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.
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.
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 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.
A priori knowledge on large-scale sub-surface conductivity structure is required in many applications investigating electrical properties of the lithosphere. A map on crustal conductivity for the ...Fennoscandian Shield and its surrounding oceans, sea basins and continental areas is presented. The map is based on a new database on crustal conductance, i.e. depth integrated conductivity, where all available information on the conductivity of the bedrock, sedimentary cover and seawater are compiled together for the first time for the Fennoscandian Shield. The final model consists of eight separate layers to allow a 3D description of conductivity structures. The first three layers, viz. water, sediments and the first bedrock layer, describe the combined conductance of the uppermost 10 km. The other five bedrock layers contain the data of the crustal conductance from the depth of 10 km to the depth of 60 km. The database covers an area from 0°E to 50°E and 50°N to 85°N. Water conductances are estimated from bathymetric data by converting depths to conductances and taking into account the salinity variations in the Baltic Sea. Conductance of the sedimentary cover includes estimates on the conductance of both marine and continental sediments. Bedrock conductances are extrapolated from 1D- and 2D-models. Extrapolations are based on data from magnetometer array studies, airborne electromagnetic surveys and other electromagnetic investigations as well as on other geophysical and geological data. The crustal conductivity structure appears to be very heterogeneous. Upper crust, in particular, has a very complex structure reflecting a complex geological history. Lower crust seems to be slightly more homogeneous although large regional contrasts are found in both the Archaean and Palaeoproterozoic areas.
Hydrophysical studies conducted in Petrozavodsk Bay of Lake Onego under ice-covered conditions in March 2016 and 2017 detected radiatively driven convection and revealed specific structural and ...dynamic parameters for the convectively mixed layer (CML). Analysis of time series, spectral energy distribution, and vertical velocity profiles indicated the presence of a mean current, seiches, and convective motion. Because of their similar spatial and temporal scales, these processes were investigated using progressive-vector diagrams (PVDs). Despite low water velocities, the CML hydrodynamic regime remained close to that of fully developed turbulence, and convective cells at a range of different scales were expected. Signal resolution constraints limited detection to only the largest cells. We investigated the horizontal structure of the CML using individual and combined observations from 3 acoustic velocity profilers located within a radius of a few tens of meters. This novel setup collected data indicating that the CML's large-scale horizontal flow structure consists of a continuum of quasi-deterministic cells. Cell parameters necessary for estimating turbulent transfer were derived from PVD and hodograph curves.
A field study on current structure and circulation characteristics in Lake Vendyurskoe, a small, shallow, ice-covered lake in Karelia, Russia, is presented. The current velocity magnitudes were ...generally found to be small. The most pronounced currents had an oscillating character, with velocity amplitudes on the order of millimeters per second. The oscillation period, obtained from spectral density calculations, corresponded to that of a barotropic uninodal seiche. The seichelike nature of the current oscillations was supported by the results from analysis of ice-level fluctuations, giving identical periods and a phase shift of one-fourth the period between the two types of oscillations. Mean currents measured during the winter were on the order of millimeters per second. Because Lake Vendyurskoe does not have any significant river inflow or outflow during winter, the most probable cause of these currents is horizontal temperature (pressure) gradients. Scaling analysis indicated that these currents are geostrophic. This was supported by theoretical estimates, based on observed horizontal temperature gradients, being of the same order as the observed currents. The mean current velocities increased considerably after spring convection from < 1 to several millimeters per second.
PDF
