1. TURBULENCE CLOSURE PROBLEM FOR STABLY STRATIFIED GEOPHYSICAL FLOWS S. Zilitinkevich 1-4, N. Kleeorin 5, I. Rogachevskii 5 1 Finnish Meteorological Institute, Helsinki, Finland 2 Atmospheric Sciences, University of Helsinki, Finland 3 Nizhniy Novgorod State University, Russia 4 Institute of Atmospheric Physics RAS, Moscow, Russia 5 Ben-Gurion University of the Negev, Beer Sheba, Israel February 2012

2. Main-stream in turbulence closure theory Boussinesq (1877) Turbulent transfer is similar to molecular transfer but much more efficient  down-gradient transfer  K -theory  eddy viscosity, conductivity, diffusivity Richardson (1920, 1922) stratification Ri (and concept of the energy cascade) Keller - Fridman (1924) a chain of b udget equations for statistical moments Problem: to express higher-order moments through lower-order moments Prandtl (1930s) mixing length l ~ z, velocity scale u T ~ ldU/dz, viscosity K ~ lu T Kolmogorov (1941) (quantified the cascade) closure as a problem of energetics: budget equation for turbulent kinetic energy (TKE) TKE dissipation rate expressed through the turbulent-dissipation length scale u T ~ (КЭТ) 1/2, K ~ l ε u T underlies further efforts until the end of 20 th century Obukhov (1946) TKE-closure extended to stratified flows, Obukhov length scale L Monin-Obukhov (1954) alternative  similarity theory for the surface layer z /L Mellor-Yamada (1974) Hierarchy of K - closures. The problem of turbulence cut-off

3. Turbulence cut-off problem Buoyancy b = (g/ρ 0 )ρ ( g – acceleration due to gravity, ρ –density ) Velocity shear S = dU/dz ( U – velocity, z – height ) Richardson number characterises static stability The higher Ri (or z/L ), the stronger suppression of turbulence Key question What happens with turbulence at large Ri ? Traditional answer Turbulence degenerates, and at Ri exceeding a critical value (Ri critical < 1) the flow becomes laminar (Richardson, 1920; Taylor, 1931; Prandtl, 1930,1942; Chandrasekhar, 1961;…‏) In fact field, laboratory and numerical (LES, DNS) experiments show that GEOPHYSICAL turbulence is maintained by shear at least up to Ri ~ 10 2. Modellers are forced to preclude the turbulence cut-off ARTIFICIALLY

4. Milestones Prandtl-1930’s followed Boussinesq’s idea of the down-gradient transfer ( K -theory), determined K ~ lu Т, and expressed u T heuristically through the mixing length l Kolmogorov-1942 (for neutrall stratication) followed Prandtl’s concept of eddy viscosity K M ~ lu Т ; determined u T = (ТКЕ) 1/2 through TKE budget equation with dissipation ε ~ (TKE) /t T ~ (TKE) 3/2 / l ε ; and assumed l ε ~ l (grounded in neutral stratification) Obukhov-1946 and then the entire turbulence community extended Kolmogorov’s closure to stratified flows keeping it untouched. They only included in the TKE equation the buoyancy term that caused cutting off TKE in “supercritical” stable stratification In doing so, they missed turbulent potential energy (TPE interacted with TKE); overlooked inapplicability of Prandtl’s relation K ~ lu Т to the eddy conductivity K H ; and disregarded principal deference between l ε and l For practical applications Mellor and Yamada (1974) developed heuristic corrections preventing unacceptable turbulence cut-off in “supercritical” static stability

5. Energy- & flux-budget (EFB) theory (2007-2012) Budget equations for major statistical moments Turbulent k inetic energy (TKE) E K Turbulent p otential energy (TPE) E P Vertical flux of temperature F z = [ or buoyancy (g/T)F z ] Vertical flux of momentum τ iz = (i = 1,2) New relaxation equation for the dissipation time scale t T = l ε / (TKE) Accounting for TPE vertical flux of buoyancy (that killed TKE in Kolmogorov’s type closures) drops out from the equation for total turbulent energy (TTE = TKE + TPE) The heat-flux equation reveals a self-limitation of the vertical heat/buoyancy flux causing essential self-preservation of turbulence up to Ri ~ 10 2 Physical mechanisms and concepts Kolmogorov’s model for the effective dissipation of the turbulent flux of momentum Non-gradient generation of the buoyancy flux  self-preservation of turbulence New, physically consistent model of the turbulent dissipation time / length scales Fully revised inter-component energy exchange (instead of “return to isotropy”) Finally we got rid of misleading analogies with molecular transfer

