A dual mass flux framework for boundary layer convection

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A dual mass flux framework for boundary layer
convection
Roel Neggers, Martin Köhler, Anton Beljaars
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Contents
Convective boundary layers at Current
scheme some issues Reassessment of
PBL physics introducing more complexity
The dual mass flux framework
A bimodal cloud scheme
SCM evaluation Idealized cases
ARM SGP
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SCM - issues
BOMEX marine steady state shallow cumulus
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4
0
Time days
2
First order cloud statistics (amount, depth) are
more or less ok
Height km
Problems
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Intermittency Unrealistic vertical structure
Drift on medium-range timescales Abrupt
convective regime transitions
time averaged
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SCM - issues
ARM SGP transient continental shallow cumulus
21 June 1997
Vertical structure and time-development of
cloudiness are unrealistic Subcloud mixed layer
and PBL deepen too fast
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Reassessment
Convective PBL framework more or less unchanged
since Tiedtke (1989) Recent new
insights detailed measurements of PBL
clouds (ARM)
large-eddy simulation (GCSS
case studies)
Method Increase complexity of PBL physics in the
SCM Introduction of more degrees of
freedom in key bulk closures
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I. The dual mass flux scheme
Eddy diffusivity - mass flux (EDMF) decomposition
of turbulent transport
Siebesma et al. (Submitted to JAS)
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Moist and dry updrafts can coexist at any time
Flexible updraft area fractions allow for gradual
regime transitions
W
E
LCL
M2
LCL
E
E
LCL
h
M2
h
M1
M1
Stratocumulus Transition
Shallow cumulus
Dry convection
a1
a2
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II. A bimodal cloud scheme
EDMF decomposition is extended into cloud scheme
One updraft PDF, one diffusive PDF
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Consistent treatment of transport and cloudiness
throughout SCM
Representation of positive skewness
Offline test of bimodal framework in LES BOMEX
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Allowed scenarios
Diffusive cloudiness
Updraft cloudiness
LCL
Independent treatment of cloudiness associated
with updraft PDF and diffusive PDF (Multiple)
passive cloud layers can exist anywhere
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SCM evaluation
BOMEX marine steady state shallow cumulus
time averaged
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No more medium-range drift! Continuous
convection, with diurnal cycle Vertical
structure of cloud fraction is reproduced Bimodal
partitioning between updraft and passive
cloudiness in SCM is realistic
Height km
1
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ARM SGP transient continental shallow cumulus
21 June 1997
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ARM SGP transient continental shallow and deep
cumulus July 1999
deep convection scheme
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Summary
Coexisting dry and moist updrafts combined with
flexible updraft area fractions allow gradual
regime transitionsThe independent updraft PDF,
introduced into the cloud scheme to represent
skewness, now accounts for all cloudiness in the
top of the shallow cumulus cloud layer in the
SCM.The dependence of the moist updraft
fraction (and the associated mass flux) on cloud
base transition layer properties represents a
stabilizing feedback mechanism, always acting to
maintain the PBL state close to
equilibrium.The evaluation against long
time-series of ARM SGP cloud observations
illustrates the robustness of the scheme in
reproducing representative cloudy PBLs for
different large-scale conditions.
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Outlook
Implementation into the ECMWF Integrated Forecast
System (IFS) SCM interactive with
larger scale dynamics Evaluate
cooperation of new framework with other
physics Towards an operational
versionFurther evaluation against ARM
continuous forcings datasetsExtension of the
multiple updraft framework to deep convection
Does the updraft entrainment closure (1 /
t w) still apply? Do we need to make
t flexible?
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PBL transition into deep convection
Can the PBL updraft model do deep convection too?
Shallow stage t0.35
Deep stage t1.42
Moist updraft qt excess g/kg
Time day