Small Lakes Mixing by Surface Cooling: Efficiency Estimates

The mixing of waterbodies and mixing efficiency estimates for different forcing mechanisms remain one of the main challenges in limnological studies, with a deep theoretical background and a wide range of practical applications. This paper examines the mixing that is triggered by surface cooling during the period of open water. The cooling often occurs at night and is largely determined by infrared radiation fluxes, in this regard, this type of forcing is usually positioned as the radiative mixing mechanism. To calculate the mixing efficiency η, an integral energy method was used, within which this parameter is defined as that portion of the external forcing that is spent on mixing itself, as opposed to viscous dissipation. Specific features of energy fluxes between different energy pools have been revealed for radiative type of forcing. For each identified mixing episode the changes of background potential energy were assessed, together with energy sink due to dissipation. Long-term temperature series for different depths at two small water bodies – a forest lake and a city pond – were used as initial data. Calculations carried out for several hundred mixing episodes showed that the mixing efficiency value, on average, significantly exceeds the canonical threshold 0.17. The correlation has also been identified between efficiency and CML thickness: vertical mixing resistance increases with CML deepening. This result introduces a new challenge to the “universality vs variability” dilemma: the efficiency may depend not only on the type and intensity of forcing, but also on the parameters of the initial temperature profile.