REDUCED DRAG COEFFICIENT FOR HIGH
WIND SPEEDS IN TROPICAL CYCLONES
MARK D. POWELL, PETER J. VICKERY & TIMOTHY A. REINHOLD
AIR-SEA EXCHANGE IN HURRICANES
PETER G. BLACK, ERIC A. D’ASARO, WILLIAM M. DRENNAN, JEFFERY R. FRENCH, PEARN P. NILLER, THOMAS B.
SANFORD, ERIC J. TERRILL, EDWARD J. WALSH, AND JUN A. ZHANG
By: Travis Perry
Major: Atmospheric Science
TROPICAL CYCLONES
(STUFF YOU SHOULD ALREADY KNOW)
•
Also known as hurricanes, typhoons and cyclones
TROPICAL CYCLONE
DESTRUCTION AND FATALITIES
• Hurricane-scale winds
• Rainfall
• Storm Surge (winds
blowing coastward +
lower atmospheric
pressure)
• Fine-Scale Tornadoes
SAFFIR-SIMPSON SCALE
HURRICANE INTENSITY SCALE
POP QUIZ
(HOPE YOU PAID ATTENTION IN 520)
• What are 6 necessary environmental conditions for tropical
cyclone formation?
• 1. SST>27ºC (about 80ºF)
• 2. Warm ocean mixed layer thick enough to supply energy
• 3. Unstable atmosphere with a moist lower/middle troposphere
• 4. Low vertical windshear
• 5. Coriolis force (do not form between 5N-5S)
• 6. Pre-existing low-level rotating circulations.
GLOBAL POSITIONING SYSTEM DROPWINSONDE
IMPORTANCE AND PROBLEMS USING THEM
•
GPS sondes are used to get measurements inside of hurricanes.
•
Measures pressure, temperature, humidity and position every 0.5 seconds.
•
Accuracy: wind 0.5-2.0 ms^-1 and height within 2 m
•
Dropped from 1.5-3 km or higher and falls at 10-15ms^-1
•
Problems – Turbulence and intense rainfall can cause signal interruptions or failures
to report values.
DRAG COEFFICIENT
IMPORTANCE AND EQUATIONS
•
In strong winds, momentum exchange at the sea surface is described by a sea-statedependent drag coefficient. (Cd)
•
Never been observed in tropical cyclones but used in prediction models
•
Forecast track, intensity, surface wind speeds, geographic distribution of extreme wind,
storm surge and wave forecast.
•
U=mean wind speed, Ustar=friction velocity, Z=height, Zo=surface roughness length,
tao=surface momentum flux, rho=air density, U10= 10-m wind speed, g=gravity
MEAN WIND SPEED
• Logarithmic increase to
max around 500m.
• Decrease due to the
weakening of the Horizontal
PG in warm core of
cyclone.
• Variability due to convective
scale features in eye wall
as well as the location of
the GPS sonde launch.
WIND SPEEDS IN DIFFERENT MEAN BOUNDARY
LAYERS
REDUCTION OF DRAG COEFFICIENT
REASON FOR REDUCTION OF DRAG
COEFFICIENT
• Foam patches
• Formed from steep
waves being sheared
off by high winds.
• Creates a ‘slip’
surface.
• GPS sonde errors
• Sampling strategy
AIR SEA EXCHANGE IN HURRICANES
• CBLAST – Coupled Boundary Layer Air-Sea Transfer
• CBLAST used to improve TC track and intensity forecast.
• Two observational components
• 1) airborne in situ and remote sensing instrumentation flown
into hurricanes by the two NOAA WP-3D aircraft
• 2) air-deployed surface-drifting buoys and subsurfaceprofiling floats.
FLIGHT PATTERN
FLIGHT PATTERN
SINGLE PLANES
DRAG AND MOIST EXCHANGE
COEFFICIENTS
• Red dashed lines are
values from previous paper.
• Gray circles are CBLAST
values.
• Each show leveling off or
decrease but CBLAST
shows leveling off near 2223ms^-1 while previous
paper showed leveling off
around 33ms^-1
SURFACE WAVE OBSERVATIONS
•
Wave height – dashed black contours
•
Wave steepness – thin blue contours
•
Swell direction – solid black
‘streamlines”
•
I. Unimodal, short-wavelength and
waves move with wind
•
II. Bimodal longer wavelengths,
outward relative to dominant waves
moving outward up to 45 degrees
relative to wind direction.
•
III. Unimodal spectra with peak longwavelegnth waves moving outward
relative to the wind by 60-90 degrees.
FLOATS AND DRIFTERS
• SOLO floats profiles
temperature, salinity and
oxygen
• EM-APEX profiles
temperature, salinity and
velocity
• Lagrangian profiles
temperatue, salinity and
gas concentration
• Drifters profile temperature
and wind speeds and
directions.
RELATIONSHIP BETWEEN WIND SPEED, WAVE
HEIGHT AND BUBBLE LAYER DEPTH
MIXED LAYER DEPTH
• Rapid deepening of the mixed
layer associated with high
shear across the thermocline.
• Strong wind and wave forcing
directly generates turbulence in
the upper 20-40m of the ocean.
• SST front created with
temperature range of about
27.5-30 degrees Celsius. Cold
wake formed from passage with
a max decrease of about 3.2
degrees Celsius.
SUMMARY AND MY THOUGHTS
• Cd leveling off near 2223ms^-1 most likely due to a
bubble layer.
• Drifter and buoy
deployments give us first
time ever boundary layer
observations.
• Cold wakes are formed from
passage of Tropical
Cyclones.
• Great Start in TC research.
• Not a lot to disagree with.
QUESTIONS?