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

5.2 Frequency distributionsSeveral evidences, from radar or in-situ measurements, show that the distribution functions ofdissipations rates - as well as the inferred diffusivity - are (approximately) log-normal (Nastromand Eaton, 1997; Alisse and Sidi, 2000; Dole et al., 2001).5.3 Persistent layers of enhanced reflectivitySeveral authors have reported the existence of persistent layers of enhanced radar reflectivity inthe lower stratosphere (Nastrom and Eaton, 2001; Luce et al., 2002). Intense turbulent eventsin the lower stratosphere were also described from in situ measurements (Pavelin et al., 2002;Luce et al., 2002). Such layers are likely associated with intense mixing (K θ ∼ 1 m 2 s −1 inthe lower stratosphere). A climatological study (Nastrom and Eaton, 2001) indicates that theselayers are present about 1−2% of time. The impact on vertical transport has to be evaluated.6 An effective diffusivity: Theoretical and semi-empiricalapproachesThe issue of the effective diffusivity of patchy turbulence was addressed by Garrett (1979);Dewan (1981); Woodman and Rastogi (1984); and Vaneste and Haynes (2000) among others.6.1 Semi-empirical approachesThe question was first to know if intermittent turbulence can be considered as a diffusive process.The response is yes, but in the long-time limit. From a very simple numerical modelDewan (1981) shows that mixing resulting from random layers can be described formally asdiffusion by comparing the results of numerical simulations of random mixing layers to theknown analytical solutions for different cases (initial conditions):K effθ = F Td 28τ m(18)An estimation based on a flux calculation was proposed by Woodman and Rastogi (1984).Considering an arbitrary level z, the flux of a tracer is evaluated across that level by assumingcomplete mixing. The local diffusivity is a function of the layer thickness d and of the life-timeof the patch.Kθ local = < d2 >(19)12TAn extension for sweeping layers is also considered. Woodman and Rastogi (1984) then inferan effective diffusivity profile by combining their flux estimates with high resolution radarobservations They obtain Kθeff ∼ 0.2 − 0.3 m 2 s −1 in the UTLS.6.2 A Lagrangian approachA Lagrangian approach was recently proposed by Vaneste and Haynes (2000). They modelthe diffusion process as a continuous-time random walk. At random time, a fluid particleencounters a turbulent patch, it is then vertically displaced. The variance of the displacementσ 2 z is related to the local diffusivity (flux per unit gradient):K localθ= σ2 z2τ m(20)199