TROPICAL CYCLONE TRACK PREDICTION REF: DEMARIA, M., 1997: SUMMARY OF THE TPC/NHC TC TRACK AND INTENSITY GUIDANCE MODELS. INFORMAL REFERENCE, AVAILABLE ON THE INTERNET AT http://www.nhc.noaa.gov/ aboutmodels.html Presented by Richard Pasch, NHC 16 December 1999
TROPICAL CYCLONE TRACK PREDICTION. Presented by Richard Pasch, NHC 16 December 1999. REF: DEMARIA, M., 1997: SUMMARY OF THE TPC/NHC TC TRACK AND INTENSITY GUIDANCE MODELS. INFORMAL REFERENCE, AVAILABLE ON THE INTERNET AT http://www.nhc.noaa.gov/aboutmodels.html. - PowerPoint PPT Presentation
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
TROPICAL CYCLONE
TRACK PREDICTION
TROPICAL CYCLONE
TRACK PREDICTION
REF: DEMARIA, M., 1997: SUMMARY OF THE TPC/NHC TC TRACK AND INTENSITY GUIDANCE MODELS. INFORMAL REFERENCE, AVAILABLE ON THE INTERNET AT
http://www.nhc.noaa.gov/aboutmodels.html
Presented by Richard Pasch, NHC16 December 1999
TRACK FORECASTING
• GUIDANCE MODELS
• USE OF INITIAL MOTION, CLIMATOLOGY, AND PERSISTENCE
• CONTINUITY FROM PREVIOUS FORECAST
• SYNOPTIC FLOW & EXAMPLES
• NON-METEOROLOGICAL FACTORS
• OFFICIAL FORECAST SKILL
• GUIDANCE MODELS
• USE OF INITIAL MOTION, CLIMATOLOGY, AND PERSISTENCE
• CONTINUITY FROM PREVIOUS FORECAST
• SYNOPTIC FLOW & EXAMPLES
• NON-METEOROLOGICAL FACTORS
• OFFICIAL FORECAST SKILL
Factors Affecting TC Motion
• Zero Order - Cons. of relative vorticity– Vortex Moves with “Steering Flow”
• First Order - Cons. of absolute vorticity– Vortex induces beta-gyres and affects motion
• General Model– Vertical structure is important– Interaction with orography, friction, convection
Track Guidance Models
• Zero Order - CLIPER, NHC90/91/98
• First Order - BAM, LBAR, VICBAR
• General Models - GFDL, AVN/MRF, NOGAPS, UKMET, ECMWF, ETA, NGM
• Required Input: – Current/12 h old speed/direction of motion– Current latitude/longitude– Julian Day, Storm maximum wind
• Avg. 24, 48, 72 h errors: 210, 450, 650 km
• Used as benchmark for other models; forecasts having errors greater than CLIPER are considered to have no skill.
CLIPER EXAMPLE: HURRICANE DANIELLE, 1998
INITIAL12-HR FORECAST
24-HR FORECAST
36-HR FORECAST
48-HR FORECAST
72-HR FORECAST
NHC90 (Atlantic), NHC91 (East Pacific)
• Statistical/Dynamical track model
• Required Input – CLIPER forecast tracks– Analyzed and forecast deep layer mean heights (1000-
100 mb) from NCEP or UKMET global model
• Input used as predictors for TC motion (along- & cross-track components) in a multiple regression
• New version for Atlantic with vortex removal scheme (NHC98)
Typical Predictor Locations for NHC90
THE SIMPLIFIED STEERING (OR TRAJECTORY) MODEL, BAM (BETA AND ADVECTION MODEL)
-USES STEERING (TRAJECTORY) GIVEN BY LAYER-AVERAGED WINDS FROM AVN MODEL (HORIZONTALLY SMOOTHED TO T25 RESOLUTION), PLUS A CORRECTION TERM TO SIMULATE THE SO-CALLED “BETA EFFECT”
-THREE DIFFERENT LAYER AVERAGES:
SHALLOW (850-700 MB) - BAMS
MEDIUM (850-400 MB) - BAMM
DEEP (850-200 MB) - BAMD
THE BETA DRIFT:
THE CIRCULATION OF A TROPICAL CYCLONE, COMBINED WITH THE NORTH-SOUTH VARIATION OF THE CORIOLIS PARAMETER, INDUCES ASYMMETRIES KNOWN AS BETA GYRES.
THESE GYRES PRODUCE A NET STEERING CURRENT ACROSS THE TC, GENERALLY TOWARD THE NW AT A FEW KNOTS. THIS MOTION HAS COME TO BE KNOWN AS THE BETA DRIFT.
