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047
NOUS41 KWBC 281958
PNSWSH
PUBLIC INFO. STATEMENT...TECHNICAL INFORMATION NOTICE 00-07
NATIONAL WEATHER SERVICE HEADQUARTERS WASHINGTON DC
300 PM EST TUE MAR 28 2000
TO: FAMILY OF SERVICES /FOS/ SUBSCRIBERS...NOAA WEATHER
WIRE SERVICE /NWWS/ SUBSCRIBERS...OTHER NATIONAL
WEATHER SERVICE /NWS/ CUSTOMERS...NWS EMPLOYEES
FROM: LEROY SPAYD...CHIEF...SCIENCE AND TRAINING CORE
SUBJECT: CHANGES TO THE NCEP ETA ANALYSIS / FORECAST
SYSTEM...EFFECTIVE MARCH 29 2000
SEVERAL CHANGES TO THE NCEP MESOSCALE ETA ANALYSIS/FORECAST
SYSTEM WILL BE IMPLEMENTED AT 1200 UTC 29 MARCH 2000.
SUMMARY OF CHANGES
1. MODIFICATION OF MODEL CONVECTION SCHEME TO REDUCE
PRECIPITATION BIASES:
DRY OVER HIGHER TERRAIN IN WESTERN U.S.
WET OVER COASTAL REGIONS OF SOUTHERN U.S.
2. ETA 3DVAR ANALYSIS MODIFIED TO ACCOUNT FOR RAWINSONDE
AND PIBAL BALLOON DRIFT; ACTUAL BALLOON LOCATION USED
IN PLACE OF STATION LOCATION.
3. VAD WINDS REINTRODUCED IN THE ETA 3DVAR ANALYSIS WITH
NEW COMPLEX QUALITY CONTROL (CQC) ALGORITHM.
4. FORECAST TIME EXTENSION: 00Z/12Z TO 60 HOURS AND
06Z/18Z TO 48 HOURS
5. NEW PRODUCTS / CHANGES TO EXISTING PRODUCTS
A. GRIDS 212 AND 215 AVAILABLE FOR 00Z AND 12Z RUNS
B. 3-HOURLY PRECIPITATION BUCKETS FOR THE HIGHER
RESOLUTION GRIDS FROM THE 00/12Z RUNS
C. FOR ALL ETA OUTPUT GRIDS...OCTET 6 OF THE GRIB
PRODUCT GENERATION SECTION (PDS)...THE MODEL GENERATING
ID...WILL BE CHANGED FROM 89 TO 84 FOR THE 00Z AND 12Z
RUNS...AND FROM 85 TO 84 FOR 06/18Z RUNS.
D. SEA LEVEL PRESSURE COMPUTED FROM BOTH THE STANDARD
NCEP REDUCTION AND THE ETA REDUCTION ALGORITHM FOR ALL
ETA GRIDS AVAILABLE TO THE AWIPS SBN
E. STORM MOTION VECTOR COMPUTATION CHANGED FROM DAVIES
AND JOHNS TO BUNKERS METHOD.
DETAILED DESCRIPTION
MODEL CHANGES
THE BETTS-MILLER-JANJIC CONVECTION SCHEME HAS BEEN
MODIFIED TO REDUCE TWO WELL-KNOWN BIASES IN THE ETA MODEL :
TOO LITTLE PRECIPITATION OVER THE HIGHER TERRAIN IN THE
WESTERN UNITED STATES...AND TOO MUCH PRECIPITATION OVER THE
COASTAL REGIONS OF THE SOUTHERN UNITED STATES.
TO ADDRESS THE WESTERN U.S. BIAS...THE MINIMUM CLOUD
DEPTH REQUIRED FOR DEEP CONVECTIVE PROCESSES (WHICH GENERATE
PRECIPITATION) HAS BEEN CHANGED FROM A FIXED VALUE OF 290 MB
TO "200 * (SURFACE PRESSURE / 1000)" MB. OVER HIGHER TERRAIN
PRECIPITATING CONVECTIVE CLOUDS OFTEN HAVE HIGH BASES AND WOULD
NOT SATISFY THE 290 MB CRITERIA. THE PRESSURE
DEPTH USED TO SEARCH FOR THE MOST BUOYANT LAYER AT A GRID POINT
HAS BEEN CHANGED FROM THE LOWEST 130 MB TO THE LOWEST
"200 * (SURFACE PRESSURE / 1000)" MB.
TO ADDRESS THE COASTAL U.S. BIAS...THE USE OF DRIER RH
REFERENCE PROFILES OVER LAND AND MOISTER RH PROFILES OVER WATER
HAS BEEN ELIMINATED...AND REPLACED BY SEA PROFILES EVERYWHERE. BY
ELIMINATING THIS DISTINCTION...AIR PARCELS WITH A LONG OVER-WATER
FETCH WILL NOT REACT TO THE DRIER RH PROFILES OVER LAND BY
PRODUCING EXCESSIVE PRECIPITATION ALONG THE COAST.
ANALYSIS/OBSERVATION CHANGES
THE ETA 3DVAR ANALYSIS HAS BEEN MODIFIED SO THAT RAWINSONDE
AND PIBAL DATA ARE ANALYZED AT THE LOCATION OF THE DRIFTING
BALLOON INSTEAD OF THE STATION LOCATION. THE HORIZONTAL DRIFT
IS CALCULATED ON EACH VALID WIND AND HEIGHT LEVEL BASED ON THE
MEAN VECTOR WIND IN THE LAYER BENEATH...AND THE ESTIMATED TIME
OVER WHICH THE BALLOON TRAVERSED THIS LAYER. THE DRIFT TIME IS
CALCULATED FROM THE LAYER THICKNESS AND AN ASSUMED ASCENT RATE
OF 5 METERS PER SECOND.
