1. Scientific Advisory Committee Meeting, November 25-26, 2002 Large-Eddy Simulation Andreas Chlond Department Climate Processes

2. Scientific Advisory Committee Meeting, November 25-26, 2002 Andreas Chlond: Large-Eddy Simulation Outline Motivation LES-Tool Strategy Examples of LESs  LES of cold air outbreaks  LES of the stratocumulus topped boundary layer  LES of shallow cumulus How can LES be used to improve Large- Scale Models? Conclusions Outlook

3. Scientific Advisory Committee Meeting, November 25-26, 2002 Andreas Chlond: Large-Eddy Simulation Motivation Sub-grid scale processes need to be parameterized in GCMs and CTMs Diurnal variation of LWP (FIRE) Boundary layer structure is not well represented

4. Scientific Advisory Committee Meeting, November 25-26, 2002 Andreas Chlond: Large-Eddy Simulation Boundary layer processes provides surface-boundary conditions interacts with the large-scale flow interacts with other parameterization schemes A realistic representation of the boundary layer is an essential ingredient of climate models because Scientific question: Which physical processes determine the state of the cloud-topped boundary layer? cloud cover entrainment organization Objective: evaluate and improve methods of representing shallow cloud systems in GCMs (GCSS WG1)

5. Scientific Advisory Committee Meeting, November 25-26, 2002 Andreas Chlond: Large-Eddy Simulation Large-Eddy Simulation Concept: only the important part of the turbulent motions is calculated the net effect of the small eddies is modelled by a subfilter model „dissipate“ energy eddy-viscosity-mixing-length model Strengths: excellent tool for studying turbulence generate database of PBL regimes

6. Scientific Advisory Committee Meeting, November 25-26, 2002 Andreas Chlond: Large-Eddy Simulation Strategy Observations Large-Eddy Simulations Parameterizations (SCM) Climate Model (GCM)

7. Scientific Advisory Committee Meeting, November 25-26, 2002 Andreas Chlond: Large-Eddy Simulation Examples of LESs LES of cold air outbreaks LES of the stratocumulus topped boundary layer LES of shallow cumulus Flow regimes:

8. Scientific Advisory Committee Meeting, November 25-26, 2002 Andreas Chlond: Large-Eddy Simulation LES of cold air outbreaks Goals: Pattern transition Scale broadening Strategy: Lagrangian simulation with an adaptive grid

9. Scientific Advisory Committee Meeting, November 25-26, 2002 Andreas Chlond: Large-Eddy Simulation LES of cold air outbreaks Main result: Cell broadening of atmospheric MCC is caused by latent heating/cooling due condensation/evaporation and due to radiative cloud top cooling

10. Scientific Advisory Committee Meeting, November 25-26, 2002 Andreas Chlond: Large-Eddy Simulation LES of stratocumulus part of the general circulation important modulators of the earth's radiation budget of importance to our understanding of the physics of the atmosphere Marine stratocumulus clouds are

11. Scientific Advisory Committee Meeting, November 25-26, 2002 Andreas Chlond: Large-Eddy Simulation LES of stratocumulus (ASTEX) Goals and objectives: How well can turbulence and cloud structure be reproduced by LES? Investigation of the statistical significance of LES- derived data products. Sensitivity analysis with respect to the treatment of subgrid-scale processes and microphysical processes Examination of the sensitivity of our LES results with respect to various external, environmental conditions

12. Scientific Advisory Committee Meeting, November 25-26, 2002 Andreas Chlond: Large-Eddy Simulation LES of stratocumulus (ASTEX) Representation of precipitation is difficult

13. Scientific Advisory Committee Meeting, November 25-26, 2002 Andreas Chlond: Large-Eddy Simulation LES of stratocumulus (FIRE) Challenge: diurnal cycle Which factors determine: cloud cover, LWP, BL-height entrainment rate turbulence structure microphysical properties

14. Scientific Advisory Committee Meeting, November 25-26, 2002 Andreas Chlond: Large-Eddy Simulation LES: Diurnal variation of LWP (FIRE)

15. Scientific Advisory Committee Meeting, November 25-26, 2002 Andreas Chlond: Large-Eddy Simulation LES: Sensitivities with respect to inversion properties (FIRE)

16. Scientific Advisory Committee Meeting, November 25-26, 2002 Andreas Chlond: Large-Eddy Simulation LES of shallow cumulus determines the vertical thermo- dynamic structure is part of the large-scale circulation is part of the feeder system for deep convection

17. Scientific Advisory Committee Meeting, November 25-26, 2002 Andreas Chlond: Large-Eddy Simulation LES of shallow cumulus (ATEX) Question: Are results obtained for BOMEX case still valid under conditions of high cloud cover? Difference: Existence of a layer of high relative humidity near the inversion

18. Scientific Advisory Committee Meeting, November 25-26, 2002 Andreas Chlond: Large-Eddy Simulation LES of shallow cumulus (ATEX) Results: Many elements of the turbulent structure are robust Representation of stratiform cloud amount is sensitive Part of this sensitivity is due to a physically realistic positive radiative feedback

19. Scientific Advisory Committee Meeting, November 25-26, 2002 Andreas Chlond: Large-Eddy Simulation How can LES be used to improve Large- Scale Models? advance the understanding of the physical processes evaluate and improve methods of representing shallow clouds produce comprehensive 4-D data sets using LESs use of LES data sets to investigate deficiencies in GCMs using the Single Column Model (SCM) version as a test bed correct and improve parameterizations in ECHAM

20. Scientific Advisory Committee Meeting, November 25-26, 2002 Andreas Chlond: Large-Eddy Simulation Diurnal cycle of shallow cumulus over land (ARM case) Questions: Do models reproduce correct timing? Do scaling laws still apply? How is sub-cloud layer affected by cu?

21. Scientific Advisory Committee Meeting, November 25-26, 2002 Andreas Chlond: Large-Eddy Simulation LES vs SCM: Diurnal variation of cloud cover (ARM case)

22. Scientific Advisory Committee Meeting, November 25-26, 2002 Andreas Chlond: Large-Eddy Simulation Conclusions LES the best technique we have today for studying turbulence and cloud processes in the boundary layer SUBROUTINE vdiff (kidia,kfdia,klon,klp2,ktdia,klev,klev, & & paclcm,paphm1,papm1,pgeom1,pum1,pvm1,pxm1) ! Description: !-- Computation of the exchange coefficients IMPLICIT NONE ! Scalar arguments with intent(In): INTEGER, INTENT (IN) :: kfdia, kidia, klev, klevm1, klevp1,& & klp2, ktdia, ktrac DO jl = kidia, kfdia zdu2 = MAX(zepdu2,pum1(jl,klev)**2+pvm1(jl,klev)**2) zqmitte = (pqm1(jl,klev)+zqs(jl)*zhsoil(jl))/2. zmult4 = zfux*zmult5 - 1. zcons = zcons12*paphm1(jl,klevp1)/(ptm1(jl,klev)* & & (1.+vtmpc1*pqm1(jl,klev)-pxm1(jl,klev))) END DO From Nature To its Representation LESs can be used in developing/calibrating PBL parameterizations

23. Scientific Advisory Committee Meeting, November 25-26, 2002 Andreas Chlond: Large-Eddy Simulation Outlook Extend analysis for the full range of boundary layer regimes precipitating sc cloud PBL  Cloud microstructure  Cloud-aerosol relationships turbulence over complex terrain PBL with chemically reacting species transition from shallow to deep convection

24. Scientific Advisory Committee Meeting, November 25-26, 2002 Andreas Chlond: Large-Eddy Simulation The End Questions