Sergey Golosov

Physics of seasonally ice-covered lakes: a review

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.

Physical background of the development of oxygen depletion in ice-covered lakes

The effect of the heat interaction between a water column and sediments on the formation, development, and duration of existence of anaerobic zones in ice-covered lakes is estimated based on observational data from five frozen lakes located in northwestern Russia and North America. A simple one-dimensional model that describes the formation and development of the dissolved oxygen deficit in shallow ice-covered lakes is suggested. The model reproduces the main features of dissolved oxygen dynamics during the ice-covered period; that is, the vertical structure, the thickness, and the rate of increase of the anaerobic zone in bottom layers. The model was verified against observational data. The results from the verification show that the model adequately describes the dissolved oxygen dynamics in winter. The consumption rates of DO by bacterial plankton and by bottom sediments, which depend on the heat transfer through the water–sediment interface, are calculated. The results obtained allow the appearance of potentially dangerous anaerobic zones in shallow lakes and in separate lake areas, which result from thermal regime changes, to be predicted.

Software, Data and Modelling News: FLake-Global: Online lake model with worldwide coverage

An online lake-modeling tool FLake-Global allows virtually instantaneous estimation of the seasonal cycle of temperature and mixing conditions in any shallow freshwater lake around the world. The tool is built on the basis of the lake model FLake (www.lakemodel.net) and a year- long near-surface meteorological data from the NCEP/NCAR Reanalysis Project (http://www.esrl.noaa.gov/psd/data/reanalysis). FLake-Global gives estimates of the surface and bottom water temperatures, mean temperature of the water column, surface mixed layer depth, and ice cover duration and thickness, using geographical coordinates, mean depth to the bottom and water transparency of the lake in question, and the NCEP atmospheric forcing as input. The tool is suitable for a wide spectrum of applications, including general limnology, lake management and restoration, fisheries, and recreation.

Hydrothermodynamic features of mass exchange across the sediment-water interface in shallow lakes

Long-term field observations on different shallow lakes revealed a sharp increase in concentration of dissolved inorganic nutrients (phosphorus and nitrogen) during the autumn cooling. It was found that a source of nutrients were bottom sediments. The phenomenon has been studied by applying a new approach based on the assumption of interrelation between heat and mass transfer across the sediment–water interface. During the autumn cooling, the upper sediment layer is warmer than the near-bottom water that may cause a convective instability in pore water with high nutrient concentration. Under certain conditions, the Rayleigh number (the main parameter characterizing a density convection) becomes higher than the critical one. As a result, the viscous density convection starts to develop in the pore water. In this case nutrients transfer across the sediment–water interface by thermal macrovolumes with positive buoyancy. The effectiveness of such mass transfer is several orders greater than that of molecular diffusion. This hypothesis was checked by special field and laboratory experiments which were carried out on shallow Lake Krasnoye. The mathematical model of this phenomenon was developed and verified against numerous experimental data.

Some features of the thermal and dissolved oxygen structure in boreal, shallow ice-covered Lake Vendyurskoe, Russia

The 5-year-long (2001–2005) studies of the winter thermal structure and the dissolved oxygen (DO) dynamics in Lake Vendyurskoe, Russia, a typical boreal shallow mesotrophic lake of glacial origin, revealed still poorly studied features of lake-wide dynamics, such as net lateral heat flux towards deeper parts of a lake and development of the anaerobic zone over the deepest points of the lake basin. We estimated magnitude of the heat transport along the bottom slope based on scaling analysis. The seasonal changes in DO concentration appear to be controlled mostly by biochemical consumption. We identify four factors controlling the extent of anoxic zones in shallow ice-covered lakes: (1) the amount of organic matter stored in the bottom layers, including the sediments surface during the autumnal bloom; (2) the length of the ice-covered period; (3) heat content of bottom sediments; and (4) the initial water temperatures at the time of the ice cover formation.

