RECENT ADVANCES IN TROPICAL CYCLONE TRACK FORECASTING TECHNIQUES THAT IMPACT DISASTER PREVENTION AND MITIGATION Russell L. Elsberry Graduate School of Engineering and Applied Sciences Department of Meteorology Naval Postgraduate School Monterey, California USA Source for most materials and illustrations on the Sixth International Workshop on Tropical Cyclones (IWTC-VI) website: hetp://severe.worldweather.wmo.int/iwtc International Training Workshop on Tropical Cyclone Disaster Reduction Guangzhou, China 26 March 2007
Transcript
RECENT ADVANCES IN TROPICAL CYCLONE TRACK FORECASTING TECHNIQUES THAT IMPACT
DISASTER PREVENTION AND MITIGATION
Russell L. ElsberryGraduate School of Engineering and Applied Sciences
Department of MeteorologyNaval Postgraduate SchoolMonterey, California USA
Source for most materials and illustrations on the Sixth International Workshop on Tropical Cyclones (IWTC-VI)
website: hetp://severe.worldweather.wmo.int/iwtc
International Training Workshop on Tropical Cyclone Disaster ReductionGuangzhou, China 26 March 2007
ORGANIZATION1.0 Importance of the track prediction
2.0 Requirements for tropical cyclone track forecast3.0 Some basic concepts in tropical cyclone motion forecasting4.0 Advances in operational track prediction5.0 Future track prediction emphases6.0 Concluding remarks
*Topic numbers are from the keynote session at IWTC-VI
International Training Workshop on Tropical Cyclone Disaster ReductionGuangzhou, China 26 March 2007
NEED TO IMPROVE TRACK FORECASTS
FIRST PRIORITY OF PARTICIPANTS AT INTERNATIONAL WORKSHOP
ON TROPICAL CYCLONE LANDFALL
RELATIVE CONTRIBUTIONS OF TRACK ERRORS, INTENSITY ERRORS,
AND STRUCTURE ERRORS TO DeMARIA and KNAFF TRACK PROBABILITY DISTRIBUTIONS
• TRACK PERTURBATIONS ONLY
IWTC-VI 21-30 November 2006
TROPICAL CYCLONE MOTION
SIZE PERTURBATIONS ONLY INTENSITY PERTURBATIONS ONLY
IWTC-VI 21-30 November 2006
Effect on Storm Motion
• Friction due to proximity to land also induces large-scale asymmetries in surface convergence
• Causes a landward drift of ~ 1 m/s when storm is 150 km offshore
• Factor to be considered in estimating the rate of increase in wind magnitude for an approaching TC From Wong & Chan
(2006)
Dots denote 12-hrly TC positions
• Terrain-induced accelerations – in the form of shear lines, reverse flow, vortices, streaks and downslope winds
Fine-scale Surface Wind Features
• Doppler radar obs. (radial wind) during T. Maggie (1999)
• High speed streaks (MI, MII, MIII) and traveling vortices (A) identified above the Hong Kong International Airport -> low-level wind shear and turbulenceFrom Shun et al. (2003)
Airport
Hydrologic Effects from Hydrologic Effects from Tropical CyclonesTropical Cyclones
The hydrological/hydraulic The hydrological/hydraulic forecast depends on:forecast depends on:– Storm TrackStorm Track
Effects on basin locationEffects on basin location– RainfallRainfall
RateRate
Time distributionTime distribution– Storm Surge Storm Surge
Forecasting StrategyForecasting Strategy
Precipitation
Hydraulic Model
Hydrologic Model
Tidal Model
Storm Track
Four Top Torrential Rains Caused by Typhoon
1672 mm/24h XinLiao/Taiwan Province
Typhoon Carla (6718) 17 Oct. 1967
1248 mm/24h BaiXin/Taiwan Province
Typhoon Gloria (6312) 11 Sep. 1963
1062 mm/24h LinZhuang/Henan Province
Typhoon Nina (7503) 6 Aug. 1975
1887 mm/24h Ali mountain/Taiwan province
Typhoon Herb (9608 31-1 Sep 1996)
Chang et al. (1993) MWR: rain rates v.s. storm center, based on 1971-1990, 82 TCs, every 3h, 1446 cases, 22 surface stations rainfall data. 20x20 rain maps.
