need to improve track forecasts

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

1.1 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 Summary

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


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


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

Tides/Surge

Inland Flooding Landslide/Debris Flow

http://www.vdem.state.va.us/library/archive/images/hurrgaston04/lr102.JPG

SummarySummary

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


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


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


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


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


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)

250

300

350

400

450

500

550

600

650

700

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

Year

5-y

ea

r ru

nn

ing

me

an

err

or

(km

)

RSMC-Tokyo

NHC-EPC

NHC-Atlantic

JTWC-WNP

JTWC-NIO

JTWC-SH

CPHC

72-hour forecast errors

4.0 ADVANCES IN OPERATIONAL TRACK PREDICTION

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


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


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


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


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


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

IWTC-VI Recommendations: ● Facilitate dissemination of ensemble forecasts ● Develop probabilistic forecast products


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


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


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


need to improve track forecasts

Documents

tropical cyclones topic

tropical cyclones iwtc

track prediction1

storm tidesurge topic

rainfall effects topic

international workshop

storm trackeffects

storm center