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