Understanding how lakes exchange heat and moisture with the atmosphere is essential for managing regional water resources and promoting sustainable development. A research team from China and Finland analyzed energy fluxes and the impact of cold fronts over Lake Vanajavesi in southern Finland using eddy covariance measurements. Their study, which provides valuable insights into this dynamic process, was published in Advances in Atmospheric Sciences.

"The findings indicate that the lake serves as a heat sink during spring and summer but is a heat source in winter," said Dr. XU Lujun from the Institute of Atmospheric Physics of the Chinese Academy of Sciences, the lead author of the study.

The study emphasizes that lakes can regulate regional energy exchange at both daily and seasonal timescales. The researchers discovered that latent heat flux—energy associated with evaporation—reaches its lowest point in the morning and peaks in the afternoon, whereas sensible heat flux follows the opposite trend. Key factors influencing sensible heat flux include the temperature difference between the lake and the air and its interaction with wind speed. For latent heat flux, the primary drivers are vapor pressure deficit and wind speed interaction.

One of the study's notable findings was that cold fronts play a substantial role in enhancing heat exchange. They induce rapid increases in wind speed and drops in temperature. Due to the lake's high heat capacity, it cools more slowly than the surrounding atmosphere, creating larger temperature and vapor pressure differences. This results in sharp increases, or pulses, in both sensible and latent heat exchange. During the ice-free season, researchers observed 28 sensible heat pulses and 17 latent heat pulses, accounting for 50.59% and 34.89% of the total heat exchanges at Lake Vanajavesi, respectively.

"Lakes influence local and regional climates, yet their heat exchange with the atmosphere remains underrepresented in weather forecasts and climate models," remarked Professor Timo Vesala from the University of Helsinki's Institute for Atmospheric and Earth System Research and co-author of the study.

The research team also examined surface roughness lengths and bulk transfer coefficients, discovering that the coefficients for heat and water vapor transfer differ significantly. This finding challenges the conventional assumption in many numerical models that these coefficients are identical, underscoring the necessity for more accurate parameterization of lake-atmosphere interactions.

These findings highlight the dynamic nature of energy fluxes and the critical role of cold fronts in lake energy exchange. However, the implications may differ depending on factors such as lake size, depth, mixing patterns, and nutrient levels, which can influence energy exchange processes.

The measurement platform over Lake Vanajavesi in Finland (Image by XU Lujun)