Momentum and heat fluxes over lakes Tamnaren and Raksjo determined by the bulk-aerodynamic and eddy-correlation methods

Micrometeorological measurements made concurrently over two boreal lakes, L ake Tamnaren (surface area 37 km(2), depth 2 m) and Lake Raksjo (1.5 km(2), 4 m), during the NOPEX observational campaigns provided long-term latent a nd sensible heat fluxes determined with the bulk aerodynamic method. The tu rbulence transfer coefficients were verified with short-term eddy correlati on runs over Lake Tamnaren. The drag coefficient and the Stanton number att ained a stability dependence close to the Businger-Dyer form. During weak o r moderate wind conditions, the latent heat flux determined from eddy corre lation 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 f rom 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 uns table, 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 o r daily basis was strongly correlated with wind speed. This was also demons trated by the higher rates of evaporation on the larger Lake Tamnaren compa red to the sheltered Lake Raksjo during strong wind conditions. Towards the end of the summer of 1995, the lake surface temperature attained somewhat higher values on Lake Raksjo, attributable to its larger depth. This also r esulted in higher daily average sensible and latent heat fluxes compared to Lake Tamnaren in August-September. Thus, in the long-term, the observed da ily deviations between the lakes nearly compensated each other. (C) 1999 El sevier Science B.V. All rights reserved.