PARAMETERIZATION FOR CO_2 TRANSFER VELOCITY AT THE SURFACE OF WIND WAVES

摘要

Laboratory experiments were made to parameterize accurately the gas transfer velocity of carbon dioxide (CO_2) at the surface of wind waves. The local flux of CO_2 was estimated at several fetches in a wind-wave tank by fitting logarithmic laws to vertical profiles of the mean wind speed and the mean concentration of CO_2 in the air. The concentration of CO_2 dissolved in the water was quantified by means of a gas-liquid equilibrator made of a hydrophobic porous tube. The local gas transfer velocity at the water surface was obtained from the local flux and the difference between the bulk concentration in the water and the equilibrium concentration at the water surface. The transfer velocity increases steadily with the friction velocity, and also increases with the fetch under the condition of the same friction velocity. For the parameterization, two dimensionless parameters gx/u_(*a)~2 and u_(*a)~3/gv_a, were derived from a dimensional analysis, where u_(*a) is the friction velocity on the air side, g the acceleration of gravity, x the fetch and v_a the kinematic viscosity of the air. On the basis of the experimental results and the dimensional analysis, we proposed a new type of empirical expression for the transfer velocity in consideration of the fetch dependence. With the fetch relations for wind waves, the expression can be described in terms of local dimensionless wind-wave parameters, i.e., gE~(1/2)/u_(*a)~2 or ω_pu_(*a)/g instead of gx/u_(*a)~2, where E represents the total energy of the surface displacement and ω_p the spectral peak angular frequency of wind waves.