Abstract
Five one-dimensional (1D) lake models were run for the open water season in 2006 for Lake Valkea-Kotinen (Finland) using on-lake measured meteorological forcing. The model results were validated using measurements of water temperature and of eddy covariance (EC) fluxes. The surface temperature is satisfactorily simulated by all models showing slight overestimation (by 0.1–1.1°C). Both sensible and latent heat fluxes are positively biased in respect to EC data, consistent with earlier studies. However, correlation coefficients between EC-fluxes and those simulated are relatively high ranging from 0.55 to 0.74. The skill to simulate vertical temperature profiles by different models is assessed as well. It is found that the lake models underestimate the EC-derived surface drag coefficient, however providing realistic temperature profiles. It is argued that the real momentum flux from the atmosphere is larger than simulated, however it is split up between the wave development and the acceleration of lake currents. Adopting the simple parameterisation for momentum flux partitioning in one of the models showed that this mechanism can be significant. Finally, the effect of including the lake bathymetry data in k-ɛ models was the drastic overheating of water below the thermocline. This is likely to be caused by omitting the heat flux at the lake margins. Thus, the parameterisation of heat flux at the lake's margins should be included in the models; otherwise it is recommended to neglect bathymetry effects for such small water bodies as the Lake Valkea-Kotinen.
Highlights
The significance of lakes affecting both local and regional atmospheric conditions at a wide range of timescales has been extensively recognised (Samuelsson et al, 2010; Balsamo et al, 2012)
The instant deviations of modelled temperature from the measured one are typically less than 28C that may be regarded as sufficient accuracy for atmospheric applications. This value coincides with the diurnal lake surface temperature range whereas the diurnal range of the land temperature in summer is several times larger
This error allows for realistic reproduction of the lakeÁland temperature difference during ice-free period, which is crucial for properly aggregating surface fluxes in atmospheric models
Summary
The significance of lakes affecting both local and regional atmospheric conditions at a wide range of timescales has been extensively recognised (Samuelsson et al, 2010; Balsamo et al, 2012). Majority of lake models captured well both the seasonal and the diurnal cycles of surface temperature, whereas the discrepancy between water temperatures below the surface calculated by different models increased with depth. This suggested that the surface energy fluxes controlling the surface temperature are relatively well reproduced by surface flux schemes used in the models, most of which are based on the Monin-Obukhov similarity theory (MOST). For other lake types new model capabilities and limitations may be encountered related to validity of fundamental assumptions underlying the 1D model formulations (e.g. neglecting horizontal gradients, using MOST theory for surface fluxes, including or omitting the explicit treatment of bottom sediments)
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