6. Turbulent potential energy (analogy with Lorenz’s available potential energy) Fluctuation of buoyancy Fluctuation of potential energy (per unit mass)

7. Turbulent energy budgets Kinetic energy Potential energy Total energy Buoyancy flux βF z drops out from the turbulent total energy budget

8. Budget equation for the turbulent flux of momentum Effective dissipation

9. LES verification of Kolmogorov closure for effective dissipation of the turbulent flux of momentum

10. Budget equation for the vertical turbulent flux of potential temperature The “pressure term” is shown to be proportional to the mean squared fluctuation of potential temperature On the r.h.s. of the equation, the 1 st term (generating positive heat flux) counteracts to the 2 nd term (generating negative heat flux) and assures self-preservation of turbulence in very stable stratification

11. LES verification of our parameterization of the pressure term

12. Turbulent dissipation time and length scales By definition, time scale:, length scale: The steady-state TKE budget Flux Ri. Obukhov number length Shear: neutral, extreme stable (TKE) Interpolation yields empirical law valid in any stratification Combining the law with TKE budget equation yields where is master length scale

13. Relaxation equation for dissipation time scale Evolution of t T is controlled by the tendency towards equilibrium: and distortion by non-stationary processes and, in heterogeneous flows, by the mean-flow and turbulent transports. Their counteraction is described by the RELAXATION EQUATION: where is the relaxation constant.

14. EFB closure and M-O similarity theory Substituting the above empirical law into definition of flux Richardson number yields CONVERTOR between Ri f and z/L EFB closure yields CONVERTOR between Ri f and Ri : where at Ri >1 (see below empirical Ri -dependence of turbulent Prandtl number Pr T )

15. Major results The concept of turbulent potential energy (Z et al., 2007) analogous to Lorenz’s available potential energy (both proportional to squared density) New treatment of and relaxation equation for turbulent dissipation time scale Disproved widely recognised, erroneous conclusion (from traditional turbulence- closure theory) that shear-generated turbulence cuts off and flow becomes laminar at Richardson numbers Ri exceeding a critical value Ri c ~ 0.25-1. Instead, in the EFB theory, a threshold value of Ri separates two regimes of the stably stratified turbulence of principally different nature: “Strong turbulence” K M ~ K H typical of boundary layers (at Ri >Ri c ) – unknown until now A hierarchy of closure models of different complexity – for use in research and operational modelling atmospheric and oceanic flows Principal revision of the Monin-Obukhov similarity theory (transitional asymptote) Field, laboratory and numerical (LES, DNS) experiments confirm our theory up to Ri ~ 10 2 – for conditions typical of the free atmosphere and deep ocean

16. Examples of empirical verification of the steady-state version of the EFB closure

17. Turbulent Prandtl number Pr T = K М /K H versus Ri Atmospheric data: (Kondo et al., 1978), (Bertin et al., 1997); laboratory experiments: (Rehmann & Koseff, 2004), (Ohya, 2001), (Strang & Fernando, 2001); DNS: (Stretch et al., 2001); and LES: (Esau, 2009). The curve sows our EFB theory. The “strong” turbulence ( Pr T  0.8 ) and the “weak” turbulence ( Pr T ~ 4 Ri ) separate at Ri ~ 0.25. …

18. Longitudinal A x, transverse A y & vertical A z TKE shares vs. z/L Experimental data from Kalmykian expedition 2007 of the Institute of Atmospheric Physics (Moscow). Theoretical curves are plotted after the EFB theory. The traditional “return-to-isotropy” model overlook the stability dependence of A y clearly seen in the Figure, where the strongest stability, z/L =100, corresponds to Ri = 8.