LOWER VALUES OF EARTH’S VORTICITY
HIGHER VALUES OF EARTH’S VORTICITY
v>0v<0
N
H
L
INDUCED STEERING
LBAR (Limited-area BARotropic)
• Barotropic dynamics, i.e. 2-d motions• Shallow water equations on Mercator
projection solved using sine transforms• Initialized with 850-200 mb average
winds/heights from NCEP global model• Sum of idealized vortex and current motion
vector added to large-scale analysis• Boundary conditions from global model
4) GFDL (MULTIPLY-NESTED, LIMITED AREA; SPECIFICALLY DESIGNED FOR TC PREDICTION)
5) ECMWF
THESE MODELS INCLUDE DIABATIC PROCESSES AND BOUNDARY LAYER EFFECTS
6) ETA, NGM (LIMITED AREA GRID-POINT)
EXPLICIT USE OF GLOBAL MODELS FOR TC PREDICTION
• USUALLY EMPLOY A “BOGUSSING” SCHEME TO INITIALIZE THE TC VORTEX (SYNTHETIC DATA SYSTEM)
• MODEL RESOLUTION IS INADEQUATE TO DEFINE THE INNER CORE (EYE, EYEWALL, RADIUS OF MAXIMUM WINDS)
• GLOBAL MODEL HAS NO LATERAL B.C.; GOOD FOR LONGER-RANGE GUIDANCE
THE BOGUSSING OF THE TC INTO THE GLOBAL MODELS USES INITIAL CONDITIONS AS PROVIDED BY THE HURRICANE SPECIALIST FOR EACH SYNOPTIC TIME:
-LOCATION, INTENSITY, CENTRAL PRESSURE, RADIUS OF OUTERMOST CLOSED ISOBAR
-RADII OF 34-, 50-KT AND MAXIMUM WINDS
-CYCLONE DEPTH
THE AVN RUN OF THE MRF PROVIDES INITIAL AND BOUNDARY CONDITIONS FOR THE GFDL MODEL (HOWEVER THE BOGUS VORTEX HAS TO BE REMOVED IN THE GFDL INITIALIZATION PROCEDURE)
AVN 1000 mb Initial Wind/Vorticity 9/21/98 00 UTC (X indicates observed center of Hurricane Georges)
X X
HURRICANE FLOYD 9/9/99 1200 UTC
HURRICANE FLOYD 24 H FCST FROM 9/9/99 1200 UTC
HURRICANE FLOYD 48 H FCST FROM 9/9/99 1200 UTC
GLOBAL MODELS SUCH AS THE NOGAPS (NAVY OPERATIONAL GLOBAL ATMOSPHERIC PREDICTION SYSTEM) AND THE UKMET (UNITED KINGDOM METEOROLOGICAL ) OFFICE MODELS, ALSO USE A BOGUSSING TECHNIQUE TO SPECIFY THE TC CIRCULATION, BUT THE METHOD IS DIFFERENT THAN THAT USED FOR THE AVN / MRF.
CURRENTLY, THE ECMWF (EUROPEAN CENTRE FOR MEDIUM-RANGE WEATHER FORECASTS) GLOBAL MODEL DOES NOT HAVE A TC BOGUS (ALTHOUGH THIS MODEL IS CAPABLE OF SIMULATING SOME (TYPICALLY THE STRONGER) TCs.
THE GEOPHYSICAL FLUID DYNAMICS LABORATORY (GFDL) MODEL:
ALSO KNOWN AS THE MMM (MULTIPLY-NESTED MOVABLE MESH MODEL), IT USES THREE NESTED GRIDS.
OVERALL DOMAIN IS 75° LONGITUDE x 75° LATITUDE; THE INNERMOST NESTED GRID HAS A RESOLUTION OF 1/ 6 °.
SINCE THE RESOLUTION IS HIGH ENOUGH TO SIMULATE SOME OF THE INNER TC STRUCTURE, THIS MODEL HAS SOME SKILL IN INTENSITY PREDICTION.
GFDL MODEL (CONTINUED):
THE INITIALIZATION SCHEME ATTEMPTS TO MORE ACCURATELY SPECIFY THE TC CIRCULATION, AS WELL AS OTHER DISTURBANCES IN THE TC ENVIRONMENT THAT COULD INFLUENCE THE TRACK AND INTENSITY.
FIRST, THE BOGUS VORTEX IS REMOVED FROM THE AVN MODEL USING A FILTER.
THE SPECIFIED TC VORTEX IS OBTAINED FROM A SEPARATE MODEL WHICH IS “NUDGED”TOWARD THE OBSERVED INITIAL CONDITIONS (INTENSITY, WIND RADII) AS SPECIFIED BY THE HURRICANE FORECASTER.
