Stratocumulus-topped Boundary LayerStratocumulus-topped Boundary Layer
Review of last lectureReview of last lecture
The turbulent closure problem: Number of The turbulent closure problem: Number of unknowns > Number of equationsunknowns > Number of equations
Surface layer: related to gradientSurface layer: related to gradient Mixed layer: Mixed layer:
Local theories (K-theory): < wLocal theories (K-theory): < w’’aa’’ >= - Ka dA/dz >= - Ka dA/dz always down-gradientalways down-gradient
Non-local theories: organized eddies filling the Non-local theories: organized eddies filling the entire BL, could be counter-gradiententire BL, could be counter-gradient
Hadley circulation and cloud typesHadley circulation and cloud types
Deep convection Trade wind cumulus Stratocumulus
HL
Tropics
Walker circulation and cloud typesWalker circulation and cloud types
Deep convection Trade wind cumulus Stratocumulus
Stratocumulus clouds Stratocumulus clouds
Stratocumulus, from the Latin Stratocumulus, from the Latin stratus meaning ‘‘layer,’’ and stratus meaning ‘‘layer,’’ and cumulus meaning ‘‘heap,’’ is a cumulus meaning ‘‘heap,’’ is a genus of low clouds composed genus of low clouds composed of an ensemble of individual of an ensemble of individual convective elements that convective elements that together assume a layered together assume a layered form. form.
The cloud top pressure is The cloud top pressure is defined to be between 680-defined to be between 680-1000 mb (surface-3 km) in the 1000 mb (surface-3 km) in the ISCCP cloud classification ISCCP cloud classification schemescheme
Video: StratocumulusVideo: Stratocumulus
https://www.youtube.com/watch?v=4dBBTz8xKps https://www.youtube.com/watch?v=4dBBTz8xKps
Satellite image of stratocumulus cloudsSatellite image of stratocumulus clouds
Global distribution of stratocumulus cloudsGlobal distribution of stratocumulus clouds
Do we notice any patterns?Do we notice any patterns? From Wood (2012)
Thickness of stratocumulus cloudsThickness of stratocumulus clouds
Generally less than 600 mGenerally less than 600 m From Wood (2012)
Clouds are both good reflectors Clouds are both good reflectors of solar radiation (cooling effect) of solar radiation (cooling effect) and good absorbers of earth and good absorbers of earth emitted longwave radiation emitted longwave radiation (warming effect). (warming effect).
The net effect (cooling or The net effect (cooling or warming) depends on the type warming) depends on the type of cloudof cloud
In a changing climate, increases In a changing climate, increases in some types of clouds would in some types of clouds would promote warming, while promote warming, while increases in others would cause increases in others would cause coolingcooling
Climate models have difficulties Climate models have difficulties in simulating cloudsin simulating clouds
Conclusion: Clouds cause the Conclusion: Clouds cause the largest uncertainty in model largest uncertainty in model simulations of future climate. simulations of future climate.
Why do clouds constitute a wildcard for climate change?
Strongerwarming effect
Stronger cooling effect
Low cloud feedback is the leading component Low cloud feedback is the leading component of the total cloud feedback of the total cloud feedback
From Soden (2011)
The model scatter in total cloud feedback is The model scatter in total cloud feedback is mainly caused by scatter in low cloud feedbackmainly caused by scatter in low cloud feedback
From Soden (2011)
Recent field experimentsRecent field experiments
Video: VOCALS field experimentVideo: VOCALS field experiment
http://www.youtube.com/watch?http://www.youtube.com/watch?v=r9Mwz0HNxaQ&list=PL28B8C70D55751A08&index=1v=r9Mwz0HNxaQ&list=PL28B8C70D55751A08&index=13 3
Vertical structure and formation mechanism of Vertical structure and formation mechanism of stratocumulus-topped boundary layer (STBL)stratocumulus-topped boundary layer (STBL)
Intense longwave Intense longwave radiative cooling at radiative cooling at cloud top drives eddies cloud top drives eddies in BLin BL
Eddies pick up Eddies pick up moisture and maintain moisture and maintain cloudcloud
Eddies also entrain Eddies also entrain warm, dry air from warm, dry air from above the inversionabove the inversion
Entrainment lifts the Entrainment lifts the cloud, large-scale cloud, large-scale subsidence lowers itsubsidence lowers it
(From Lin et al. 2014)
Vertical structure of STBLVertical structure of STBL
Cloud base
Cloud top
Dots: observation; dotted line: expected for a well-mixed BL
Modeling of STBLModeling of STBL
Three generations of BL modelsThree generations of BL modelsLocal (good for stable BL)Local (good for stable BL)Non-local forced by surface heating (good for convective Non-local forced by surface heating (good for convective BL without clouds)BL without clouds)Non-local forced by surface heating and cloud-top cooling Non-local forced by surface heating and cloud-top cooling (good for STBL)(good for STBL)
Simulation by IPCC AR5 modelsSimulation by IPCC AR5 models
Model simulated low cloud coverModel simulated low cloud cover
Observation
Model simulated cloud vertical structureModel simulated cloud vertical structure
Observation
Model simulated cloud feedbackModel simulated cloud feedback
Observation
SummarySummary
Definition of stratocumulus cloudsDefinition of stratocumulus clouds Global distribution Global distribution Importance for global warmingImportance for global warming Vertical structure and formation mechanism of Vertical structure and formation mechanism of
STBLSTBL Modeling of STBLModeling of STBL :: non-local forced by surface non-local forced by surface
heating and cloud-top coolingheating and cloud-top cooling
Works citedWorks cited http://capita.wustl.edu/capita/datasets/modis/globfused/http://capita.wustl.edu/capita/datasets/modis/globfused/
glob3d.html glob3d.html http://www.ssec.wisc.edu/sos/wvsst/wvsst.html http://www.ssec.wisc.edu/sos/wvsst/wvsst.html http://www.arm.gov/news/facility/post/1025http://www.arm.gov/news/facility/post/1025 http://apollo.lsc.vsc.edu/classes/met130/notes/http://apollo.lsc.vsc.edu/classes/met130/notes/
chapter4/es_temp.html chapter4/es_temp.html http://www.windows2universe.org/earth/Atmosphere/http://www.windows2universe.org/earth/Atmosphere/
clouds/stratocumulus.html clouds/stratocumulus.html