Topic 0.2 – Observations and Forecasts of Wind Distribution

Kevin CHEUNG (USA)Akhilesh GUPTA (India)Bruce HARPER (Australia)Jeff KEPERT (Australia)Kenichi KUSUNOKI (Japan)

S.T. CHAN(Hong Kong)

Slide #2IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

AgendaRecent researches on TC wind distribution evolution arising from land-sea contrasts

Land-induced asymmetric friction on boundary layer winds

Observational issues Fine-scale surface wind features in landfalling TC Deployment of surface wind observation network Wind speed averaging standards

Forecasting issues Progress in modeling of related BL processes in NWP Deployment of empirical & parametric wind field models

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Part I New Research Developments

Slide #4IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

Land-induced Asymmetric Friction

Provides a Wave-No. 1 forcing as does motion-induced asymmetry -> asymmetric boundary-layer wind structureWind maximum for TC making landfall

Right forward quadrant (motion-induced) Offshore flow to the left of track (landfall-induced)

Kepert 2006 suggested land at ~3X RMW could produce marked asymmetry in the inner coreEnhanced inflow near land extends over offshore gives increased angular momentum advection to cause strong winds in the offshore-flow side of storm

Slide #5IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

Hurricane Mitch (1998)

Surface wind max located to left rear of stormStrongest winds rotated anticyclonically with heightStrongest inflow 90° of azimuth upstream

From Kepert (2006)

Slide #6IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

Effect on Inner-core Structure

Another modeling study by Chen & Yau (2003)Diagnosis utilizing PV flux analysesBand of PV develops along the coastlineInteraction with eyewall PV ring leads to an observed 2-hr weakening & re-intensifying cycle Responsible for eyewall replacement cycles?

Slide #7IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

Effect on Storm Motion

Friction due to proximity to land also induces large-scale asymmetries in surface convergenceCauses 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

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Part II Observational Issues

Slide #9IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

BL rolls commonly observed in TC BL may produce damaging winds at surfaceLorsolo et al. (2006) and Wurman et al. (2006) find the rolls coherent through the depth of BL and circulation extended to surface, though with attenuationKusunoki & Mashiko (2006)‘s observational study of Typhoon Songda (2004) landing on Okinawa Island

Fine-scale Surface Wind Features

Slide #10IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

Fine-scale Features – T. SongdaThe perturbation reflectivity fields reveals small-scale features spiraling outward from eyewallAverage band wavelength ~7 km and width ~3 kmShort time-scale (~10 min) wind perturbations (~6 m/s) during passage of bands

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Slide #11IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

Extreme wind gusts of landfalling TC - in both horizontal and vertical components Although rare, caused extensive damage & not adequately represented by broad-brush scales like Saffir Simpson ScaleBetter characterization of their nature needed, say statistically using increasing sample (GPS dropsonde obs. collected in the past decade, Doppler radar, tower wind measurements, etc.)

Fine-scale Surface Wind Features

Slide #12IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

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 turbulence

From Shun et al. (2003)

Airport

Slide #13IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

Engineering models correlating topographic speed-up factors and observed building damagesMueller et al. (2006) -Bermuda during Hurricane Fabian Similar analysis being undertaken in Australia following extremely damaging landfall of Severe TC Larry in 2006Useful work for design of structures and climatological risk analysis

Fine-scale Surface Wind Features

Slide #14IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

Maintaining and expanding surface wind measurement networks in TC-prone areas remains a critical need for wind hazard monitoring; and for verification of forecast/modeled winds Robust instrumentation that can withstand high winds (rugged structure, backup power, data storage)Opportunities: emergence of wireless networking protocols, low-cost low-power electronicsAlternatives to costly conventional height towers: mobile wind sensing systems & “infrastructure of opportunity”, e.g. power transmission line/communications towers

Surface Wind Observation Network

Slide #15IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

Standardization of wind analysis needed for building reliable ground truthPowell et al. (2004) presented a US project to photographically document exposures of hundreds of AWSRoughness lengths for each octant of wind direction estimated for conversion of wind measurements to open terrainDemonstrated that wind measurements associated with significant terrain upstream may underestimate open-terrain wind by ~30%

