BEYOND TWO-WEEK PREDICTIONS OF TROPICAL CYCLONE
EVENTS IN WESTERN NORTH PACIFIC AND ATLANTIC
Russell L. Elsberry
Trauma, Health, and Hazards Center
University of Colorado
Colorado Springs, Colorado
Hsiao-Chung Tsai & Mary S. Jordan
Department of Meteorology
Naval Postgraduate School
Monterey, California
• 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
Acknowledgements: ONR Marine Meteorology and ECMWF
International Conference on Subseasonal & Seasonal Prediction, 10-13 February 2014
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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
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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 cyclonelike 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”
International Conference on Subseasonal & Seasonal Prediction, 10-13 February 2014
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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)
Dend
Best-Track
Dshort
Dform
Ensemble Storm Track
International Conference on Subseasonal & Seasonal Prediction, 10-13 February 2014
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TYPHOON MEGI
(red: ensemble storm track; white: JTWC best-track)
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
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OVERALL PERFORMANCE
DURING THE 2012 WPAC TYPHOON SEASON
• 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)
Hit
TD25: skip
too short (~ 1 day)
International Conference on Subseasonal & Seasonal Prediction, 10-13 February 2014
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CONTINGENCY TABLE
FOR THE 2012 WPAC TYPHOON SEASON
• 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
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2012 WPAC Hits
As 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.
Week-1
Week-2
Week-3
Week-4
International Conference on Subseasonal & Seasonal Prediction, 10-13 February 2014
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2012 WPAC False Alarms
In 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.
Week-1
Week-2
Week-3
Week-4
International Conference on Subseasonal & Seasonal Prediction, 10-13 February 2014
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2012 Atlantic Basin Tropical Storm and Hurricane Tracks - I
Baroclinic-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 – II
African Easterly Wave (AEW) origin
with either predominantly westward or recurver tracks
1. Predominantly westward
2. AEW with recurvature central Atlantic
– 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
–
–
–
–
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
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OVERALL 2012 ATLANTIC SEASON PERFORMANCE
EXAMPLES OF MISSES IN ALL FOUR WEEKS
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.
Hit
CHRIS
MICHAEL
PATTY
TONY
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°N
3) Tropical Storm Patty (AL16) was a small, short-lived system that formed at 25°N at southern end of
north-south oriented cloud band
4) 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
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Ensemble storm tracks starting in Week 1 that were HITS
(i.e., could be matched with a NHC storm)
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.
International Conference on Subseasonal & Seasonal Prediction, 10-13 February 2014
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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)
(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
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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 alarmtype 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.
International Conference on Subseasonal & Seasonal Prediction, 10-13 February 2014
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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.
International Conference on Subseasonal & Seasonal Prediction, 10-13 February 2014
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References
• 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
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