Characteristics of Energy Fluxes and Cold Frontal Effects on Energy Exchange over a Boreal Lake

Abstract

Understanding the characteristics and variations of heat exchange and evaporation of lakes is important for regional water resource management and sustainable development. Based on eddy covariance measurements over Lake Vanajavesi in southern Finland, characteristics of energy fluxes and cold frontal effects on energy exchange were investigated. The lake acted as a heat sink in spring and summer and a heat source in winter. The latent heat flux reached its minimum value in the morning and peaked in the afternoon. The diurnal variation of sensible heat flux was opposite to that of latent heat flux. Impact factors for the sensible heat flux were mainly the lake-air temperature difference and the product of lake-air temperature difference and wind speed. The latent heat flux was mainly affected by the vapor pressure deficit and the product of vapor pressure deficit and wind speed. The annual mean values of bulk transfer coefficients for momentum, heat, and water vapor were 1.98 × 10−3, 1.62 × 10−3, and 1.31 × 10−3, respectively. Bulk transfer coefficients for heat and water vapor were not equal, indicating that the parameterization of energy exchange in numerical models, where the assumption that the heat coefficient equals the water vapor coefficient needs improvement. During the ice-free season, cold fronts resulted in 28 sensible heat pulses and 17 latent heat pulses, contributing to 50.59% and 34.89% of sensible and latent heat exchange in Lake Vanajavesi. These results indicate that cold fronts significantly impact the surface energy budget and evaporation over lakes.