Dynamics of Atmospheres and Oceans 73 (2016) 1–9 Contents lists available at ScienceDirect Dynamics of Atmospheres and Oceans journal homepage: www.elsevier.com/locate/dynatmoce Foam input into the drag coefficient in hurricane conditions Ephim Golbraikh a,∗ , Yuri M. Shtemler b a Department of Physics, Ben-Gurion University of the Negev, P.O. Box 653, Beer-Sheva 84105, Israel b Department of Mechanical Engineering, Ben-Gurion University of the Negev, P.O. Box 653, Beer-Sheva 84105, Israel a r t i c l e i n f o Article history: Received 10 February 2015 Received in revised form 3 October 2015 Accepted 26 October 2015 Available online 10 November 2015 Keywords: Hurricane Ocean–atmosphere exchange Ocean boundary layer Foam a b s t r a c t A semi-empirical model is proposed for the estimation of the foam impact on the variation of the effective drag coefficient, C d , with the reference wind speed U 10 in stormy and hurricane conditions. The proposed model treats the efficient air–sea aerodynamic roughness length as a sum of two weighted aerodynamic roughness lengths for the foam-free and foam- covered conditions. On the basis of available optical and radiometric measurements of the fractional foam coverage and partitioning of the ocean surface into foam-covered and foam-free areas, the present model yields the resulting dependence of C d vs. U 10 within the range from low to hurricane wind speeds. This dependence is in fair agreement with those obtained from both open-ocean and laboratory measurements of the vertical variation of the mean wind speed. The velocity value, at which the fractional foam coverage is saturated, is found to be responsible for the difference of C d behavior in the laboratory and open-ocean conditions. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Saturation/reduction of the effective drag coefficient, C d , for the air–sea interface with wind speed rising up to hurricane (typhoon) conditions has been a focus of intensive experimental investigation over the last decade. Many field experiments (Powell et al., 2003; Black et al., 2007; Edson et al., 2007; Jarosz et al., 2007; Holthuijsen et al., 2012), laboratory (Donelan et al., 2004; Reul et al., 2008; Troitskaya et al., 2012), and theoretical studies (Bye and Jenkins, 2006; Kudryavtsev and Makin, 2007; Bye and Wolff, 2008; Mueller and Veron, 2009; Soloviev and Lukas, 2010; Suzuki et al., 2013; Soloviev et al., 2014, etc.) have been conducted to study variations of the ocean-surface momentum transfer and effective drag coefficient with wind speed in hurricane conditions. A reduction of the ocean-surface drag coefficient in hurricane conditions instead of its monotonic growth with wind speed (predicted by the Charnock relation that is commonly employed in moderate wind conditions Charnock, 1955) has been found by Powell et al. (2003). As conjectured by Powell et al. (2003) and Holthuijsen et al. (2012), the foam cover increases due to wave breaking and forms a slip surface on the atmosphere–ocean interface that leads to a saturation/reduction of the effective drag coefficient in hurricane conditions. Saturation in the drag coefficient growth has been observed in laboratory experiments by Donelan et al. (2004) who note that “one may expect a qualitatively different behavior in its properties than that suggested by observations in moderate wind conditions”. The principal role of the air–sea foam layer has been first suggested by Newell and Zakharov (1992). According to empirical data, foam formation is highly correlated with wind speed and sea gravity waves breaking (Stogryn, 1972; Monahan and O’Muircheartaigh, 1980; Monahan and Woolf, 1989; Reul and Chapron, 2003; Callaghan et al., 2007, etc.). The foam fractional ∗ Corresponding author. E-mail address: golbref@bgu.ac.il (E. Golbraikh). http://dx.doi.org/10.1016/j.dynatmoce.2015.10.005 0377-0265/© 2015 Elsevier B.V. All rights reserved.