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TROPICAL CYCLONE TRACK PREDICTION

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.

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TROPICAL CYCLONE TRACK PREDICTION

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  1. 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

  2. TRACK FORECASTING • GUIDANCE MODELS • USE OF INITIAL MOTION, CLIMATOLOGY, AND PERSISTENCE • CONTINUITY FROM PREVIOUS FORECAST • SYNOPTIC FLOW & EXAMPLES • NON-METEOROLOGICAL FACTORS • OFFICIAL FORECAST SKILL

  3. 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

  4. Track Guidance Models • Zero Order - CLIPER, NHC90/91/98 • First Order - BAM, LBAR, VICBAR • General Models - GFDL, AVN/MRF, NOGAPS, UKMET, ECMWF, ETA, NGM

  5. Another way to classify models: Statistical - CLIPER Statistical/Dynamical - NHC/90/91/98 Purely Dynamical - GFDL, AVN/MRF, NOGAPS, UKMET, ECMWF, ETA,NGM

  6. CLIPER (CLIMatology and PERsistence) Statistical track model developed in 1972 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.

  7. 72-HR FORECAST 48-HR FORECAST 36-HR FORECAST 24-HR FORECAST 12-HR FORECAST INITIAL CLIPER EXAMPLE: HURRICANE DANIELLE, 1998

  8. 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)

  9. Typical Predictor Locations for NHC90

  10. 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

  11. 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.

  12. HIGHER VALUES OF EARTH’S VORTICITY INDUCED STEERING H v<0 v>0 L N LOWER VALUES OF EARTH’S VORTICITY

  13. 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

  14. THREE-DIMENSIONAL (MULTI-LEVEL) DYNAMICAL MODELS (PRIMITIVE EQUATION MODELS): 1) MRF (GLOBAL SPECTRAL) 2) UKMET (GLOBAL GRID-POINT) 3) NOGAPS (GLOBAL SPECTRAL) 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)

  15. 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

  16. 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)

  17. AVN 1000 mb Initial Wind/Vorticity 9/21/98 00 UTC (X indicates observed center of Hurricane Georges) X X

  18. HURRICANE FLOYD 9/9/99 1200 UTC

  19. HURRICANE FLOYD 24 H FCST FROM 9/9/99 1200 UTC

  20. HURRICANE FLOYD 48 H FCST FROM 9/9/99 1200 UTC

  21. 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.

  22. 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.

  23. 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.

  24. 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

  25. Vortex from global Analysis Vortex from spin-up procedure

  26.  (=p/ps) LEVELS OF THE GFDL MODEL

  27. Inner Mesh of GFDL Model (Hurricane Georges 1998)

  28. Outer Mesh of GFDL Model (Hurricane Georges 1998)

  29. SAMPLE 10 M WIND SWATH FROM GFDL MODEL FORECAST

  30. GFDL MODEL CAN PRODUCE USEFUL OBJECTIVE RAINFALL GUIDANCE

  31. 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.

  32. 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).

  33. 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.

  34. 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)

  35. 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

  36. 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.

  37. 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.

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