BEYOND TWO-WEEK PREDICTIONS OF TROPICAL CYCLONEEVENTS IN WESTERN NORTH PACIFIC AND ATLANTIC

Russell L. ElsberryTrauma, Health, and Hazards Center

University of ColoradoColorado Springs, Colorado

International Conference on Subseasonal & Seasonal Prediction, 10-13 February 2014 1

• An objective of S2S is a seamless tropical cyclone (TC) prediction from the present 10-day deterministic and 15-day ensemble track forecasts through the seasonal timescales for TC activity (numbers of TCs only)

• One approach to fill the “sub-seasonal gap” is to apply advanced seasonal models to forecast the activity in that timescale

• Alternate approach is to extend the ensemble forecasts beyond 15 days to predict formations and subsequent tracks on the extended-range (5-30 days) and hopefully beyond

Hsiao-Chung Tsai & Mary S. Jordan

Department of Meteorology Naval Postgraduate School

Monterey, California

Acknowledgements: ONR Marine Meteorology and ECMWF

OBJECTIVES

Demonstrate forecastability by the 51-member ECMWF 32-day ensemble model of tropical cyclone events (formations and subsequent tracks)

• Western North Pacific: Predict out to 3-4 weeks in advance most of the typhoons and even many of the tropical storms and tropical depressions

• Atlantic: Only a few tropical cyclones from the African easterly waves are forecastable to 4 weeks, and very limited (< 1 week) forecastability for baroclinically-influenced tropical cyclone formation and subsequent track

International Conference on Subseasonal & Seasonal Prediction, 10-13 February 2014 2

DATA

• ECMWF 32-day ensemble forecasts

• 51 members: 50 members plus one control

• horizontal resolution (CY36R1): ~32 km to Day 10 and then ~65 km

• coupling with ocean begins at 10 days

• the real-time forecasts issued every Monday and Thursday (Mondays since 10 Oct. 2011)

• A TC tracker is used by ECMWF to obtain the 12-h tropical cyclone-like vortex track positions predicted by each of the members in the Atlantic and western North Pacific

• relative vorticity

• surface wind speed

• warm core anomaly in upper lever

• track must be longer than one day

• Weighted Mean Vector Motion technique (Elsberry et al. 2008) is used to combine similar member vortex tracks and form “ensemble storm tracks”

3International Conference on Subseasonal & Seasonal Prediction, 10-13 February 2014

TRACK ANALOG APPROACH TO ENSEMBLE TRACK VERIFICATION

• Basic criteria to select an ensemble storm as an analog to a best-track:• Time diff. at matched points:

± 3 days for Week-1, ± 4 days for Week-2, and ± 5 days for Week-3 and Week-4. • Total matched points:

at least 2 days or 1/3×LifespanBest-Track

• Four metrics are used:• average distance of the matched points (Davg), shortest distance (Dshort), distance

at formation time (Dform), and distance at ending time (Dend)

Ensemble Storm Track

Best-Track

Dend

Dform

Dshort

International Conference on Subseasonal & Seasonal Prediction, 10-13 February 2014 4

TYPHOON MEGI(red: ensemble storm track; white: JTWC best-track)

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Week-1 Week-2

Week-3 Week-4

*A Week-1 storm starts in the forecast on the Monday (or Thursday) before the JTWC declares the system to be a Tropical Depression. A storm starting in the Week-2 (3 or 4) forecast is two (three or four) weeks before the corresponding JTWC Tropical Depression existed.

International Conference on Subseasonal & Seasonal Prediction, 10-13 February 2014

OVERALL PERFORMANCE DURING THE 2012 WPAC TYPHOON SEASON

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TD25: skiptoo short (~ 1 day)

Hit

• Objectively-determined quality measure is maximum LikeliHood Value (LHV), which measures the overall similarity of the best ensemble storm track to the NHC working best-track in terms of four metrics: (1) average distance; (2) shortest distance; (3) distance at formation time; and (4) distance at ending time

LHV = MFavg(Davg)×0.3 + MFshort(Dshort)×0.25 + MFform(Dform)×0.23 + MFend(Dend)×0.22

• Similar to the previous seasons, the ECMWF 32-day ensemble can provide skillful TC formation and track forecasts during the 2012 season

•Only a small number of Misses (LHV<0.2)

International Conference on Subseasonal & Seasonal Prediction, 10-13 February 2014

CONTINGENCY TABLE FOR THE 2012 WPAC TYPHOON SEASON

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• Probability of detection (POD):• Week-1: 93.3%• Week-2: 83.7%• Week-3: 85.4%• Week-4: 85.0%

Hits FAs Misses CNs

Week-1 (58 fcsts)

42 109 3 4

Week-2(58 fcsts)

36 70 7 8

Week-3(56 fcsts)

35 76 6 4

Week-4(54 fcsts)

34 98 6 4

International Conference on Subseasonal & Seasonal Prediction, 10-13 February 2014

2012 WPAC HitsAs in Typhoon Megi example, multiple ensemble storms starting at slightly different times may be

representing the same TC. An objective method being developed for combining these similar ensemble storms will reduce the number of HITS and increase the reliability.

