Irina Sandu , Jean-Louis Brenguier, Olivier Geoffroy

1. CNRM/GMEI/ MNPCA Ambiguous results of Large-Eddy Simulations of marine stratocumulus clouds Irina Sandu, Jean-Louis Brenguier, Olivier Geoffroy Joint GCSS-GPCI/BLCI-RICO Workshop , NASA/GISS New York USA, 18-21 September 2006

2. CNRM/GMEI/ MNPCA I. Introduction II. Application of the LES technique for studying the indirect effect of aerosols Méso-NH as a LES code Set-up of the simulations for different cases of marine stratocumulus clouds III. Analysis of the results IV. Conclusions Joint GCSS-GPCI/BLCI-RICO Workshop , NASA/GISS New York USA, 18-21 September 2006

3. CNRM/GMEI/ MNPCA I. Introduction II. Application of the LES technique for studying the indirect effect of aerosols Méso-NH as a LES code Set-up of the simulations for different cases of marine stratocumulus clouds III. Analysis of the results IV. Conclusions Joint GCSS-GPCI/BLCI-RICO Workshop , NASA/GISS New York USA, 18-21 September 2006

4. I. ? ? diurnal variation of the cloud and of the boundary layer aerosols Boundary layer clouds: marine stratocumulus Considerable coverage + optical properties Negative radiative forcing r significant impact on the planetary radiative budget Complexe interaction between dynamics - microphysics - radiation Joint GCSS-GPCI/BLCI-RICO Workshop , NASA/GISS New York USA, 18-21 September 2006

5. I. Twomey (1977): LWP constantNCCN Nc albedo Albrecht (1989): NCCN Nc re Suppression of drizzle formation The cloud life time LWP constant dynamical feedbacks: LWP non constant inseparability of the indirect effects the indirect effects of aerosols 1st indirect effect 2nd indirect effect Joint GCSS-GPCI/BLCI-RICO Workshop , NASA/GISS New York USA, 18-21 September 2006

6. CNRM/GMEI/ MNPCA I. Introduction II. Application of the LES technique for studying the indirect effect of aerosols Méso-NH as a LES code Set-up of the simulations for different cases of marine stratocumulus clouds III. Analysis of the results IV. Conclusions Joint GCSS-GPCI/BLCI-RICO Workshop , NASA/GISS New York USA, 18-21 September 2006

7. II. Méso-NH as a LES code Turbulence scheme – 3D scheme(Cuxart-2000) – TKE equation +1.5 closure + Deardorff mixing length Advection scheme – positively defined second order centered scheme Non-hydrostatic model, anelastic system of equations(Lipps and Hemler, 1982) Microphysics – Khairoutdinov &Kogan 2000(O. Geoffroy ) –2 moments bulk scheme developed for Sc. • Radiative code (ECMWF) –recentcode improvements (O. Thouron) • LW – Savijarvi (1997) • cloud optical • properties SW - Fouquart (1987) not suited for • LES of marine stratocumulus New parametrisation for the single scattering albedo based on aerosol chemical composition for LES modelling of boundary layer clouds Sandu I., P. Tulet and J.L. Brenguier, Geophys. Res. Lett., 32, L19814, 2005 set of parametrisations adequate and coherent Complexe interaction dynamics – microphysics - radiation Joint GCSS-GPCI/BLCI-RICO Workshop , NASA/GISS New York USA, 18-21 September 2006

8. II. Summer situation over Nord-Est Pacific, similar toFIRE I case CCN impact drizzle, the diurnal cycle amplitude, the LWP, the surface and radiative fluxes, the boundary layer energetics Set-up similar to EUROCS intercomparison case (Duynkerke and al. 2004) 2.5kmX2.5kmX1.2km – 50mX50mX10m Different subsidence rates, large scale advection terms how the system response depends on the external conditions (large scale horizontal advection, subsidence, etc…) Large-Eddy Simulations of different cases of marine stratocumulus 36 hours simulations initialized with various CCN concentrations, for several large scale forcings = the impact of CCN on the stratocumulus capped boundary layer evolution for a time scale longer then one diurnal cycle Joint GCSS-GPCI/BLCI-RICO Workshop , NASA/GISS New York USA, 18-21 September 2006

9. CNRM/GMEI/ MNPCA I. Introduction II. Application of the LES technique for studying the indirect effect of aerosols Méso-NH as a LES code Set-up of the simulations for different cases of marine stratocumulus clouds III. Analysis of the results IV. Conclusions Joint GCSS-GPCI/BLCI-RICO Workshop , NASA/GISS New York USA, 18-21 September 2006

10. LWP (g/m2) Nc re Nc LWP Drizzle is suppressed Cloud droplet sedimentation weakens Entrainment enhances Nc LWP 0 LT 12 LT 0 LT 12 LT W6 : subsidence w = - 6X10-6 z (m/s) Nccn= 50, 200, 600 cm-3 no large scale advection Nc ~40, 120, 220 cm-3

11. Nc LWP Nc LWP W4 : subsidence w = - 4X10-6 z (m/s) Nccn= 50, 600 cm-3 no large scale advection Nc ~ 40, 220 cm-3 subsidence w = - 6X10-6 z (m/s) Nccn= 50, 600 cm-3 1K/day cooling 1K Nc ~ 40, 220 cm-3 LWP (g/m2) 0 LT 12 LT 0 LT 12 LT W6 : subsidence w = - 6X10-6 z (m/s) Nccn= 50, 200, 600 cm-3 no large scale advection Nc ~40, 120, 220 cm-3

12. 700 m  budget: 50cm-3 700 m  budget: 600cm-3 CB 12LT CB 700 m d/dt (K/ h) 700 m 700 m drv/dt (g/kg/h) 700 m 700 m TKE (m2/s2) 9 LT 9 –12 LT 9 –12 LT 9 LT CB CB 12LT CB 9LT 50 cm-3 CB CB 600 cm-3 CB 50 cm-3 600 cm-3 12LT 9LT 12 LT 12 LT 9 –12 LT 9 –12 LT CB 9LT CB Cloud Droplets evaporation Cloud Droplets evaporation Radiative heating Radiative heating Drizzle evaporation Vertical advection Vertical advection

13. LWP (g/m2) Nc LWP Nc LWP 0 LT 12 LT 0 LT 12 LT W6 : subsidence w = - 6X10-6 z (m/s) Nccn= 50, 200, 600 cm-3 no large scale advection Nc ~40, 120, 220 cm-3

14. CNRM/GMEI/ MNPCA I. Introduction II. Application of the LES technique for studying the indirect effect of aerosols Méso-NH as a LES code Set-up of the simulations for different cases of marine stratocumulus clouds III. Analysis of the results IV. Conclusions Joint GCSS-GPCI/BLCI-RICO Workshop , NASA/GISS New York USA, 18-21 September 2006

15. short-term response  long-term response IV. Conclusions for specific initial conditions the system response is not unique strong coupling between dynamics - microphysics - radiation the system response to an increase of the cloud droplet concentration reverses from the first diurnal cycle to the next one the trap of 1st order hypotheses !

16. Questions?