Turbulence in a Katabatic Flow |
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| Authors: | Ernst Van Der Avoird Peter G Duynkerke |
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| Institution: | (1) Institute for Marine and Atmospheric Research Utrecht (IMAU), Princetonplein 5, 3584 CC Utrecht, The Netherlands |
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| Abstract: | Turbulence measurements performed in a stable boundary layer over the sloping ice surface of the Vatnajökull in Iceland are described. The boundary layer, in which katabatic forces are stronger than the large-scale forces, has a structure that closely resembles that of a stable boundary layer overlying a flat land surface, although there are some important differences. In order to compare the two situations the set-up of the instruments on an ice cap in Iceland was reproduced on a flat grass surface at Cabauw, the Netherlands. Wind speed and temperature gradients were calculated and combined with flux measurements made with a sonic anemometer in order to obtain the local stability functions  m and h as a function of the local stability parameter z/L. Unlike the situation at Cabauw, where m was linear as a function of z/L, in the katabatically forced boundary layer, the dependence of m on stability was found to be non-linear and related to the height of the wind maximum. Thermal stratification and the depth of the stable boundary layer however seem to be rather similar under these two different forcing conditions.Furthermore, measurements on the ice were used to construct the energy balance. These showed good agreement between observed melt and components contributing to the energy balance: net radiation (supplying 55% of the energy), sensible heat flux (30%) and latent heat flux (15%).Local sources and sinks in the turbulent kinetic energy budget are summed and indicate a reasonable balance in near-neutral conditions but not in more stable situations. The standard deviation of the velocity fluctuations  u, v, and w, can be scaled satisfactorily with the local friction velocity u* and the standard deviation of the temperature fluctuation  with the local temperature scale *. |
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| Keywords: | Energy balance Glacier Katabatic flow Stable boundary layer Turbulence |
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