Proceedings with Extended Abstracts (single PDF file) - Radio ...

where τ m is the mean waiting time between successive encounters.The effective diffusivity will depend on three parameters, (i.e. on the P.D.F. of these quantities):• the height h of the turbulent layers;• the vertical displacement within a turbulent patch of height h (i.e. turbulence intensity);• the waiting time τ m .In the long time limit, they obtain (for complete mixing within the patches , i.e. the Dewan’shypotheses):where H is the height of the considered atmosphere.K effθ = h312Hτ m(21)6.3 An energetic point of viewIt is long time recognised that mixing results in an increase of potential energy (e.g. Thorpe,1973). Winters et al. (1995) showed how to relate the mixing properties of turbulence to theenergy budget. First, following (Lorenz, 1955) they consider the background potential energyof a fixed domain. They show how diapycnal mixing, through destratification, leads to anincrease of that background potential energy: the APE (buoyancy flux) is partly irreversiblyconverted into background potential energy. This is a way to quantify a bulk mixing efficiencyof turbulence (already used in laboratory and numerical experiments) for a considered volume.However, this method cannot be applied directly to the atmosphere, as the density in not aconservative quantity.7 Concluding remarksSince the early developments of the MST radar technique (Woodman and Guillen, 1974) considerableprogresses have been made concerning the physics of measurements (understandingof scattering processes), and signal processing (inference of turbulence parameters). MSTradars allow now fairly consistent and reliable estimations of the energetic parameters of smallscale turbulence: KE and dissipation rates, even though some assumptions might be questioned.Also, MST radar measurements have learned to us a lot about the morphology of atmosphericturbulence. However, if concerned with the mixing properties of turbulence, several questionsremains. In the order:• What is the correct expression for the effective diffusivity of intermittent turbulence?Theoretical considerations suggest that an estimator of the effective diffusivity depends on parametersdescribing the space and time characteristics of turbulence events, (i.e. intermittency):height of the layers, waiting-time and turbulence intensity as well.• How to relate the (measured) energetic parameters of turbulence to a local diffusion coefficient?Several hypotheses seems questionable: γ = Const., or L m /L B = Const..• What should (can) we measure with MST radars ? It appears that a need exists for highresolution measurements in order to better describe the space and time distribution of turbulentevents. Also, it seems that simultaneous KE and C 2 n estimations are needed as it allows toestimate the mixing efficiency γ.Finally, an energetic approach, such the one proposed by Winters et al. (1995), is likelypromising on the way to estimate the effective diffusivity of small scale turbulence in the atmosphere.200