Comparison of latent and sensible heat fluxes over boreal lakes with concurrent fluxes over a forest: implications for regional averaging

The seasonal and diurnal variation of latent, and sensible, heat fluxes abo ve two boreal-zone lakes and an adjacent coniferous forest was studied usin g data from the NOPEX field campaigns in 1994 and 1995. Heat fluxes over th e lakes were estimated with the bulk aerodynamic method and those over the forest with the eddy-correlation method. The night-time latent heat Aux, wh ile non-existent from the forest, was significant from the lakes during all summer months. The day-time flux from the lakes peaked, on average, in the afternoon, while the maximum flux from the forest occurred around noon. Th e differences in the latent heat supply between the two surface types varie d within +/-(60-80)W m(-2), depending on the month and time of day. The mon thly mean latent heat flux from the forest was higher than that from the la kes in May, in June and July the fluxes were about equal; and in August-Sep tember, the lake values exceeded those of the forest. The sensible heat Bur reached its maximum values over the forest near noon, but over the lakes d uring the early morning. An excess supply of heat over the forest of up to 200 W m(-2) was found in comparison with the lakes during the mid-day hours . From May to August, the monthly mean sensible heat flux was higher from t he forest than from the lakes. The difference in the heat exchange of lakes and forest was also estimated based from airborne measurements made from a n aircraft flying at a height of 100 m. For Right legs consisting of a 15% portion over the lake, the leg-averaged latent heat Aux was, on average, 2% lower and the sensible heat flux 9% lower, compared to a Right leg over fu lly-forested terrain. (C) 1999 Elsevier Science B.V. All rights reserved.