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288
NOUS41 KWBC 201934
PNSNMC
NATIONAL TECHNICAL INFORMATION MESSAGE 95-46
NATIONAL WEATHER SERVICE HEADQUARTERS
1800 PM EDT FRI OCT 20 1995
TO: USERS OF NT4S GLOBAL SPECTRAL MODELS (AVN
AND MRF)
FROM: JOSEPH R. BOCCHIERI...
CHIEF... SERVICES DEVELOPMENT BRANCH
SUBJECT: CHANGES TO THE GLOBAL MODELS
REFERENCE: CHANGES TO THE 1995 NCEP OPERATIONAL MRF
MODEL ANALYSIS/FORECAST SYSTEM
(THE FOLLOWING INFORMATION WAS PROVIDED BY PETER
CAPLAN... NCEP... ENVIRONMENTAL MODELING CENTER)
I. SYSTEM CHANGES: AT 1200 UTC ON 25 OCTOBER 1995...
CHANGES IN THE NCEP GLOBAL SPECTRAL FORECAST MODEL AND
ANALYSIS SYSTEM WILL BE MADE. THESE CHANGES APPLY TO
BOTH THE MRF RUN ITSELF AND TO THE 0000 UTC AND 1200 UTC
AVIATION (AVN) RUNS OF THE MRF MODEL. IN THE ANALYSIS
SYSTEM THERE IS A MAJOR CHANGE - THE USE OF SATELLITE
RADIANCES INSTEAD OF TEMPERATURE RETRIEVALS - AND TWO
MINOR CHANGES: A CONSTRAINT ON THE INCREMENT OF THE
DIVERGENCE AND THE USE OF NEAR-SURFACE WINDS DERIVED FROM
ERS-1 SATELLITE SCATTEROMETER OBSERVATIONS. IN THE
FORECAST SYSTEM... THERE ARE TWO AREAS OF CHANGE:
BOUNDARY LAYER PARAMETERIZATION AND CONVECTION
PARAMETERIZATION.
II. EVALUATION OF CHANGES: THE NEW MODEL WAS FOUND TO
PRODUCE MORE ACCURATE FORECASTS OF GEOPOTENTIAL HEIGHTS
THROUGHOUT THE TROPOSPHERE IN BOTH HEMISPHERES... WITH
MARKED IMPROVEMENT NOTED IN THE SOUTHERN HEMISPHERE. THE
PRECIPITATION FORECASTS OVER THE U.S. (EVALUATED ONLY IN
SUMMER) SHOWED A SMALL OVERALL IMPROVEMENT IN SKILL.
THERE WAS A REDUCTION IN THE AREA COVERED BY SMALL
AMOUNTS (LESS THAN .25-.50 INCHES/24 HOURS) AND A
TENDENCY TO INCREASE THE HIGHER AMOUNTS AND PROVIDE MORE
DETAIL IN HEAVY PRECIPITATION AREAS. IN THE TROPICS...
THE SAME CHANGE IN PRECIPITATION DISTRIBUTION WAS
NOTED... ALONG WITH A TENDENCY TO PRODUCE STRONGER SMALL-
SCALE CIRCULATION SYSTEMS AND OCCASIONAL SPURIOUS BULLS-EYES;
ESPECIALLY IN CONVECTIVE SITUATIONS. THE LARGE-SCALE
CIRCULATIONS WERE MORE ACCURATELY FORECAST IN THE NEW SYSTEM.
III. DESCRIPTION OF CHANGES:
A. RADIANCES IN THE ANALYSIS: THE TOVS NOAA-12 SATELLITE
RADIANCES ARE NOW USED DIRECTLY IN THE NEW ANALYSIS
SYSTEM. NEITHER CONVENTIONAL NOR INTERACTIVE SATELLITE
TEMPERATURE RETRIEVALS ARE USED.
B. ERS-1 WINDS: NEAR-SURFACE WIND VECTORS DERIVED FROM
SCATTEROMETER OBSERVATIONS FROM THE ERS-1 POLAR-ORBITING
SATELLITE HAVE BEEN ADDED TO THE DATA BASE.
C. DIVERGENCE CONSTRAINT IN THE ANALYSIS: THE VERTICAL
INTEGRAL OF THE DIVERGENCE TENDENCY INCREMENT WAS SET TO
ZERO FOR THE LARGEST SCALES TO REMOVE SMALL ERRORS IN THE
SEMI-DIURNAL ATMOSPHERIC TIDE.
D. PLANETARY BOUNDARY LAYER SCHEME: INSTEAD OF
CALCULATING DIFFUSION COEFFICIENTS ON THE BASIS OF LOCAL
VERTICAL GRADIENTS OF WIND AND TEMPERATURE... THIS SCHEME
ATTEMPTS TO REPRESENT THE GRADIENTS OVER THE ENTIRE
PLANETARY BOUNDARY LAYER. THIS HELPS TO PREVENT TRAPPING
OF SURFACE MOISTURE. ANOTHER CHANGE... WHICH IMPROVES
THE RESPONSE OF THE BOUNDARY LAYER TO CLOUDS... IS THE
UPDATING OF SHORT-WAVE RADIATION EVERY HOUR INSTEAD OF
EVERY THREE HOURS.
E. CONVECTION: THE ADJUSTMENT OF THE CAPE (ATMOSPHERIC
CONVECTIVE AVAILABLE POTENTIAL ENERGY) BY CONVECTION HAS
BEEN MODIFIED. FORMERLY... THE CONVECTION CONTINUED ONLY
UNTIL THE CAPE REACHED TOWARD A CLIMATOLOGICAL (NON-ZERO)
VALUE UNDER ALL CONDITIONS... WHICH PRODUCED INSUFFICIENT
PRECIPITATION SPREAD OVER TOO WIDE AN AREA. THE NEW
VERSION PRODUCES STRONGER PRECIPITATION IN DISTURBED
AREAS (AREAS HAVING LARGE-SCALE UPWARD MOTION AT CLOUD
BASE) BY ALLOWING CONVECTION TO PROCEED UNTIL THE CAPE
REACHES ZERO.
END
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