Understanding how lakes exchange heat and moisture with the atmosphere is crucial for regional water resource management and sustainable development. A China-Finland research team has shed light on this dynamic process by analyzing energy fluxes and the effects of cold fronts over Lake Vanajavesi in southern Finland, based on eddy covariance measurements.

“Our findings show that the lake acts as a heat sink during spring and summer but becomes a heat source in winter,” said Dr. XU Lujun from the Institute of Atmospheric Physics, Chinese Academy of Sciences. the lead author of the study published in Advances in Atmospheric Sciences.

The study highlights that lakes can regulate regional energy exchange on both daily and seasonal timescales. The latent heat flux—the energy associated with evaporation—was found to reach its lowest point in the morning and peak in the afternoon, while the sensible heat flux followed an opposite trend. Key factors influencing sensible heat flux included the temperature difference between the lake and the air, as well as the interaction of this temperature difference with wind speed. For latent heat flux, vapor pressure deficit and its interaction with wind speed were the primary drivers.

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

One of the study’s most striking findings was the significant role of cold fronts in enhancing heat exchange. Cold fronts triggered rapid increases in wind speed and decreases in temperature. Due to the lake's high heat capacity, it cooled more slowly than the atmosphere, creating larger temperature and vapor pressure differences. These differences resulted in sharp increases—or pulses—in sensible and latent heat exchange. Over the ice-free season, 28 sensible heat pulses and 17 latent heat pulses were observed, accounting for 50.59% and 34.89% of the respective total heat exchanges at Lake Vanajavesi. 

“Lakes influence local and regional climates, but their heat exchange with the atmosphere remains poorly understood and inadequately represented in weather forecast and climate models,” said Professor Timo Vesala from the University of Helsinki’s Institute for Atmospheric and Earth System Research, coauthor of the study. 

The research team also analyzed surface roughness lengths and bulk transfer coefficients, finding that heat and water vapor transfer coefficients were not equal. This challenges the conventional assumption used in many numerical models that these coefficients are identical, highlighting the need for improved parameterization of lake-atmosphere interactions.

These findings underscore the complexities of energy fluxes and the role of cold fronts in lake energy exchange. However, the implications may vary depending on factors such as lake size, depth, mixing patterns, and nutrient levels. Future research will focus on strengthening long-term lake observations from space and refining models to better capture lake-atmosphere interactions, advancing our understanding of how lakes respond to and influence climate change.