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

Another way to classify models:

Statistical - CLIPER

Statistical/Dynamical - NHC/90/91/98

Purely Dynamical - GFDL, AVN/MRF, NOGAPS, UKMET, ECMWF, ETA,NGM

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.

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

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)

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.

AVILA FORECAST POSITIONS AND M AX WINDS INITIAL 16/1500Z 15.3N 69.8W 85 KTS12HR VT 17/0000Z 16.3N 67.9W 90 KTS (verifying: 100 kts)24HR VT 17/1200Z 18.1N 65.3W 95 KTS (verifying: 105 kts)36HR VT 18/0000Z 20.0N 62.5W 90 KTS (verifying: 130 kts)48HR VT 18/1200Z 21.5N 61.0W 85 KTS (verifying: 120 kts)72HR VT 19/1200Z 24.0N 56.0W 75 KTS (verifying: 70 kts)

U.K. MET. OFFICE MODELU.K. MET. OFFICE MODEL

L

L

VERIFYING POSITION

EARL MODEL GUIDANCE 9/01/98 12Z

HURRICANE EARL--SYNOPTIC SURVEILLANCE

INPUT DATA FOR 9/02/98 00Z MODEL RUNS

STEERING FLOW

EARL MODEL GUIDANCE 9/02/98 00Z

EARL MODEL GUIDANCE 9/02/98 06Z

06Z TRACK MODELS RESPOND TO ADDED DATA

(MAKE USE OF 00Z GLOBAL MODEL RUNS)

MITCH MODEL GUIDANCE 10/24/98 18Z

MODELS SHOW WESTWARD TURN

LACK OF OBS. OVER CENTRAL AMERICA & MEXICO

MITCH MODEL GUIDANCE 10/26/98 18Z

NO MODEL SHOWS TRACK INTO HONDURAS

-15

-10

-5

0

5

10

15

20

25

0 24 48 72 96 120

GFDLAVIATIONVICBARCONSENSUS

Mea

n F

ore

cast

Imp

rove

men

t (%

)

Forecast Time (h)

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.

0

2000

4000

6000

8000

10000

100 110 120 130 140 150 160 170

Hei

ght

(ft)

Wind Speed (kt)

Hurricane Mitch - EyewallGPS Dropsonde Wind Profile

2337 UTC 27 October 1998

* Maximum flight-level windfrom reconaissance aircraft

MAXIMUM FLIGHT-LEVEL WIND 130 KT

0

500

1000

1500

2000

2500

3000

80 90 100 110 120 130 140 150

Hurricane Georges - EyewallGPS Dropsonde Wind Profiles

19 September 1998

1918 UTC1959 UTC

Hei

ght (

ft)

Wind Speed (kt)

HIGH-RISES

0

500

1000

1500

2000

2500

3000

0.70 0.80 0.90 1.00 1.10 1.20

Bonnie (N=23-34)Georges (N=6-15)Mitch (N=10-39)

He

ight

(m

)

Normalized Wind Speed(WS/WS

2km)

Mean Wind Speed ProfileEyewall - By Storms

WS10m

/WS2km

= 0.770.890.92

0

500

1000

1500

2000

2500

3000

120 130 140 150 160 170 180 190

Hei

gh

t (m

)

Wind Speed (kt)

Hurricane Lenny - EyewallGPS Dropsonde Wind Profile1652 UTC 17 November 1999

FACTORS DETERMINING TC INTENSITY CHANGE

• SST AND UPPER OCEANIC HEAT CONTENT• VERTICAL WIND SHEAR• STATIC STABILITY• INTERACTION WITH LAND• EYEWALL REPLACEMENT CYCLES*• TROUGH INTERACTIONS*

• SST AND UPPER OCEANIC HEAT CONTENT• VERTICAL WIND SHEAR• STATIC STABILITY• INTERACTION WITH LAND• EYEWALL REPLACEMENT CYCLES*• TROUGH INTERACTIONS*

* HOW TO USE OPERATIONALLY?* HOW TO USE OPERATIONALLY?

