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NOUS41 KWBC 241231
PNSNMC
NATIONAL TECHNICAL INFORMATION MESSAGE 96-20
NATIONAL WEATHER SERVICE HEADQUARTERS WASHINGTON DC
845 AM EDT WED JUL 24 1996
TO: USERS OF NCEP ETA MODELS
FROM: JOSEPH R. BOCCHIERI
CHIEF... SCIENCE AND TRAINING CORE
SUBJECT: CHANGES TO THE ETA MODEL
(THE FOLLOWING INFORMATION WAS PROVIDED BY TOM BLACK...
NCEP... ENVIRONMENTAL PREDICTION CENTER)
TWO CHANGES TO THE EARLY ETA ARE BEING IMPLEMENTED AT 1200 UTC
ON WEDNESDAY... JULY 24.
THE FIRST CHANGE IS THE CORRECTION OF AN ERROR IN INITIAL SOIL
MOISTURE. THE GLOBAL SPECTRAL MODEL PROVIDES THE ETA'S
ASSIMILATION SYSTEM (EDAS) WITH ITS INITIAL SOIL MOISTURE
FIELD. TO COMPENSATE FOR A TENDENCY FOR THE GLOBAL VALUES
TO BE TOO HIGH... NO INCOMING VALUE TO THE EDAS WAS PERMITTED
TO EXCEED A SPECIFIED AMOUNT. THIS TRUNCATION WAS INCORRECTLY
APPLIED IN A MULTIPLE FASHION... LEADING TO TOO LITTLE SOIL
MOISTURE IN REGIONS WHERE THE AMOUNTS SHOULD HAVE BEEN LARGER.
A SINGLE TRUNCATION OF THE VALUE COMING FROM THE GLOBAL IS
NOW BEING DONE.
THE SECOND CHANGE INVOLVES ONE OF THE QUANTITIES USED BY THE
MODEL'S GROUND SURFACE SCHEME... NAMELY... THE GREEN
VEGETATION FRACTION WHICH INFLUENCES THE AMOUNT OF WATER VAPOR
ENTERING THE ATMOSPHERE FROM THE GROUND. RECENT DATA
INDICATED THAT THE VALUES BEING USED IN THE MODEL WERE
GENERALLY TOO SMALL AND THUS THE ENTIRE FIELD HAS BEEN
INCREASED BY 1/.85 OR 17.6% IN ORDER TO BETTER AGREE WITH THE
OBSERVATIONS.
END
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