Thermohydrodynamic processes in Lake Ladoga

Abstract

The results of adapting a three-dimensional mathematical model of the hydrodynamics of an inland sea to the conditions of Lake Ladoga are presented, followed by its verification using field and remote measurements. During adaptation, a spherical B-type computational grid with a spatial resolution of 1 km × 1 km was constructed. The equation of state for seawater was replaced by an equation for weakly mineralized water with a maximum density at a temperature close to 4 °C. A procedure was implemented to calculate the inflow of heat and dissolved substances into the lake through liquid boundaries with the waters of tributaries. The flow of the Neva River was taken into account. Atmospheric forcing from NCEP/NCAR reanalysis with a spatial resolution of 2.5°×2.5° has been replaced with the European equivalent ERA5 with a resolution of 0.25°×0.25°. Verification of the new model version was performed for three areas of the lake: the southern shallow area (≤ 40 m), the central area (~50 m), and the northern deep area with depths up to 230 m. To assess the impact of climate change on thermohydrodynamic processes, calculations were performed from 1990 to 2020. Both contact measurements from the Institute of Limnology of the Russian Academy of Sciences and images from NOAA and MODIS Terra/Aqua satellites were used for verification. The verification results showed that the model adequately reproduces the main features of thermohydrodynamic processes in Lake Ladoga, namely: the vertical thermal structure of the lake’s water mass, the beginning of the formation and spread of the vertical frontal zone - the thermobar. Accounting for inflow waters allows tracking their further distribution in the lake. Replacing atmospheric forcing has led to a significant improvement in calculations of ice conditions in the lake. The results of verification of the new version of the model should be considered satisfactory. Some shortcomings of the model have also been noted.