Climatological-statistical rainfall prediction model for TC near Taiwan
9:02 UTC 10 Aug 2006 Typhoon Saomai’s reflectivity in 2.4 km
(Shadow)
9-10 UTC 10 Aug 2006 one hour QPE for Saomai
(Shadow)
9:02 UTC 10 Aug 2006 Typhoon Saomai’s reflectivity in 2.4 km
(Shadow)
9-10UTC 10 Aug 2006 Observation of one hour
precipitation (Shadow and digits-mm)
Quantitative Precipitation Forecasting (QPF) of Tropical Cyclone Rainfall
1. Meso Scale Numerical Model Advanced data assimilation Fine grid space High Vertical and horizontal resolution Topographic Physics Non-hydrostatic Process
2. Statistical-dynamical Scheme Model out put Statistics
3. Statistical Methods Climatic background
Very Short Range Forecast
observation(Radar + raingauge)
Meso-scale NWP
combination
Hourly, 5km grid, 1-hr precipitation forecast up to 6 hours
Radar2.5 km grid spacingspatially continuous
AMeDAS( nearly 17km spacing ) +Raingauges of other agencies( 5200 data )
Very Short-Range Forecast
From Mannoji (2005)
Ocean WavesOcean Waves
MWLWave Setup SWL
ExtremeExtremeWindsWinds
CurrentsCurrents
Wave Runup
HAT
MSL datumExpectedHigh Tide
StormStormTideTide
Surge
after Harper (2001)
Highlight the progress and improvements of the storm tide models (including inundation)
ObjectivesObjectives
Developments are proposed in effective operational numerical storm tide prediction facility for forecasting the total water level by including storm surge, breaking wave set-up and astronomical tides
Maximum surge envelopes simulated with different typhoon tracks. (unit: cm) (a) Typhoon track used in the simulations. (b) The case in
which a typhoon takes the westernmost path. (c) The same as (b) but for the easternmost path
1.5 SUMMARY
Warnings of a tropical cyclone disaster depend critically on the accuracy of the track forecast
● Localized wind damages, including orographic effects ● Hydrological effects (flooding, landslides, debris flow)● Heavy precipitation location and timing● Storm tide/surge location and timing
If the track is not predicted accurately, even a perfect wind structure, precipitation field, or storm surge forcing will not result in an accurate warning
International Training Workshop on Tropical Cyclone Disaster ReductionGuangzhou, China 26 March 2007
2.0 REQUIREMENTS FOR TRACK FORECASTS
Accuracy of TC path requirements depend on horizontal scale of disaster event
● Width of localized wind damage● Scale of heavy precipitation relative to rainshed size ● Width of maximum storm surge
Timing (along-track) accuracy depends on the time required for preparedness activities
● Reservoir control, NASA Space Shuttle – 5 days● Sortie ship from port – 3-4 days ● Remove oil drilling rig personnel – 2 days ● Coastal area preparations/evacuations – 1 day
International Training Workshop on Tropical Cyclone Disaster ReductionGuangzhou, China 26 March 2007
3.0 BASIC CONCEPTS IN TC MOTION FORECASTING -- I
● First-order steering concept of small vortex advected by large-scale environmental flow ● Requirement is to observe, analyze, and predict this environmental flow ● Strength and positions of monsoon trough, subtropical anticyclone, and adjacent synoptic-scale circulations
● Recently improved TC track predictions from global/regional models ● Better use of satellite observations ● Advanced data assimilation techniques ● Improved horizontal/vertical resolution NWP models ● Improved physics representations
International Training Workshop on Tropical Cyclone Disaster ReductionGuangzhou, China 26 March 2007
3.0 BASIC CONCEPTS IN TC MOTION FORECASTING – II
● Second-order motion effect is TC vortex interaction with environment ● Requirement is an accurate specification of outer wind structure
● Beta-effect propagation due to two counter-rotating gyres● Westward and poleward motion component
● Intensity effect on motion ● Stronger (weaker) TCs are advected over deeper
(shallower) steering layer
International Training Workshop on Tropical Cyclone Disaster ReductionGuangzhou, China 26 March 2007
3.0 BASIC CONCEPTS IN TC MOTION FORECASTING – III
● TC track prediction is difficult because environment and vortex change in time and interact nonlinearly
● Time variations on synoptic, intraseasonal (10-25 d and Madden-Julian Oscillation), seasonal, interannual (El Nino/ La Nina), and multi-decadal periods
● Interactions include with asymmetric convection, boundary layer, vertical shear, etc. ● Because TC track prediction is a multi-scale, non-linear interaction problem, only numerical models have great skill
International Training Workshop on Tropical Cyclone Disaster ReductionGuangzhou, China 26 March 2007
Figure 3.1.5(c) 5-year running mean error of 72-hour tropical cyclone track forecasts by different operationaltropical cyclone warning centres (1990-2005)
In addition to improvements in dynamical model track guidance, application of consensus track forecasting approach has contributed to reduced track forecast errors
● Topic 3.1, and Topic 3.2 reports on IWTC-VI website ● Special focus session 3a at IWTC-VI was organized to inform forecasters on how to do consensus track forecasting IWTC-VI Recommendation: Facilitate the dissemination of all TC-related NWP products so that consensus forecasting can be applied at more warning centers
