air-sea exchange in hurricanes by peter g. black & hurricane intensity and eyewall replacement...
TRANSCRIPT
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Air-Sea Exchange in Hurricanes
by Peter G. Black
&Hurricane Intensity and Eyewall
Replacementby Robert A. Houze Jr.
Lynsie M. SchwererAtmospheric Science
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What’s the big deal?
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● A new era with greater number of hurricanes
● Over 15 years tracks of hurricanes are better forecasted -not intensity
Houston, we have a problem...
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C- CoupledB- BoundaryL- LayerA- AirS- SeaT- Transfer
CBLAST Experiment (2000-2005)
Brought to you by:
● HRD● ONR● NOAA● U.S. AFSC’s 53rd
WRS
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Where to start?● Energy exchange at the air-sea
interface is one of three processes dictating hurricane intensity
● Specifically exchange of heat,moisture and momentum
● Goal-provide new understanding to improve forecasting
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1)Airborne in situ and remote sensing instrumentation
2)Air deployed surface-drifting “drifters” and “floats”
The Plan
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Drifters and FloatsDrifters● Aim to follow
ocean current at 15m depth
● Measure temp profile to 100m depth
● Estimate wind speed and direction as well
Floats● Measure velocity,
temperature, salinity
● Change volume & density
● Allows them to change depth
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Results (Aircraft Component)● Estimation of surface
momentum and enthalpy flux from direct eddy correlation measurements
● First direct-flux measurements ever made in a hurricane
● Evidence for secondary boundary layer circulations
● Surface wave observations
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● CBLAST Cd measurements agree with recorded gale force, open ocean wind measurements (17-22m/s)
● Values are lower than Cd observations for limited wave conditions
● Little dependence on storm quadrant
● However, natural variability shows little confidence in this result
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● Ratio is very important in estimating hurricane intensity
● Average is around .7 for tropical storm conditions-close to the Emanuel (1986) .75 threshold for hurricanes \
● These levels indicate “superintense” conditions leading to category 4 and 5 conditions
● Strong air-sea interaction possibly explains why Isabel was category 5 for 3 days
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Surface Wave Height
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Results (Oceanographic Sensor)● New information on ocean
structure and structure changes induced by a hurricane below ocean mixed layer
● First ever 4D ocean structure observations beneath a hurricane
● Goal: understand properties of sea-air interface and upper ocean at wind speeds greater than 30m/s to determine air-sea fluxes
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Howze and Hurricane Intensity
● Similarly concerned with lack of intensity forecasting abilities, particularly following Hurricane Katrina (2005)
● Slightly different approach
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Structure of a Hurricane (crash course)
● Radius of maximum wind is located in a circular region of heavily precipitating cloud called the “eyewall”
● Eyewall encircles relatively calm “eye”
● Outside eyewall are convective cloud lines that spiral in towards the eyewall called “rainbands”
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What is Eyewall Replacement?● Key process in hurricane
intensity change● New outer eyewall forms and
takes over from inner (original) eyewall
● Storm deintensifies as original eyewall is “choked off”
● New eyewall may shrink in radius so that the storm reintensifies
● Occurs naturally in hurricanes
Hurricane Rita
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The Mission● Goal of learning when
eyewall replacement would and would not occur to better forecasting
● Aircraft focused on documenting specific aspects of rainbands and eyewalls that appear most likely to interact and affect intensity changes
● Hurricane Rita studied as an example
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What’s going on?● Old eyewall draws in near surface air, causing air in the eye to
sink as compensation for the transfer● Air inside eye warms and dries, creating characteristic “cloud
free” hole● Near the surface, drying is counteracted by turbulent mixing in
the ocean. This transfers sea spray and and water vapor into the air to maintain high humidity.
● Result: hurricanes have a 2 layer structure- moist at lower levels and dry at higher levels
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Detailed modeling of eyewall replacement helps forecasters predict hurricane intensity!
Results
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● Impact of CBLAST hurricane research has just begun!
● Support already obtained from NOAA and ONR to continue operations
● Specific focus on impact of sea spray on air-sea enthalpy
● Cloud seeding endeavors- attempt to induce eyewall reduction
Looking to the Future...
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Questions?