Surface Wind Observation Network

Slide #16IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

Local preferences in reporting wind strength: 1, 2, 3 and 10-min averageLack of a “gust” wind standard under WMO associationsExistence of different “standards”

adds uncertainties in wind measurements/estimation introduces difficulties in transferring forecast

techniques from one region to anotherFollowing IWTC-V, WMO commissioned a review to recommend refined conversion factors between different averaging periods -> WMO Global Guide to TC ForecastingDraft report prepared; some outstanding work to complete the review

Wind Speed Averaging Standards

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Part IIIForecasting Issues

Slide #18IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

Successful prediction of structural changes of a landfalling TC depends much on adequacy in simulating BL processesPowell et al. (2003) shows that drag coefficients would decrease with wind speed -> impact TC intensity evolution -> improved parametrization scheme. How about land surface?High-resolution models could reproduce observed eyewall evolution over land but physical processes involved still not yet fully understood (Wang & Wu, 2004)

Modeling of BL Processes in NWP

Slide #19IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

Downward transfer of momentum significant in maintaining high winds observed on the lee sides of high terrain (Kasheta & Chang, 2002)Use of a very fine grid (say, 1 km mesh) and a refined surface roughness length scheme based on surface canopy as well as terrain height could capture the details of such terrain-induced downdraftsIn coming years, studies on sensitivities to various parameters in a land surface model are needed

Modeling of BL Processes in NWP

Slide #20IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

Parametric models (e.g. Holland 1980), relying on having some surface observations, provide spatial context thus usefully augment DvorakOperation boosted in recent years by availability of scatterometer data to provide outer spatial scale of storms (e.g. radius of gale winds)Willoughby et al. (2005) analysed several decades of reconnaissance flights and refined the description of TC wind field & data dependencies of spatial scale, intensity and latitudinal variationsKossin et al. (2006) introduces algorithms to estimate spatial scale parameters & entire 2-D wind distribution within 200 km of storm based on EIR imageries

Parametric Wind Field Modeling

Slide #21IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

Empirical models developed for various regions (e.g. Vickery 2005 for coast of US and Roy Bhowmik et al. 2005 for east coast of India)Rate of storm filling proportional to central pressure difference and translation speed, inversely proportional to RMWNote: relatively flat coastal areas considered. TC may be deflected and wind distribution would be much different when orographic influence is effective (e.g. Taiwan)

Empirical Models

Slide #22IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

SHIPS (Stat. Hurricane Intensity Prediction Scheme), a multiple linear regression model for operational intensity forecasting in Atlantic & East PacificAn empirical exponential decay model introduced in 2000 to account for decay over landA modified decay model [DeMaria et al. (2006)], which includes a factor equal to the fraction of storm over land, was further introduced

Empirical Models

Slide #23IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

New scheme reduced the intensity forecast errors by ~8% relative to original model (2001-2004)

STIPS - a variant of SHIPS was developed and operationally deployed in W North Pacific basin at JTWC in 2002 and updated in 2003

Empirical Models

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Summary & Recommendations

Slide #25IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

Summary & Recommendations

Contributions to asymmetry in TC structure due to motion and proximity to land could be comparable. Interaction between them may be important-> full investigation needed

Damaging extreme wind gusts induced by convective, coherent or vortex-related features not adequately represented by broad-brush scales in use-> better characterization using increased observational data (Doppler radar, tower winds, GPS dropsondes) should become possible

Slide #26IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

Summary & Recommendations (Cont’d)

Recent use of exposure-based engineering model to quantify destructive potential of topographic effects noted-> foundational to design of structures and to climat. risk analysis; extensive verification desired

Recent progress made in understanding air-sea exchange under high winds, similar validation of land surface schemes also required -> sensitivity studies on various parameters in land surface model

Slide #27IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

Summary & Recommendations (Cont’d)

More extensive use of parametric/empirical wind field models recommended for their

quality surface wind estimates compared with Dvorak relatively low cost & effort compared with NWP

Need for enhancing surface wind networks in TC-prone areas. Innovative alternatives to conventional weather stations could be explored

Need for improved wind measurement and reporting standards to ensure consistency across various forecast techniques

