Pääjärvi representative basin in Finland: heat balance of a lake

Abstract

The heat balance of Lake Pääjärvi in Southern Finland was studied. Net radiation was measured at a level of 2 m above the lake. Heat storage in the lake was surveyed once a week. Air temperature, humidity and wind velocity were measured at four levels (1, 2, 4 and 6 m) above the lake sur­face from June to October in 1969 and 1970. The upper parts of the wind profiles in near‑neutral cases were used to determine the drag coefficient (C_u10) which was found to have a mean value of 1.3 X 10^‑3 with no dependence on wind velocity. The scatter was large. A bulk aerodynamic method including the effect of stability was used to estimate the fluxes of sensible and latent heat. Thermal stratification was usually unstable at night and stable or near‑neutral in the afternoon over the lake in summer; most (80 %) of the cases (5208 mean hourly profiles) were unstable. The greatest component (absolute value) of the heat balance of the lake in summer was net radiation; in autumn it was the heat loss from the water body of the lake. Over the ground, heat flux into the ground is typically the smallest term in summer. The ratio of the monthly mean flux of sensible heat to that of latent heat was very small in summer: e.g. 0.10 in June 1970 and about 0.50 in autumn. The main components of the heat balance of the lake had large day‑to‑day variations caused by different weather conditions. The mean diurnal course of the flux of latent heat was fairly smooth. The mean fluxes of sensible heat were nearly half those of latent heat and heat was transferred from the water body into the air at night. During summer daylight hours, however, sensible heat was transferred from the air into the water. In autumn sensible beat was, on average, transferred throughout the day from the water body into the air.