VAD WINDS...ABSENT FROM THE 3DVAR ANALYSIS SINCE JANUARY
1999 DUE TO NUMEROUS QUALITY CONTROL PROBLEMS...WILL BE
REINTRODUCED THROUGH APPLICATION OF A NEW VAD COMPLEX QUALITY
CONTROL (CQC) ALGORITHM. THIS ALGORITHM IS DESIGNED TO REMOVE
WINDS WITH VERY SMALL MAGNITUDE (ABOUT 8% OF ALL WINDS)..."ROUGH"
ERRORS WITH VERY LARGE MAGNITUDE (7%)...AND RADAR SIGNAL "ERRORS"
FROM MIGRATING BIRDS (5% OF DATA DURING MIGRATION SEASONS).
EXTENSION OF 00Z/12Z FORECAST TO 60-H AND 06Z/18Z
FORECAST TO 48-H
THE ETA ANALYSIS/FORECAST SYSTEM HAS BEEN RUN TO 48-H
FOR ALL FOUR CYCLE TIMES (00Z...06Z...12Z...18Z) SINCE
IMPLEMENTATION ON THE IBM-SP IN NOVEMBER 1999. HOWEVER...THE 06Z
AND 18Z CYCLES WERE ONLY PROCESSED TO 33-H AND
30-H...RESPECTIVELY FOR DISTRIBUTION TO THE AWIPS SATELLITE
BROADCAST NETWORK (SBN)...REFLECTING THE FORECAST LENGTH OF THE
PRE-IBM OFF-TIME RUNS. WITH THIS IMPLEMENTATION...PROCESSING OF
THE 06/18Z FORECASTS WILL BE DONE TO 48-H AND THE FORECASTS WILL
BE DISSEMINATED TO AWIPS VIA THE SBN. ADDITIONALLY...THE 00/12Z
FORECASTS WILL BE
EXTENDED FROM 48-H TO 60-H.
- NEW PRODUCTS / CHANGES TO EXISTING PRODUCTS
CURRENTLY...THE 06Z AND 18Z ETA OUTPUT SENT TO THE AWIPS SBN
IS ON GRID 212 (40-KM RESOLUTION) AND GRID 215 (20-KM
RESOLUTION). THESE GRIDS WILL NOW BE AVAILABLE FROM THE
00Z AND 12Z ETA RUNS...IN ADDITION TO THE LOWER RESOLUTION OUTPUT
(GRIDS 104...207...211) CURRENTLY AVAILABLE. THE FORECAST
PRECIPITATION BUCKETS FOR THE HIGHER RESOLUTION GRIDS FROM THE
00/12Z ETA RUNS WILL BE 3-HOURLY AS THEY ARE FOR THE 06/18Z ETA.
THE LOW RESOLUTION GRIDS (207...211...104) WILL CONTINUE TO HAVE
12-H PRECIPITATION BUCKETS.
FOR ALL ETA OUTPUT GRIDS...OCTET 6 OF THE GRIB PRODUCT
GENERATION SECTION (PDS)...THE MODEL GENERATING ID...WILL
BE CHANGED FROM 89 TO 84 FOR THE 00Z AND 12Z RUNS...AND FROM
85 TO 84 FOR 06/18Z ETA RUNS. THE ONE EXCEPTION TO THIS
IS THE INTERACTIVE COMPUTER WORDED FORECASTS (ICWF) GRIDS
PRODUCED FROM GRID 212 OUTPUT FOR THE 00/12Z ETA RUNS; THEY
WILL CONTINUE TO USE 89 AS THE MODEL GENERATING ID.
ALL ETA GRIDS AVAILABLE TO THE AWIPS SBN WILL NOW CONTAIN
THE SEA LEVEL PRESSURE COMPUTED FROM THE STANDARD NCEP
REDUCTION IN ADDITION TO THE SLP FROM THE ETA REDUCTION
ALGORITHM.
THE METHOD TO COMPUTE STORM MOTION VECTORS IN THE ETA
MODEL HAS BEEN CHANGED TO USE THE BUNKERS DYNAMIC METHOD.
THE PREVIOUSLY USED DAVIES AND JOHNS METHOD WAS BASED ON
A CLIMATOLOGY OF CENTRAL AND EASTERN U.S. SUPERCELL
THUNDERSTORMS. THE BUNKERS METHOD IS MORE PHYSICALLY-BASED...
USING THE INFLUENCE OF THE MEAN WIND ON THE INITIAL CELL
MOTION AND THE INTERACTIONS OF THE CONVECTIVE UPDRAFT
WITH THE ENVIRONMENT.
THE DRAFT NWS TECHNICAL PROCEDURES BULLETIN (#465) DESCRIBING
THESE CHANGES IN MORE DETAIL CAN BE FOUND AT
FTP://FTP.NCEP.NOAA.GOV/PUB/EMC/WD20ER/ETATPB/ETA60TPB.HTM
/LOWER CASE/
TECHNICAL DETAILS REGARDING IMPLEMENTATION CAN BE FOUND AT
RC # 2453 ON THE WEB PAGE
HTTP://WWW.NWS.NOAA.GOV/OSO/OSO1/OSO11/OSO112/DRG/DRGRPTY.HTM
/LOWER CASE/
END
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