A dual mass flux scheme for

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A dual mass flux scheme for shallow cumulus
convection Results for GCSS RICO
Roel Neggers, Martin Köhler, Anton Beljaars
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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
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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?
moist updraft
dry updraft
K
K
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The dual mass flux scheme main EDMF upgrades
I. Multiple updrafts
N2 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
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Moist and dry updrafts can co-exist at any time
Flexible updraft area fractions facilitate
representation of gradual regime transitions
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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
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RICO results mean state
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RICO results cloudy state
DualM
cy30r1
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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
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RICO results - updrafts
Lateral entrainment closure makes differently
initialized updrafts diverge with height.
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RICO results - fluxes
LES
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RICO results variance I
Used in statistical cloud scheme
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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)
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RICO results variance II
Closure of updraft variance
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RICO results bimodal clouds
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Updraft clouds
Passive clouds
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RICO results - precipitation
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RICO results - precipitation
How important is it?
RICO
BOMEX
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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
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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