Synoptic variability of the energy parameters in an ice-covered lake was studied during the period of spring radiatively-driven convection. Measurements of water temperature (a chain with 14 ...temperature sensors TR-1060 RBR), solar radiation fluxes (two star-shaped pyranometer Theodor Friderich & Co, Germany for recording the incident and reflected radiation and one pyranometer M80 for recording radiation penetrating the ice) in an ice-covered small boreal lake carried out from 28 March to 6 April 2020. During the measurement period, there was a sharp change in the weather - first there were five sunny days, then heavy snow fell for two days, the next three days there was partly cloudy weather, and on the last day of measurements there was sunny weather. The flux of under-ice radiation reacted sensitively to snowfall and decreased from 100 W/m2 (day maximum) to 5-10 W/m2 after snowfall. This flux increased to 50-60 W/m2 as the snow melted. Changes in background potential energy (BPE), and buoyancy flux were calculated using TR-chain and pyranometer M80 data. A well-pronounced increase in BPE and buoyancy flux against the sunny weather and a sharp slowdown in the growth of these parameters after snowfall were revealed.
Recently, the attention to the ice season in lakes has been growing remarkably amongst limnological communities, in particular, due to interest in the response of mid- and high-latitude lakes to ...global warming. We review the present advances in understanding the governing physical processes in seasonally ice-covered lakes. Emphasis is placed on the general description of the main physical mechanisms that distinguish the ice-covered season from open water conditions. Physical properties of both ice cover and ice-covered water column are considered. For the former, growth and decay of the seasonal ice, its structure, mechanical and optical properties are discussed. The latter subject deals with circulation and mixing under ice. The relative contribution of the two major circulation drivers, namely heat release from sediment and solar heating, is used for classifying the typical circulation and mixing patterns under ice. In order to provide a physical basis for lake ice phenology, the heat transfer processes related to formation and melting of the seasonal ice cover are discussed in a separate section. Since the ice-covered period in lakes remains poorly investigated to date, this review aims at elaborating an effective strategy for future research based on modern field and modeling methods.
Early-spring under-ice convection in the Petrozavodsk Bay of Lake Onega (Russia) was investigated as part of an interdisciplinary research project conducted during March 2015. Measurements performed ...using a thermistor chain and vertical profiling sensors were used to examine temperature dynamics in the convectively mixed and stratified layers of the lake. Radiative transfer through the ice was high leading to a large convective mixed layer (up to 20 m deep) during daytime. Convective velocity was evaluated using two different methods. It is shown that convective velocity (a maximum value of ~7.4 mm s
−1
, and daytime average of 3.9 mm s
−1
) is completely damped during the restratifying night hours. We observed internal waves in the thermocline below the convective mixed layer with intriguing variations between night and day. Maximum of internal wave energy was found to start in the afternoon and continue long after the end of solar radiation forcing. Our analysis indicates that local convective processes are key forcing mechanisms for the generation of internal waves in ice-covered lakes. We also hypothesize that spatial differential heating between the nearshore regions and the centre of the bay (e.g. density current intruding the thermocline) could be a source of internal waves in ice-covered lakes.
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.
The intensity of vertical heat and mass transfer remains among the challenging topics in the study of ice-covered lakes. Presumably, internal waves (IWs) make a significant contribution to the heat ...transfer in the water column. However, the mechanisms of mixing enhancement by generation, interaction, and breaking of IWs of different scales, especially short-wavelength ones, have not been sufficiently studied. Furthermore, the experimental data required for estimating the key parameters of IWs (wavelengths, propagation velocities) are rather fragmentary, which makes it difficult to quantify the turbulent transfer caused by IWs. This paper presents the estimates of these IW parameters based on data obtained in the winter months of 2014 and 2016 in a small boreal ice-covered lake. Having analyzed horizontally spaced thermistor chain data, we managed to detect the presence of short standing and propagating IWs, and to estimate their length (from several meters to several tens of meters) and phase and group velocities (from several mm/s to several tens of mm/s). Also, their vertical mode structure was detected. It was shown that IW generation events were characterized by a high degree of spatial localization, and the IW energy was unevenly distributed through the water column.
