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NEED TO IMPROVE TRACK FORECASTS

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NEED TO IMPROVE TRACK FORECASTS

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  1. RECENT ADVANCES IN TROPICAL CYCLONE TRACK FORECASTING TECHNIQUES THAT IMPACT DISASTER PREVENTION AND MITIGATIONRussell L. ElsberryGraduate School of Engineering and Applied SciencesDepartment of MeteorologyNaval Postgraduate SchoolMonterey, California USASource 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

  2. ORGANIZATION1.0 Importance of the track prediction1.1 Wind damage associated with tropical cyclones (Topic 0.2)* 1.2 Hydrological effects (Topic 0.5) 1.3 Rainfall effects (Topic 0.3) 1.4 Storm tide/surge (Topic 0.4) 1.5 Summary2.0 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

  3. 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

  4. TROPICAL CYCLONE MOTION SIZE PERTURBATIONS ONLY INTENSITY PERTURBATIONS ONLY IWTC-VI 21-30 November 2006

  5. Dots denote 12-hrly TC positions From Wong & Chan (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

  6. Airport From Shun et al. (2003) Fine-scale Surface Wind Features • Terrain-induced accelerations – in the form of shear lines, reverse flow, vortices, streaks and downslope winds • 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 turbulence

  7. Hydrologic Effects from Tropical Cyclones Tides/Surge Inland Flooding Landslide/Debris Flow

  8. Summary • The hydrological/hydraulic forecast depends on: • Storm Track Effects on basin location • Rainfall Rate Time distribution • Storm Surge

  9. Forecasting Strategy Storm Track Precipitation HydrologicModel Hydraulic Model TidalModel

  10. Four Top Torrential Rains Caused by Typhoon 1887 mm/24h Ali mountain/Taiwan province Typhoon Herb (9608 31-1 Sep 1996) 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

  11. Climatological-statistical rainfall prediction model for TC near Taiwan 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.

  12. 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)

  13. 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

  14. Radar 2.5 km grid spacing spatially continuous AMeDAS ( nearly 17km spacing ) + Raingauges of other agencies ( 5200 data ) Very Short-Range Forecast Meso-scale NWP combination Very Short Range Forecast Hourly, 5km grid, 1-hr precipitation forecast up to 6 hours observation (Radar + raingauge) From Mannoji (2005)

  15. Ocean Waves Extreme Winds MWL Wave Runup Wave Setup SWL Currents Surge HAT Storm Tide Expected High Tide MSL datum after Harper (2001)

  16. Objectives • Highlight the progress and improvements of the storm tide models (including inundation) 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

  17. a b c 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

  18. 1.5 SUMMARYWarnings 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 timingIf 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

  19. 2.0 REQUIREMENTS FOR TRACK FORECASTSAccuracy 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 surgeTiming (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

  20. 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

  21. 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●Synthetic TC observations (bogus vortex) required● 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

  22. 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

  23. 72-hour forecast errors

  24. 4.0 ADVANCES IN OPERATIONAL TRACK PREDICTIONIn 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 forecastingIWTC-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

  25. Western North Pacific (1992-2006) 72-h TC Track Forecast Error (nm)

  26. TOPIC 3 SUMMARY REPORT OPTIMAL 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

  27. 5.0 FUTURE SHORT-TERM TRACK PREDICTION EMPHASESForecasters 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

  28. Average track forecast errors (nm) for the 1 to 5 day period before landfall (2001-2005)

  29. 5.0 FUTURE LONG-TERM TRACK PREDICTION EMPHASESFurther 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

  30. 5.0 FUTURE TRACK EMPHASIS - CONFIDENCE MEASURE-IEspecially 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

  31. 5.0 FUTURE TRACK EMPHASIS - CONFIDENCE MEASURE-IIAn 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) programIWTC-VI Recommendations: ● Facilitate dissemination of ensemble forecasts ● Develop probabilistic forecast products International Training Workshop on Tropical Cyclone Disaster ReductionGuangzhou, China 26 March 2007

  32. 5.0 FUTURE TRACK EMPHASIS – TARGETED OBSERVATIONSTopic 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 valueFour 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

  33. 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 agoMajor 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

  34. 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

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