© 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.

© Crown copyright Met Office

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

© Crown copyright Met Office

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

© Crown copyright Met Office

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

© Crown copyright Met Office

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)

© Crown copyright Met Office

Increments from Convectiond (qcl) / dt

d (Cl) / dt

d (qcf) / dt

d (Cf) / dt

© Crown copyright Met Office

Microphysical effects on d(qcf)/dtDeposition Sublimation

Autoconversion of ice crystals to snow Fall of ice

0.036 g/kg/hr

© Crown copyright Met Office

Effects on d(qcf)/dt

All microphysics

Advection by compensating

subsidence

© Crown copyright Met Office

Effects on d(Cf)/dtFall of ice

Sublimation

Advection by compensating

subsidence

© Crown copyright Met Office

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

subsidenceAdvection by compensating subsidence

Large-scale forcing Boundary-layer processes

© Crown copyright Met Office

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

subsidence

Boundary-layer processesLarge-scale forcing

Advection by compensating subsidence

© Crown copyright Met Office

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

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

© Crown copyright Met Office

Comparing Tendencies

d (qcl) / dt

d (qcf) / dt

d (Cl) / dt

d (Cf) / dt

“Truth” from LEM Parametrization

© Crown copyright Met Office

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.

© Crown copyright Met Office

Questions and answers

© Crown copyright Met Office

Extra figures

Massflux