ИСТИНА

Lakes occupy a significant part of land in many northern regions, e.g. in Northern Siberia, Karelia, Finland and Canada. The importance of thermodynamic interaction between lakes and the atmosphere in these regions led to inclusion of lake parameterizations into climate models and numerical weather prediction systems. However, these lake parameterizations are still confined to heat and momentum exchange at the lake-atmosphere interface, whereas observational evidence is growing on the importance of greenhouse gases emissions from lakes. In order to extend our current knowledge on the dynamics of these emissions and gain a capability of making future projections of climate taking into account lake carbon fluxes, suitable modelling framework is to be developed. However, the modelling task faces two kinds of problems here. First, a lake model involving explicit treatment of both key biotic and abiotic controls of methane and carbon dioxide emissions is to be developed. And second, air surface layer parameterizations are needed that are adequate to lake-adjacent typical landscapes met in high latitudes. From this point a special attention must be paid to a case of a lake, surrounded by bluff topography (a forest), that violates the hypotheses underlying Monin-Oboukhov similarity theory. Large Eddy Simulation (LES) occurs to be an only feasible tool to explicitly reproduce turbulent flow and fluxes over such a landscape. This abstract presents first results of a project, addressing the above issues. We adopt the one-dimensional model LAKE (Stepanenko et al., 2011) containing a module of methane production, transport and sink processes in bottom sediments and in a water column to one of thermokarst lakes in Seida place, Komi Republic, Russia. The meteorological forcing and validation data on water temperature and methane fluxes were measured in situ by the University of Eastern Finland during the 2007-2008 seasons (Marushchak et al., 2013; Repo et al ., 2009). Chamber method was used for measuring methane fluxes from the thermokarst lakes. The measurements missed atmospheric radiation and shortwave radiation extinction coefficient in the water column. Therefore, the sensitivity of the model to specification of these variables was studied. It is shown that the “optimal” choice of atmospheric radiation parameterization allows a good match of water surface temperature with observations. The surface temperature was found to be almost insensitive to extinction coefficient, due to intense vertical mixing down to the lake bottom (2 m depth). However, the sensitivity of methane fluxes to extinction coefficient was high, pointing at an exponential dependence of methane production on sediment temperature. The modelled methane fluxes agreed well with the available measured data by calibrating a parameter describing organic substrate amount and quality in methane production formula. An LES study by INM RAS – RCC MSU LES model (Glazunov et al., 2010), has been performed for an idealized case of an elliptic lake surrounded by forest. The “forest” is represented by a regularly spaced set of vertically elongated blocks. The reference boundary layer stratification, wind speed and direction were varied. It is shown, that the statistics of the flow (including fluxes) are almost insensitive to stratification resulting from turbulent kinetic energy predominantly produced by shear generation at heights around the tree canopy. This strongly questions the applicability the Monin-Oboukhov similarity to this case. Another important conclusion derived is that the constant-flux layer does not exist in the first meters above the lake surface, i.e. the flux at 1.5 m height may differ from that at the surface by 1.6-1.7 times. This has a direct implication for eddy covariance measurements above lakes, that are typically performed at this height. References 1. Glazunov A.V., Dymnikov V.P., Lykossov V.N. Mathematical modelling of spatial spectra of atmospheric turbulence. - Russ. J. Numer. Anal. Math. Modelling, 2010, v. 25, No. 5, p. 431 – 451. 2. Marushchak, M.E., Kiepe, I., Biasi, C., Elsakov, V., Friborg, T., Johansson, T., Soegaard, H., Virtanen, T., Martikainen, P.J., 2013. Carbon dioxide balance of subarctic tundra from plot to regional scales. Biogeosciences 10 (1), 437–452. 3. Repo, M.E., Susiluoto, S., Lind, S.E., Jokinen, S., Elsakov, V., Biasi, C., Virtanen, T. & Martikainen, P.J. (2009). Large N2O emissions from cryoturbated peat soil in tundra. Nature Geoscience. 4. Stepanenko, V. M., Machul’skaya, E. E., Glagolev, M. V. and Lykosov, V. N. 2011. Numerical modeling of methane emissions from lakes in the permafrost zone. Izv. AN. Fiz. Atmos. Ok+ 47, 275–288.