GFDL MODEL (CONTINUED):
INITIALIZATION
initial field
= global analysis - globally analyzed NCEP vortex
+ specified GFDL vortex
global analysis = basic field + disturbance field
disturbance field = globally analyzed NCEP vortex
+ nonhurricane component
environmental field = basic field
+ nonhurricane component
Vortex from global Analysis
Vortex from spin-upprocedure
(=p/ps) LEVELS OF THE GFDL MODEL
Inner Mesh of GFDL Model (Hurricane Georges 1998)
Outer Mesh of GFDL Model (Hurricane Georges 1998)
SAMPLE 10 M WIND SWATH FROM GFDL MODEL FORECAST
GFDL MODEL CAN PRODUCE USEFUL OBJECTIVE RAINFALL GUIDANCE
NOTE: THE GFDL MODEL IS GENERALLY THE MOST ACCURATE AND RELIABLE OF OUR TC TRACK FORECAST MODELS.
HOWEVER, THE GFDL MODEL REQUIRES AN ACCURATE SPECIFICATION OF THE INITIAL SIZE, WIND DISTRIBUTION, AND STRENGTH OF THE CYCLONE FROM THE HURRICANE FORECASTER, AND REALISTIC ENVIRONMENTAL WIND FIELDS FROM THE AVN RUN OF THE NCEP GLOBAL MODEL.
DEFICIENCIES IN EITHER OF THE ABOVE WILL LEAD TO DEGRADED GFDL FORECASTS.
AVAILABILITY OF MODELS FOR OPERATIONAL TC FORECASTING:
“LATE” VS. “EARLY”:
DYNAMICAL MODELS SUCH AS THE AVN, UKMET, NOGAPS AND THE GFDL ARE NOT AVAILABLE UNTIL ABOUT 4 TO 6 HRS AFTER THE INITIAL SYNOPTIC TIME (I.E. THE 12Z RUN IS NOT AVAILABLE UNTIL 18Z IN REAL TIME).
SIMPLER MODELS SUCH AS LBAR, NHC90, BAM, AND CLIPER ARE AVAILABLE SHORTLY AFTER THE SYNOPTIC TIME (USING 6- THROUGH 78-HR GLOBAL MODEL FORECAST FIELDS).
UP UNTIL JUST A FEW YEARS AGO, STATISTICAL/DYNAMICAL MODELS, SUCH AS THE NHC90, WERE TYPICALLY THE MOST RELIABLE, AND HAD THE LOWEST FORECAST ERRORS OF ALL OBJECTIVE TRACK PREDICTION TECHNIQUES
HOWEVER IN RECENT YEARS (IN THE ATLANTIC BASIN), DYNAMICAL MODELS HAVE BECOME THE MAINSTAY FOR TC TRACK PREDICTION, AND OFFER THE PROMISE FOR CONTINUED REFINEMENTS SUCH AS IMPROVED PHYSICS, HIGHER RESOLUTION AND A MORE REALISTIC INITIALIZATION.
INITIAL (CURRENT) MOTION
• VERY IMPORTANT FOR THE SHORT-TERM FORECAST
• 12-HOUR FORECAST TRACK IS WEIGHTED HEAVILY BY THE INITIAL MOTION
• NOT ALWAYS EASY TO DETERMINE…PARTICULARLY FOR SYSTEMS WITH POORLY-DEFINED CENTERS (E.G., EARL 1998)
• VERY IMPORTANT FOR THE SHORT-TERM FORECAST
• 12-HOUR FORECAST TRACK IS WEIGHTED HEAVILY BY THE INITIAL MOTION
• NOT ALWAYS EASY TO DETERMINE…PARTICULARLY FOR SYSTEMS WITH POORLY-DEFINED CENTERS (E.G., EARL 1998)
CONTINUITY
• PREVIOUS OFFICIAL FORECAST PROVIDES CONSTRAINTS ON THE CURRENT FORECAST
• CREDIBILITY CAN BE HURT BY MAKING BIG CHANGES FROM ONE OFFICIAL FORECAST TO THE NEXT
• SO, CHANGES TO PREVIOUS FORECAST ARE NORMALLY MADE IN SMALL INCREMENTS
• PREVIOUS OFFICIAL FORECAST PROVIDES CONSTRAINTS ON THE CURRENT FORECAST
• CREDIBILITY CAN BE HURT BY MAKING BIG CHANGES FROM ONE OFFICIAL FORECAST TO THE NEXT
• SO, CHANGES TO PREVIOUS FORECAST ARE NORMALLY MADE IN SMALL INCREMENTS
ADDITIONAL GUIDLINES
• DO NOT MAKE SUDDEN CHANGES IN DIRECTION FROM ONE FORECAST INTERVAL (12-24 HR, 24-36 HR, etc.) TO THE NEXT.
• DO NOT MAKE DRASTIC CHANGES IN FORWARD SPEED FROM ONE FORECAST INTERVAL TO THE NEXT. HOWEVER, WHEN DECELERATION OR ACCELERATION IS PREDICTED, MAKE THE CHANGES GRADUAL.
