The dependence of bulk evaporation coefficients on air‐water interfacial conditions as determined by the isotopic method

Abstract

The analysis of the natural distribution of deuterium and oxygen 18 in moisture inside the turbulent boundary layer developed above a water surface makes possible the investigation of the mechanism of evaporation. The distribution of isotopes in water vapor allows the calculation of the relative contributions of molecular and turbulent transfer to the total mass transport (Merlivat and Coantic, 1975). We have applied this method to assess the influence of the conditions at the liquid surface on the evaporation flux. In particular, the influence of wind waves and superposed artificially generated waves was studied. Experiments have been carried out at the Institut de Mécanique Statistique de la Turbulence air‐water tunnel, specially designed for the simulation of ocean‐atmosphere energy exchanges. Here the wind velocities could be varied from 0.7 to 6 m/s. Waves were generated by a motor‐driven paddle. The wave frequency was 1 Hz, and the maximum height 12 cm, while the surface roughness Reynolds numberResvaried from 0.02 to 2. Mean wind velocity profiles and isotopic profiles were measured. Experimental drag coefficients are described quite well by Charnock's relationZ0=u*2/ag., wherea= 81.1. No specific difference is observed when waves are artificially generated. Agreement between experiment and theoretical isotopic distribution is checked for six models proposed to describe evaporation processes. Quite good agreement is found with Brutsaert's model (Brutsaert, 1975a,b) for a smooth surface ifRes< 1 and for a rough surface ifRes> 1. Again, no specific effect due to the presence of artificially generated waves is observed. The above observations allow, then, the calculation of the drag and bulk evaporation coefficients,CDandCq, as a function of the surface roughness Reynolds number: these coefficients, as well as their ratio, when calculated for a height ofz= 10 m, vary from 0.99 × 10−3to 1.24 × 10−3and from 1.31 to 0.84, respectively, whenResincreases from 0.02 to 10, corresponding to a mean wind speed range extending from 2 to 13 m/s. These evaporation bulk transfer coefficients are given for near‐neutral stability conditions of the atmospheric layer. In the case of moderate instability, frequently observed over the sea, we would expect, and we observe, that the transfer coefficients are generally higher.