Anisotropic Turbulence in the Radiatively Driven Convective Layer in a Small Shallow Ice-Covered Lake: An Observational Study

Convectively mixed layer (CML) forms due to radiatively driven convection (RDC) in the upper part of the water column in shallow ice-covered lakes. The spatial structure of this layer has been very poorly studied. The long-standing hypothesis postulates a continuum of convective cells in this layer. The invariant analysis is used to reveal the spatial inhomogeneity of the turbulence parameters within CML and their evolution during the daily cycle of RDC based on solar radiation, water temperature and current measurements in a small shallow ice-covered lake. The values of all six components of the Reynolds-stress tensor are estimated using the method suggested by Bogdanov et al. (Fundam Prikl Gidrofiz 14:17–28, 2021). A high level of turbulence anisotropy within CML was observed throughout the entire measurement period (10 days). Anisotropy invariant maps demonstrate multiple transitions between prolate (rod-like) and oblate (disk-like) types of axisymmetry, without reference to the diurnal cycle of RDC. The dynamics of anisotropy tensor eigenvalues, in contrast to that of the stresses per se, also exhibited no connection with the diurnal cycle of RDC. Considering the presence of mean geostrophic drift in the studied lake, the revealed changes of axisymmetry types and anisotropy tensor eigenvalues are most likely associated with the spatial inhomogeneity of turbulence within the convective cells moving through the measurement zone. The absence of an explicit dependence of turbulence anisotropy on the diurnal cycle of RDC suggests that convective cells "survive" at night and, together with geostrophic drift, maintain the turbulence of the mixed layer.