Momentum and heat fluxes over lakes Tämnaren and Råksjö determined by the bulk-aerodynamic and eddy-correlation methods

Abstract

Micrometeorological measurements made concurrently over two boreal lakes, Lake Tämnaren (surface area 37km2, depth 2m) and Lake Råksjö (1.5km2, 4m), during the NOPEX observational campaigns provided long-term latent and sensible heat fluxes determined with the bulk aerodynamic method. The turbulence transfer coefficients were verified with short-term eddy correlation runs over Lake Tämnaren. The drag coefficient and the Stanton number attained a stability dependence close to the Businger–Dyer form. During weak or moderate wind conditions, the latent heat flux determined from eddy correlation was close to that calculated with the bulk aerodynamic method using typical values of the Dalton number. However, the independent verification of the Dalton number under conditions of moderate or strong wind suffered from the poor performance of the fast-response hygrometer due to movement of the float. Measurements of the surface roughness suggested a value of the Charnock constant near 0.02.On average, the stratification of the surface layer over both lakes was unstable, but stable conditions occurred almost daily due to advection of warm air from adjacent land areas. High evaporation rates occurred even during stable stratification. The variation of the latent heat flux on an hourly or daily basis was strongly correlated with wind speed. This was also demonstrated by the higher rates of evaporation on the larger Lake Tämnaren compared to the sheltered Lake Råksjö during strong wind conditions. Towards the end of the summer of 1995, the lake surface temperature attained somewhat higher values on Lake Råksjö, attributable to its larger depth. This also resulted in higher daily average sensible and latent heat fluxes compared to Lake Tämnaren in August–September. Thus, in the long-term, the observed daily deviations between the lakes nearly compensated each other.