Topic 0.2 – Observations and Forecasts of Wind Distribution

1. 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)

2. Agenda • Recent 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 IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

3. Part I New Research Developments IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

4. Land-induced Asymmetric Friction • Provides a Wave-No. 1 forcing as does motion-induced asymmetry -> asymmetric boundary-layer wind structure • Wind 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 core • Enhanced inflow near land extends over offshore gives increased angular momentum advection to cause strong winds in the offshore-flow side of storm IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

5. Azimuthal Radial From Kepert (2006) Hurricane Mitch (1998) • Surface wind max located to left rear of storm • Strongest winds rotated anticyclonically with height • Strongest inflow 90° of azimuth upstream IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

6. Effect on Inner-core Structure • Another modeling study by Chen & Yau (2003) • Diagnosis utilizing PV flux analyses • Band of PV develops along the coastline • Interaction with eyewall PV ring leads to an observed 2-hr weakening & re-intensifying cycle • Responsible for eyewall replacement cycles? IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

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

8. Part II Observational Issues IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

9. Fine-scale Surface Wind Features • BL rolls commonly observed in TC BL may produce damaging winds at surface • Lorsolo et al. (2006) and Wurman et al. (2006) find the rolls coherent through the depth of BL and circulation extended to surface, though with attenuation • Kusunoki & Mashiko (2006)‘s observational study of Typhoon Songda (2004) landing on Okinawa Island IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

10. Fine-scale Features – T. Songda • The perturbation reflectivity fields reveals small-scale features spiraling outward from eyewall • Average band wavelength ~7 km and width ~3 km • Short time-scale (~10 min) wind perturbations (~6 m/s) during passage of bands IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

11. Fine-scale Surface Wind Features • 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 Scale • Better characterization of their nature needed, say statistically using increasing sample (GPS dropsonde obs. collected in the past decade, Doppler radar, tower wind measurements, etc.) IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

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

13. Fine-scale Surface Wind Features • Engineering models correlating topographic speed-up factors and observed building damages • Mueller et al. (2006) -Bermuda during Hurricane Fabian • Similar analysis being undertaken in Australia following extremely damaging landfall of Severe TC Larry in 2006 • Useful work for design of structures and climatological risk analysis IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

14. Surface Wind Observation Network • 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 electronics • Alternatives to costly conventional height towers: mobile wind sensing systems & “infrastructure of opportunity”, e.g. power transmission line/communications towers IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

15. Surface Wind Observation Network • Standardization of wind analysis needed for building reliable ground truth • Powell et al. (2004) presented a US project to photographically document exposures of hundreds of AWS • Roughness lengths for each octant of wind direction estimated for conversion of wind measurements to open terrain • Demonstrated that wind measurements associated with significant terrain upstream may underestimate open-terrain wind by ~30% IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

16. Wind Speed Averaging Standards • Local preferences in reporting wind strength: 1, 2, 3 and 10-min average • Lack of a “gust” wind standard under WMO associations • Existence of different “standards” • adds uncertainties in wind measurements/estimation • introduces difficulties in transferring forecast techniques from one region to another • Following IWTC-V, WMO commissioned a review to recommend refined conversion factors between different averaging periods -> WMO Global Guide to TC Forecasting • Draft report prepared; some outstanding work to complete the review IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

17. Part IIIForecasting Issues IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

18. Modeling of BL Processes in NWP • Successful prediction of structural changes of a landfalling TC depends much on adequacy in simulating BL processes • Powell 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) IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

19. Modeling of BL Processes in NWP • 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 downdrafts • In coming years, studies on sensitivities to various parameters in a land surface model are needed IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

20. Parametric Wind Field Modeling • Parametric models (e.g. Holland 1980), relying on having some surface observations, provide spatial context thus usefully augment Dvorak • Operation 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 variations • Kossin et al. (2006) introduces algorithms to estimate spatial scale parameters & entire 2-D wind distribution within 200 km of storm based on EIR imageries IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

21. Empirical Models • 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 RMW • Note: relatively flat coastal areas considered. TC may be deflected and wind distribution would be much different when orographic influence is effective (e.g. Taiwan) IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

22. Empirical Models • SHIPS (Stat. Hurricane Intensity Prediction Scheme), a multiple linear regression model for operational intensity forecasting in Atlantic & East Pacific • An empirical exponential decay model introduced in 2000 to account for decay over land • A modified decay model [DeMaria et al. (2006)], which includes a factor equal to the fraction of storm over land, was further introduced IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

23. Empirical Models • 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 IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

24. Summary & Recommendations IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

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

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

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

28. References • Chen, Y. and M.K. Yau, 2003: Asymmetric structures in a simulated landfall hurricane. J. Atmos. Phys., 60, 2294-2312 • DeMaria, M., J.A. Knaff and J.Kaplan, 2006: On the decay of tropical cyclone winds crossing narrow landmasses. J. Appl. Met. and Clim., 45, 491-499 • Holland, G.J., 1980: An analytical model of the wind and pressure profiles in hurricanes. Mon. Wea. Rev., 108, 1212-1218 • Kasheta, T.E. and C.B. Chang, 2002: Development of a hurricane boundary-layer wind model. Meteorology and Atmospheric Physics, 79, 259-273 • Kepert, J.D., 2006: Observed boundary-layer wind structure and balance in the hurricane core. Part II: Hurricane Mitch. J. Atmos. Sci., 63, 2194-2211 • Kossin, 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 P4.10 • Lorsolo, 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 IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

29. References • Mueller, 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 5A8 • Powell, M., P.J. Vickery and T.A. Teinhold, 2003: Reduced drag coefficient for high wind speeds in tropical cyclones. Nature, 422, 279-283 • Powell, 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, 179-185 • Vickery, 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, 1807-1826 • Wang, Y. and C.C. Wu, 2004: Current understanding of tropical cyclone structure and intensity changes – a review. Meteor. and Atmos. Physics, 87, 257-258 • Willoughby, 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, 1102-1120 • Wong, M.L.M. and J.C.L. Chan, 2006: Tropical cyclone motion in response to land surface friction. J. Atmos. Sci., 63, 1324-1337 • Wurman, 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 IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution

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