Convection - University of British...

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Convection Convection

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in ECHAM in ECHAM 55

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er Francesco Francesco IsottaIsotta, Peter , Peter SpichtingerSpichtinger, Ulrike , Ulrike LohmannLohmann0808thth June June 20092009

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Outline2

► Parametrisation of convection in ECHAM5 GCM

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► Implementation of a different shallow convection scheme and further developments

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► Results

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Introduction Results Conclusion 3

ECHAM5 GCM:

mass flux scheme of Tiedtke (1989), with modifications (Nordeng, 1994):

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► Single entraining/detraining plume. Bulk values are calculated.► Distinction between deep, shallow and mid-level convection. Unimodal!► Entrainment (E) and detrainment (D) are subdivided into an organised and a

turbulent part

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ce turbulent part. ► Tiedtke: large scale moisture convergence.Nordeng: cloud base mass flux related to the available potential energy (CAPE).

► D t i d li id t d i f l l l d

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► Many developments since Tiedtke. Critical analysis of delicate aspects as entrainment/detrainment rates, homogeneity of cloud ensemble, life cycle...

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4 Introduction Results Conclusion

A newer shallow cumulus convection scheme, presented by von Salzen and Mc Farlane (2002), is now implemented in ECHAM5-GCM.

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Main properties:► Test parcels are lifted from the boundary layer through the LFC until the LNB is

reached. The ascent is affected by entrainment/detrainment.

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ce reached. The ascent is affected by entrainment/detrainment.

► Cloud base closure: simplified turbulent kinetic energy (TKE) budget

cloud base mass flux (Grant, 2001).

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► Varying entrainment/detrainment rates due to difference of buoyancy.

► Account for life cycle of cumuli (time dependent cloud cover).

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y ( p )

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i: top of cloud


LNB

s: lateral boundariesf: final

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LNB

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LFC

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von Salzen and Mc Farlane, 2002; modified

-Cloud scheme: Tompkins 2002i i d kCloud cover


-Convection: Tiedtke 1989-1 year + 3 months, T63L31-Δt=5min

Cloud cover

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1 STEP


► New shallow convection scheme.

1. STEP

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Stop if LNB above freezing level.

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Search the level of maximum moist static energy in the lowest

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r: Mixing ratio (vapour) [kg/kg], rt: total water mixing ratio (=r+rl) Conserved under hydrostatic adiabatic saturated/unsaturated transformation, in which mass is conserved (rt*cl<<cpd and rt<<1)

Cloud cover


Cloud coverZonal mean

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2 STEP


► Ice phase

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Microphysic following Lohmann et al. (1996):- Homogeneous freezing below -35°C- Heterogeneous freezing above -35°C-0°C:

# acti ated aerosol follo ing Lin and Leaitch (1997) b t

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ce - # activated aerosol following Lin and Leaitch (1997) but differently from Lohmann (2002) the updraft velocity is directly found from the parcel ascent.- freezing dependent on aerosol (immersion and contact freezing)

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- freezing dependent on aerosol (immersion and contact freezing).- autoconversion (cloud droplets precipitation), change from Beheng (1994) to Khairoutdinov and Kogan (2000) .- aggregation (cloud ice snow).

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gg g ( )- accretion (cloud droplet & rain/snow, ice crystal and snow) .

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► 300hPa

2 STEP


► New moist static energy (Emanuel,1994):

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cpd: heat capacity at constant pressure for dry air, cpv: of water vapour [J/(kg K)]rT: Net water mixing ratio (r+rl+ri) [kg/kg]T: temperature [K]

≈1

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p [ ]g: gravitational acceleration [m/s²]Lv: latent heat of vaporization [J/kg]rl: liquid water mixing ratio [kg/kg]Ls: latent heat of sublimation [J/kg]r : ice mixing ratio [kg/kg]

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Conserved in adiabatic, hydrostatic transformation, in which mass is conserved. (rT<<1)

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Cloud cover

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Diff t ISCCPStandard ECHAM

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Without microphysics

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3 STEP


►Tracer

3. STEP

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Passive transport of all tracers.Scavenging as in ECHAM5 (Stier, 2005):

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Ci: tracer “i” mixing ratio [kg/kg]

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Without tracers

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Cloud cover-Differences to ISCCP14 Introduction Results Conclusion

Winter Spring

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Summer Autumn

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Standard ECHAM with new developments15

Ship and land observations 1971-1996.

