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A dual mass flux scheme for shallow cumulus convection Results for GCSS RICO . Roel Neggers, Martin Köhler, Anton Beljaars. Contents. The DualM scheme An extension to EDMF for shallow cumulus convection A *VERY* short overview (more on Thursday) RICO results

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A dual mass flux scheme for

shallow cumulus convection

Results for GCSS RICO

Roel Neggers, Martin Köhler, Anton Beljaars


Contents

The DualM scheme

An extension to EDMF for shallow cumulus convection

A *VERY* short overview (more on Thursday)

RICO results

Mean state

Equilibration

Updraft profiles & fluxes

Variances

Cloudy state & precipitation

Experiments with an entrainment efficiency closure for cumulus


The current PBL scheme (cy30r1)

Eddy diffusivity - mass flux (EDMF) decomposition for turbulent transport

Siebesma et al. (Submitted to JAS)

Currently applied to dry PBL and stratocumulus topped PBL onlyGoal: upgrade EDMF to enable representation of i) shallow cumulus convection ii) transitions What is needed?

dry updraft

moist

updraft

K

K


Moist updrafts

Dry updrafts

h

a2up

a1up

Flexible area

partitioning

K diffusion

z0

1 - aup

aup

The dual mass flux scheme – main EDMF upgrades

I. Multiple updrafts

N=2: dry updrafts stopping at cloud base,

moist updrafts continuing as cumulus clouds.

II. Flexible updraft area fractions

Closure as a function of model state

Facilitates representation of i) gradual regime transitions, ii) coupling of cumuli to mixed layer

III. Extension of EDMF partitioning into the statistical cloud scheme

A matching bimodal PDF (passive clouds and updraft clouds) Consistent treatment of PBL transport and cloudiness

IV. Top-entrainment efficiency closures for cumulus

Explicit treatment of transport across strong unresolved gradients ( cu inversion, transition layer)

V. … and many more …


W

E

M2

LCL

LCL

E

E

LCL

h

M2

M1

h

M1

Stratocumulus Shallow cumulus Dry convection

a1

a2

Moist and dry updrafts can co-exist at any time

Flexible updraft area fractions facilitate representation of gradual regime transitions


III. Vertical integration of updraft budgets then reconstructs the joint-PDF

I. First guess of cloud core fraction at cloud base, using local variance scaling

II. These fractions are subsequently used in surface initialization of updrafts

Closure of the flexible updraft area fractionsExplicit representation of the coupling between cumulus updrafts and mixed-layer

Knowledge of how many updrafts manage to reach LFC is `back-stitched` into the partitioning and initialization of the updrafts

This carries information on cloud base transition layer properties




RICO results - equilibration

Area fraction is retained in the updraft mass flux

The flexibility of a and its closure introduce a valve mechanism in M, always acting to equilibrate the mixed layer budgets


RICO results - updrafts

Lateral entrainment closure makes differently initialized updrafts diverge with height.



RICO results – variance I

Used in statistical cloud scheme


RICO results – variance II

Extension of EDMF into the modelling of clouds – a bimodal distribution

qt

Updraft PDF

Diffusive PDF

l

Relations between moments

Lewellen & Yoh (JAS,1993)


RICO results – variance II

Closure of updraft variance



Updraft clouds

Passive clouds



RICO results - precipitation

How important is it?

RICO

BOMEX


RICO results – entrainment efficiency experiments

All flux into cu inversion is explicitly modelled through the K-component of EDMF

Wyant et al. (1997)

RICO

ATEX


Summary

A shallow cumulus extension is formulated for EDMFMore complexity is added to the system of equations, in various new ways: - multiple transporting updrafts - flexible updraft area fractions - EDMF partitioning is extended into the PBL cloud scheme (a bimodal PDF)Assuming flexible updraft area fractions facilitates representation of gradual regime transitions Closure of the updraft area fractions allows explicit representation of the coupling between cumuli and the mixed layer, introducing a valve mechanism in the cumulus mass fluxThe mean, turbulent and cloud structure of the RICO short composite case is reproducedPrecipitation appears to be an important player in the SCM thermodynamic budgetsA more detailed discussion of model physics on Thursday…


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