1 / 28

Parametrization of PBL outer layer Irina Sandu

Parametrization of PBL outer layer Irina Sandu. Overview of models Bulk models Local K-closure ED/MF closure K-profile closure TKE closure Current closure in the ECMWF model. Reynolds equations. Reynolds Terms. Parametrization of PBL outer layer (overview).

vlora
Download Presentation

Parametrization of PBL outer layer Irina Sandu

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Parametrization of PBL outer layer Irina Sandu • Overview of models • Bulk models • Local K-closure • ED/MF closure • K-profile closure • TKE closure • Current closure in the ECMWF model

  2. Reynolds equations Reynolds Terms

  3. Parametrization of PBL outer layer (overview)

  4. Parametrization of PBL outer layer • Overview of models • Bulk models • Local K-closure • ED/MF closure • K-profile closure • TKE closure • Current closure in the ECMWF model

  5. An example: Mixed layer (bulk) model of day time BL Bulk - Slab - Integral - Mixed Layer Models energy mass inversion entrainment This set of equations is not closed. A closure assumption is needed for entrainment velocity or for entrainment flux.

  6. Closure for mixed layer model Buoyancy flux in inversion scales with production in mixed layer: CE is entrainment constant (0.2) Cs represents shear effects (2.5-5) but is often not considered The closure is based on the turbulent kinetic energy budget of the mixed layer:

  7. Parametrization of PBL outer layer • Overview of models • Bulk models • Local K closure • ED/MF closure • K-profile closure • TKE closure • Current closure in the ECMWF model

  8. Local K closure Levels in ECMWF model K-diffusion in analogy with molecular diffusion, but 91-level model Diffusion coefficients need to be specified as a function of flow characteristics (e.g. shear, stability,length scales).

  9. Diffusion coefficients according to MO-similarity Use relation between and to solve for .

  10. Stable boundary layer in the IFS: current problems SFMO 1/l=1/kz+1/λ , λ=150m since 36R4 Surface layer – SFMO Above: f = α* fLTG + (1- α) * fMO α = exp(-H/150) SFMO As in other NWP models the diffusion maintained in stable conditions is stronger than what LES or observations indicate

  11. Stable boundary layer in the IFS: current problems Wind turning is underestimated Mean nocturnal bias over Europe 2m T is too low despite too strong diffusion obs 200m Mean annual wind speed at Cabaw model 80 m 10 m 2011 OPERATIONAL Time (UTC)

  12. Impact of reducing the diffusion in stable conditions ST: long tails short tails LT30: λ=150m λ=30m 1/l=1/kz+1/λ , λ=150m Almost halves the errors in low level jet, also increases the wind turning

  13. Impact of reducing the diffusion in stable conditions BAD GOOD Bias (FC-AN) T2m CTL ST-CTL ST: long tails short tails LT30: λ=150m λ=30m LT30-CTL

  14. K-closure with local stability dependence (summary) flux • Scheme is simple and easy to implement. • Fully consistent with local scaling for stable boundary layer. • A sufficient number of levels is needed to resolve the BL i.e. to locate inversion. • Entrainment at the top of the boundary layer is not represented (only encroachment)!

  15. Parametrization of PBL outer layer • Overview of models • Bulk models • Local K closure • ED/MF closure • K-profile closure • TKE closure • Current closure in the ECMWF model

  16. K-diffusion versus Mass flux method K-diffusion method: analogy to molecular diffusion Mass-flux method: mass flux entraining plume model detrainment rate

  17. ED/MF framework a M sub-core flux env. flux M-flux Siebesma & Cuijpers, 1995

  18. BOMEX LES decomposition M-flux sub-core flux total flux env. flux M-flux covers 80% of flux Siebesma & Cuijpers, 1995

  19. Parametrization of PBL outer layer • Overview of models • Bulk models • Local K-closure • ED/MF closure • K-profile closure • TKE closure • Current closure in the ECMWF model

  20. K-profile closure Troen and Mahrt (1986) h Heat flux Profile of diffusion coefficients: Find inversion by parcel lifting with T-excess: such that:

  21. K-profile closure (ECMWF up to 2005) • Inversion interaction was too aggressive in original scheme and too much dependent on vertical resolution. • Features of ECMWF implementation: • No counter gradient terms. • Not used for stable boundary layer. • Lifting from minimum virtual T. • Different constants. • Implicit entrainment formulation. Entrainment fluxes h Moisture flux Heat flux ECMWF entrainment formulation: ECMWF Troen/Mahrt C1 0.6 0.6 D 2.0 6.5 CE 0.2 -

  22. K-profile closure (summary) • Scheme is simple and easy to implement. • Numerically robust. • Scheme simulates realistic mixed layers. • Counter-gradient effects can be included (might create numerical problems). • Entrainment can be controlled rather easily. • A sufficient number of levels is needed to resolve BL e.g. to locate inversion.

  23. Parametrization of PBL outer layer • Overview of models • Bulk models • Local K closure • ED/MF closure • K-profile closure • TKE closure • Current closure in the ECMWF model

  24. TKE closure (1.5 order) Eddy diffusivity approach: With diffusion coefficients related to kinetic energy:

  25. Closure of TKE equation Pressure correlation TKE from prognostic equation: Storage Shear production Buoyancy Turbulent transport Dissipation with closure: Main problem is specification of length scales, which are usually a blend of , an asymptotic length scale and a stability related length scale in stable situations.

  26. TKE (summary) • TKE has natural way of representing entrainment. • TKE needs more resolution than first order schemes. • TKE does not necessarily reproduce MO-similarity. • Stable boundary layer may be a problem.

  27. Parametrization of PBL outer layer • Overview of models • Bulk models • Local K closure • ED/MF closure • K-profile closure • TKE closure • Current closure in the ECMWF model

  28. Current closure in the ECMWF model K-diffusion closure with different f(Ri) for stable and unstable layers K-diffusion closure with different f(Ri) for stable and unstable layers ED/MF (K-profile + M flux)

More Related