1. IACETH Institute for Atmospheric and Climate Science Boundary Layer parametrisation in the climate model ECHAM5-HAM Colombe Siegenthaler - Le Drian, Peter Spichtinger, Ulrike Lohmann CFMIP/GCSS Boundary Layer WG Workshop, 8 th -12 th June, 2009

2. IACETH Institute for Atmospheric and Climate Science Layout Improve the representation of the boundary layer in order to study the interaction aerosols – clouds and its impact on the future climate in the GCM ECHAM5- HAM Aim: Introduction ECHAM today – problems - solution diffusion on moist conserved variables explicit entrainment parametrisation

3. IACETH Institute for Atmospheric and Climate Science LES study A. Ackermann et al., 2004, ‘The impact of humidity above stratiform clouds on indirect aerosol climate forcing’, Nature)‏ N drizzle N LWP Sensitivity of LWP with CCN related with the entrainment rate, and relative humidity above the cloud

4. IACETH Institute for Atmospheric and Climate Science ECHAM5-HAM : Standard T63 x L31, Δ t = 15 min TKE model with prognostic TKE Turbulent diffusion on non-conserved variables Statistical cloud cover scheme (Tompkins, 2002)‏ 2 moment cloud microphysical scheme coupled with double moment aerosol scheme (Lohmann, 2008)‏ Moist Richardson number, modified Blackadar mixing length (Brinkop, 1995)‏

5. IACETH Institute for Atmospheric and Climate Science Diffusion on conserved variables Standard versionDiffusion on moist conserved variables qlql qlql qvqv qtqt

6. IACETH Institute for Atmospheric and Climate Science q t, q c, cc,... _ __ q t PDF ab Grid box variables q sat (T)‏ ba ba Variance threshold : C liquid vapour corresponds does not correspond < C< C= C= C C : “tuning parameter” standard version : 10 % moist version : 0.1 % Retrieve the condensate from the PDF Statistical cloud scheme (Tompkins, 2002)‏

7. IACETH Institute for Atmospheric and Climate Science Total cloud cover standard (C =10 %)‏ reduced minimal width PDF (RED), C =0.1 % cloud cover [%] ISCCP

8. IACETH Institute for Atmospheric and Climate Science Total cloud cover - C = 0.1 % reduced minimal width PDF (RED) difference CMO-RED cloud cover [%] d iffusion on moist conserved variable (CMO)‏ difference cloud cover [%]

9. IACETH Institute for Atmospheric and Climate Science TKE scheme robust on resolution ? very high resolution (LES)SCM resolution L31 3 hours simulation of a nocturnal Stratocumulus, idealisation based on ASTEX campaign In the low resolution experiment, the structure of the TKE is not well represented, particularly at the cloud top Lenderink et al.,1999,‘Evaluation of the Kinetic Energy Approach for modeling turbulent fluxes in stratocumulus’)‏ TKE [m 2 s -2 ] In ECHAM5

10. IACETH Institute for Atmospheric and Climate Science low tropospheric stability ( ) > Criterion for onset explicit entrainment S. Klein & D. Harmann, 1993, J. Climate)‏ explicit entrainment if: cloud is present positive subsidence region cloud top below 700hPa 15 K 20 K = frequency of occurrence of the onset criterion over one year [%]:

11. IACETH Institute for Atmospheric and Climate Science Mean fluxes above cloud/clear sky ω'χ' ___ c ω'χ' ___ d buoyancy flux: 11

12. IACETH Institute for Atmospheric and Climate Science Entrainment parametrisation Replace at the top of the boundary layer (i=interface)‏ Turton and Nicholls (1987) entrainment parametrisation: Buoyancy production into the TKE equation : 12

13. IACETH Institute for Atmospheric and Climate Science Resulting profiles - SCM SCM version of ECHAM5, averaged over 1 hour of simulations for the ASTEX case 13

14. IACETH Institute for Atmospheric and Climate Science Effect of entrainment on LWP, SCM First results with NT87 entrainment parametrisation promising! Same figure as Ackermann (2004), but with SCM version of ECHAM5, averaged over 1 hour of simulations for the ASTEX case

