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Boundary Layer parametrisation in the climate model ECHAM5-HAM

Colombe Siegenthaler - Le Drian, Peter Spichtinger, Ulrike Lohmann

CFMIP/GCSS Boundary Layer WG Workshop, 8th-12th June, 2009

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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

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ceLES study

A. Ackermann et al., 2004, ‘The impact of humidity above stratiform clouds on indirect aerosol climate forcing’, Nature)

N

drizzle

N

drizzle

LWP

Sensitivity of LWP with CCN related with the entrainment rate, and relative humidity above the cloud

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ceECHAM5-HAM : Standard

• T63 x L31, Δt = 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)

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ceDiffusion on conserved variables

Standard version Diffusion on moist conserved variables

ql

ql

qv q

t

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qt , q

c , cc , ...

_ __

qt PDF

a b

Grid box variables

qsat

(T)

baba

Variance threshold : C

liquid

vapo

ur corresponds

does not correspond

< C = C

C : “tuning parameter” standard version : 10 %

moist version : 0.1 %

Retrieve the condensate from the PDF Statistical cloud scheme (Tompkins, 2002)

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ceTotal cloud cover

standard (C =10 %) reduced minimal width PDF (RED) , C =0.1 %

cloud cover [%]

ISCCP

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reduced minimal width PDF (RED)

difference CMO-RED

cloud cover [%]

diffusion on moist conserved variable (CMO)

difference cloud cover [%]

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ceTKE 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

[m2s-2]

In ECHAM5

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low tropospheric stability ( ) >

Criterion for onset explicit entrainmentS. 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 [%]:

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ceMean fluxes above cloud/clear sky

ω'χ' ___

cω'χ' ___

d

• buoyancy flux:

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ceEntrainment parametrisation

Replace at the top of the boundary layer (i=interface)

Turton and Nicholls (1987) entrainment parametrisation:

Buoyancy production into the TKE equation :

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ce Resulting profiles - SCM

SCM version of ECHAM5, averaged over 1 hour of simulations for the ASTEX case

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ceEffect 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

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ceDiurnal cycle, SCM

Dec06/nbhbv1Dec06/nbhbv1Dec06/nbhbv1

,

we NT

Local Time Local TimeLocal Time

STD

CMO

TKE [m/s] Cloud cover [%] Cloud liquid [kg/kg¨]

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ceOutlook

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

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ceCross-section-Very Preliminary results

CMO we 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

we NT

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ceLES 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

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Now in ECHAM (without entrainment parametrisation)

In nature

LWPLWP

Number of aerosols

drizzle drizzle , entrainment ( )

144

U. Lohmann, 2007, ACP

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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 inSoutheast Pacific Stratocumulus”, J. Atm. Sci.

~ 10-3 [ms-1]

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ceEntrainment 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)

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ceIn 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

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ceEddy diffusivities – standard/equivalent

Version with entrainment :

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ceWhat 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-strat ECHAM5-conv ECHAM5-apc

ISCCP

surf. obs.

(zonal mean)

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ceDiffusion on conserved variables - 3D

Sundqvist scheme

Difference of annual mean total cloud cover (MOIST-REF)[%]

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ceTKE scheme.....

1.5 order closure introduction 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)

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ceVertical cloud cover

vertical cloud cover (%) averaged over jja

Method seems to find clouds in the good locations

even without perfect profiles