Slide #28IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

ReferencesChen, Y. and M.K. Yau, 2003: Asymmetric structures in a simulated landfall hurricane. J. Atmos. Phys., 60, 2294-2312DeMaria, M., J.A. Knaff and J.Kaplan, 2006: On the decay of tropical cyclone winds crossing narrow landmasses. J. Appl. Met. and Clim., 45, 491-499Holland, G.J., 1980: An analytical model of the wind and pressure profiles in hurricanes. Mon. Wea. Rev., 108, 1212-1218Kasheta, T.E. and C.B. Chang, 2002: Development of a hurricane boundary-layer wind model. Meteorology and Atmospheric Physics, 79, 259-273Kepert, J.D., 2006: Observed boundary-layer wind structure and balance in the hurricane core. Part II: Hurricane Mitch. J. Atmos. Sci., 63, 2194-2211Kossin, J.P., J.A. Knaff, H.I. Berger, D.C. Herndon, T.A. Cram, C.S. Velden, R.J. Murnane and J.D. Hawkins, 2006: Estimating hurricane wind structure in the absence of aircraft reconnaissance. Weather and Forecasting, submitted.Kusunoki, K. and W. Mashiko, 2006: Doppler radar investigations of the inner core of Typhoon Songda (2004) – polygonal / elliptical eyewalls, eye contraction, and small-scale spiral bands. Extended abstracts, 27th Conference on Hurricanes and Tropical Meteorology. Amer. Meteorol. Soc., Monterey, CA, April 24-28. Paper P4.10Lorsolo, S. and J.L. Schroeder, 2006: Tower and Doppler radar observations from the boundary layer of Hurricane Isabel (2003) and Frances (2004). Extended abstracts, 27th Conference on Hurricanes and Tropical Meteorology. Amer. Meteorol. Soc., Monterey, CA, April 24-28. Paper 10C5

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ReferencesMueller, K.J., C. Miller, K. Beatty and A. Boissonnade, 2006: Correlation of topographic speed-up factors and building damage ratios for Hurricane Fabian in Burmuda. Extended abstracts, 27th Conference on Hurricanes and Tropical Meteorology. Amer. Meteorol. Soc., Monterey, CA, April 24-28. Paper 5A8Powell, M., P.J. Vickery and T.A. Teinhold, 2003: Reduced drag coefficient for high wind speeds in tropical cyclones. Nature, 422, 279-283Powell, M., D. Bowman, D. Gilhousen, S. Murillo, N. Carrasco and R. St. Fleur, 2004: Tropical cyclone winds at landfall: The ASOS-C-MAN wind exposure documentation project. Bull. Amer. Meteor. Soc., 85, 845-851 Shun, C.M., S.Y. Lau, and O.S.M. Lee, 2003: Terminal Doppler Weather Radar Observation of atmospheric flow over complex terrain during tropical cyclone passages. J. Appl. Meteor., 42, 1697-1710 Roy Bhowmik, S.K., S.D. Kotal and S.R. Kalsi, 2005: An empirical model for predicting the decay of tropical cyclone wind speed after landfall over the Indian region. J. Appl. Meteor., 44, 179-185Vickery, P.J., 2005: Simple empirical models for estimating the increase in the central pressure of tropical cyclones after landfall along the coastline of the United States. J. Appl. Meteor., 44, 1807-1826Wang, Y. and C.C. Wu, 2004: Current understanding of tropical cyclone structure and intensity changes – a review. Meteor. and Atmos. Physics, 87, 257-258Willoughby, H.E., R.W.R. Darling and M.E. Rahn, 2005: Parametric representations of the primary hurricane vortex. Part II: A new family of sectionally continuous profiles. Mon. Wea. Rev., 134, 1102-1120Wong, M.L.M. and J.C.L. Chan, 2006: Tropical cyclone motion in response to land surface friction. J. Atmos. Sci., 63, 1324-1337Wurman, J., C. Alexander, P. Robinson and F. Masters, 2006: Preliminary comparison of DOW and in situ wind measurements in Hurricane Rita. Extended abstracts, 27th Conference on Hurricanes and Tropical Meteorology. Amer. Meteorol. Soc., Monterey, CA, April 24-28. Paper 10C6

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