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Week-1 Week-2

Week-3 Week-4

International Conference on Subseasonal & Seasonal Prediction, 10-13 February 2014

2012 WPAC False AlarmsIn Week-1, a large fraction of the 109 FAs began below 10°N and had similar tracks. The objective method for combining similar ensemble storm tracks will considerably reduce the numbers of these

easily recognized FAs, and some of the FAs that resemble real storms.

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Week-1 Week-2

Week-3 Week-4

International Conference on Subseasonal & Seasonal Prediction, 10-13 February 2014

2012 Atlantic Basin Tropical Storm and Hurricane Tracks - IBaroclinic-affected formations with northward tracks (10 of 19 of 53%)

1. Subtropical formation locations– Alberto (AL01) starts 32°N, loops, and then NE– Beryl (AL02) starts 30°N, off E coast, LANDFALL

– Chris (AL03) starts 29°N, NE– Debby (AL04) starts 25°N, G of Mex, LANDFALL– Gordon (AL08) starts 28°N, Central Atlantic– Michael (AL13) starts 26°N, Central Atlantic– Patty (AL16) starts 25°N, mesoscale  

2. Low-latitude formation locations– Rafael (AL17) North from 15°N, 63°W– Sandy (AL18) North from 12°N, 78°W– Tony (AL19) North to 30°N and decays

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2012 Atlantic Basin Tropical Storm and Hurricane Tracks – IIAfrican Easterly Wave (AEW) origin

with either predominantly westward or recurver tracks

1. Predominantly westward– Ernesto (AL 5) starts 11°N, 47°W to Mexico – Florence (AL 6) starts 12°N, 23°W to 19°N, 55° W– Helene (AL 7) long track along 14°W, TS for 36 h– Isaac (AL 9) long track to New Orleans

– Joyce (AL10) short track to 20°N, 50°W  

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2. AEW with recurvature central Atlantic– Kirk (AL11) Northwest to 24°N, 43°W before a TS– Leslie (AL12) Classical recurvature track to NE Canada– Nadine (AL14) At first classical, but then multiple loops– Oscar (AL15) Short from 10°N, 30°W to 24°N, 37°W

OVERALL 2012 ATLANTIC SEASON PERFORMANCEEXAMPLES OF MISSES IN ALL FOUR WEEKS

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Objectively-determined quality measure is maximum LikeliHood Value (LHV), which measures the overall similarity of the best ensemble storm track to the NHC working best-track in terms of four metrics: (1) average distance; (2) shortest distance; (3) distance at formation time; and (4) distance at ending time.

1) Hurricane Chris (AL03) was a baroclinic system that first became a tropical storm at 37°N 2) Hurricane Michael (AL13) was a non-tropical system that became a tropical storm at 25°N3) Tropical Storm Patty (AL16) was a small, short-lived system that formed at 25°N at southern end of

north-south oriented cloud band4) Tropical Storm Tony (AL19) was a short-lived system that formed at 27°N and moved

northeastward *Note also the number of misses (LHV = 0) in Weeks 2, 3, and 4 for other storms

Hit

MICHAEL PATTY TONYCHRIS

Ensemble storm tracks starting in Week 1 that were HITS (i.e., could be matched with a NHC storm)

International Conference on Subseasonal & Seasonal Prediction, 10-13 February 2014 13

1) For the 34 African wave-type 1 tracks, three quality measures of the HITS are indicated by colors (see inset).

2) Note that only four Week 1 HITS were not of the African wave-type.

Ensemble storm tracks starting in Week 1 that were False Alarms (FAs)(i.e., could not be matched with a NHC storm with a LHV ≥ 0.2. Four types of tracks based on geographical considerations are indicated by colors and the numbers of FAs in each type are indicated)

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(1) Note similarity of Type 1 FAs (red) with Type 1 HITS tracks in previous PPT, except many FA tracks are short.(2) For Type 2 FA tracks (blue) and Type 4 FA tracks (black) no corresponding HITS tracks existed for AL 3 – AL 19.(3) For Type 3 FA tracks (green), only two (three) corresponding HITS tracks originated near Central (South) America.

