Features of mixing the water mass of a small lake during night cooling

The article presents the results of a study of the mixing regime of a small lake, the wind effect on the water column of which is significantly weakened by the forest cover of the adjacent coastal area. Temperature measurements using a chain of thermistors were carried out during two open seasons, in 2021 and 2023. The main attention was paid to periods of night cooling, since it is this mechanism that can be considered prevalent in the mixing of the water column and the formation of a convectively mixed surface layer. Wherein, in the summer months, despite significant nighttime heat fluxes on the surface, complete mixing does not occur. The analysis of the dynamics of the temperature profile and the calculation of the efficiency of mixing were carried out using the integral energy method, based on the identification of mixing episodes and calculations of the background potential energy and buoyancy flux. For mixing efficiency, estimates of 0.45 and 0.49 were obtained for two seasons, significantly exceeding the canonical value of 0.17. It was also revealed that the efficiency value depends on the shape of the density profile and, in particular, on the thickness of the mixed layer