Timescales and structures in vertical transport in the atmospheric boundary layer

J. Schalkwijk, F. Bosveld and P. Siebesma, Koninklijk Nederlands Meteorologisch Instituut
De Bilt : KNMI

A combination of high frequency measurements from the Cabauw measurement site and LES results is used to evaluate timescales and structures of turbulent transport of heat in the atmospheric boundary layer. On the basis of the cospectrum of vertical velocity and temperature, a dominant timescale of transport is introduced. An emperical equation is proposed to easily determine this timescale as a function of wind velocity, height and Obukhov length. Using this timescale, loss in Eddy Covariance measurements due to finite averaging time can be predicted on the basis of a model for the cospectrum shape. Eddy Covariance measurements from the Cabauw site show behaviour confirming this predicted loss behaviour. No evidence is found for the possible presence of unpredicted systematic flux on very large timescales. Attention is also given to the presence of a certain gap scale seperating turbulent and mesoscale motions, but it is found that these motions overlap in unstable conditions, making seperation impossible.

65 p.
Fig., tab. ; 30 cm
(Scientific report = Wetenschappelijk rapport ; WR 2010-02)
With a bibliogr.