Topic 0.2 – Observations and
Forecasts of Wind Distribution
S.T. CHAN
(Hong Kong)
Kevin CHEUNG
Akhilesh GUPTA
Bruce HARPER
Jeff KEPERT
Kenichi KUSUNOKI
(USA)
(India)
(Australia)
(Australia)
(Japan)
Agenda
Recent researches on TC wind distribution
evolution arising from land-sea contrasts
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Land-induced asymmetric friction on boundary layer winds
Observational issues
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Fine-scale surface wind features in landfalling TC
Deployment of surface wind observation network
Wind speed averaging standards
Forecasting issues
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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
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Part I
New Research
Developments
IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution
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Land-induced Asymmetric Friction
Provides a Wave-No. 1 forcing as does motioninduced asymmetry -> asymmetric boundary-layer
wind structure
Wind maximum for TC making landfall
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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
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Hurricane Mitch
(1998)
From
Kepert (2006)
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
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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
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Effect on Storm Motion
Friction due to proximity
to land also induces largescale 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
Dots denote 12-hrly
TC positions
From Wong & Chan (2006)
IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution
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Part II
Observational
Issues
IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution
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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
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Fine-scale Features – T. Songda
dBZ
Distance from the radar (km)
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
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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
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Fine-scale Surface Wind Features
Terrain-induced accelerations – in the form
of shear lines, reverse flow, vortices,
streaks and downslope winds
Airport
From Shun et al. (2003)
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
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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
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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
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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
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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”
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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
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Part III
Forecasting
Issues
IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution
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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
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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
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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
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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
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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
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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
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Summary &
Recommendations
IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution
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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
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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
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Summary & Recommendations
(Cont’d)
More extensive use of parametric/empirical wind
field models recommended for their
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quality surface wind estimates compared with Dvorak
relatively low cost & effort compared with NWP
Need for enhancing surface wind networks in TCprone 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
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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
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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, 845851
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
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IWTC-VI Topic 0.2 - Observations and Forecasts of Wind Distribution
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