Low clouds in the atmosphere: Never a dull moment
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Low clouds in the atmosphere:Never a dull moment
Stephan de Roode (GRS)
stratocumulus cumulus
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Stratocumulus off the coast of California
1100
km
1100 km
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Low clouds above the North Sea (and a part of the coast)
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Outstanding questions
1. Stratocumulus and climate changeRandall et al. 1984:
"A 4% increase in global stratocumulus extend would offset 2-3K global warming from CO2
doubling"
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100 200 300 400 500 c lo u d la y e r th i c k n e s scloud layer geometric thickness [m]
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Outstanding questions/problems
2. Poor representation of stratocumulus in weather forecast models
Stevens et al. 2007:
"ECMWF products are more deficient in their representation of the stratocumulus topped boundary
layer itself...... cloud liquid water paths still tend to be a factor of 2–4 less than observed, and
boundary layer temperatures are found to correlate poorly with the observations."
* Can we improve the prediction with a high-resolution model?
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Contents
* Stratocumulus formation
* Stratocumulus climate feedback
* High-resolution modeling of stratocumulus
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Global stratocumulus amount
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Necessary ingredients for stratocumulus formation
high pressure area &
cold surface water
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Stratocumulus frequently present in the subsiding branch of Hadley circulation
deep convection shallow cumulus stratocumulus
warm and dry
cold and moist
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Stratocumulus frequently present in the Arctic area(but we can hardly observe them from satellite)
Infrared: very low thermal contrast between ice surface and low clouds
Shortwave: albedo ice and cloud similar (and long dark winters)
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Stratocumulus clouds, 31 January 2011
Radiosonde observations for De Bilt
31 January 2011
00 UTC12 UTC
Sharp inversion layer structure cannot be captured by weather forecast models
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Longwave radiative cooling at the cloud top
Duynkerke et al. (1995)
radiative cooling tendency at cloud top ~ -8 K/hr
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Turbulence causes entrainment of warm and dry inversion air into the cloud layer
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Cloud dynamics
10 m 100 m 1 km 10 km 100 km 1000 km 10000 km
turbulence Cumulusclouds
Cumulonimbusclouds
Mesoscale Convective systems
Extratropical Cyclones
Planetary waves
Large Eddy Simulation (LES) Model
Limited Area Weather Model (LAM)
Numerical Weather Prediction (NWP) Model
Global Climate Model
Atmospheric Models
DNS
mm
Cloud microphysics
Fundamental Engineering
Slide courtesy Harm Jonker and Pier Siebesma
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Cloud dynamics
10 m 100 m 1 km 10 km 100 km 1000 km 10000 km
turbulence Cumulusclouds
Cumulonimbusclouds
Mesoscale Convective systems
Extratropical Cyclones
Planetary waves
Large Eddy Simulation (LES) Model
Current developments
DNS
mm
Cloud microphysics
Limited Area Weather Model (LAM)
Global Climate Model
Numerical Weather Prediction (NWP) Model
Fundamental Engineering
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Feedback effects in a changing climate
Dufresne & Bony, Journal of Climate 2008
Radiative effects only
Water vapor feedback
Surface albedo feedback
Cloud feedback
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Neg. Feedback
Pos. Feedback
Present Perturbed Future
Strong Pos. Feedback
slide Pier Siebesma
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Can models faithfully represent low clouds?Simulations of the ASTEX Lagrangian stratocumulus
experiment
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Modeling results for ASTEX case
Van der Dussen et al. (2012)
• Case set-up based on De Roode and Duynkerke (1997)
• 20 international institutions participate in this model intercomparison case
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Results of single-column model versions of climate models: cloud fraction
Plot by Roel Neggers (KNMI)
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What did we learn from the Lagrangian transition?
Movie by Johan van der Dussen
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The CGILS project (Zhang et al. 2013)
Equilibrium states are computed for three selected columns in the Hadley circulation
* Single-column model (SCM) versions of climate models
* Large-eddy simulation (LES) models
S11 cumulus under stratocumulus
S6cumulus
S12 stratocumulus
• Run to steady state with diurnally-averaged insolation
• Vertical profiles of temperature, humidity and ozone above the LES domain are used for radiative transfer
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Cloud Radiative Feedback (CRF) if the large-scale forcing is perturbed (higher sea surface temperature)
S11 cumulus under stratocumulus
S12stratocumulus
red =positive feedback
blue=negative feedback
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"phase space” experiments with a conceptual model:how do inversion jumps control equilibrium solutions?
0 = surface flux +entrainment flux - source
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Liquid water path (LWP) response to a perturbation in the sea surface temperature q0
warmer inversion layer
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De Roode et al. (2013)
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Cloud regimes: Results from two different transport schemes used in weather and climate models ("dualM" and "TKE")
Dal Gesso et al. (2013)
cloud cover CC liquid water path LWP
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Cloud response to warming of atmosphere
Dal Gesso et al. (manuscript in preparation)
cloud cover (CC)liquid water path (LWP)
warmer inversion layer
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Cold air outbreak
mean wind
Satellite
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Cold air outbreak case simulated with the LES model
Thomas Frederikse (MSc student)
simulated cloud liquid water path (kg/m2)
MSc thesis defense: Tuesday 2 April, 15.00 pm
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Summary
• Physics of stratocumulus
Small scale processes (longwave radiation, entrainment across thin inversion layer)
• Weather forecast models
Have a coarse vertical resolution causing difficulties to capture small-scale processes
• High-resolution large-eddy simulation models
Invaluable tool for studying dynamics of stratocumulus
Ambition: Detailed weather prediction for wind and solar energy
• Current model intercomparison projects
Phase space experiments, cold air outbreak