© Crown copyright Met Office Cloudier Evaluating a new GCM prognostic cloud scheme using CRM data Cyril Morcrette, Reading University, 19 February 2008.

© Crown copyright Met Office

CloudierEvaluating a new GCM prognostic cloud scheme using CRM dataCyril Morcrette, Reading University, 19 February 2008

© Crown copyright Met Office

The need for a cloud scheme

• Clouds exists well before grid-box reaches 100% relative humidity.

• But clouds exists on scales much smaller than GCM grid boxes.

• Can’t represent them explicitly.

• Need to parameterize them.

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Summary of moisture variables in our Cloud Scheme

Variable Current scheme PC2

qcl (Liquid Water Content) Diagnostic Prognostic

qcf (Ice Water Content) Prognostic Prognostic

qt (Total water content) Prognostic qt=qcl+qcf

Cl (Liquid cloud fraction) Diagnostic Prognostic

Cf (Ice cloud fraction) Diagnostic Prognostic

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Fields from LEM simulation of TOGA-COARE (Tropical convection)

Mean qcl in env Mean qcf in env

Liquid cloud fraction Ice cloud fraction

Hei

ght

(km

)

Time (hours)

Hei

ght

(km

)

Hei

ght

(km

)H

eigh

t (k

m)

20

144

0.1 g/kg

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Tendencies from the LEM fields

d (qcl) / dt d (qcf) / dt

d (Cl) / dt d (Cf) / dt

0.36 / hr

0.036 g/kg/hr

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Increments from Convection

Detrainment

dx/dt=D(xplume-xenv)

Detrainment

Also consider:

• vertical transport by compensating subsidence

• evaporation following warming due to compensating subsidence.

(These are small effects)

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Increments from Convectiond (qcl) / dt

d (Cl) / dt

d (qcf) / dt

d (Cf) / dt

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Microphysical effects on d(qcf)/dtDeposition Sublimation

Autoconversion of ice crystals to snow Fall of ice

0.036 g/kg/hr

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Effects on d(qcf)/dt

All microphysics

Advection by compensating

subsidence

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Effects on d(Cf)/dtFall of ice

Sublimation

Advection by compensating

subsidence

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Effects on d(qcl)/dt Adiabatic warming by compensating

subsidenceAdvection by compensating subsidence

Large-scale forcing Boundary-layer processes

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Effects on d(Cl)/dt Adiabatic warming by compensating

subsidence

Boundary-layer processesLarge-scale forcing

Advection by compensating subsidence

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Parameterized tendenciesd (qcl) / dt d (qcf) / dt

d (Cl) / dt d (Cf) / dt

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Comparing Tendencies

d (qcl) / dt

d (qcf) / dt

d (Cl) / dt

d (Cf) / dt

“Truth” from LEM Parametrization

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Conclusions (work in progress)

• General methodology seems promising.

• Source from convective detrainment appears to be too high. (This may be due to way detrainment is calculated from LEM data)

Future work• Consider using a diagnostic cloud fraction for shallow convection which doesn’t have large anvils.

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Questions and answers

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Extra figures

Massflux