Prof. Julian Hunt
University College London
The usual non-standard behaviour of the atmospheric boundary layer
Abstract: Turbulent boundary layers (bl) have different structures depending on the wind speed and the surface heat flux, which are usually characterised by the ratio of the bl depth to the Monin-Obukhov length and it is usually assumed in meteorological and environmental models that ,even if the atmospheric conditions are changing, the bl profiles are the same as those in a steady state conditions. Many observations have shown since the 1920's that this assumption is often quite erroneous-but no general concepts or modelling methods have emerged to describe these situations.In this talk recent theoretical, numerical and field studies (involving collaboration between Potsdam, London, Delft, Tempe and Uppsala) are described concerning atmospheric bls where the surface heat flux changes in time on a flat and sloping surface. A noticeable feature is the occurrence of maxima or 'jets' in the vertical mean velocity profile, both with heated and cooled surfaces. On cooled sloping surfaces stagnation points and local mixing occurs that is significant for local mixing eg for orchards in 'frost pockets'. Measurements of the turbulence in these bls over the Baltic sea by Profs Smedman and Hogstrom shows , surprisingly, that these jets can reduce the energy of the largest scale eddies in the spectrum, with consequent reductions in the air-sea mass/heat transfer, that are significant, at least for the Baltic.
| Zeit: | Freitag, 31. Oktober 2003, 16.00 (Kaffee/Tee um 15.30 p.m.) |
| Ort: | FU Berlin, Arnimalle e 2-6, Raum 032 im EG |