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

1. MOTIVATION FOR CBLAST CBLAST was an effort conceived by the Office of Naval Research (ONR) to fill a gap in our understanding of air-sea transfer processes at extreme wind conditions. Developed under an Accelerated research Initiative, CBLAST Hurricane was conceived to improve understanding of air-sea flux processes in extreme hurricane wind conditions. It was widely recognized that approximation of air-sea flux of momentum and enthalpy (heat and moisture) by bulk aerodynamic formulas involving exchange coefficients was well established only for winds slightly in excess of gale, i.e. tropical storm, force (17 m/s). Dependence of the drag coefficient and enthalpy exchange coefficient on wind speed was only well established to winds of 20 m/s (Large and Pond, 1976; Smith, 1980; Fairall et al, 2003). Yet extrapolations to winds over 60 m/s is common in hurricane models, both operational and experimental. These extrapolations have taken on a wide range of values, given the lack of observations. CBLAST was therefore conceived as a first step to reduce this gap in measurements and basic understanding. The motivation of CBLAST was thus to 1) improve air-sea flux physical understanding, 2) improve air-sea flux parameterization and 3) through this effort to improve Tropical Cyclone (TC) intensity prediction. While significant progress has been made in improving forecasts of TC track over the past 30 years (Franklin et al 2003), little improvement in forecasts of TC intensity has been achieved (DeMaria et al. 2005). While many factors have contributed to the lack of intensity forecast skill, lack of basic understanding of three major areas is key: 1) environmental interactions with the surrounding atmosphere and underlying ocean, 2) internal dynamics such as eyewall replacement cycles and 3) physics of air-sea flux at the ocean surface. This latter is the focus of CBLAST, especially air-sea exchange at extreme winds and how it depends upon surface wave spectral characteristics, spray and foam production and secondary boundary layer circulations. This undertaking has taken on added significance and urgency in light of the new era of increased numbers of hurricanes (Goldenberg, et al, 1998) and the increased numbers of landfalling hurricanes in the past two years.