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Parametrization of surface fluxes: Outline

Parametrization of surface fluxes: Outline. Surface layer (Monin Obukhov) similarity Surface fluxes: Alternative formulations Roughness length over land Definition Orographic contribution Roughness lengths for heat and moisture Ocean surface fluxes

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Parametrization of surface fluxes: Outline

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  1. Parametrization of surface fluxes: Outline • Surface layer (Monin Obukhov) similarity • Surface fluxes: Alternative formulations • Roughness length over land • Definition • Orographic contribution • Roughness lengths for heat and moisture • Ocean surface fluxes • Roughness lengths and transfer coefficients • Low wind speeds and the limit of free convection • Air-sea coupling at low wind speeds: Impact training course: boundary layer; surface layer

  2. Mixing across steep gradients Stable BL Dry mixed layer Cloudy BL Surface flux parametrization is sensitive because of large gradients near the surface. training course: boundary layer; surface layer

  3. Boundary conditions for T and q have different character over land and ocean Surface fluxes of heat and moisture are proportional to temperature and moisture differences: Lowest model level T1,q1 z1 Ts, qs Surface Ocean boundary condition Land boundary condition training course: boundary layer; surface layer

  4. Parametrization of surface fluxes: Outline • Surface layer (Monin Obukhov) similarity • Surface fluxes: Alternative formulations • Roughness length over land • Definition • Orographic contribution • Roughness lengths for heat and moisture • Ocean surface fluxes • Roughness lengths and transfer coefficients • Low wind speeds and the limit of free convection • Air-sea coupling at low wind speeds: Impact training course: boundary layer; surface layer

  5. Surface layer similarity (Monin Obukhov similarity) For z/h << 1 flux is approximately equal to surface flux. Flux profile • Considerations about the nature of the process: • z/zo >> 1 • distance to surface determines turbulence length scale • shear scales with surface friction rather than with zo Scaling parameters: training course: boundary layer; surface layer

  6. MO similarity for gradients is a universal function of dimensionless shear Stability parameter is a universal function of dimensionless potential temperature gradient Stability parameter Note that with we obtain: training course: boundary layer; surface layer

  7. MO gradient functions Observations of as a function of z/L, with Empirical gradient functions to describe these observations: unstable stable training course: boundary layer; surface layer

  8. Parametrization of surface fluxes: Outline • Surface layer (Monin Obukhov) similarity • Surface fluxes: Alternative formulations • Roughness length over land • Definition • Orographic contribution • Roughness lengths for heat and moisture • Ocean surface fluxes • Roughness lengths and transfer coefficients • Low wind speeds and the limit of free convection • Air-sea coupling at low wind speeds: Impact training course: boundary layer; surface layer

  9. Integral profile functions for momentum Dimensionless wind gradient (shear) or temperature gradient functions can be integrated to profile functions: with: integration constant (roughness length for momentum) wind profile function, related to gradient function: Profile functions for temperature and moisture can be obtained in similar way. training course: boundary layer; surface layer

  10. Integral profile functions: Momentum, heat and moisture Profile functions for surface layer applied between surface and lowest model level provide link between fluxes and wind, temperature and moisture differences. training course: boundary layer; surface layer

  11. MO wind profile functions applied to observations Unstable Stable training course: boundary layer; surface layer

  12. Transfer coefficients Surface fluxes can be written explicitly as: Lowest model level U1,V1,T1,q1 z1 0, 0, Ts, qs Surface training course: boundary layer; surface layer

  13. Numerical procedure: The Richardson number The expressions for surface fluxes are implicit i.e they contain the Obukhov length which depends on fluxes. The stability parameter z/L can be computed from the bulk Richardson number by solving the following relation: • This relation can be solved: • Iteratively; • Approximated with empirical functions; • Tabulated. training course: boundary layer; surface layer

  14. Louis scheme Initially, the empirical stability functions, , were not related to the (observed-based) Monin-Obukhov functions. The older Louis formulation uses: With neutral transfer coefficient: And empirical stability functions for training course: boundary layer; surface layer

  15. Stability functions for surface layer unstable stable Land Louis et al (dash) MO-functions (solid) Sea training course: boundary layer; surface layer

  16. Surface fluxes: Summary • MO-similarity provides solid basis for parametrization of surface fluxes: • Different implementations are possible (z/L-functions, or Ri-functions) • Surface roughness lengths are crucial aspect of formulation. • Transfer coefficients are typically 0.001 over sea and 0.01 over land, mainly due to surface roughness. training course: boundary layer; surface layer