UPPER OCEAN HEAT CONTENT

1 SEPT, 1998

Fig. 4. Sequence of microwave images of Hurricane Floyd at: (a) 0116 UTC, (b) 1122 UTC, (c) 1347 UTC, and (d) 2240 UTC 13 September, 1999.

a b

c d

Fig. 5. Sequence of microwave images of Hurricane Floyd at: (a) 2240 UTC 13 September, (b) 0104 UTC, (c) 1110 UTC, and (d) 2228 UTC 14 September, 1999.

a b

c d

20

30

40

50

60

70

80

90

100

110

120

130

140

150

9/6 9/8 9/10 9/12 9/14 9/16 9/18 9/20

Hurricane Floyd September 1999

BEST TRACKSat (TAFB)Sat (SAB)Sat (AFWA)3 Hr Obj T-NumAC (sfc)AC (flt>sfc)AC (DVK P>W)SurfaceDrop (sfc)Drop (xtrp)Drop (MBL xtrp)

Win

d S

pee

d (

kt)

Date (Month/Day)

EYEWALL REPLACEMENT

TC INTENSITY FORECAST MODELS• SHIFOR (STATISTICAL HURRICANE INTENSITY FORECAST):

CLIMATOLOGY & PERSISTENCE

• SHIPS (STATISTICAL HURRICANE INTENSITY PREDICTION SCHEME): CLIMATOLOGY & PERSISTENCE PLUS SYNOPTIC AND DYNAMICAL DATA

• GFDL*

• UK MET*

• MRF/AVN*

• ECMWF*

*DYNAMICAL MODEL

THE SHIPS MODEL

• (+) SST POTENTIAL (VMAX-V) - DIFFERENCE BETWEEN CURRENT INTENSITY AND MAXIMUM POTENTIAL INTENSITY (FUNCTION OF SST)

• (-) 200 MB TEMPERATURE

• (-) 850-200 MB SHEAR (& MODEL FORECAST FIELDS)

• (+) PERSISTENCE (PREVIOUS 12 HR V)

• (-) 200 MB ZONAL WIND

• (+) 200 MB EDDY ANGULAR MOMENTUM FLUX

• (-) CLIMATOLOGY (JULIAN DAY - 253|)

MULTIPLE REGRESSION RELATING TC INTENSITY CHANGE TO CLIMATOLOGY/PERSISTENCE,

SYNOPTIC AND DYNAMIC VARIABLES:

DYNAMICAL MODELS HAVE VERY LIMITED SKILL IN FORECASTING TC INTENSITY (THE STATISTICAL/DYNAMICAL MODEL, SHIPS, IS SLIGHTLY BETTER).

-SPARSE OBSERVATIONAL DATA

-INCOMPLETE UNDERSTANDING OF INTENSITY CHANGE PROCESS (INNER CORE DYNAMICS, EDDY FLUXES, ETC.)

-INADEQUATE MODEL RESOLUTION (MODEL’S GRID/WAVE SCALE CANNOT RESOLVE EYEWALL STRUCTURE) AND PHYSICS

-BIASES IN GLOBAL MODEL UPPER-TROPOSPHERIC WIND PREDICTIONS

SUBJECTIVE (OFFICIAL) INTENSITY FORECAST

-TENDS TO BE CONSERVATIVE

-PERSISTENCE (EXTRAPOLATION OF CURRENT TREND) IS MOST COMMONLY USED

-UNFAVORABLE SIGNS IN FUTURE ENVIRONMENT (LAND, COOLER WATERS) ARE TAKEN INTO CONSIDERATION

-SHIPS OR DVORAK CAN BE USED QUANTITAVELY, DYNAMICAL MODELS USUALLY CANNOT.

-TENDS TO BE CONSERVATIVE

-PERSISTENCE (EXTRAPOLATION OF CURRENT TREND) IS MOST COMMONLY USED

-UNFAVORABLE SIGNS IN FUTURE ENVIRONMENT (LAND, COOLER WATERS) ARE TAKEN INTO CONSIDERATION

-SHIPS OR DVORAK CAN BE USED QUANTITAVELY, DYNAMICAL MODELS USUALLY CANNOT.

INLAND WIND DECAY MODEL: EXPONENTIAL DECREASE OF INTENSITY WITH TIME

0

5

10

15

20

25

12hr 24hr 36hr 48hr 72hr

OFCL

SHFR

SHIPS

GFDI

GFDL

1998 AVERAGE MODEL ABSOLUTE WIND SPEED ERRORS (KT)

ATLANTIC BASIN

EXPOSED CENTEREXPOSED CENTER

72-HR INTENSITY FORECAST?72-HR INTENSITY FORECAST?

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.

PASCH FORECAST POSITIONS AND MAX WINDS INITIAL 04/2100Z 15.5N 55.0W 50 KTS12HR VT 05/0600Z 16.3N 57.5W 50 KTS24HR VT 05/1800Z 17.7N 60.3W 50 KTS36HR VT 06/0600Z 19.0N 62.5W 50 KTS48HR VT 06/1800Z 20.0N 64.5W 50 KTS72HR VT 07/1800Z 22.0N 67.0W 50 KTS

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>

AVILA FORECAST POSITIONS AND MAX WINDS INITIAL 05/0300Z 16.0N 56.3W 50 KTS12HR VT 05/1200Z 16.8N 58.6W 50 KTS24HR VT 06/0000Z 18.2N 61.5W 50 KTS36HR VT 06/1200Z 19.5N 63.5W 50 KTS48HR VT 07/0000Z 21.0N 66.0W 50 KTS72HR VT 08/0000Z 22.5N 68.0W 50 KTS NNNN

UPPER TROUGH

72 HR LATER.

MAX WINDS 90 KT AND INCREASING.

