The art of modeling stratocumulus clouds

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The art of modeling stratocumulus clouds
Stephan de Roode
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Cloud regimes The Hadley circulation
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ISCCP stratocumulus cloud climatologyECMW
F net shortwave radiation error
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Stratocumulus-Topped Boundary Layer - Vertical
structure
Entrainment mixing of relatively warm and dry
air from above the inversion into the cloud layer
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Stratocumulus-Topped Boundary Layer - Vertical
structure
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The representation of clouds in most GCMs
K-diffusion Massflux
K-diffusion Mass flux
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Eddy diffusivity coefficient K
ECMWF RACMO 2
K-profile prescribed K(z) TKE(z)1/2 l(z) 1
stability controls K-profiles controls TKE and l
entrainment K we Dz we from parameterization by Eq. 1
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Strategy Adapt TKE scheme for moist convection

• Develop moist TKE scheme for RACMO2
• no need for explicit entrainment
  parameterization
• dry TKE scheme (Lenderink/Holtslag) is already
  present in RACMO2

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Outline of the talk
Introduction stratocumulus presence model
representation of stratocumulus Moist TKE
scheme some details Evaluation - GCSS
cases - entrainment rates - vertical mixing
cloud liquid water path - problems
Conclusions outlook
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TKE scheme
turbulent transport
pressure transport
buoyancy production
mechanical shear
dissipation
diffusion on conserved variables sl and qt
(liquid water static energy)
(total specific humidity)
The difference between the dry and moist TKE
scheme is incorporated in the prefactors A and B
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Buoyancy flux moist thermodynamics
cf cloud fraction
,
,
for cf gt 0 , latent heat release effects
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Length scale
Length scale depends on Richardson
number buoyancy gradient in cloud computed as
in TKE scheme
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Call tree
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Call tree
suphec1c.F90 RVTMP2 0
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Outline of the talk
Introduction stratocumulus presence model
representation of stratocumulus Moist TKE
scheme some details Evaluation - GCSS
cases EUROCS FIRE I (diurnal cycle) ASTEX
A209 (entrainment) - vertical mixing cloud
liquid water path - problems Conclusions
outlook
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GCSS intercomparison cases
Stratocumulus cases based on observations
Prescribe in SCM version of RACMO2 - initial
state - large-scale horizontal advection -
large-scale subsidence rate - surface fluxes or
SST Today focus on - EUROCS FIRE I diurnal
cycle case - ASTEX A209
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1D results from General Circulation Models
-Cloud liquid water path (LWP) for EUROCS FIRE I