Observations

Introduction Results Conclusion

pAdapted&interpolated.Row data from: Climatic Atlas of Clouds Over Land and Ocean(S. G. Warren, C. J. Hahn)

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Frequency deep convection

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Frequency mid level convection

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Frequency cumulus convection

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Frequency shallow convection


Δ Convective precipitation (mm/d) Δ LWP (std vs new) (kg/m2) e

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Large-scale precipitation Total precipitation (mm/d)

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Convective precipitation

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Cloud coverDifference to ISCCP (with new scheme)

Zonal mean17 Introduction Results Conclusion

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Difference to ISCCP (no shallo con )

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Standard ECHAM

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Standard ECHAM with new developments18

without shallow convectionIntroduction Results Conclusion

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Frequency mid level convection

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Frequency shallow convection


Δ Convective precipitation (mm/d) Δ LWP (std vs nosh) e

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Δ Frequency deep convection Δ Large-scale precipitation (mm/d)

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Conclusions


Conclusions

The introduction of shallow convection and the further development of the scheme for the ice phase (heterogeneous freezing, homogeneous freezing, precipitation)

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for the ice phase (heterogeneous freezing, homogeneous freezing, precipitation) change the cloud cover in ECHAM5 in a drastic way.

► The results suggest the necessity for a fine tuning of the shallow convective

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► The overall results seems to show an improvement (cloud cover,...) but the

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p ( , )interaction of the different convection types and the precipitation efficiency is to control in more detail.

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er ► Next steps: Detailed analysis of the output and understanding of the effects of the new scheme.

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e Thank you!

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Large-scale precipitation, difference ECHAM all to std

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Introduction Results Problems Conclusion CloudsIntroduction Model Results Conclusion 24

TKE Budget (Turbulent kinetic energy)

e I II III VIIV VIIV

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► I Local storage or tendency term (Assumption: stationary).► II Advection of TKE by mean flow (Assumption: horizontally homogeneous) ► III Buoyancy production of consumption

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► V Turbulent transport term by eddies► VI Pressure correlation term (redistribution of TKE by pressure perturbations).

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Closure


Closure (Grant, 2001)

► TKE budget for the convective boundary layer (simplified):g: gravity acceleration, θ : virtual potential temperature,

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buoyancy term turbul. transp. term dissipation

►Approximation of the buoyancy-flux profile (good assumption):

θv: virtual potential temperature,Є: viscous dissipation rate, E: TKE,zi: ~ height of PBL,mb: cloud-base mass flux,α, Aε: assumed to be constant (0.2 and 0.37, from observations).

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at surface

► Integration between surface and zi (~ the height of PBL):

turb. flux of θv

flux of TKE

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where: sub-cloud conv.velocity scale

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estimation of the TKE flux at zi:

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► Values of α and Aε from simulations

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Grant (2001), modifiedfive simulations from ATEX

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Introduction Model Results Conclusion Introduction Model Results Conclusion Introduction Model Results Conclusion e

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Lohmann 2008

Introduction Model Results Conclusion Introduction Model Results Conclusion

I l d titi li i tIn clouds quantities: linear mixture

Hori ontal a erage of f (fraction of

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Horizontal average of f (fraction of environmental air in a mixture)

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Probability density function: increasing prob. of dilution at top of cloud with increasing time during ascent At base: undiluted

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ascent. At base: undiluted. fmax:most diluted

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von Salzen et al 2002

Introduction Model Results Conclusion Introduction Model Results Conclusion

Simple parameterization of the subcloud-scale contribution in terms of cloud-scale variables

Choose X˜ so that alpha depends

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only on the mean value of the mixing fraction in the cloud. This is done by assuming:

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It follows:

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Lohmann 1996

ρρρρ

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Grid box 1 Grid box 2Introduction Life cycle Global run Conclusion

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- Same life cycle (in same grid box) - In a certain moment different stadium of development of the single cloud in the ensemble- Next second/minute: the clouds decaying are no more there, other clouds grow/decay

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