72-HR WIND FCST ERROR= -40 KT.

72 HR LATER.

MAX WINDS 90 KT AND INCREASING.

72-HR WIND FCST ERROR= -40 KT.

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.

<ADDITIONAL DISCUSSION DELETED HERE>

AVILA FORECAST POSITIONS AND MAX WINDS INITIAL 23/1500Z 12.7N 77.9W 50 KTS12HR VT 24/0000Z 12.9N 78.0W 50 KTS24HR VT 24/1200Z 14.0N 78.2W 50 KTS36HR VT 25/0000Z 14.8N 78.6W 65 KTS48HR VT 25/1200Z 16.0N 79.0W 75 KTS72HR VT 26/1200Z 17.0N 81.0W 80 KTS

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.

PASCH FORECAST POSITIONS AND M AX WINDS INITIAL 15/0300Z 16.1N 78.3W 70 KTS12HR VT 15/1200Z 16.1N 77.2W 75 KTS24HR VT 16/0000Z 16.4N 74.9W 80 KTS36HR VT 16/1200Z 16.8N 72.5W 85 KTS48HR VT 17/0000Z 17.5N 70.0W 85 KTS (verifying: 100 kts)72HR VT 18/0000Z 19.0N 65.0W 80 KTS (verifying: 130 kts)

20

30

40

50

60

70

80

90

100

110

120

130

140

150

11/13 11/14 11/15 11/16 11/17 11/18 11/19 11/20 11/21

Best Track Intensity - Hurricane LennyNovember 1999

OPERATIONAL

Win

d S

pee

d (

kt)

Date (Month/Day)

80

kt

72-h forecast

VERTICAL SHEAR & TENDENCY FROM UWISC/CIMSS

CONCLUDING REMARKSCONCLUDING REMARKS

ACCURATE INTENSITY FORECASTS ARE CRUCIAL FOR EMERGENCY MANAGEMENT DECISION-MAKING.

INTENSITY FORECASTS HAVE VERY LIMITED SKILL OUT TO 2 DAYS; 3 DAYS AND BEYOND THERE IS ESSENTIALLY NO SKILL.

STATISTICAL/DYNAMICAL MODELS SUCH AS SHIPS CAN PROVIDE USEFUL GUIDANCE.

INCREASES IN RESOLUTION SHOULD IMPROVE INTENSITY FORECASTS IN DYNAMICAL MODELS.

ACCURATE INTENSITY FORECASTS ARE CRUCIAL FOR EMERGENCY MANAGEMENT DECISION-MAKING.

INTENSITY FORECASTS HAVE VERY LIMITED SKILL OUT TO 2 DAYS; 3 DAYS AND BEYOND THERE IS ESSENTIALLY NO SKILL.

STATISTICAL/DYNAMICAL MODELS SUCH AS SHIPS CAN PROVIDE USEFUL GUIDANCE.

INCREASES IN RESOLUTION SHOULD IMPROVE INTENSITY FORECASTS IN DYNAMICAL MODELS.

CONCLUDING REMARKS (cont.)CONCLUDING REMARKS (cont.)

WE NEED TO FURTHER OUR UNDERSTANDING OF (AMONG OTHER THINGS):

•INNER-CORE DYNAMICS (EYEWALL REPLACEMENT CYCLES, CONVECTIVE ASYMMETRIES)

•ENVIRONMENTAL INTERACTIONS (ROLE OF VERTICAL SHEAR, UPPER-LEVEL TROUGH INTERACTIONS, LOW-LEVEL FORCING)

•ROLE OF THE OCEAN (UPPER OCEANIC HEAT CONTENT, WARM EDDIES, TC FEEDBACKS)

WE NEED TO FURTHER OUR UNDERSTANDING OF (AMONG OTHER THINGS):

•INNER-CORE DYNAMICS (EYEWALL REPLACEMENT CYCLES, CONVECTIVE ASYMMETRIES)

•ENVIRONMENTAL INTERACTIONS (ROLE OF VERTICAL SHEAR, UPPER-LEVEL TROUGH INTERACTIONS, LOW-LEVEL FORCING)

•ROLE OF THE OCEAN (UPPER OCEANIC HEAT CONTENT, WARM EDDIES, TC FEEDBACKS)