Entrainment in stratocumulus clouds

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Entrainment in stratocumulus clouds
Stephan de Roode (KNMI)
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stratocumulus vertical structure
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stratocumulus vertical structure
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Key questions
How well is stratocumulus represented in
models? Entrainment - what is it? - why
important? - how parameterized?
Boundary-layer mixing and cloud liquid water
path - perfect boundary-conditions, perfect
cloud structure? FIRE I observations
revisited - a different view on entrainment
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ISCCP stratocumulus cloud climatology
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ECMWF RE-ANALYSIS shortwave radiation errors
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GCSS intercomparison cases
Stratocumulus case based on observations (FIRE
I) Prescribe - initial state -
large-scale horizontal advection - large-scale
subsidence rate Simulation of diurnal
cycle - 1D versions of General Circulation
Models - Large-Eddy Simulation Models (LES)
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GCSS intercomparison cases
Stratocumulus case based on observations (FIRE
I) Prescribe - initial state -
large-scale horizontal advection - large-scale
subsidence rate Simulation of diurnal
cycle - 1D versions of General Circulation
Models - Large-Eddy Simulation Models (LES)
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GCSS FIRE I intercomparison participants

• Fine-scale turbulence models Large-Eddy
  Simulation Models (LES) DxDy50m, Dz10m
• IMAU - Peter G. Duynkerke, Stephan de Roode, M.
  C. van Zanten and P. Jonker
• MPI - Andreas Chlond, Frank Müller, and Igor
  Sednev
• WVU - David Lewellen
• INM - Javier Calvo, Joan Cuxart, Dolores Olmeda,
  Enrique Sanchez
• UKMO - Adrian P. Lock
• NCAR - Chin-Hoh Moeng (NCAR)
• 1D versions of General Circulation Models
  Single-Column Models (SCM)
• LMD - Sylvain Cheinet
• MPI - Andreas Chlond, Frank Müller, and Igor
  Sednev
• Meteo France I - Hervé Grenier
• Meteo France II - Jean-Marcel Piriou
• ECMWF - Martin Köhler
• CSU - Cara-Lyn Lappen
• KNMI - Geert Lenderink
• UKMO - Adrian P. Lock
• INM - Javier Calvo, Joan Cuxart, Dolores Olmeda,
  Enrique Sanchez

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3D results from Large-Eddy Simulation results
-The cloud liquid water path
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What is entrainment?Why is entrainment important?

• Entrainment
• mixing of relatively warm and dry air from above
  the inversion into the cloud layer
• - important for cloud evolution

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3D results from Large-Eddy Simulation results
-Entrainment rates
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Boundary-layer representation
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1D results from General Circulation Models -The
cloud liquid water path (LWP)
Single Column Model liquid water path results
very sensitive to entrainment rate drizzle
parameterization convection scheme (erroneous
triggering of cumulus clouds)
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Key questions
How well is stratocumulus represented in
models? Entrainment - what is it? - why
important? - how parameterized?
Boundary-layer mixing and cloud liquid water
path - perfect boundary-conditions, perfect
cloud structure? FIRE I observations
revisited - a different view on entrainment
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The clear convective boundary layer (CBL)
-Entrainment scaling from observations
Entrainment rate we scales as A
0.2 H boundary-layer height (g/q0)
Dqv buoyancy jump across the inversion
w convective velocity scale vertically
integrated buoyancy flux
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Buoyancy flux in stratocumulus
convective velocity scale w depends on
entrainment rate we
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Solve entrainment rate
solve for entrainment rate
?
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Solve entrainment rate
solve for entrainment rate
?
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Solve entrainment rate
solve for entrainment rate
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Solve entrainment rate
solve for entrainment rate
?
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Stability jumps
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Stability jumps
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Stability jumps
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Entrainment parameterizations for stratocumulus
-Results based on LES results
Based on observations of clear CBL
Nicholls and Turton (1986)
Stage and Businger (1981) Lewellen and
Lewellen (1998) VanZanten et al. (1999)
Lock (1998)
Lilly (2002)
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Sensitivity of entrainment parameterizations to
inversion jumps
observations from ASTEX Flight 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 liquid.
water content 0.5 g/kg LWP 100 g/m2
Compute entrainment rate from
parameterizations as a function of inversion jumps
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Entrainment rate cm/s sensitivity to inversion
jumps
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Entrainment rate cm/s parameterizationsof
observed cases
Parameterization ? Case ? Observed Moeng Lock Lilly Nicholls-Turton Lewellen
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
Entrainment results mirror the LES results where
they are based on
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Entrainment parameterizations -Implementation in
K-diffusion schemes
Turbulent flux at the top of the boundary layer
due to entrainment ("flux-jump"
relation) Top-flux with K-diffusion
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Key questions
How well is stratocumulus represented in
models? Entrainment - what is it? - why
important? - how parameterized?
Boundary-layer mixing and cloud liquid water
path - perfect boundary-conditions, perfect
cloud structure? FIRE I observations
revisited - a different view on entrainment
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Compute eddy- diffusivity coefficients from FIRE
I LES
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K-coefficients from FIRE I LES
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Importance of eddy-diffusivity coefficients on
internal boundary-layer structure
Change magnitude K profiles Compute
vertical profiles ql and qt from integration
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Total water content profiles for different
K-profiles but identical vertical flux
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Liquid water content profiles for different
K-profiles
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 liquid water content!
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Key questions
How well is stratocumulus represented in
models? Entrainment - what is it? - why
important? - how parameterized?
Boundary-layer mixing and cloud liquid water
path - perfect boundary-conditions, perfect
cloud structure? FIRE I observations
revisited - a different view on entrainment
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FIRE I stratocumulus over the Pacific Ocean
-Aircraft lidar observations of cloud-top height
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Thermodynamic structure of clear air above cloud
top depressions
mean in-cloud value
clear air value
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Evaporation of cloud top by turbulent mixing
horizontal winds
turbulence
turbulence
vertical velocity
evaporation
liquid water content
liquid water potential temperature
total water content
12 km
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Observations of moist and cold layers on top of
stratocumulus
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Entrainment mixing scenario
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Conclusions
Entrainment parameterizations - extrapolation
of Large-Eddy Simulation results - considerable
differences ? different heat and moisture
budgets Cloud liquid water path and
K-diffusion turbulence schemes - different
solutions for identical surface and cloud-top
fluxes ? different albedo Entrainment
observations - may induce the formation of
moist layers above cloud top ? opposes general
view on the entrainment process
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stability jumps