Synthesis of major results from the Coupled Boundary Layer Air-Sea Transfer Experiment (CBLAST) in hurricanes (2003–2004)

The purpose of Hurricane CBLAST was to investigate the mechanisms for air-sea transfer in the high wind environment of hurricanes and to extend the range of observations for exchange coefficients of momentum and enthalpy to hurricane force winds and beyond. The experimental design consisted of two major components: 1) an aircraft component and 2) an air-deployed drifting buoy and float component. The aircraft component had two modules: a) an aircraft stepped descent module and b) a survey/eyewall multisonde deployment module. The former was to focus on in-situ flux and spray measurements, while the latter was to focus on large-scale structure and eyewall flux budget measurements. Both modules were complemented with an array of airborne remote and in-situ sensors. Buoy/float air-deployments consisted of arrays of measurements of surface and upper ocean conditions before, during and after hurricane passage. Together the aircraft and buoy/float array provided a unique description of air-sea fluxes, surface wave and upper ocean conditions in hurricane conditions never before achieved.

Measurements were made primarily in Hurricanes Fabian and Isabel in 2003 and in Frances, Jeanne and Ivan in 2004. Observations for the aircraft component of CBLAST were made by two NOAA WP-3D Orion aircraft flown by the NOAA Aircraft Operations Center (AOC) and manned by personnel from the Hurricane Research Division, AOC, NESDIS/ORA and CBLAST PI's. The observations obtained from the WP-3D aircraft was a collaborative effort between CBLAST PI's and the NESDIS Ocean Winds project. Collaborative operational Synoptic Surveillance flights from the NOAA G-IV and reconnaissance flights from the Air Force Reserve Command (AFRC) WC-130H Hercules aircraft flown by the 53rd Weather Reconnaissance Squadron were conducted together with most of the CBLAST flights, thus documenting environmental impacts and storm evolution. A total of 87 flights were flown in support of CBLAST, with 31 being direct CBLAST/ Ocean Winds flights: Fabian03 (3 days- 13 flights), Isabel03 (3 days-13 flights), Frances04 (5 days- 25 flights), Ivan04 (4 days- 24 flights) and Jeanne (3 days- 12 flights). Observations for the drifting buoy/ ocean float component of CBLAST were provided by air deployments of 16 drifting buoys and four floats in Hurricane Fabian (2003) and by the deployment of 38 drifting buoys (30 Minimet; 8 ADOS) and 16 floats (10 ARGO/SOLO; 2 gas flux; 2 Lagrangian; 2 EM/APEX) in Hurricane Frances. Deployments were carried out by the 53rd Weather Reconnaissance Squadron of AFRC using WC-130J and C-130J ‘stretch' aircraft.

The big picture of the collective impact of CBLAST initial results is presented in this paper. The CBLAST stepped descent flight segments have resulted in a new description of the behavior of the surface drag and enthalpy exchange coefficients for high winds from 18 to 32 m/s, nearly doubling the prior range of wind speeds for which measurements were available. Detailed bulk profiles of PBL structure in this wind regime have also been obtained concurrently with dropsondes and SFMR surface winds. The rapid-deployment sequence of 12 dropsondes (involving coordinated drops between the two P3 aircraft) across the hurricane eyewall in several quadrants of Fabian and Isabel have resulted in budget-based estimates of exchange coefficients at extreme winds speeds over 50 m/s. These observations together with data from the Tail (TA) Doppler radar and the Integrated Wind and Rain Atmospheric Profiler (IWRAP), a vertical Doppler profiler/scatterometer instrument, have provided documentation in finer detail than ever before of the boundary layer structure in hurricane eyewalls.

Airborne directional wave spectra were obtained from the Scanning Radar Altimeter (SRA) which have resulted in defining three sectors of the hurricane which have uniquely different wave spectra and surface roughness conditions. This characterization allows direct flux observations to be stratified by wave regime for the first time. Wave spectra from the ARGO/SOLO floats in the Frances buoy/float array provided high frequency wave spectral measurements in extreme winds for the first time, which were concurrent with the SRA measurements.

A detailed four-dimensional evolution of the upper ocean temperature structure has also been derived from the Frances buoy/float deployment. Detailed mixed layer float measurements of mean currents and mixing processes were achieved concurrently with the SOLO float observations. The mix of buoy and float data provided unprecedented detail in sea surface temperature time/space evolution during hurricane passage. The collective effect of this defining data set on our new view of air-sea transfer processes will be described in detail as well as the initial impact on evolving hurricane coupled modeling efforts.

Concurrent surface winds from the Stepped Frequency Microwave Radiometer (SFMR) were measured throughout all CBLAST/Ocean Winds flights. This together with the unprecedented SST observations and GPS sonde observations of surface air temperature and humidity have also provided data for computation of surface flux variability due to different flux parameterization schemes, including the CBLAST high wind parameterization.