Eddy covariance measurements of carbon exchange and latent and sensible heat fluxes over a boreal lake for a full open-water period

	Become an AGU Member Subscribe to AGU Journals



Read Full Article (file size: 698219 bytes) Cited by JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, D11101, doi:10.1029/2005JD006365, 2006 Eddy covariance measurements of carbon exchange and latent and sensible heat fluxes over a boreal lake for a full open-water period Timo Vesala Department of Physical Sciences, University of Helsinki, Helsinki, Finland Jussi Huotari Department of Ecological and Environmental Sciences, University of Helsinki, Lahti, Finland Üllar Rannik Department of Physical Sciences, University of Helsinki, Helsinki, Finland Tanja Suni CSIRO Marine and Atmospheric Research, Canberra, ACT, Australia Sampo Smolander Department of Physical Sciences, University of Helsinki, Helsinki, Finland Andrey Sogachev Department of Physical Sciences, University of Helsinki, Helsinki, Finland Samuli Launiainen Department of Physical Sciences, University of Helsinki, Helsinki, Finland Anne Ojala Department of Ecological and Environmental Sciences, University of Helsinki, Lahti, Finland Abstract Long-term measurements of sensible and latent heat and carbon dioxide fluxes were performed over a boreal lake in southern Finland using the direct micrometeorological eddy covariance (EC) technique. The water column was sampled weekly for dissolved carbon dioxide, and the CO₂ flux was estimated also applying the concentration gradient method. Temperature and oxygen profiles of the lake were measured twice a week. The measurements covered one full open-water period from April to November 2003, and it is the longest continuous CO₂ record ever measured over a lake by EC. The sensible heat flux H was positive, that is, from the lake to the atmosphere, except in May, when it was >0 W/m² at night and <0 W/m² in daytime. The latent heat flux dominated clearly over H in spring and summer; that is, the Bowen ratio was less than 1. Higher-moment turbulence statistics proved to be efficient in detection of frequent nonstationary situations. Applying the statistical criteria for CO₂ concentration and vertical wind speed, averaging over a 5-min period and selecting only the wind direction with longest fetch, we could obtain lake-representative CO₂ fluxes. Footprint analysis based on a closure model revealed that the source areas were relatively short because of the presence of turbulence generated by the surrounding forest, compared to a larger lake with an extended smooth surface. We observed a net CO₂ source of 0.2–0.4 μmol m⁻² s⁻¹ excluding July, when the flux was closer to zero. The results are consistent with the gradient method, based on more infrequent sampling, and both methods gave the same average flux, 0.2 μmol m⁻² s⁻¹, over the whole open-water period. Received 14 June 2005; accepted 1 February 2006; published 2 June 2006. Keywords: carbon dioxide exchange; boreal lake; eddy covariance. Index Terms: 0315 Atmospheric Composition and Structure: Biosphere/atmosphere interactions (0426, 1610); 0746 Cryosphere: Lakes (9345); 0793 Cryosphere: Biogeochemistry (0412, 0414, 1615, 4805, 4912); 1818 Hydrology: Evapotranspiration; 1840 Hydrology: Hydrometeorology. Read Full Article (file size: 698219 bytes) Cited by Citation: Vesala, T., J. Huotari, Ü. Rannik, T. Suni, S. Smolander, A. Sogachev, S. Launiainen, and A. Ojala (2006), Eddy covariance measurements of carbon exchange and latent and sensible heat fluxes over a boreal lake for a full open-water period, J. Geophys. Res., 111, D11101, doi:10.1029/2005JD006365. Copyright 2006 by the American Geophysical Union.

Abstract