Entrainment in stratocumulus clouds

1. Entrainment in stratocumulus clouds Stephan de Roode (KNMI)

2. stratocumulus vertical structure

3. stratocumulus: vertical structure

4. Key questions • How well is stratocumulus represented in models? • Entrainment - what is it? - why important? - how parameterized? • Boundary-layer mixing and cloud liquid water path - perfect boundary-conditions, perfect cloud structure? • FIRE I observations revisited - a different view on entrainment

5. ISCCP stratocumulus cloud climatology

6. ECMWF RE-ANALYSIS shortwave radiation errors

7. GCSS intercomparison cases • Stratocumulus case based on observations (FIRE I) •Prescribe - initial state - large-scale horizontal advection - large-scale subsidence rate •Simulation of diurnal cycle - 1D versions of General Circulation Models - Large-Eddy Simulation Models (LES)

8. initial jumps for three GCSS stratocumulus cases GCSS intercomparison cases • Stratocumulus case based on observations (FIRE I) •Prescribe - initial state - large-scale horizontal advection - large-scale subsidence rate •Simulation of diurnal cycle - 1D versions of General Circulation Models - Large-Eddy Simulation Models (LES)

9. GCSS FIRE I intercomparison participants Fine-scale turbulence models [Large-Eddy Simulation Models (LES)] : Dx=Dy=50m, Dz=10m IMAU - Peter G. Duynkerke, Stephan de Roode, M. C. van Zanten and P. Jonker MPI - Andreas Chlond, Frank Müller, and Igor Sednev WVU - David Lewellen INM - Javier Calvo, Joan Cuxart, Dolores Olmeda, Enrique Sanchez UKMO - Adrian P. Lock NCAR - Chin-Hoh Moeng (NCAR) 1D versions of General Circulation Models [Single-Column Models (SCM)] LMD - Sylvain Cheinet MPI - Andreas Chlond, Frank Müller, and Igor Sednev Meteo France I - Hervé Grenier Meteo France II - Jean-Marcel Piriou ECMWF - Martin Köhler CSU - Cara-Lyn Lappen KNMI - Geert Lenderink UKMO - Adrian P. Lock INM - Javier Calvo, Joan Cuxart, Dolores Olmeda, Enrique Sanchez

10. 3D results from Large-Eddy Simulation results -The cloud liquid water path

11. What is entrainment?Why is entrainment important? • Entrainment • mixing of relatively warm and dry air from above the inversion into the cloud layer • - important for cloud evolution

12. 3D results from Large-Eddy Simulation results -Entrainment rates

13. Boundary-layer representation

14. 1D results from General Circulation Models -The cloud liquid water path (LWP) Single Column Model liquid water path results very sensitive to • entrainment rate • drizzle parameterization • convection scheme (erroneous triggering of cumulus clouds)

15. Key questions • How well is stratocumulus represented in models? • Entrainment - what is it? - why important? - how parameterized? • Boundary-layer mixing and cloud liquid water path - perfect boundary-conditions, perfect cloud structure? • FIRE I observations revisited - a different view on entrainment

16. The clear convective boundary layer (CBL) -Entrainment scaling from observations Entrainment rate we scales as • A ≈ 0.2 • H boundary-layer height • (g/q0) Dqv buoyancy jump across the inversion • w* convective velocity scale: vertically integrated buoyancy flux

17. Buoyancy flux in stratocumulus convective velocity scale w* depends on entrainment rate we

18. forcing WNE we __________ "jumps" Solve entrainment rate solve for entrainment rate 

19. forcing WNE we __________ "jumps" Solve entrainment rate solve for entrainment rate 

20. forcing WNE we __________ "jumps" Solve entrainment rate solve for entrainment rate 

21. forcing WNE we __________ "jumps" Solve entrainment rate solve for entrainment rate 

22. Stability jumps

23. Stability jumps

24. Stability jumps

25. • Based on observations of clear CBL Entrainment parameterizations for stratocumulus -Results based on LES results • Nicholls and Turton (1986) • Stage and Businger (1981) Lewellen and Lewellen (1998) VanZanten et al. (1999) • Lock (1998) • Lilly (2002)

26. Sensitivity of entrainment parameterizations to inversion jumps observations from ASTEX Flight A209 __________________________________ cloud base height = 240 m cloud top height = 755 m sensible heat flux = 10 W/m2 latent heat flux = 30 W/m2 longwave flux jump = 70 W/m2 max liquid. water content = 0.5 g/kg LWP = 100 g/m2 Compute entrainment rate from parameterizations as a function of inversion jumps

27. Entrainment rate [cm/s] sensitivity to inversion jumps

28. Entrainment rate [cm/s] parameterizationsof observed cases   high low Entrainment results mirror the LES results where they are based on

29. Entrainment parameterizations -Implementation in K-diffusion schemes • Turbulent flux at the top of the boundary layer due to entrainment: ("flux-jump" relation) • Top-flux with K-diffusion:

30. Key questions • How well is stratocumulus represented in models? • Entrainment - what is it? - why important? - how parameterized? • Boundary-layer mixing and cloud liquid water path - perfect boundary-conditions, perfect cloud structure? • FIRE I observations revisited - a different view on entrainment

31. Compute eddy- diffusivity coefficients from FIRE I LES

32. K-coefficients from FIRE I LES

33. same change Importance of eddy-diffusivity coefficients on internal boundary-layer structure • Change magnitude K profiles • Compute vertical profiles ql and qt from integration

34. Total water content profiles for different K-profiles but identical vertical flux

35. Liquid water content profiles for different K-profiles Magnitude K-coefficient in interior BL important for liquid water content!

36. Key questions • How well is stratocumulus represented in models? • Entrainment - what is it? - why important? - how parameterized? • Boundary-layer mixing and cloud liquid water path - perfect boundary-conditions, perfect cloud structure? • FIRE I observations revisited - a different view on entrainment

37. FIRE I stratocumulus over the Pacific Ocean -Aircraft lidar observations of cloud-top height

38. Thermodynamic structure of clear air above cloud top depressions mean in-cloud value clear air value

39. horizontal winds Evaporation of cloud top by turbulent mixing turbulence turbulence vertical velocity evaporation liquid water content liquid water potential temperature total water content 12 km

40. Observations of moist and cold layers on top of stratocumulus

41. Entrainment mixing scenario

42. • Entrainment parameterizations - extrapolation of Large-Eddy Simulation results - considerable differences  different heat and moisture budgets • Cloud liquid water path and K-diffusion turbulence schemes - different solutions for identical surface and cloud-top fluxes  different albedo • Entrainment observations - may induce the formation of moist layers above cloud top  opposes general view on the entrainment process Conclusions

43. stability jumps