RESPONSE OF A SHALLOW LAKE TO WIND LOAD DURING THE OPEN WATER PERIOD (3D NUMERICAL EXPERIMENTS)

Abstract

Based on the results of numerical calculations on a three-dimensional hydrodynamic model, the response of a shallow lake to wind action during the open water period was studied. The estimate of the atmospheric impact on the lake is based on ERA5 re-analysis data. The fields of currents and water temperature are analyzed for the period from May to December with alternating weekly intervals of calm and wind action at a constant speed of 3 m/s (the first and third weeks of the month are west and east winds, respectively, the second and fourth are calm). The model calculations showed that the spring-summer heating stage in the lake lasted from early May to mid-August. The periods of strong and weak stratification alternated against the background of increased wind load and changes in air temperature. From mid-August onwards the lake cooled down in a state of homothermia. According to the model calculations, stable circulation is formed in the lake already on the first day of the wind. On the second and third days, the current velocities increased reaching a maximum of 0.05–0.15 m/s, and changed very little during the next 4–7 days of wind. Under calm conditions, the currents decayed within 2–3 days. With homothermy, wind circulation engaged the entire water column, with currents greater than during stratification. During periods of stratification under the influence of wind, a surface mixed layer was formed, the thickness of which reached 3–4 m by the seventh day of the wind load. The velocities of currents in the mixed layer were noticeably higher than under the thermocline. In the first days of the calm, stratification began to recover. The alternation of periods of wind and calm at the stage of spring-summer heating of the lake contributed to the accumulation of heat in the surface layers of the water column and its transfer to the hypolimnion, and then to the sediments. Simulated and observed data on water temperature and current velocities in the lake showed good agreement.