Arkady Terzhevik

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.

Radiatively driven convection in ice-covered lakes: Observations, scaling, and a mixed layer model

Penetrative convection is discussed where the instability is driven by radiative heating of water below the temperature of maximum density. Convection of this type occurs in ice-covered freshwater lakes in late spring, when the snow cover vanishes and solar radiation is absorbed beneath the ice cover. The vertical temperature structure, bulk mixed layer scaling, and mixed layer deepening are examined for a number of temperate and polar lakes. A bulk mixed layer scaling for this type of convection is based on energy arguments underlying the classical Deardorff convective scaling. The depth of the convective layer serves as an appropriate length scale. However, a modified scale that takes account of the energetics of a distributed radiation source term replaces the surface buoyancy flux velocity scale used by Deardorff. The scaling compares favorably with large-eddy simulations of turbulence kinetic energy (TKE) and with both observations and large-eddy simulations of the TKE dissipation rate. Mixed layer deepening is simulated with a model of convection beneath lake ice. The model describes the structure of the stably stratified layer just beneath the ice with a stationary solution to the heat transfer equation; the structure of the entrainment layer is parameterized with a zero-order jump approach. The entrainment equation is derived from the mixed layer TKE budget and bulk mixed layer scaling. Entrainment regimes characteristic of convection beneath ice are analyzed. It is shown that if the Deardorff convective velocity scale is replaced with a scale incorporating the distributed buoyancy flux, the entrainment equation describing atmospheric and oceanic convective boundary layers also applies beneath the ice. Model predictions compare well with data from observations in a number of lakes. We propose and compare with observations an extension of the mixed layer model that allows for the inclusion of salinity. Although the salt concentration is low in most temperate and polar lakes, its dynamical effect can be significant close to the temperature of maximum density. (Less)

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.

Radiatively driven convection in an ice-covered lake investigated by using temperature microstructure technique

[1] Convection in an ice-covered lake, driven by the absorption of solar radiation, is investigated by means of temperature microstructure technique. This type of convection typically occurs in spring, when melting snow on the ice cover enables solar radiation to penetrate into the water body. The diurnal dynamics of the stratification system of five distinct layers is analyzed by means of consecutive CTD profiles and with the aid of a one-dimensional model. The model solves the transfer equation of heat and salinity and includes convective procedures to react on density instabilities. This study is focused on the turbulent kinetic energy (TKE) balance. The stratification analysis reveals the importance of several processes for the TKE balance, namely: (1) the entrainment into the top layer from the convective layer below, (2) the inflow of water from melted ice, and (3) the volumetric solar heating. Enabled by the analysis of the temperature microstructure profiles, two TKE budgets are presented. The temporally averaged budget reveals a vertical distribution of generation and dissipation rate similar to the case of cooling-induced convection in a surface boundary layer. But contrary to this reference regime, a transition layer was found in the upper convective layer, where both rates drop back to zero toward the layer above. The second TKE budget is spatially averaged over the convective layer but resolves the diurnal dynamics. The generation rate and dissipation rate feature similar diurnal dynamics, where the dissipation lags on average by 1.5 hours. The temporal change rate of TKE was found to be on the same order of magnitude as the generation rate and the dissipation rate, while the export rate of TKE out of the convective layer was found to be less significant.

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.

Preface: European large lakes-ecosystem services and management in a changing world : The 4th European large lakes symposium

Large lake ecosystems are ecologically, culturally, and economically very important ecosystems providing many valuable services to mankind. Although the large lakes share many common size-related features, the ecosystems also have specific characters due to differences in their geological history, landscape topography, drainage area properties, and regional climatic factors as well as past and present human activities. Thus, the biota, ecosystem functions, and services are differently challenged by changing climate, nutrient and organic matter loading, and other anthropogenic factors. Multidisciplinary research and reliable monitoring of the ecological status are essential for planning sustainable use of resources and maintaining ecosystem services of the large lakes. Moreover, many large lakes or their drainage areas are shared by several nations. There has been a long tradition of international co-operation in environmental planning, research, and monitoring of transboundary lakes. Further development of this cooperation is encouraged within the present EU water policy, which calls for the establishing of International River Basin Districts and for the agreement of International River Basin Management Plans. European Large Lakes Symposia (ELLS) have been a forum for scientists, environmental planners, and authorities from different countries to present research results, advance international co-operation, and develop ideas for improving management of the large lakes. The first symposium took place in 2006 in Tartu, Estonia and was followed by every 3rd year symposia in Norrtälje, Sweden (2009), and Konstanz, Germany (2013). Before these meetings, there had been four International Lake Ladoga Symposia during the years 1992–2002. The 4th European Large Lakes Symposium (ELLS 2015) was held in Joensuu (University of Eastern Finland) on 24–28 August 2015. The symposium was Guest editors: Paula Kankaala, Tiina Nõges, Martti Rask, Dietmar Straile & Arkady Yu. Terzhevik / European Large Lakes IV. Ecosystem Services and Management in a Changing World

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.