Field study of winter hydrodynamics in Lake Vendyurskoe (Russia)

A.Yu. Terzhevik

Institute of Limnology, Russ. Acad. Sci., St. Petersburg, Russia,

P.M. Boyarinov, N.N. Filatov, A.V. Mitrokhov, N.I. Palshin, M.P. Petrov

Northern Water Problems Research Institute, Karelian Center, Russ. Acad. Sci., Petrozavodsk, Russia

T. Jonas, M. Schurter

Swiss Institute for Environmental Science and Technology, Dübendorf, Switzerland,

and

O. Ali Maher

Department of Water Resources Engineering, Lund University, Lund, Sweden

The international project on winter hydrodynamics in ice-covered lakes was launched under the INTAS financial support. The main project objectives were formulated as

· to provide qualitative and quantitative description of the temperature distribution, quasi-steady circulation and reciprocating seiche-type currents during the period of ice cover, to provide direct estimates of the turbulence kinetic energy,

· to quantify the role of the heat fluxes at the water-bottom sediments and the water-ice interfaces in the evolution of the temperature and velocity fields, to develop and validate usable parameterisations of these fluxes,

· to quantify the structure, energetics and transport properties of convection driven by the vertically distributed flux of solar radiation that penetrates through the ice down to the water when the snow covering the ice disappears, and to develop and test a physically realistic model of convection.

Measurements were performed in Lake Vendyurskoe, Karelia, Russia during December 1998 – April 1999 period. At the beginning of winter 1998-1999, three thermistor chains were installed along one cross-section. Also, a ‘combined’ thermistor chain that registers temperatures in ice, water and sediments simultaneously has been installed. Then, a first survey (4-8.12.98) has been performed. Along four cross-sections, 56 station measurements have been performed with use of the bathythermograph (BTG) included registrations of temperature, conductivity, ice and snow thickness, also temperature of bottom sediments and its gradients within a 10-cm layer (40 stations). Measurements of the temperature and conductivity distribution in the thin water layer below ice (16 stations) were made along one cross-section.

During the second survey (19.02-31.03.99), measurements of the temperature and conductivity distribution along four cross-sections (80 stations) have been made at the beginning and in the end of the period; also temperatures of bottom sediments and its gradients within a 10-cm layer were registered (40 stations). Besides, the thickness of ice, snow, and water on the ice was measured. Along one cross-section, the temperature and conductivity distribution in the under-ice layer was measured. Directed, reflected, and penetrated-into-water components of solar radiation were registered. Mean currents were measured by means of the acoustic current meter on 5 stations (depths 1, 2, 3, 5, 7, 10 m and 10 cm from the bottom). Besides, long-term measurements of currents were made (3 to 16 hr long) to evaluate a variability of seiche-like currents.

During the last survey (8-24.04.99), registrations of the vertical temperature and conductivity distribution were made three times along one cross-section (42 stations). More four surveys were made with use of the bathythermograph (61 stations). The same device was used for measurements during two daily-long registrations (19 profiles). On 15 stations, measurements of the temperature of bottom sediments and its gradients were made. Registrations of ice, snow, and water on the ice thickness were made. Directed, reflected, and penetrated-into-water components of solar radiation were measured for 10 days. Mean currents were measured by means of the acoustic current meter on 4 stations. A summary of the measurements is given in a Table below.

Type of measurements	December-98	February-March-99	April-99
WCP, profiles	56	80	88
UIP, profiles	16	14
PVB, profiles		16
TCG, readings	40	40	15
SIT, measurements	56	93	63
CUR, stations		7	4
SR, days	2		10

WCP - water column profiling, (temperature, conductivity); UIP - under-ice profiling; PVB - profiling in the vicinity of bottom; TCG - temperature/conductivity gradients in the 10-cm layer of sediments; SIT - snow and ice thickness; CUR – currents; SR – solar radiation.

The Swiss team took part in the field campaign in Karelia with its equipment to register the temperature microstructure during the period of spring convection. Measurements have been made at one location, namely N 62°13′01.4″, E 33°16′52.0″, with accuracy of positioning ±12m. The profiler was installed underneath an undisturbed area of the ice. Besides in situ measurements within a water column, registrations of several meteorological parameters were performed. Ten temperature-logging units were installed in the water column, as well as an acoustic current meter.

Current velocity magnitudes have been registered at numerous occasions at 10 stations during the winter. These measurements were combined with registrations of the air pressure field and wind regime. A permanent feature in the current regime is atmospherically induced (air pressure variation induced) oscillatory currents, having a main period corresponding to that of a uni-nodal seiche. The oscillating current velocity amplitude was found to be about mm×s^-1, vertically uniform, and has little influence on vertical water exchange. Besides forth-back water movements, currents and the circulation structure are results of thermal and mass forcing at the lake boundaries.

Estimates of salt dynamics during the period of the ice formation have been made. During freeze-up, the salt from the freshly formed ice is educed into the water beneath it. It usually results in a certain increase of salt content in the upper layers of a water column (Fig. 1). As one can see, from the depth of about 1 m a vertical distribution of the salt content can be characterized as well mixed. Then, a salt content in the layer between the lower boundary of ice and upper boundary of a mixed layer can be evaluated from measurements. Besides, one can estimated a salt release, S_i, from the ice under the unit square (1 m²) as

,

where r_w and r_i are densities of water and ice, respectively; h_i ice thickness; C_w and C_i average salt contents in lake water and in water of melted ice, respectively. Fig. 2 represents a spatial distribution of the ratio between the salt content in the upper layer and salt release from the ice. A certain tendency to divergence of values directed towards shallower depths can be distinguished.

Fig. 1. Vertical distribution of the salt content in 0-5 m layer of Lake Vendyurskoe (3-7 December, 1998). Numbers stand for: 1 – st. 4-11; 2 – st. 5-7; 3 – st. 3-7; 4 – st. 6-3.

Spring 1999 was characterised by extreme heating due to the solar radiation. For most days in April, it was cloudless that led to rather high values of solar radiation beneath the ice (up to 120-130 W×m^-2 in daytime). As a result, under-ice convection started rather early, comparing to previous years. In late April, the water temperature in the upper layer exceeded that of maximum density and reached 5°C (Fig. 3).

Fig. 2. Spatial distribution of the ratio between the salt content in the upper layer of water column and salt release from the ice.

Fig. 3. Successive water temperature profiles in Lake Vendyurskoe (st. 4-6) during 11-24 April, 1999.