Air-Sea Exchange in Hurricanes by Peter G. Black & Hurricane Intensity and Eyewall Replacement by Robert A. Houze Jr. Lynsie M. Schwerer Atmospheric Science

What’s the big deal?

● A new era with greater number of hurricanes ● Over 15 years tracks of hurricanes are better forecasted - not intensity Houston, we have a problem...

C- Coupled B- Boundary L- Layer A- Air S- Sea T- Transfer CBLAST Experiment ( ) Brought to you by: ●HRD ●ONR ●NOAA ●U.S. AFSC’s 53rd WRS

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

1) Airborne in situ and remote sensing instrumentation 2) Air deployed surface-drifting “drifters” and “floats” The Plan

Drifters and Floats Drifters ● 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

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

●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

●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

Surface Wave Height

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

Howze and Hurricane Intensity ● Similarly concerned with lack of intensity forecasting abilities, particularly following Hurricane Katrina (2005) ● Slightly different approach

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”

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

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

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

Detailed modeling of eyewall replacement helps forecasters predict hurricane intensity! Results

● 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...

Questions?