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

These two last relations are the ones widely used for most turbulence studies.The inertial domain ranges from the viscous scale (10 −2 – 10 m) to the outer scale L m(10 – 1000 m). Typically, turbulent energy lies in the range 10 −2 to 10 J/kg (KE and APE). Thedissipation rates of fluctuations variances σ 2 (velocity, temperature ...) scale as σ 2 N.Two estimates for the turbulent diffusivity have been proposed, either from the dissipationrate of temperature variance (related to the thermal conductivity), or from the KE dissipationrate, by assuming a given efficiency of mixing.Φ T = w ′ T ′ = K θdθdz⎧⎪⎨⎪⎩K θ = ɛ θdθ/dzK θwhere the mixing efficiency γ reads:γ =(= ɛ pN 2 )(Osborn and Cox, 1972)= γ ɛ kN 2 (Lilly et al., 1974)BP − B =2.1 A turbulent diffusivity estimate ?(5)R f1 − R f= ɛ pɛ k(6)An estimation of the mixing properties of turbulence raises several difficulties. The turbulentdiffusivity is likely related to the intensity of turbulence (KE or ɛ k ), but not only:• The diffusivity will depend on that fraction of energy that is not dissipated into heat (i.e.ɛ p rather than ɛ k ). Now, ɛ k is the commonly evaluated turbulent quantity.• The turbulence is intermittent in space and time. An effective diffusivity is likely dependenton the space-time distribution of turbulent events, and not only on the turbulenceintensity.• Some assumptions are likely questionable (local homogeneity, stationarity, constant mixingefficiency, ...)3 Radar measurements of turbulence parametersSeveral scattering mechanisms are important for atmospheric radars operating from the MF tothe UHF bands: Rayleigh scattering from hydro-meteors, insects, etc. . . (UHF); Fresnel scattering,from thin refractive-index interfaces (operating frequencies under about 450 MHz) (e.g.Röttger and Liu, 1978; Gage and Green, 1978); and Bragg scattering induced by refractiveindexfluctuations at λ r /2 (MF to UHF) (Tatarskii, 1961). The Bragg scattering is the relevantprocess for studies of small scale turbulence in the atmosphere. Actually, VHF measurementshave to be obtained from tilted beams, at least 15 ◦ from zenith (e.g. Tsuda et al., 1997). UHFmeasurements are useful if Rayleigh scattering is negligible, i.e. in the UTLS.MST radar measurements consist of backscattered power, Doppler shift and Doppler width(zeroth, first and second moments of the Doppler spectrum). The radial wind velocity, aswell as the velocity variance (provided that the non-fluctuating causes of broadening can beevaluated), are directly estimated, without any calibration. An important point is that theseestimates are both range-and-reflectivity weighted averages. On the other hand, the estimationof reflectivity, and related C 2 n, requires a calibrated radar. Furthermore, the reflectivity estimateis a volume average, a uniform fluctuating field being implicitly assumed.Two methods were proposed for the estimation of turbulence parameters from clear-airradar measurements. Both methods assume that the turbulence is inertial, isotropic, locallyhomogeneous and quasi-stationary.195