Mixing height over water and its role on the correlation between temperature and humidity fluctuations in the unstable surface layer

Results from an experimental investigation of the mixing height over inner Danish waters carried out from September 1990 to October 1992, are discusse d. The statistical analysis of the mixed-layer height (z(i)) over the sea does not exhibit the daily variation that is characteristic of the mixed layer over land, but it is nearly constant over a 24-hour cycle. During summer, t he mixed layer is higher than during winter. A second inversion was often o bserved. A case study of the development of the mixed layer over the sea under near- neutral and unstable atmospheric conditions during six consecutive days is presented. A zero-order mixed-layer height model is applied. In addition to momentum and heat fluxes the effect of subsidence was found to be importan t for the evolution of the mixed layer over the sea. The modelled evolution of z(i) compared successfully with measurements. We have investigated the influence of the mixed-layer height on the correla tion coefficient RqT between temperature and humidity fluctuations using th e values obtained with the model. We found that the evolution of RqT follow s the evolution of the mixing height. An empirical model linking the surfac e values of RqT to z(i) and the Obukhov scaling length L has been suggested . The model reproduces the experimental features.