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

4.1 The mixing efficiency γThe mixing efficiency appears when expressing the dissipation rate, ɛ k , as a function of aquantity related to the turbulent APE, C 2 n (eq. 8) or heat flux (i.e. K θ , eq. 5).• A “canonical” R f = 0.25 gives γ = 1/3 (e.g. Lilly et al., 1974, based on Thorpe (1973)experiments).• Numerous evaluations from measurements in the ocean and lakes, or from laboratoriesexperiments, usually from the ratio ɛ p /ɛ k , (e.g. Rohr et al., 1984; Imberger and Ivey, 1991;Ruddick et al., 1997; St Laurent and Schmitt, 1999). or DNS as well (Itsweire et al., 1993)indicate 0 ≤ γ ≤ 0.2• In situ or radar estimations for the stratosphere give 0.06 ≤ γ ≤ 0.3 (Alisse and Sidi,2000; Dole et al., 2001).• Various expressions for γ can be found in the literature, for instance by writing R f =Ri/P T r (Ri being the gradient Richardson number, P T r the turbulent Prandl number), γ reads:γ = 1P T rP T rRi− Ri(17)• Several authors suggested however that γ should not be treated as a constant (Weinstock,1992; Hocking and Mu, 1997; Gossard et al., 1998; Smyth et al., 2001).4.2 The turbulent fractionAn estimate of the turbulent fraction, F T , appears necessary when expressing the local Cn 2 fromthe volume-averaged reflectivity (eq. 9). As will be further discussed, F T is also a fundamentalquantity when expressing an effective diffusivity from local estimates.• In-situ measurements in the UTLS indicate 0.02 ≤ F T ≤ 0.05 (Lilly et al., 1974)• Radar measurements in the lower stratosphere show 0.05 ≤ F T ≤ 0.2 (Dole et al., 2001).• A parameterisation based on a statistical model for the wind shear (VanZandt et al., 1978)gives 0.03 ≤ F T ≤ 0.1.• A simplified version of this parameterisation suggests F 1/3TN 2 ∼ Const. (Gage et al.,1980).5 Radar observations5.1 Climatologies of turbulence diffusivitySeveral climatologies of turbulence diffusivity inferred from radar measurements are published(Hocking, 1988; Fukao et al., 1994; Kurosaki et al., 1996; Nastrom and Eaton, 1997; Rao et al.,2001). All these climatologies are based on wind variance estimates. The median diffusivityestimates compare very well in the lower stratosphere, Kθlocal ∼ 0.2 − 0.5 m 2 s −1 . Interestingdifferences where observed in the upper troposphere, the turbulent diffusivity being increasingwith height in one case (Kurosaki et al., 1996), and decreasing for an other case (Nastrom andEaton, 1997). The annual maximum in the UTLS is observed during winter over Shigaraki,during summer at the WSMR, and during the monsoon and post-monsoon months at Bangalore.In the mesosphere, Hocking (1988) does not observe any clear annual variation. Fukao et al.(1994) and Kurosaki et al. (1996) observe a semi-annual variability with solstice maxima. Raoet al. (2001) found a maximum of diffusivity around about 75 km, the annual maximum beingobserved during summer.198