SUBJECT:  CORRECTIONS TO THE GLOBAL MODELS 
 
(THE FOLLOWING INFORMATION WAS PROVIDED BY HUA-LU PAN, NCEP -
ENVIRONMENTAL PREDICTION CENTER)
 
THE OCTOBER 20..1995 IMPLEMENTATION OF CHANGES TO THE GLOBAL
SPECTRAL MODEL (MRF/AVN) (NTIM 95-46) LED TO FREQUENT
FORECASTS OF PRECIPITATION BULLS-EYE PATTERNS IN THE TROPICS.
THIS RAISED SOME CONCERN WITH TROPICAL PREDICTION CENTER (TPC)
FORECASTERS EVEN THOUGH THE NEW VERSION OF THE MRF/AVN HAD
SIGNIFICANTLY IMPROVED FORECASTS IN THE TROPICS (ESPECIALLY
THE TROPICAL STORM TRACK FORECASTS AND THE GFDL HURRICANE
MODEL FORECASTS).  THE PROBLEM WAS DIAGNOSED AS DERIVING FROM
THE OCTOBER 1995 CHANGES IN THE PARAMETERIZATION OF DEEP
CONVECTION AND IN THE PLANETARY BOUNDARY LAYER (PBL) SCHEME. 
ON JUNE 5..1996 AT 1200 UTC... CORRECTIONS WERE MADE TO THOSE
PORTIONS OF THE MRF/AVN AS FOLLOWS:
 
1.)  PARAMETERIZATION OF DEEP CONVECTION - TOO MUCH CUMULUS
DOWNDRAFT WAS ALLOWED TO REACH THE SURFACE.  OVER LAND... THE
COOLING CUTS DOWN THE AMOUNT OF CONVECTION.  OVER WATER...
BECAUSE OF THE FIXED SEA SURFACE TEMPERATURE... THE REVERSE
HAPPENS AND WE GET MUCH ENHANCED EVAPORATION AND SENSIBLE HEAT
FLUX.  THIS FEEDBACK WAS DIMINISHED BY REDUCING THE PORTION OF
THE DOWNDRAFT ALLOWED TO REACH GROUND.
     
2.)  PLANETARY BOUNDARY LAYER SCHEME - THE PLANETARY BOUNDARY
LAYER (PBL) PARAMETERIZATION WAS SLIGHTLY TOO ACTIVE IN THE
TRANSPORT OF MOISTURE TO THE TOP OF THE PBL.  TO CORRECT
THIS... THE COUNTER-GRADIENT TERM IN THE MOISTURE TRANSPORT
WAS REMOVED.
     
     IN TESTS ON DATA FOR AUGUST-SEPTEMBER... 1995 AND APRIL
1996... THE NORTH AMERICA PRECIPITATION SCORES AND THE
HEMISPHERIC SCORES WERE INDISTINGUISHABLE FROM THE CONTROL
SCORES OTHER THAN THE REDUCTION OF THE PRECIPITATION BULLS-EYE
PATTERNS IN THE TROPICS.  SUBJECTIVE EVALUATIONS ABOUT THE
CHANGE BY BOTH THE TPC AND THE HYDROLOGIC PREDICTION CENTER~S
INTERNATIONAL DESK WERE POSITIVE.
     
END