A parameterized model of heat storage by lake sediments

A model of seasonal heat storage by lake sediments is proposed oriented at applications in climate modeling and at lake parameterization in numerical weather prediction. The computational efficiency is achieved by reformulating of the heat transfer problem as a set of ordinary differential equations for evolution of the temperature wave inside the upper sediment layer. Arising temperature and depth scales completely replace the conductivity of the sediment in the heat transfer equation and can be easily achieved from the lake water temperature observations without any data on the sediment thermal properties. The method is proposed for the scales estimation from the inverse solution of the model equations in special case of the constant water-sediment heat flux in ice-covered lakes. The method is tested on data from sediments of Lake Krasnoye, North-Western Russia. The long-term (1961-2002) modeling of temperature in German lakes Muggelsee and Heiligensee with a coupled one-dimensional model of lake water column and sediments has demonstrated an appreciable effect of the sediment heat storage on near-bottom temperatures in both lakes. Thus, incorporation of the sediment layer into lake temperature models can essentially improve, at low computational costs, the model performance, especially for shallow lakes. In addition, a better forecast of near-bottom temperature evolution on climatic scales can provide a better understanding of the response of lake benthic communities to global warming.

Basin-scale internal waves in the bottom boundary layer of ice-covered Lake Müggelsee, Germany

We performed high-resolution temperature measurements under ice cover in Lake Müggelsee, Germany, during the winter of 2005–2006. Intense seiche-like temperature oscillations developing after the ice-on have been encountered in a thin water layer above the sediments. The oscillations were initiated immediately after lake freezing by the release of the potential energy of the thermocline slope and existed for several weeks without appreciable external forcing. The oscillations were associated with a basin-scale internal waves existing in the lower stratified part of the water column. The weakness of the density stratification under ice ensured the long wave periods, exceeding the period of geostrophic inertial oscillations at the lake’s latitude. As a result, two frequency peaks were present in the oscillations corresponding to two rotational waves, one of Kelvin-wave type and another of Poincaré type wave. The rotational character provided long dissipation times of the waves and allowed the oscillations to persist in lake several weeks. Temperature measurements in the upper several centimeters of the sediment demonstrated that oscillations of the near-bottom temperature produced vertical density instability and pore-water convection in the upper sediments.

Hydrophysical aspects of oxygen regime formation in a shallow ice-covered lake

Long-term observational data on a small, shallow Lake Vendyurskoe (Karelia) were used to analyze the space and time dissolved-oxygen dynamics in winter. Biochemical consumption was found to play a leading role in the reduction of dissolved-oxygen concentration in lake water in winter. The total decrease in the amount of dissolved oxygen since the beginning of under-ice period until mid-April was shown to amount to one third of the initial value. The year-to-year variations in winter oxygen consumption are ~10%, suggesting the process to be stable in the years of observations. The rate of oxygen consumption and variations in dissolved oxygen content of lake water in winter were evaluated. The analysis and literary data allow us to conclude that the hydrophysical processes taking place in shallow lakes in winter have a considerable effect on their oxygen regime.

Modified parameterization of the vertical water temperature profile in the FLake model

Abstract The lake model FLake is currently widely used in numerical weather prediction and in climate models to parameterize the effect of freshwater lakes on the state of the boundary atmospheric layer. The model is based on a two-layer parametric representation of the evolving temperature profile (ETP) and on the integral budget of energy for the layers in question. The structure of the stratified layer between the upper mixed layer and the basin bottom is described using the concept of self-similarity of the temperature-depth curve. Capacity of a function of such type to simulate ETP accurately defines the model quality, that is, the extent of correspondence between numerical results and observational data. Several self-similar temperature-depth curves either obtained analytically or resulted from observational data handling, have been used in earlier FLake modifications with different extent of advance. The main shortcoming of parameterizations used previously was their inability to reproduce all types of the ETP known from observations. In the present study, a new parameterization of ETP in frames of FLake, also based on self-similarity of the temperature-depth curve, is proposed. It is demonstrated that a new parameterization is capable to reproduce most of the ETP types observed, whereas the self-similar functions proposed earlier are found to be its particular cases.