• DO NOT MAKE SUDDEN CHANGES IN DIRECTION FROM ONE FORECAST INTERVAL (12-24 HR, 24-36 HR, etc.) TO THE NEXT.
• DO NOT MAKE DRASTIC CHANGES IN FORWARD SPEED FROM ONE FORECAST INTERVAL TO THE NEXT. HOWEVER, WHEN DECELERATION OR ACCELERATION IS PREDICTED, MAKE THE CHANGES GRADUAL.
INITIAL INPUT DATA
AS SOON AFTER THE SYNOPTIC TIME (00Z, 06Z, 12Z, 18Z) AS POSSIBLE, THE FORECASTER PROVIDES THE INITIAL CONDITIONS OF THE TC FOR INPUT INTO THE PREDICTION MODELS.
THESE DATA ARE THE CURRENT & T-12 HR POSITION / MOTION, T-24 HR POSITION, CURRENT MAXIMUM WINDS / MINIMUM PRESSURE, RADIUS OF MAXIMUM WINDS, RADIUS AND VALUE OF THE OUTERMOST CLOSED ISOBAR, AND RADII (BY QUADRANT) OF 34- AND 50-KNOT WINDS.
AS SOON AFTER THE SYNOPTIC TIME (00Z, 06Z, 12Z, 18Z) AS POSSIBLE, THE FORECASTER PROVIDES THE INITIAL CONDITIONS OF THE TC FOR INPUT INTO THE PREDICTION MODELS.
THESE DATA ARE THE CURRENT & T-12 HR POSITION / MOTION, T-24 HR POSITION, CURRENT MAXIMUM WINDS / MINIMUM PRESSURE, RADIUS OF MAXIMUM WINDS, RADIUS AND VALUE OF THE OUTERMOST CLOSED ISOBAR, AND RADII (BY QUADRANT) OF 34- AND 50-KNOT WINDS.
HOW TO USE TRACK PREDICTION MODELS TO MAKE A FORECAST??
• DO NOT SIMPLY “GO WITH” A GIVEN MODEL.
• A “CONSENSUS” OF ALL THE MODELS (I.E. ROUGHLY THE AVERAGE OF ALL TRACKS) IS OFTEN A GOOD BET.
• CONTINUITY FROM PREVIOUS FORECAST IS A STRONG CONSTRAINT (AVOIDANCE OF THE “WINDSHIELD WIPER” EFFECT).
• EVALUATE LARGE-SCALE SYNOPTIC ENVIRONMENT (CONVENTIONAL DATA AND SATELLITE, E.G. WATER VAPOR, INFORMATION) AND TRY TO ASSESS THE STEERING INFLUENCES. IN OTHER WORDS, BE A GOOD METEOROLOGIST, NOT A MECHANICAL “MODEL FOLLOWER”.
USE OF MODELS (CONT.)
• IN SOME CASES, WHEN A GIVEN MODEL APPEARS TO BE “DOING WELL” , AND THAT MODEL’S TRACK LOOKS REASONABLE FOR THE SYNOPTIC SITUATION, THE OFFICIAL FORECAST MIGHT FOLLOW THIS MODEL FAIRLY CLOSELY.
• NON-METEOROLOGICAL FACTORS ALSO HAVE TO BE TAKEN INTO CONSIDERATION (!!!)
• THE OVERALL SPREAD OF THE OBJECTIVE TRACKS MAY GIVE SOME MEASURE OF FORECAST CONFIDENCE.
• EVEN THE MOST RELIABLE MODELS CAN FAIL MISERABLY.
ENSEMBLE FORECASTS
IN THE ENSEMBLE FORECAST APPROACH, A NUMBER OF FORECASTS ARE MADE WITH A MODEL USING PERTURBED INITIAL CONDITIONS THAT (HOPEFULLY) REPRESENT THE LIKELY INITIAL ANALYSIS ERROR DISTRIBUTION.
THE ENSEMBLE APPROACH HOLDS PROMISE IN PROVIDING A MEASURE OF CONFIDENCE IN A FORECAST.
IF THERE IS A RELATIVELY SMALL SPREAD AMONGST MEMBERS OF THE ENSEMBLE, THEN ONE MIGHT EXPECT TO BE ABLE TO MAKE A RELATIVELY “HIGH CONFIDENCE” TC TRACK FORECAST.
ENSEMBLE FORECASTS
IN THE ENSEMBLE FORECAST APPROACH, A NUMBER OF FORECASTS ARE MADE WITH A MODEL USING PERTURBED INITIAL CONDITIONS THAT (HOPEFULLY) REPRESENT THE LIKELY INITIAL ANALYSIS ERROR DISTRIBUTION.
