12 Air-sea Interaction under Hurricane Wind Conditions Yuliya Troitskaya, Daniil Sergeev, Alexander Kandaurov and Vasilii Kazakov Institute of Applied Physics Russia 1. Introduction One of the main characteristics appearing in the models of forecasting wind over the sea is the roughness of the sea surface determined by the parameters of the wind waves, quantitatively parameterised by the sea surface drag coefficient C D . To define it we introduce the turbulent shear stress or turbulent momentum flux far from the sea surface 2 * () turb a z u τ ρ = , (1) where ρ a is the air density, u * is the wind friction velocity. Wind is the turbulent boundary layer with the logarithmic mean velocity profile: () * ln 0 u z Uz z κ = , (2) Similar to the resistance law of the wall turbulent flow the sea surface drag coefficient is introduced as follows: 2 * 2 2 10 10 u turb C D U U a τ ρ = = , (3) where U 10 -the wind velocity at a standard meteorological height ǻ 10 =10 m. which relate this coefficient to U 10 are obtained either by generalizing empirical data (Garratt, 1977; Large & Pond, 1981, Taylor & Yelland, 2002; Fairall et al., 2003) or by numerical models (see, for example, Janssen, 1989; Janssen, 1991, Makin et.al, 1994; Hara & Belcher, 2004). Numerous field measurements give increasing dependencies of C D on wind speed, which relates to increasing of wave heights with the wind. The aerodynamic drag coefficient of the sea surface is a critical parameter in the theory of tropical hurricanes (Emanuel, 1995). To illustrate it we consider here the ideas of theory of energy balance in a tropical cyclone suggested by (Emanuel, 1986; Emanuel, 1995, Emanuel, 2003). According to this theory the mature tropical cyclone may be idealized as a steady, axisymmetric flow whose energy cycle is very similar to that of an ideal Carnot engine, www.intechopen.com