A dual mass flux framework for boundary layer convection Explicit representation of cloud base...
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A dual mass flux framework A dual mass flux framework for boundary layer convectionfor boundary layer convection
Explicit representation of cloud base Explicit representation of cloud base coupling mechanismscoupling mechanisms
Roel Neggers, Martin Köhler, Anton Beljaars
Contents
The coupling between cumulus clouds and the mixed layer
Mass flux feedbacks
The dual mass flux scheme
The eddy diffusivity mass flux (EDMF) approach
An extension for shallow cumulus
Introducing more complexity
New parameterizations
SCM evaluation
Idealized prototype cases
Daily evaluation against ARM and CloudNet data
Interactive with the larger scale dynamics
II. Retaining the area fraction in the cumulus mass flux
waM
Neggers,Siebesma, Lenderink and Holtslag, 2004: “An Evaluation of Mass Flux Closures for Diurnal Cycles of Shallow Cumulus”. MWR, 132, 2525-2538.
I. Using a constant pre-factor
* 03.0 wM Grant, 2001: “Cloud-base fluxes in the cumulus-capped boundary layer”. Quart. J. Roy. Met. Soc., 127, 407-422.
Closures staying close to thís structure:
* waM cloud
* (CIN) wfM
Cumulus mass flux at cloud base
Definition
Bretherton, McCaa and Grenier, 2004: “A new parameterization for shallow cumulus convection and its application to marine subtropical cloud-topped boundary layers. Part I: Description and 1D results”. Mon. Wea. Rev., 132, 864-882.
A *SIMPLE* equilibrium model including a mass flux closure with flexible fraction
Neggers, Stevens and Neelin, 2006: “A Simple Equilibrium Model for Shallow Cumulus Topped Mixed Layers”. Special Issue of the Theor. Comp. Fluid-Dynamics, in press, September 2006.
humidity
heat
mass
Variance scaling at mixed layer topNeggers, Stevens and Neelin, 2006: “Variance Scaling in Shallow Cumulus Topped Mixed Layers”. Submitted to QJRMS, August 2006.
Equilibrium model solution on ERA40 forcings and boundary conditions
50 years of ship based synops observations (Norris, J.Clim. 1998)
The dual mass flux scheme – main EDMF upgrades
Moist updrafts
Flexible area partitioning
aup
Dry updrafts
K diffusion
a1up a2
up
h
z0
1 - aup
I. Multiple updrafts
N
i
upi
upi
upi
upup wawa1
)(´´
II. Flexible updraft area fractions – LCL coupling
VII. Extension of EDMF into statistical cloud modelling (a bimodal PDF)
III. Updraft initialization
upupK waz
Kaw ´´´´
},{ tl q
Eddy diffusivity - mass flux (EDMF) approach
IV. Updraft budgets
V. Mass transport
VI. Diffusion – an entrainment efficiency closure for cumulus
Flexible updraft area fractions facilitate representation of gradual regime transitions
E
LCL
h
LCL
h
LCL E E
M2 M1
Stratocumulus Transition Shallow cumulus Dry convection
M1
M2
a2
a1
Moist and dry updrafts can coexist at any time
I. Multiple updrafts with flexible area fractions
II. Closure of the flexible updraft area fractions
Explicit representation of the coupling between cumulus updrafts and mixed-layer
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
Knowledge of how many updrafts manage to reach LFC is `back-stitched` into the partitioning and initialization of the updrafts
Variance scaling at mixed layer top
This gives an implicit solution for the area fraction that carries mass flux:
Neggers, Stevens and Neelin, 2006: “Variance Scaling in Shallow Cumulus Topped Mixed Layers”. Submitted to QJRMS, August 2006.
shape parameter of Beta function
SCM evaluation
I. For idealized prototype cases (LES)
II. Daily evaluation against ARM and CloudNet data
III. Interactive with the large scale dynamics
Daily evaluation against ARM and CloudNet data
Setup:
- initialize with ECMWF analysis state at midnight (local time)
- 2 day time integration
- relaxation towards forecast state (on a 6 hour timescale)
- LS advective forcings are also applied
At multiple sites around the world where relevant observational data is available
Summary
An extension for EDMF is formulated that allows representation of shallow cumulusAn extension for EDMF is formulated that allows representation of shallow cumulus convection convection
Key component is the explicit representation of the coupling of shallow cumulusKey component is the explicit representation of the coupling of shallow cumulus clouds to the mixed layer clouds to the mixed layer
This is done through flexible mass flux area fractions, which introduces cloudThis is done through flexible mass flux area fractions, which introduces cloud base valve-mechanisms base valve-mechanisms
The DualM scheme brings significant improvements for shallow cumulus concerning The DualM scheme brings significant improvements for shallow cumulus concerning i) mixed layer stability i) mixed layer stability ii) structure of mean, turbulent and cloudy state ii) structure of mean, turbulent and cloudy state iii) mixed layer depth and 2m humidity iii) mixed layer depth and 2m humidity
Further evaluation against ARM and CloudNet data illustrates that these Further evaluation against ARM and CloudNet data illustrates that these results are representative of nature and robust for different large-scale scenarios. results are representative of nature and robust for different large-scale scenarios.
Outlook: Implementation and testing in IFSOutlook: Implementation and testing in IFS
Implementation completed Implementation completed Evaluation in full progress Evaluation in full progress