Acoustic Doppler current profilers (ADCP) are widely used in geophysical studies for mean velocity profiling and calculation of energy dissipation rate. On the other hand, the estimation of turbulent ...stresses from ADCP data still remains challenging. With the four-beam version of the device, only two shear stresses are derivable; and even for the five-beam version (Janus+), the calculation of the full Reynolds stress tensor is problematic currently. The known attempts to overcome the problem are based on the “coupled ADCP” experimental setup and include some hard restrictions, not to mention the essential complexity of performing experiments. In this paper, a new method is presented which allows to derive the stresses from single-ADCP data. Its essence is that interbeam correlations are taken into account as producing the missing equations for stresses. This method is applicable only for the depth range, for which the distance between the beams is comparable to the scales, where the turbulence is locally isotropic and homogeneous. The validation of this method was carried out for convectively-mixed layer in a boreal ice-covered lake. The results of computations turned out to be physically sustainable in the sense that realizability conditions were basically fulfilled. The additional verification was carried out by comparing the results, obtained by the new method and “coupled ADCPs” one.
The quality of lake ice is of uppermost importance for ice safety and under-ice ecology, but its temporal and spatial variability is largely unknown. Here we conducted a coordinated lake ice quality ...sampling campaign across the Northern Hemisphere during one of the warmest winters since 1880 and show that lake ice during 2020/2021 commonly consisted of unstable white ice, at times contributing up to 100% to the total ice thickness. We observed that white ice increased over the winter season, becoming thickest and constituting the largest proportion of the ice layer towards the end of the ice cover season when fatal winter drownings occur most often and light limits the growth and reproduction of primary producers. We attribute the dominance of white ice before ice-off to air temperatures varying around the freezing point, a condition which occurs more frequently during warmer winters. Thus, under continued global warming, the prevalence of white ice is likely to substantially increase during the critical period before ice-off, for which we adjusted commonly used equations for human ice safety and light transmittance through ice.
The results of a numerical simulation of radiatively driven convection (RDC) in a small ice-covered lake with a lateral pressure gradient are shown. RDC influences aquatic ecosystems as convective ...flow transfers heat and dissolved and suspended matter through the water column. There is a hypothesis that a continuum of convective cells with areas of ascending and descending water flows exists in a convective mixed layer (CML). Until now, little has been known about how the structure of the CML changes in lakes with lateral transport. In this work, the evolution of the CML in the computational domain with a lateral pressure gradient over several days is reproduced using an Implicit Large Eddy Simulation. We show that after a few days of lateral pressure gradient occurrence, convective cells are replaced by rolls oriented along the lateral transport direction. The change in the CML’s turbulence patterns under a lateral pressure gradient is confirmed by Anisotropic Invariant Map analysis. The study revealed a heterogeneity of pulsations of the horizontal and vertical velocity components over the entire depth of the CML and showed that when a horizontal gradient is present, the velocity pulsations generally increase.
The calculation of the turbulent stress matrix using acoustic Doppler current profiler (ADCP) data remains a challenging problem in the study of geophysical flows. One of the ways to overcome the ...problem is to use a system of two coupled ADCP with pairs of beams intersecting at a certain depth. When device configuration is symmetric in horizontal, this setting makes it possible to estimate the stresses only for a small range of depths, close to the depth of beam intersection point. To overcome this restriction, in this paper the modified setting is proposed, when both devices are symmetrically turned in the horizontal plane. The X axes of the devices are not collinear for such setting, and two pairs of beams intersect at two different depths, which depend on the distance between the emitters and devices’ rotation angle, and can be chosen in advance. At each of these depths, six beam velocity variances can be directly calculated, as well as the correlation of those velocity components, which correspond to the intersecting beams. As a result, an overdetermined system of equations is derived for unknown stresses, for both depths. The method was approbated during the processing of two series of field data obtained in lakes during open water and ice-covered periods. In most cases, calculations lead to physically consistent results; in particular, the stress matrix turns out to be positive definite. The method’s limitations and perspectives of its development are discussed.
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