Convective Boundary Layers: Modeling and Parameterization

1. Title Convective boundary layers Jordi Vilà Collaboration and discussions: David Pino, Olaf Vellinga, Reinder Ronda, Harm Jonker, Bert Holtslag UCLA, Lake Arrowhead, June 2005

2. Forum on modeling the atmospheric boundary layer Forum on modeling the atmospheric boundary layer (summary in BAMS (2005) by R. Pielke and T. Vukicevic) “The third conclusion is that parameterizations can be replaced with lookup tables or analytical formulations...” “With this approach the existing parameterizations of sub-grid scale fluxes, radiative fluxes, ... would be replaced by lookup tables or analytical formulations.”

3. My approach to the talk: Critical user of a research/operational mesoscale model with particular interest on the performance of the planetary boundary layer. Here, the majority of the results are based on runs carried out by the mesoscale MM5 (PSU-NCAR), but the results are similar to other mesoscale models.

4. Advantages of using MM5 • Severalparameterizations/boundary layer schemes are implemented. • Possibility to investigate and to teach • the impact of physical assumptions on • the representation of the CBL • c) Possibility to study feedbacks (but • sometimes this is also a disadvantage)

5. Essential processes to be represented in the CBL (3) Entrainment (exchange fluxes) (1) Turbulent mixing (turbulent fluxes) (2) Land/surface processes (surface fluxes)

6. (1) Turbulent mixing • Two main schools/approaches to represent • turbulent mixing in the CBL: • Non-local + convective scaling • Local + prognostic TKE equation

7. Non-local approach Main goal: to include the influence of coherent structure (organized eddies) which fill the entire boundary layer Argot: parcel method, countergradient, transilient, ...

8. Local approach Main goal: to represent the turbulent characteristics of the CBL by calculating its main relevant variable: the TURBULENT KINETIC ENERGY Argot: master turbulent length scale, Mellor-Yamada levels,...

9. Motivation a) Study of the capabilities of a widely used mesoscale model (NWP and regional climate modeling) to simulate the evolution of a convective boundary layer b) Evaluating with observations the performance of several boundary layer schemes derived with different physical assumptions

10. Case study Synoptic situation (2/3 May 1995) High pressure over Denmark Sunny day (cloudless) Low horizontal easterly winds Suitable conditions for the formation of a well mixed convective boundary layer

11. Numerical experimental setup Four domains (27, 9, 3, 1 km grid length) Two way nesting Initial and boundary conditions provided by ECMWF (every 6 hours) Available observations at the Cabauw mast and De Bilt.

12. MM5 model has several options to • parameterize the turbulent flux in the • ABL (boundary layer schemes) • Two main categories: • 1-order non local (MRF, Blackadar) • 1-1/2-order local (ETA, BRT)

13. Vertical profiles of potential temperature Non-Local Local

14. Vertical profiles of specific humidity Non-local Local

15. Vertical profile of wind Local Non-local

16. Time evolution of the convective boundary layer height Non-local Non-local Local

17. Other MM5/RAMS research studies which evaluated the boundary layer height + MM5 o RAMS Observations Model Model Observations Zhong and Fast (2003) Hanna and Yang (2001)

18. Discussion points (I) The first-order non local schemes reproduce better the vertical structure of the convective boundary layer and the boundary layer growth. Wind is poorly represented by all schemes

19. Discussion points (II) • All the BL schemes reproduce normally colder and moister boundary layer compared to observations • soil moisture content • lower mixing height

20. (2) Land/surface processes Is the surface forcing represented by the surface turbulent fluxes consistent with the previous results?

21. Time evolution of the surface fluxes Sensible heat flux Non-local Local

22. Sensible heat flux: MM5 results compare to observations (Berg and Zhong, 2004) Model (non-local) (non local-TKE) (non-local) Observed

23. Time evolution of the surface fluxes Latent heat flux Local Non-local

24. Time evolution of the surface fluxes Friction velocity Non-local Local

25. Discussion points (III) Normally there is an overestimation of the momentum, sensible and latent heat flux compared with observations

26. Discussion points (IV) Still (at least for me) not well understood the interaction and feedbacks between the surface schemes and boundary layer schemes on 3D models SL scheme BL scheme

27. (3) Entrainment The third essential process is entrainment Top-down diffusion of an scalar. Laboratory experiment by Harm Jonker (TUD)

28. How do we represent the entrainment processes in the boundary layer schemes? Majority of the analyzed and discussed boundary layer schemes represent implicitly the entrainment of warm and dry air from the free troposphere

29. Mixed-layer models prescribed or calculated explicitly the entrainment flux at the top of the boundary layer. Commonly, Free convection case  0.2

30. The ECMWF model also prescribes explicitly that the entrainment flux is equal to -20% the surface buoyancy flux. Beljaars and Betts (1993)

31. Is it possible to improve/generalize • this closure assumption? • To allow departures from the constant 0.2 • To introduce other relevant processes • at the entrainment zone (shear, dissipation)

32. Entrainment process is closely related to the energy that is made available from turbulence to overcome the buoyancy forces at the inversion • analysis by scaling the terms in theTKE equation at the entrainment interface

33. TKE at the entrainment interface TE B S T+P D

34. Previous research studies: Lilly (1968) Tennekes (1973) Betts (1973) Carson (1973) Zeman (1975) Mahrt and Lenschow (1976) Driedonks (1981) ..... Can we add something?

35. LES can provide new insight and quantification B: buoyancy only BG: buoyancy and shear (initial Ug=Vg=10 m/s)

36. TKE budget in an ABL only driven by buoyancy (B)

37. TKE budget in an ABL driven by buoyancy andshear (BG)

38. Terms taking into account in the TKE equation at the entrainment interface (so far) TE = B + S + P + T +D

39. Ratio of the entrainment to the surface flux Entrainment parameterization implemented in a mixed-layer model and compared to LES. N P LES * MXL

40. Boundary layer schemes have also an impact on atmospheric chemistry studies!!!

41. Some considerations of the influence of boundary layer schemes on atmospheric chemistry studies a) Boundary layer controls the diurnal variability of atmospheric compounds Example transition cloudless BL to cloudy BL: - Enhancement of vertical transport - As a result, reactant concentration are more diluted

42. Enhancement of vertical transport. Higher dilution of atmospheric compounds. Scatter clouds appear Cloudless Wind profile measurements H. Klein-Baltink (KNMI)

43. b) Turbulent mixing can control/limit the reactivity of atmospheric compounds Testing the impact and validity of assumptions such as: - Homogeneous mixing (asymmetry of the transport in the CBL) - Validity of K-theory

44. Da=10-2 Da=100 Turbulent reacting flows (first-order reaction) LES (Jonker et al., 2004) Da < 1 slow chemistry Da = 1 moderate chemistry Da > 1 fast chemistry Mixing ratio reactant

45. Last discussion/summary points Non-local schemes perform better than local-TKE schemes. Need to do a systematic evaluation. Study the feedback/interaction of BL schemes with surface schemes The entrainment processes should be treated explicitly or implictly.

46. Further Answering the “Forum”. It is still necessary to parameterize the essential processes that drive the CBL evolution and characteristics Need of strong interaction between developers of BL-schemes and users (mesoscale, regional climate models,...) The parameterizations must have a balance between relevant processes and retaining simplicity (easy to say it than to do it)