The Polar boundary layer over High Arctic tundra in summer

The Polar boundary layer was studied over land in Svalbard during two consecutive summers. Changes in the aerodynamic roughness length, the radiation budget and the turbulent fluxes were focussed on during snow free conditions on High Arctic tundra. Compared to winter, the summer boundary layer in this region has received little attention. Moreover, the gradient method and the bulk method were tested and compared to the eddy-covariance method in order to evaluate if these methods can be used to increase the number of observations, which are generally sparse in the High Arctic.

The surface albedo increased throughout the snow free summer, from approximately 0.1 to 0.2, which compares well with similar studies. Great changes in the aerodynamic roughness were recorded with only small differences in wind direction, even though the surface visually appeared homogenous.

Weather conditions during the High Arctic summer are less severe than in winter but it is often more difficult to take measurements over land in this season than in the winter. Snow mobiles cannot be used on the snow free ground and measurements are hence restricted to easily accessible areas such as the coastline. The alternative is to use expensive and often unreliable transport, such as helicopters, to access and maintain the instruments. This study demonstrates that the gradient method can be employed to estimate surface fluxes using relatively cheap and robust instrumentation that can be left unattended for longer periods at remote locations during the High Arctic summer. Compared to the eddy-covariance method, the methods agree within 5% for momentum flux, while for sensible heat flux, the gradient method produces, on average, approximately 40% lower values for unstable stratification and 67% lower for stable stratification. However, this corresponds to only 20 and 12 W m-2 respectively. The bulk method, on the other hand, showed much greater uncertainty both for momentum flux and sensible heat flux.

The surface energy budget for the two summers revealed that the sensible heat flux was usually directed upwards, while the latent heat flux decreased during the summer and changed sign from upward to downward in July.