● Japan Meteorological Agency has password-protected website with tracks and fields from several NWP centers
● Topic 3.2 has a long list of global/regional models that might be used
International Training Workshop on Tropical Cyclone Disaster ReductionGuangzhou, China 26 March 2007
Western North Pacific (1992-2006)72-h TC Track Forecast Error (nm)
TOPIC 3 SUMMARY REPORTOPTIMAL USE OF GUIDANCE - II
POTENTIAL REDUCTION IN 120-H ERRORS FROM OPTIMUM USE OF GUIDANCE FOR 38 (33) LSLE CASES DURING 2004 (2005)
SELECTIVE CONSENSUS (SCON) HAS POTENTIAL TO IMPROVE TRACK FORECASTING WHEN ONLY A SMALL NUMBER OF MODEL TRACKS ARE AVAILABLE AND ONE OR TWO TRACKS ARE CLEARLY ERRONEOUS
IWTC-VI 21-30 November 2006
SCON vs CONW SCON vs JTWC
Year 2004 2005 2004 2005
Average 239 222 203 382
Maximum 626 629 932 1256
Minimum 13 64 -83 33
5.0 FUTURE SHORT-TERM TRACK PREDICTION EMPHASES
Forecasters at IWTC-VI stated further short-term track forecast improvements are required for better warnings of TC landfall impacts
● High temporal resolution TC center fixes ● Vortex wind structure and precipitation observations
are needed, along with advanced data assimilation ● High-resolution mesoscale numerical models
including air-sea-land coupling● Nowcasting techniques that update the longer track guidance with local observations
International Training Workshop on Tropical Cyclone Disaster ReductionGuangzhou, China 26 March 2007
0
50
100
150
200
250
300
350
24-h 48-h 72-h 96-h 120-h
landfall
all storms
Average track forecast errors (nm) for the 1 to 5 day period before landfall (2001-2005)
5.0 FUTURE LONG-TERM TRACK PREDICTION EMPHASES
Further improvements at longer forecast intervals are possible
● Advancements in NWP model guidance via observations, data assimilation, and improved numerical models ● Advanced global models are being developed in
several nations that could also be used if TC initial conditions were added
● Addition of more models that have skill, and optimum use of model guidance
● Characterization of when each model is likely to be erroneous is necessary for optimum use of consensus
forecasting
International Training Workshop on Tropical Cyclone Disaster ReductionGuangzhou, China 26 March 2007
5.0 FUTURE TRACK EMPHASIS - CONFIDENCE MEASURE-I
Especially longer-term track guidance can vary widely at times
● Small track guidance spread generally means high confidence
● Large track guidance spread● May indicate large uncertainty in this forecast situation● May indicate an erroneous track due to initial conditions or poor model forecast ● May have compensating track guidance errors that cancel in the consensus approach
● Worst-case scenario is that all track guidance is wrong, which requires more skillful models
International Training Workshop on Tropical Cyclone Disaster ReductionGuangzhou, China 26 March 2007
5.0 FUTURE TRACK EMPHASIS - CONFIDENCE MEASURE-II
An ensemble prediction system (EPS) is another tool to estimate track uncertainty for difficult situations
● Average of EPS member tracks is not better than deterministic model
● EPS numerical model has degraded horizontal resolution● As of 2005, track spread from ECMWF or NCEP EPSs was not a good indicator of track forecast error
● Post-processing of EPS tracks is required● Future opportunities from several nation’s EPS and from THORPEX Interactive Grand Global Ensemble (TIGGE) program
International Training Workshop on Tropical Cyclone Disaster ReductionGuangzhou, China 26 March 2007
5.0 FUTURE TRACK EMPHASIS – TARGETED OBSERVATIONS
Topic 3.3 at IWTC-VI summarized use of targeted aircraft dropsonde observations to improve TC track predictions
● USA has demonstrated value of targeted observations● Dropwindsonde Observations for Typhoon Surveillance near Taiwan Region (DOTSTAR) program has also demonstrated value
Four targeting strategies have been used to determine the most sensitive areas for deploying dropwindsondes
● Working Group formed at IWTC-VI to perform a detailed evaluation of the various techniques
International Training Workshop on Tropical Cyclone Disaster ReductionGuangzhou, China 26 March 2007
CONCLUSIONS -- I
● Importance of TC track forecasts for TC disaster warning process has been emphasized
● Dramatic improvements in 72-h track forecasts have been achieved from improved numerical guidance and consensus approaches
● Extensions to 120-h forecasts have been made – as accurate as 72-h forecasts of a decade ago
Major IWTC-VI Recommendations:● Maintain and increase remotely sensed observations● Insert TC initial conditions in other global models● Transfer this improved technology to developing nations
International Training Workshop on Tropical Cyclone Disaster ReductionGuangzhou, China 26 March 2007
CONCLUSIONS – II
● Forecast community stated requirement for improved landfall position forecasts
● Requires high temporal resolution center fixes, advanced data assimilation techniques, higher- resolution models
● Likely future focus will be on probabilistic forecasts to assist in tropical cyclone disaster risk assessment
International Training Workshop on Tropical Cyclone Disaster ReductionGuangzhou, China 26 March 2007
