Energy balance of a sparse coniferous high-latitude forest under winter conditions

Measurements carried out in Northern Finland on radiation and turbulent flu xes over a sparse, sub-arctic boreal forest with snow covered ground were a nalysed. The measurements represent late winter conditions characterised by low solar elevation angles. During the experiment (12-24 March 1997) day a nd night were about equally long. At low solar elevation angles the forest shades most of the snow surface. Therefore an important part of the radiati on never reaches the snow surface but is absorbed by the forest. The sensib le heat flux above the forest was fairly large, reaching more than 100 W m( -2). The measurements of sensible heat flux within and above the forest rev ealed that the sensible heat flux from the snow surface is negligible and t he sensible heat flux above the forest stems from warming of the trees. A s imple model for the surface energy balance of a sparse forest is presented. The model treats the diffuse and direct shortwave (solar) radiation separa tely. It introduces a factor that accounts for the shading of the ground at low solar elevation angles, and a parameter that deals with the partial tr ansparency of the forest. Input to the model are the direct and diffuse incoming shortwave radiation. Measurements of the global radiation (direct plus diffuse incoming shortwa ve radiation) above the forest revealed a considerable attenuation of the g lobal radiation at low solar elevation. A relation for the atmospheric turb idity as function of the solar elevation angle is suggested. The global rad iation was simulated for a three month period. For conditions with a cloud cover of less than 7 oktas good agreement between model predictions and mea surements were found. For cloud cover 7 and 8 oktas a considerable spread c an be observed. To apply the proposed energy balance model, the global radi ation must be separated into its diffuse and direct components. We propose a simple empirical relationship between diffuse shortwave and global radiat ion as function of cloud cover.