THE ENSEMBLE APPROACH HOLDS PROMISE IN PROVIDING A MEASURE OF CONFIDENCE IN A FORECAST.
IF THERE IS A RELATIVELY SMALL SPREAD AMONGST MEMBERS OF THE ENSEMBLE, THEN ONE MIGHT EXPECT TO BE ABLE TO MAKE A RELATIVELY “HIGH CONFIDENCE” TC TRACK FORECAST.
NOTE: A GROUP OF FORECAST TRACKS FROM DIFFERENT PREDICTION MODELS (GFDL, UKMET, NOGAPS, etc.) FROM THE SAME INITIAL TIME CAN ALSO BE CONSIDERED AS AN ENSEMBLE.
THIS, OF COURSE, IS THE TYPE OF “ENSEMBLE” THAT IS AVAILABLE TO US OPERATIONALLY
NOTE: A GROUP OF FORECAST TRACKS FROM DIFFERENT PREDICTION MODELS (GFDL, UKMET, NOGAPS, etc.) FROM THE SAME INITIAL TIME CAN ALSO BE CONSIDERED AS AN ENSEMBLE.
THIS, OF COURSE, IS THE TYPE OF “ENSEMBLE” THAT IS AVAILABLE TO US OPERATIONALLY
HURRICANE GEORGES GUIDANCE 9/22/98 18Z
OFFICIAL FORECAST
SMALL SPREAD -- HIGH CONFIDENCE?
HURRICANE GEORGES GUIDANCE 9/26/98 1200 UTC
OFFICIAL FORECAST
LARGE SPREAD -- LOW CONFIDENCE?
NOAA Gulfstream IV Jet
SYNOPTIC FLOW MISSION
0000Z
13 SEP 99
SYNOPTIC FLOW MISSION
0000Z
13 SEP 99
HURRICANE FLOYD GUIDANCE 9/13/99 0600 UTC
OFFICIAL FORECAST
FLOYD VS. ANDREW -- A SIZE COMPARISON
L
LENNY TRACK GUIDANCE 16 NOV 0600Z 1999
HURRICANE LENNY DISCUSSION NUMBER 13NATIONAL WEAT HER SERVICE MIAM I FL10 AM EST TUE NOV 16 1999
LENNY REM AINS EMBEDDED WITHIN THE WESTERLIES AND CO NSEQ UENTLY ISSTILL MO VING TOWARD THE EAST ABOUT 14 KNOTS. A RIDGE IS FO RECAST TOAM PLIFY TO THE EAST OF THE HURRICANE PROVIDING A PATTERN WHICH WOULDFAVO R A TRACK MORE TO THE NORTHEAST. IN FACT...THIS IS THE SOLUTIONINDICATED BY MO ST OF THE MODELS...WITH SOME SUGGESTING A DECREASE INFORWARD SPEED. ALTHOUG H THE OFFICIAL FORECAST TRACK HAS BEENADJUSTED A LITTLE TO THE EAST DUE TO THE INITIAL MOTION AND WAT CHESAND WARNINGS HAVE BEEN EXTENDED EASTWARD...THE HURRICANE WARNINGFOR PUERTO RICO AND THE VIRG IN ISLANDS MUST CONTINUE BECAUSE RELIABLEUK...GFDL AND GFDN MODELS BRING THE HURRICANE O VER PUERTO RICO.
A NO AA P3 PLANE HAS BEEN FLYING AND HAS RECENTLY REPO RTED A MINIMUMPRESSURE OF 973 MB WITH WINDS OF 96 KNOTS AT FLIGHT LEVEL. SINCE THEOUTFLO W REM AINS FAIR AND THERE IS NO SHEAR IN THE NEAR FUTURE...THEINTENSITY FORECAST CALLS FOR SOME STRENGTHENING DURING THE NEXT 24HO UR OR SO ...AS INDICATED IN THE PREVIO US ADVISORY.
Forecast Improvements Due toGPS Dropwindsonde Observations
Synoptic Surveillance Missions 1997-98
N = 17 17 17 15 14 14 13 1313
NON-METEOROLOGICAL CONSIDERATIONS:
AT TIMES, WE ARE FACED WITH THE DILLEMA THAT THE “BEST” FORECAST,
BASED ON METEOROLOGICAL GUIDANCE AT A GIVEN POINT IN TIME, LESSENS THE THREAT TO SOME VULNERABLE
AREAS ALONG THE COAST.
IN SUCH CASES, THE FORECAST MAY BE SLIGHTLY BIASED, TO SHOW THAT A
POTENTIAL THREAT STILL EXISTS.
OFFICIAL FORECAST TENDING TO THE LEFT OF MODELS -- “NON-METEOROLOGICAL” FACTORS??