  17. Parametrization of surface fluxes: Outline • Surface layer (Monin Obukhov) similarity • Surface fluxes: Alternative formulations • Roughness length over land • Definition • Orographic contribution • Roughness lengths for heat and moisture • Ocean surface fluxes • Roughness lengths and transfer coefficients • Low wind speeds and the limit of free convection • Air-sea coupling at low wind speeds: Impact training course: boundary layer; surface layer

  18. Surface roughness length (definition) Often displacement height is used to obtain U=0 for z=0: Example for wind: • Surface roughness length is defined on the basis of logarithmic profile. • For z/L small, profiles are logarithmic. • Roughness length is defined by intersection with ordinate. 10 1 0.1 0.01 U • Roughness lengths for momentum, heat and moisture are not the same. • Roughness lengths are surface properties. training course: boundary layer; surface layer

  19. Roughness length over land Geographical fields based on land use tables: training course: boundary layer; surface layer

  20. Roughness length over land (orographic contribution) Drag is determined by “silhouette area” per unit surface area. U Effective roughness length: • Small scale sub-grid orography contributes substantially to surface drag due to pressure forces on orographic features (form drag). • Effects are usually parametrized through orographic enhancement of surface roughness. training course: boundary layer; surface layer

  21. Roughness length over land (orographic contribution) Orographic form drag (simplified Wood and Mason, 1993): Shape parameters Drag coefficient Silhouette slope Wind speed Reference height Vertical distribution (Wood et al, 2001): training course: boundary layer; surface layer

  22. Parametrization of flux divergence with continuous orographic spectrum: Assume: 1000 m 100 m Write flux divergence as: Beljaars, Brown and Wood, 2003 training course: boundary layer; surface layer

  23. Roughness lengths for heat and moisture • Roughness lengths for heat and moisture are different from the aerodynamic roughness length, because pressure transfer (form drag) does not exist for scalars. • Vegetation: roughness lengths for heat and moisture are ~ 10x smaller than aerodynamic roughness. training course: boundary layer; surface layer

  24. Parametrization of surface fluxes: Outline • Surface layer (Monin Obukhov) similarity • Surface fluxes: Alternative formulations • Roughness length over land • Definition • Orographic contribution • Roughness lengths for heat and moisture • Ocean surface fluxes • Roughness lengths and transfer coefficients • Low wind speeds and the limit of free convection • Air-sea coupling at low wind speeds: Impact training course: boundary layer; surface layer

  25. Roughness lengths over the ocean Roughness lengths are determined by molecular diffusion and ocean wave interaction e.g. Current version of ECMWF model uses an ocean wave model to provide sea-state dependent Charnock parameter. training course: boundary layer; surface layer

  26. Transfer coefficents for moisture (10 m reference level) CEN Neutral exchange coeff for evaporation • Using the same roughness length for momentum and moisture gives an overestimate of transfer coefficients at high wind speed • The viscosity component increases the transfer at low wind speed training course: boundary layer; surface layer

  27. Low wind speeds and the limit of free convection At zero wind speed, coupling with the surface disappears e.g. for evaporation: Lowest model level U1,V1,T1,q1 z1 0 qs Surface Extension of MO similarity with free convection velocity: inversion h Surface training course: boundary layer; surface layer

  28. Air-sea coupling at low winds Revised scheme: Larger coupling at low wind speed (0-5 ms-1) training course: boundary layer; surface layer

  29. Air-sea coupling at low winds (control) Precipitation, JJA; old formulation training course: boundary layer; surface layer

  30. Air-sea coupling at low winds (revised scheme) Precipitation, JJA; new formulation training course: boundary layer; surface layer

  31. Air-sea coupling at low winds Near surface Theta_e difference: New-Old training course: boundary layer; surface layer

  32. Air-sea coupling at low winds Theta and Theta_e profiles over warm pool with old an new formulation new new old old training course: boundary layer; surface layer

  33. Air-sea coupling at low winds Zonal mean wind errors for DJF Old New training course: boundary layer; surface layer

  34. IMET-stratus buoy / ECMWF (20 S 85 W) Latent heat flux training course: boundary layer; surface layer

  35. IMET-stratus buoy vs. ECMWF (20 S 85 W) Sensible heat flux training course: boundary layer; surface layer

  36. IMET-stratus buoy vs. ECMWF (20 S 85 W) Horizontal wind speed training course: boundary layer; surface layer

  37. IMET-stratus buoy vs. ECMWF (20 S 85 W) Water/air q-difference training course: boundary layer; surface layer

  38. IMET-stratus buoy vs. ECMWF (20 S 85 W) Water/air T-difference training course: boundary layer; surface layer

  39. BL budget considerations: IMET-stratus Heat budget: +40 +10 -80 +20 +10 W/m2 Moisture budget: -3 +3.3 -0.3 mm/day training course: boundary layer; surface layer

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