International Conference on Subseasonal & Seasonal Prediction, 10-13 February 2014

SUMMARY FROM EVALUATION OF 2012 ATLANTIC EXTENDED-RANGE FORECASTS

• Overall forecastability by the ECMWF 32-day ensemble of Atlantic tropical cyclones during Weeks 1-4 for 2012 season is less than in previous evaluations in the western North Pacific

• No forecastability by the ECMWF 32-day ensemble in any of the four weeks was found for two hurricanes (AL 03 & AL 13) and two tropical storms (AL 16 & AL 19)

• ECMWF 32-day ensemble forecasts had large numbers of false alarm-type tracks during 2012 Atlantic season with some distinct geographic and seasonal characteristics

• In summary, the objective of subseasonal forecasting of tropical cyclone events (formations and subsequent tracks) by an extension beyond the extended-range (5-30 days) is more likely to be successful in the western North Pacific than in the Atlantic.

15International Conference on Subseasonal & Seasonal Prediction, 10-13 February 2014

IMPLICATIONS OF EXTENDED-RANGE PREDICTIONS FOR SEASONAL FORECASTS

1) If the proper distribution of formations and tracks is wrong in the extended-range predictions, will they also be wrong in the seasonal forecasts for the Atlantic?

2) If baroclinically-influenced tropical cyclones are not predictable in seasonal forecasts, is the all-important issue of tropical cyclone landfalls on the United States predictable?

3) If the capability of the ECMWF 32-day ensemble to forecast most of the typhoons and many of the tropical storms is due to the forecastability of the Asia summer monsoon and the Madden-Julian Oscillation, will the ECMWF seasonal model be able to forecast not just the activity but also the track types likely associated with landfalls?

* Please stop by Poster Session B to get the answers based on the performance of the ECMWF seasonal forecasts of Atlantic and western North Pacific TCs during 2012.

16International Conference on Subseasonal & Seasonal Prediction, 10-13 February 2014

• Elsberry, R. L., H.-C. Tsai, M. S. Jordan, and F. Vitart, 2014: Extended-range forecasts of Atlantic tropical cyclone events during 2012 using the ECMWF 32-day ensemble predictions. Wea. Forecasting, doi: 10.1175/WAF-D-13-00104.1

• Tsai, H.-C., and R. L. Elsberry, 2013: Opportunities and challenges for extended-range predictions of tropical cyclone impacts on hydrological predictions. J. Hydrol., 506: 9, 42-54, doi: 10.1016/j.jhydrol.2012.12.025

• Tsai, H.-C., R. L. Elsberry, M. S. Jordan, and F. Vitart, 2013: Objective verifications and false alarm analyses of western North Pacific tropical cyclone event forecasts by the ECMWF 32-day ensemble. Asia-Pacific J. Atmos. Sci., 49, 409-420. doi:10.1007/s13143-013-0038-6

• Elsberry, R.L., H.-C. Tsai, and M. Jordan, 2013: Extended-range (5-30 day) Forecasts of Tropical Cyclones with the ECMWF Ensemble: Atlantic during HS3 Field Experiment, 67th Interdepartmental Hurricane Conference. College Park, MD, USA.

• Elsberry, R.L., H.-C. Tsai, M. Jordan, and F. Vitart, 2013: Opportunities and Challenges For Probabilistic Extended Range (7-30 Day) Forecasts of Tropical Cyclone Events, 93rd AMS Annual Meeting. Austin, TX, USA.

• Tsai, H.-C., R. L. Elsberry, and M. Jordan, 2012: Western North Pacific Tropical Cyclones In The ECMWF 32-Day Ensemble Prediction System, 30th Conference on Hurricanes and Tropical Meteorology. Jacksonville, FL, USA.

• Elsberry, R. L., M. S. Jordan, and F. Vitart, 2011. Evaluation of the ECMWF 32-day ensemble predictions during 2009 season of western North Pacific tropical cyclone events on intraseasonal timescales. Asia-Pacific J. Atmos. Sci., 47, 305-318.

• Elsberry, R. L., M. S. Jordan, and F. Vitart, 2010. Predictability of tropical cyclone events on intraseasonal timescales with the ECMWF monthly forecast model. Asia-Pacific J. Atmos. Sci., 46, 135-153

References

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