HURRICANE BONNIE, 8/22/98 18Z
Atlantic Official Track Forecast Errors
1999: n = 288 at T = 0...n = 181 at T = 72
0 12 24 36 48 72Forecast Period (hours)
050
100150200250300350400
Error (nautical miles)
1999 OFCL1999 CLIP89-98 OFCL89-98 CLIP
(VERIFICATION COURTESTY OF DR. JIM GROSS, TPC / NHC)
1999 Atlantic Track Model Forecast Errors (Early: pre-fire)Errors Normalized With Respect To CLIPER (Homogeneous)
1999: n = 147 at T = 0...n = 101 at T = 72
12 24 36 48 72Forecast Period (hours)
0
-10
-20
-30
-40
-50
Normalized Error (%)
OFCLOFCIBAMDLBARGFDIUKMIOHPC
1999 Atlantic Track Model Forecast Errors (Early: post-fire)Errors Normalized With Respect To CLIPER (Homogeneous)
1999: n = 32 at T = 0...n = 17 at T = 72
12 24 36 48 72Forecast Period (hours)
0
10
20
-10
-20
-30
-40
-50
Normalized Error (%)
OFCLOFCIBAMDLBARGF2IUKMIOHPC
1999 Atlantic Track GFDL Model Forecast Errors (Early)Errors Normalized With Respect To CLIPER (Homogeneous)
1999: n = 26 at T = 0...n = 16 at T = 72
12 24 36 48 72Forecast Period (hours)
0
-10
-20
-30
-40
-50
-60
-70
Normalized Error (%)
OFCLLBARGFDIGFNIUKMI
Atlantic Official Intensity Forecast Errors
1999: n = 286 at T = 0...n = 180 at T = 72
0 12 24 36 48 72Forecast Period (hours)
0
5
10
15
20
25Error (knots)
1999 OFCL1999 SHFR90-98 OFCL90-98 SHFR
1999 Atlantic Intensity Model Forecast ErrorsErrors Normalized With Respect to SHIFOR (Homogeneous)
1999: n = 260 at T = 0...n = 161 at T = 72
12 24 36 48 72Forecast Period (hours)
0
-5
-10
-15
-20
-25
-30
Normalized Error (%)
OFCLOFCISHIP
1999 ATLANTIC VERIFICATION CONCLUSIONS
Official track errors were smaller than theirlong-term average for all forecast periods,however, so were the CLIPER errors.UKMI and LBAR provided the best early averageobjective track guidance.Use of the GFDL model was complicated by theuse of three different versions: GFDL, GFD2and GFDN.Official absolute intensity errors were neartheir long-term average, inspite of much largerSHIFOR errors than their long-term average.SHIPS model provided the best objective intensityguidance.
WARNING: EVEN IF THE AVERAGE IS LOW, LARGE INDIVIDUAL ERRORS DO OCCUR!
TROPICAL CYCLONE INTENSITY FORECASTING
TROPICAL CYCLONE INTENSITY FORECASTING
TOPICSTOPICS
• CURRENT INTENSITY ESTIMATE
• INTENSITY FORECAST MODELS
• OPERATIONAL INTENSITY FORECASTING
THE CURRENT INTENSITY ESTIMATE
DILEMMA
THE CURRENT INTENSITY ESTIMATE
DILEMMA
HOW WELL DO WE KNOW THE MAXIMUM SURFACE WINDS IN A TROPICAL CYCLONE?
GROUND TRUTH (10 METER, 1-MINUTE WIND) DATA ARE USUALLY LACKING.
SATELLITE (DVORAK) ESTIMATES ARE MOST COMMONLY EMPLOYED. THESE CAN EASILY BE IN ERROR BY +/- 15 KT (ONE SSHS CATEGORY).
HOW WELL DO WE KNOW THE MAXIMUM SURFACE WINDS IN A TROPICAL CYCLONE?
GROUND TRUTH (10 METER, 1-MINUTE WIND) DATA ARE USUALLY LACKING.
SATELLITE (DVORAK) ESTIMATES ARE MOST COMMONLY EMPLOYED. THESE CAN EASILY BE IN ERROR BY +/- 15 KT (ONE SSHS CATEGORY).
AIRCRAFT MEASUREMENTS OF WINDS, AT FLIGHT LEVELS TYPICALLY FROM 850-700 MB, ARE VERY USEFUL FOR ESTIMATING CURRENT INTENSITY.
SINCE THE HURRICANE IS A WARM-CORE VORTEX, WINDS SHOULD INCREASE FROM THE AIRCRAFT FLIGHT LEVEL DOWNWARD TO THE TOP OF THE BOUNDARY LAYER. HOWEVER, FRICTION REDUCES THE VELOCITITES BELOW THAT LEVEL. HOW MUCH DOES THE WIND DECREASE FROM THERE DOWN TO 10 METERS ABOVE THE SURFACE?
