The Influence of Waves on the Heat Exchange over Sea

Sammanfattning: The main focus of this study is the influence of waves on the heat transfer over sea. In particular, the bulk transfer coefficient CH (the Stanton number), has been investigated for possible wave influence. Measurements from the site Östergarnsholm in the Baltic Sea have been used. The site has a large sector with undisturbed over water fetch. Data during the period 1995-1999 have been used. It is shown that CH behaves differently as it approach z/L=0 from the unstable side depending on the wave state. During growing sea, CH makes a rapid drop as it passes over neutrality, strikingly different from swell conditions where CH makes a much ’smoother’ transition. This difference is also shown to exist for the kinematic heat flux. Based on the definition of CH, it is suggested that one of the reasons of CH’s different behaviour for different stratification and wave state, is ought to be sought in the kinematic heat flux itself. A comparison of the w,θ cospectra during growing sea and swell conditions, showed differences. For growing sea, the larger size eddies dominates the heat flux during unstable conditions. There is no significant difference in peak frequency for different grade of instability. The swell cases showed a more inconsistent behaviour as it approached neutrality, with the peak frequency shifting for different stability ranges. The correlation coefficient between u, the longitudinal wind component, and w, the vertical wind component, Ru,w is also investigated in this study. It is shown that Ru,w is exposed to some wave influence. A comparison of Ru,w as a function of wave age, for neutral and non-neutral stratification is made. For swell cases and non-neutral stratification Ru,w makes a rapid drop and assumes values close to zero. This is not seen for the neutral cases although there is a slight decrease. It is concluded that a certain amount of positive heat flux and inactive turbulence is needed to see this drop in the correlation coefficient.