Air-Sea Exchange in Hurricanes

by Peter G. Black

&Hurricane Intensity and Eyewall

Replacementby Robert A. Houze Jr.

Lynsie M. SchwererAtmospheric 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- CoupledB- BoundaryL- LayerA- AirS- SeaT- Transfer

CBLAST Experiment (2000-2005)

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 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

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?