19. Порог Ri = 0.25 The share of turbulent potential energy Е Р / (Е Р +Е К ) Насыщение Е Р / (Е Р + Е К ) ~ 0.2-0.4

20. The share of the energy of vertical velocity Е z / Е K

21. The dimensionless vertical flux of momentum  two plateaus corresponding to the “strong” and “weak” turbulence regimes

22. The dimensionless heat flux  sharply diminishes in the “weak” turbulence regime

23. The velocity gradient versus ζ = z/L after LES (dots) and the EFB model (curve)

24. The temperature gradient versus ζ = z/L after LES (dots) and the EFB model (curve)

25. Richardson number, Ri, versus ζ = z/L after LES (dots) and the EFB model (curve)

26.  TKE budget equation is INSUFFICIENT E K and E P are equally important  Е = E K + E P  There is no Ri c in the energetic sense; experimental data confirm this theoretical conclusion up to Ri ~ 10 2  There is a threshold Ri ~ 0.2-0.3 (quite close to the liner instability limit) – separating principally different regimes of “strong” and “weak“ turbulence  The newly discovered “weak turbulence regime” is typical of the free atmosphere and deep ocean, wherein it determines turbulent transport of the energy and momentum and diffusion of passive scalars  A hierarchy of EFB closure models – new instruments for research and modelling applications Conclusions

27. References (last decade) Zilitinkevich, S.S, Gryanik V.M., Lykossov, V.N., Mironov, D.V., 1999: A new concept of the third-order transport and hierarchy of non-local turbulence closures for convective boundary layers. J. Atmos. Sci., 56, 3463-3477. Mironov, D.V., Gryanik V.M., Lykossov, V.N., & Zilitinkevich, S.S., 1999: Comments on “A new second-order turbulence closure scheme for the planetary boundary layer” by K. Abdella, N. Mc.Farlane. J. Atmos. Sci., 56, 3478-3481. Zilitinkevich, S.S., Elperin, T., Kleeorin, N., Rogachevskii, I., 2007: Energy- and flux-budget (EFB) turbulence closure model for the stably stratified flows. Pt.I: Steady-state, homogeneous regimes. Boundary-Layer Meteorol. 125, 167- 192. Mauritsen, T., Svensson, G., Zilitinkevich, S.S., Esau, I., Enger, L., Grisogono, B., 2007: A total turbulent energy closure model for neutrally and stably stratified atmospheric boundary layers, J. Atmos. Sci., 64, 4117–4130. Zilitinkevich, S., Elperin, T., Kleeorin, N., Rogachevskii, I., Esau, I., Mauritsen, T., Miles, M., 2008: Turbulence energetics in stably stratified geophysical flows: strong and weak mixing regimes. Quart. J. Roy. Met. Soc. 134, 793-799. Sofiev M., Sofieva V., Elperin T., Kleeorin N., Rogachevskii I., Zilitinkevich S.S., 2009: Turbulent diffusion and turbulent thermal diffusion of aerosols in stratified atmospheric flows. J. Geophys. Res. 114, DOI:10.1029/2009JD011765 Zilitinkevich, S., Elperin, T., Kleeorin, N., L'vov, V., Rogachevskii, I., 2009: Energy- and flux-budget (EFB) turbulence closure model for stably stratified flows. Pt.II: The role of internal waves. Boundary-Layer Meteorol. 133, 139-164. Zilitinkevich, S.S., 2010: Comments on numerical simulation of homogeneous stably stratified turbulence. Boundary-Layer Meteorol. DOI 10.1007/s10546-010-9484-1 Zilitinkevich, S.S., Esau, I.N., Kleeorin, N., Rogachevskii, I., Kouznetsov, R.D., 2010: On the velocity gradient in the stably stratified sheared flows. Part 1: Asymptotic analysis and applications. Boundary-Layer Meteorol. 135, 505-511. Kouznetsov, R.D., Zilitinkevich, S.S., 2010: On the velocity gradient in stably stratified sheared flows. Part 2: Observations and models. Boundary-Layer Meteorol. 135, 513-517. Zilitinkevich, S.S., Kleeorin, N., Rogachevskii, I., Esau, I.N., 2011: A hierarchy of energy- and flux-budget (EFB) turbulence closure models for stably stratified geophysical flows. Submitted to Boundary-Layer Meteorol.

28. Turbulence does not degenerate up to very strong stratification to «TKE + TPE» From «only TKE»