15. IACETH Institute for Atmospheric and Climate Science Diurnal cycle, SCM Dec06/nbhbv 1, w e NT Local Time STD CMO TKE [m/s] Cloud cover [%]Cloud liquid [kg/kg¨]

16. IACETH Institute for Atmospheric and Climate Science Outlook Turbulent diffusion done on moist conserved variables. Turton and Nicholls (1987) explicit entrainment rate has been implemented only over cloudy grid box, we first observe a reduction of the LWP in SCM study. With a reduction of the minimal width of the PDF, the turbulent diffusion on moist variables shows an improvement in Scu regions. Diurnal cycle of and structure of TKE and cloud cover better represented in a SCM study because of the addition and competition of LW radiative cooling and entrainment in the buoyancy flux in SCM study. Our TKE model with coarse resolution do not represent entrainment satisfactory

17. IACETH Institute for Atmospheric and Climate Science Cross-section-Very Preliminary results CMO w e NT Cloud cover [%], JJA 2000 Radiative cooling contribution in TKE only when explicit entrainment active Radiative cooling contribution in TKE once a cloud is present w e NT

18. IACETH Institute for Atmospheric and Climate Science LES study I. Sandu et al., 2008, ‘Aerosol Impacts on the Diurnal Cycle of Marine Stratocumulus’, AMS)‏ Sensitivity of LWP with CCN related to the decoupling induced by the interaction between drizzle inhibition-SW warming-reduced sensible heat flux and entrainment rate

19. IACETH Institute for Atmospheric and Climate Science Now in ECHAM (without entrainment parametrisation) In nature LWP Number of aerosols drizzledrizzle, entrainment ( )‏ 144 U. Lohmann, 2007, ACP‏

20. IACETH Institute for Atmospheric and Climate Science Mesured entrainments The actual scheme produce equivalent entrainment rates 3 orders of magnitude too small Caldwell et al., 2005,” Mixed-Layer Budget Analysis of the Diurnal Cycle of Entrainment in Southeast Pacific Stratocumulus”, J. Atm. Sci. ~ 10 -3 [ms -1 ]

21. IACETH Institute for Atmospheric and Climate Science Entrainment parametrisation - future improvements Replace at the top of the boundary layer (i=interface)‏ Several entrainment parametrisations form: Turton & Nicholls (1987)‏ Adaptation of Bretherton (2007)‏ Lock (1998)‏

22. IACETH Institute for Atmospheric and Climate Science In ECHAM5... 144 (MODIS)‏ U. Lohmann, 2007, ‘Global anthropogenic aerosol effects on convective clouds in ECHAM5-HAM’,ACP)‏ Too strong sensitivity of the increasing LWP with increasing AOD

23. IACETH Institute for Atmospheric and Climate Science Eddy diffusivities – standard/equivalent Version with entrainment :

24. IACETH Institute for Atmospheric and Climate Science What does ECHAM do? The stratocumulus are not well resolved in ECHAM U. Lohmann, 2007, ‘Global anthropogenic aerosol effects on convective clouds in ECHAM5-HAM’,ACPD)‏ ECHAM5-stratECHAM5-convECHAM5-apc ISCCP surf. obs. (zonal mean)

25. IACETH Institute for Atmospheric and Climate Science Diffusion on conserved variables - 3D Sundqvist scheme Difference of annual mean total cloud cover (MOIST-REF)[%]

26. IACETH Institute for Atmospheric and Climate Science TKE scheme..... 1.5 order closureintroduction of a TKE equation m states for momentum, h for heat : turbulent kinetic energy (TKE)‏ : mixing length, depends on the stability, vertical position ~ “ measure of the average distance a parcel moves in the mixing process that generates flux” computed prognostically for each time step (conservation equation)

27. IACETH Institute for Atmospheric and Climate Science Vertical cloud cover vertical cloud cover (%) averaged over jja Method seems to find clouds in the good locations even without perfect profiles