Our primary goal is to improve our understanding of air-sea surface flux processes in high winds, specifically in the complex conditions of tropical hurricanes where swell, sea spray and secondary boundary layer circulations play a role. Our ultimate goal and prime motivation for this work is to parameterize these new observations and improve the accuracy of hurricane intensity prediction.

OBJECTIVES

The objective of this work is to develop a new surface wave-dependent flux parameterization for the high wind hurricane boundary layer containing secondary (roll-vortex) circulations over fetch limited seas in the presence of sea spray and one or more swell components from an airborne platform. We propose to test the following hypotheses:

1)  that surface momentum exchange coefficients increase with wind speed for moderate winds (>30 m/s),      are enhanced by fetch-limited waves or opposing swell, but level off or decrease above a high wind      threshold (>45 m/s), especially in quadrants where swell has a significant downwind component
2)  that compensating mechanisms for enhanced surface air-sea enthalpy fluxes over and above current      parameterizations must exist for storm maintenance and growth above some high-wind threshold wind
     speed
3)  that candidate mechanisms are separable and can be estimated, such as a) enhanced turbulent fluxes      due to wave interactions, b) spray evaporation and c) secondary flow circulations (roll-vortex type).

APPROACH

Our approach is to implement a multifaceted observational program among several investigators to simultaneously measure air-sea fluxes using several independent methods while at the same time measuring two-dimensional surface wave spectra, as well as spray droplet spectra , in wind speed regimes ranging from 20-40 m/s, and possibly higher. The secondary goal is to use existing data sets to inter-compare various published wave-dependent bulk flux parameterizations, with and without wave age effects and with and without spray parameterizations, using never-before-available surface inputs from GPS dropsondes, AXBTs, model-generated as well as remotely-measured wave spectra and remotely-measured surface winds in gale- and hurricane-force conditions.

Flight track design and evaluation, bulk surface parameter measurement for surface flux estimation and atmospheric and ocean profiling will be led by P. Black (HRD) and E. Uhlhorn (HRD/UM-CIMAS) will assist. This will include measurement of PBL wind, air temperature and specific humidity profiles from GPS sondes together with surface winds from the SFMR (HRD and UMASS versions) as well as mixed layer thermal profiles, sea surface temperature and upper ocean heat content from AXBTs. PBL continuous wind profiling and surface wind vector retrieval will be lead by D. McLaughlin (UMASS-MIRSL), with A. Zhang assisting, using the UMASS dual polarized, dual frequency Doppler scatterometer. Large scale surface wind retrievals will be lead by P. Chang (ORA) using satellite scatterometry. Large scale ocean heat content retreivals will be lead by G. Goni (PHOD) using satellite altimetry. Direct flux measurements will be lead by T. Crawford (FRD), J. French (FRD) and W. Drennan (UM-RSMAS). In a collaboative effort, K. Emanuel (MIT) will lead the effort to estimate boundary layer fluxes from GPS dropsonde profiles, including an initial assesment of this technique using recent data sets. R. Black (HRD) will collaborate with C. Fairall (ETL) in measurements of spray droplet spectra. Surface 2-D long-wave swell spectral measurements will be lead in a collaborative effort by E. Walsh (ETL/WFC). Measurements of surface wave dissipation due to wave breaking will be lead by Melville (SIO) in a collaborative effort using high speed video imaging. E. Terril (SIO) will lead a collaborative effort to obtain 1-D wind wave spectra using a laser altimeter, as well as ocean thermal and current profiles using subsurface floats. E. D'Asaro (UW) will lead a collaborative effort to measure ocean mixing profiles using subsurface floats.

Our strategy will be to obtain new parameterizations of momentum, heat and moisture fluxes from these observations including accurate estimates of the exchange coefficients as a function of wind speed and to estimate the modulation of the fluxes by fetch-limited wind waves, long-wavelength swell, sea spray and PBL secondary circulations, i.e.'roll vortices'.