By: Travis PerryMajor: Atmospheric Science

REDUCED DRAG COEFFICIENT FOR HIGH WIND SPEEDS IN TROPICAL CYCLONES

MARK D. POWELL, PETER J. VICKERY & TIMOTHY A. REINHOLD

AIR-SEA EXCHANGE IN HURRICANESPETER 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

TROPICAL CYCLONES(STUFF YOU SHOULD ALREADY KNOW)

• Also known as hurricanes, typhoons and cyclones

• Hurricane-scale winds

• Rainfall

• Storm Surge (winds blowing coastward + lower atmospheric pressure)

• Fine-Scale Tornadoes

TROPICAL CYCLONEDESTRUCTION AND FATALITIES

SAFFIR-SIMPSON SCALEHURRICANE 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 DROPWINSONDEIMPORTANCE 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 COEFFICIENTIMPORTANCE AND EQUATIONS

• In strong winds, momentum exchange at the sea surface is described by a sea-state-dependent 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

• 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.

MEAN WIND SPEED

WIND SPEEDS IN DIFFERENT MEAN BOUNDARY LAYERS

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

REASON FOR REDUCTION OF DRAG COEFFICIENT

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 subsurface-profiling floats.

FLIGHT PATTERN

FLIGHT PATTERNSINGLE PLANES

• 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 22-23ms^-1 while previous paper showed leveling off around 33ms^-1

DRAG AND MOIST EXCHANGE COEFFICIENTS

• 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 long-wavelegnth waves moving outward relative to the wind by 60-90 degrees.

SURFACE WAVE OBSERVATIONS

• 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.

FLOATS AND DRIFTERS

RELATIONSHIP BETWEEN WIND SPEED, WAVE HEIGHT AND BUBBLE 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.

MIXED LAYER DEPTH

• Cd leveling off near 22-23ms^-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.

SUMMARY AND MY THOUGHTS

QUESTIONS?