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

TURBULENT DIFFUSIVITY INFERRED FROMMST RADAR MEASUREMENTS: A REVIEWWilson, RichardService d’Aéronomie/IPSL, Université P. et M. Curie, Paris, France1 IntroductionThe small scale turbulence in the atmosphere - and ocean - is the sink for mechanical energy,a process inducing irreversible transport of heat, mass and chemical species, as well as theprocess inducing drag on the large scale flow through the Reynolds stress. Evaluating theactual impact of small scale turbulence in the atmosphere - for instance the diffusive properties- is still a challenging question.The main goal of this communication is to review and discuss several aspects of radarmeasurements in turbulence studies, as well as to stress some of the needs for future works.2 Turbulence in a Stratified MediumIn a stratified medium, turbulent fluctuations of velocity induce fluctuations of temperature,concentration, refractive index... Due to gravity, vertical displacements δz are related to availablepotential energy (APE). When considering the energetics of stratified turbulence, twoquantities have thus to be considered: turbulent Kinetic Energy (KE) and APE.KE = 1 2 (u′2 + v ′2 + w ′2 ) and AP E = 1 2 N 2 δz ′2 = 1 θ ′2(1)2 N 2 θwhere N is the Brunt-Väisälä frequency, and θ the potential temperature. Therefore, the dissipationrate of temperature variance ɛ θ , due to thermal conductivity, is related to the the dissipationrate of APE, ɛ p .The equations describing the time evolution for turbulent KE and APE reads (Tennekes andLumley, 1972):ddt KE = P − B −ɛ k + transport terms(2)ddt AP E = B −ɛ p + transport termswhereP = −u ′ w dU ′ and B = − g dzT w′ T ′ = − g T Φ T (3)P is the production term through Reynolds stress acting in a shear, the buoyancy flux B expressesa reversible conversion of KE into APE, B being simply related to the heat flux Φ T . Byassuming spatial homogeneity and stationarity, the transport terms and time-derivatives vanish:ɛ k = P − Bɛ p = Bg 2(4)194