AIRCRAFT MEASUREMENTS OF WINDS, AT FLIGHT LEVELS TYPICALLY FROM 850-700 MB, ARE VERY USEFUL FOR ESTIMATING CURRENT INTENSITY.
SINCE THE HURRICANE IS A WARM-CORE VORTEX, WINDS SHOULD INCREASE FROM THE AIRCRAFT FLIGHT LEVEL DOWNWARD TO THE TOP OF THE BOUNDARY LAYER. HOWEVER, FRICTION REDUCES THE VELOCITITES BELOW THAT LEVEL. HOW MUCH DOES THE WIND DECREASE FROM THERE DOWN TO 10 METERS ABOVE THE SURFACE?
RECON FLIGHT-LEVEL WINDS
HURRICANE GEORGES 9/20/98 20-23Z
A VALUABLE NEW TOOL FOR MEASURING THE WINDS IN A HURRICANE
THE GPS (GLOBAL POSITIONING SYSTEM) DROPSONDE IS CAPABLE OF MEASURING WINDS AT VERY HIGH VERTICAL RESOLUTION DOWN TO THE SURFACE.
DATA FROM THESE INSTRUMENTS REVEAL THAT THE WIND STRUCTURE IN THE EYE WALL CAN BE VERY COMPLICATED. THE WINDS CLOSE TO THE SURFACE CAN BE MUCH STRONGER THAN THOSE FOUND AT AIRCRAFT FLIGHT LEVEL.
THE GPS (GLOBAL POSITIONING SYSTEM) DROPSONDE IS CAPABLE OF MEASURING WINDS AT VERY HIGH VERTICAL RESOLUTION DOWN TO THE SURFACE.
DATA FROM THESE INSTRUMENTS REVEAL THAT THE WIND STRUCTURE IN THE EYE WALL CAN BE VERY COMPLICATED. THE WINDS CLOSE TO THE SURFACE CAN BE MUCH STRONGER THAN THOSE FOUND AT AIRCRAFT FLIGHT LEVEL.
TROPICAL STORM ERIKA DISCUSSION NUMBER 7NATIONAL WEATHER SERVICE MIAMI FL5 PM EDT THU SEP 04 1997
THE STORM REMAINS IN A SHEARED STATE...WITH THE LOW-CLOUD CENTER EXPOSED TO THE WEST-SOUTHWEST OF THE MAIN AREA OF DEEP CONVECTION THIS SHEAR IS APPARENTLY BEING PRODUCED MAINLY BY STRONG LOW- TO MID-TROPOSPHERIC EASTERLIES...AND NOT AS MUCH BY HIGH-LEVEL INDS...SINCE THE 200 MB WINDS APPEAR TO BE FAIRLY LIGHT IN THE AREA. RECENT SATELLITE IMAGES SUGGEST THAT SOME DEEP CONVECTION IS REDEVELOPING NEARER TO THE CENTER. HOWEVER...THIS TREND MAY NOT CONTINUE. ALTHOUGH ERIKA COULD STILL STRENGTHEN...IF THE SHEARING PERSISTS...IT COULD EASILY WEAKEN. I AM FORECASTING NO CHANGE IN STRENGTH THROUGH THE PERIOD...BUT THIS IS MAINLY A REFLECTION OF THE UNCERTAINTY IN INTENSITY FORECASTING.
<DELETED DISCUSSION OF TRACK FORECAST MODELS HERE>
ALTHOUGH THE LARGE 500 MB TROUGH THAT IS CURRENTLY OVER THE EXTREME WESTERN ATLANTIC IS EXPECTED TO LIFT OUT IN A DAY OR SO...A NEW TROUGH IS FORECAST TO DROP IN AND REPLACE IT A COUPLE OF DAYS LATER. THUS...IN THE LONGER TERM...A MORE NORTHWARD MOTION OF ERIKA IS LIKELY...IF IT SURVIVES.
TROPICAL STORM ERIKA DISCUSSION NUMBER 8NATIONAL WEATHER SERVICE MIAMI FL11 PM EDT THU SEP 04 1997 IT IS VERY DIFFICULT TO LOCATE THE LOW-LEVEL CENTER OF ERIKA WITH IR IMAGES. IF I HAD NOT SEEN THE EXPOSED LOW-LEVEL CENTER ON THE LAST VIS OF THE DAY A FEW HOURS AGO...I WOULD NOW PLACE THE CENTER WITHIN THE DEEP AND EXPLOSIVE CONVECTION. AT THIS TIME...I WOULD RATHER USE CONTINUITY AND ASSUME THAT THE CENTER IS TO THE WEST OF THE MID-LEVEL CIRCULATION AND CONVECTION. THERE ARE HIGH CLOUDS EXPANDING OUTWARD WHICH SOMETIMES BUT NOT ALWAYS IS AN INDICATION OF INTENSIFICATION.