Single Column Model liquid water path results
very sensitive to drizzle parameterization
convection scheme (erroneous triggering of
cumulus clouds) vertical resolution
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3D results from Large-Eddy Simulation results
-The cloud liquid water path
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FIRE I 1D and Large-Eddy Simulation results
vertical levels 60 time step 900
s precipitation on/off convection off
TKE scheme thicker cloud for FIRE I
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Outline of the talk
Introduction stratocumulus presence model
representation of stratocumulus Moist TKE
scheme some details Evaluation - GCSS
cases EUROCS FIRE I (diurnal cycle) ASTEX
A209 (entrainment) - vertical mixing cloud
liquid water path - problems Conclusions
outlook
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ASTEX A209
ASTEX A209 _________________________________ cloud
base height 240 m cloud top height
755 m sensible heat flux 10 W/m2 latent heat
flux 30 W/m2 longwave flux jump 70 W/m2 max
liq. water content 0.5 g/kg LWP 100
g/m2 Dql 5.5 K Dqt -1.1 g/kg
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Buoyancy flux for ASTEX A209
cloud top "locked" for 60 level run and less
entrainment realistic buoyancy flux profile
for high-resolution run
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Entrainment parameterizations proposed in the
literature
Nicholls and Turton (1986)
Stage and Businger (1981), Lewellen and
Lewellen (1998) VanZanten et al. (1999)
Lock (1998)
Lilly (2002)
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Buoyancy flux decomposition
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Stability jumps
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Examples of entrainment rates cm/s from
parameterizations
Parameterization ? Case ? Observed Moeng Lock UKMO, new ECMWF Lilly Nicholls-Turton MM5 Lewellen / van Zanten
North Sea NT620 0.55 0.50 0.13 0.30 0.30 0.33
North Sea NT624 0.56 0.76 0.28 0.55 0.66 0.61
ASTEX A209 0.9 0.3 1.23 0.42 0.86 1.06 0.97
ASTEX RF06 1.0 0.6 1.24 0.48 1.04 1.31 1.33
DYCOMSII RF01 0.38 0.10 0.72 0.69 0.62 0.60 0.64
FIRE I 0.58 0.08 (mean LES) 0.57 0.16 0.37 0.35 0.50
? ? high low
TKE H_Res we 1.27 cm/s
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Examples of entrainment rates cm/s from
parameterizations
Parameterization ? Case ? Observed Moeng Lock UKMO, new ECMWF Lilly Nicholls-Turton MM5 Lewellen / van Zanten
North Sea NT620 0.55 0.50 0.13 0.30 0.30 0.33
North Sea NT624 0.56 0.76 0.28 0.55 0.66 0.61
ASTEX A209 0.9 0.3 1.23 0.42 0.86 1.06 0.97
ASTEX RF06 1.0 0.6 1.24 0.48 1.04 1.31 1.33
DYCOMSII RF01 0.38 0.10 0.72 0.69 0.62 0.60 0.64
FIRE I 0.58 0.08 (mean LES) 0.57 0.16 0.37 0.35 0.50
? ? high low
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Entrainment sensitivityvary inversion jumps for
ASTEX A209
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Entrainment sensitivity to inversion jumps
Entrainment rate depends on moisture jump with
TKE scheme
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Outline of the talk
Introduction stratocumulus presence model
representation of stratocumulus Moist TKE
scheme some details Evaluation - GCSS
cases EUROCS FIRE I (diurnal cycle) GCSS
ASTEX A209 (entrainment) - vertical mixing
cloud liquid water path - problems
Conclusions outlook
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Quotes from Zhu et al. (2005) on the DYCOMS II
RF01 intercomparison case
"The large spread in LWP in part reflects the
sensitivity of liquid water content to small
changes in total humidity and temperature induced
by the different turbulent transport and
microphysics scheme employed in the SCMs."
"For two models using the same microphysics
scheme, the different turbulent transport
realized in models is a more important factor
than microphysics for causing large LWP spread..."
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K-coefficients from FIRE I LES
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K-coefficient from TKE scheme compared to LES for
FIRE I
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The role of the K-coefficient on the vertical
cloud structure -Test different K-profiles
4. Diagnose cloud liquid water content
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Total water content as a function of magnitude K
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Liquid water content as a function of magnitude K
K factor LWP g/m2
0.2 2
0.5 52
1.0 79
2.0 94
5.0 103
? 109
Magnitude K-coefficient in interior BL important
for cloud liquid water content!
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Outline of the talk
Introduction stratocumulus presence model
representation of stratocumulus Moist TKE
scheme some details Evaluation - GCSS
cases EUROCS FIRE I (diurnal cycle) GCSS
ASTEX A209 (entrainment) - vertical mixing
cloud liquid water path - problems
Conclusions outlook
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Two-layer structure for K-profile -Example for
ASTEX A209
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Fluxes of conserved variables -Example for ASTEX
A209
TKE scheme fluxes of ql and qt not linear
Effect standard (coarse) 60 level vertical
resolution is obvious
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Evaluation of TKE scheme Conclusions

• GCSS cases
• Diurnal cycle of EUROCS FIRE I is represented
  reasonably well
• Entrainment rate depends on moisture jump (as
  like some parametrizations)
• Eddy diffusivity K-profile
• K-profile controls cloud liquid water path -
  Eddy diffusivities for qt and ql usually differ
• Potential problem for K-profile below cloud
  base (two-layer structure)
• Further remarks
• Coarse vertical resolution can be problematic
• Fluxes appear not to be linear in the boundary
  layer

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Outlook
Next step(s) Combine TKE scheme with mass-flux
approach (Eddy-Diffusivity Mass-Flux gt
EDMF) Does this solve the two-layer
eddy-diffusivity structure? but also Snow
Janneke Ettema Stable boundary layer Peter
Baas Cabauw Reinder Ronda Climate Geert
Lenderink, Erik van Meijgaard, Willem-Jan van de
Berg Radiation Dave Donovan and Gerd-Jan van
Zadelhoff Cloud effective radius Gabriella de
Martino Precipitation Margreet van
Zanten Cumulus convection Pier Siebesma gt
meeting dedicated to RACMO?