Objective
Most lakes in mid-latitudes are covered by ice over a period of several months. The structure of the temperature and the velocity fields during this period is very different from their structure during the period of open water, when the lake is exposed to direct atmospheric forcing. The temperature and velocity distributions strongly affect chemical and biological processes and are of utmost importance for the lake ecosystem. There is still little knowledge, however, of the temperature and velocity fields in ice-covered lakes. Physical processes that are responsible for the evolution of these fields are poorly understood.
The proposed project is aimed at better understanding the physical processes responsible for the water temperature and current velocity distributions in ice-covered lakes, and developing improved parameterisations of these processes. The project goals will be achieved through
(i) in situ measurements of physical fields in the ice-covered lake, including measurements at the water-bottom sediments and water-ice interfaces;
(ii) analysis of data from measurements to assess the role of various processes in forming the structure of physical fields, and;
(iii) numerical experiments with hydrodynamic models of various complexity and verification of results against empirical data.
Measurements will be performed in Lake Vendyurskoe, Karelia, Russia, a typical medium-sized lake where three pilot measurement programmes were implemented in recent years. When the lake is ice-covered, two periods are clearly distinguished. They are characterised by essentially different regimes of heat transport, which ultimately determines the water currents and mixing conditions. Over most of the winter, the ice is covered by snow, no radiation penetrates down to the water, and the lake is thermally insulated from the atmosphere. However, the temperature changes do occur due to the interaction between the lake water and the bottom sediments. The sediments heat the water from below by liberating the amount of heat accumulated in the course of the previous summer. In early spring, when the snow cover overlying the ice vanishes, the solar radiation starts penetrating down through the ice. This results in the vertically inhomogeneous heating of the water column, where the upper layers gain more heat than the lower layers. A part of the water column eventually becomes hydrostatically unstable and overturns, leading to the formation of convectively mixed layer that grows in depth as the radiation heating proceeds.
Thus, the analysis of the measurement data and the modelling activities will be focused on two issues, namely, the heat exchange between the bottom sediments and the water column, and convection under the ice due to vertically distributed heating. In order to get deeper insight into the energetic of convection and to quantify and parameterise its transport properties, the in situ measurements will be supplemented with data from high-resolution numerical experiments with non-hydrostatic large-eddy model. Considering the water currents in ice covered lakes, the focus will be on the quasi-stationary motions caused by the vertical and horizontal inhomogeneity of the density filed, and on the reciprocating seiche-like motions caused by wind stress on the ice.
The project results will include
(i) a comprehensive data set on the temperature and current velocity distributions in the ice-covered lake, and on the mean and turbulence structure of spring convection under the ice;
(ii) an improved understanding of the physical processes that form the structure of temperature and velocity fields during the period of ice-cover;
(iii) advanced parameterisations of the heat fluxes at the water-bottom sediments and the water-ice interfaces, and of the energetics and the transport properties of spring convection under the ice.
Besides, a physically realistic and computationally efficient model for the temperature profile evolution and mixing conditions in the ice-covered lake will be developed. Recommendations towards practical use of the project results in lake modelling, including modelling of lake ecosystems, will be formulated.
Call for proposal
Data not availableFunding Scheme
Data not availableCoordinator
221 00 Lund
Sweden