SAB AND TAFB INCREASED THE T-NUMBERS TO 3.5 ON THE DVORAK SCALE. HOWEVER...WITH THE PREVIOUSLY OBSERVED SHEARING PATTERN BEFORE DARK...IT IS CONSERVATIVE TO KEEP THE WINDS AT 50 KNOTS UNTIL CLEAR INDICATIONS OF STRENGTHENING OR WEAKENING BECOME APPARENT. <TRACK DISCUSSION DELETED HERE>
TROPICAL STORM MITCH DISCUSSION NUMBER 7NATIONAL WEATHER SERVICE MIAMI FL11 AM EDT FRI OCT 23 1998
MITCH IS NOT STRENGTHENING AT THIS TIME. ALTHOUGH THE LOW-LEVEL CIRCULATION IS WELL DEFINED...THE CONVECTION IS WEAK. UNEXPECTED UPPER-LEVEL WESTERLY WINDS HAVE BECOME ESTABLISHED OVER THE SYSTEM DISRUPTING THE EXCELLENT OUTFLOW WHICH WAS OBSERVED YESTERDAY. INITIAL INTENSITY IS KEPT AT 50 KNOTS IN THIS ADVISORY...BUT A RECONNAISSANCE PLANE WILL DETERMINE IF MITCH HAS WEAKENED AS SUSPECTED. ALL AVAILABLE MODELS SUGGEST THAT AN UPPER-LEVEL ANTICYCLONE SHOULD FORM OVER THE WESTERN CARIBBEAN WITHIN THE NEXT 24 HOURS OR SO. SUCH A PATTERN WOULD PROVIDE EXCELLENT CONDITIONS FOR STRENGTHENING AND IN FACT...BOTH SHIPS AND THE GFDL MODELS BRING MITCH TO A 75- AND 100-KNOT HURRICANE... RESPECTIVELY. HOWEVER...IT IS DIFFICULT TO VISUALIZE SUCH INTENSIFICATION WITH THE PRESENT POORLY DEFINED PATTERN OBSERVED ON SATELLITE IMAGES AND KNOWING THAT THE GLOBAL MODELS TEND TO GET RID OF WESTERLIES TOO FAST. AT THIS TIME...THE OFFICIAL FORECAST CALLS FOR NO CHANGE IN INTENSITY DURING THE NEXT 24 HOURS AND IF MITCH SURVIVES...THEN..STRENGTHENING SHOULD BEGIN.
MITCH NEAR PEAK INTENSITYMITCH NEAR PEAK INTENSITY
HURRICANE LENNY DISCUSSION NUMBER 7NATIONAL WEAT HER SERVICE MIAM I FL10 PM EST SUN NO V 14 1999
OBVIOUSLY...THE RAPID INTENSIFICATION OF THIS TROPICAL CYCLONE WASNOT ANTICIPATED. THIS AG AIN UNDERSCORES OUR LIMITED ABILITY TOPREDICT INTENSITY CHANG E. LENNY IS DEVELOPING A FAIR UPPER-LEVELOUTFLO W ANTICYCLONE AND...PRESUMING THAT VERTICAL SHEAR DOES NOTINCREASE TOO MUCH DURING THE NEXT COUPLE OF DAYS... ADDITIONALSTRENGTHENING IS CERTAINLY POSSIBLE. THIS IS INDICATED IN THEOFFICIAL FORECAST.
THE CENTER HAS SLIPPED A LITTLE TO THE SOUTH BUT MOTION IS ESTIM ATEDTO BE GENERALLY EASTWARD AT A SLIGHTLY FASTER FORWARD SPEED...7KNOTS. LENNY IS EXPECTED TO BE STEERED BY THE FLO W ON THE SO UTHSIDE OF A DEEP-LAYER TROUGH O VER THE WESTERN ATLANTIC. CURRENTWATER VAPOR IM AGERY SHOWS A REINFORCING VORTICITY M AX MO VING OFF THEU.S. EAST CO AST...AND THE 18Z NCEP GLO BAL MODEL RUN SHOWS ADDITIO NALVO RTICITY ENTERING THE TROUGH DURING THE NEXT COUPLE OF DAYS. THISSHOULD PRODUCE SOME AM PLIFICATION OF THE TROUGH AND AN INCREASEDWESTERLY STEERING FLOW FOR LENNY. THE OFFICIAL FORECAST SHO WS AGRADUAL INCREASE IN FORWARD SPEED AND A TURN TO WARD THE EAST-NO RTHEAST...IN GENERAL AG REEMENT WITH THE U.K. MET. OFFICE AND GFDLMODELS. THIS IS ALSO VERY SIMILAR TO THE PREVIOUS OFFICIAL TRACKFORECAST.