scientific advisory committee meeting, november 25-26, 2002 large-eddy simulation andreas chlond...

Scientific Advisory Committee Meeting, November 25-26, 2002

Large-Eddy Simulation

Andreas ChlondDepartment Climate Processes


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



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



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•organizationObjective:•evaluate and improve methods of representing shallow cloud systems in GCMs (GCSS WG1)



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

4

13 )( L



Strategy

Observations

Large-Eddy Simulations

Parameterizations

(SCM)

Climate Model(GCM)



Examples of LESs

•LES of cold air outbreaks

•LES of the stratocumulus topped boundary

layer

•LES of shallow cumulus

Flow regimes:



LES of cold air outbreaks

Goals:

• Pattern transition

• Scale broadening

Strategy:

• Lagrangian simulation with an adaptive grid



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



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



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



LES of stratocumulus (ASTEX)

• Representation of precipitation is difficult



LES of stratocumulus (FIRE)

Challenge: diurnal cycleWhich factors determine:• cloud cover, LWP, BL-height • entrainment rate• turbulence structure• microphysical properties



LES: Diurnal variation of LWP (FIRE)



LES: Sensitivities with respect to inversion properties (FIRE)



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



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



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



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



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?



LES vs SCM: Diurnal variation of cloud cover (ARM case)



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 coefficientsIMPLICIT 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



Outlook

•Extend analysis for the full range of boundary layer regimes

•precipitating sc cloud PBLCloud microstructureCloud-aerosol relationships

•turbulence over complex terrain

•PBL with chemically reacting species

•transition from shallow to deep convection



The End

Questions

scientific advisory committee meeting, november 25-26, 2002 large-eddy simulation andreas chlond...

Documents

largeeddy simulation

lagrangian simulation

scientific question

large scale models

represented slide

cooling slide

energy eddyviscosity

cloudtopped boundary