Stratocumulus-topped Boundary Layer

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Presentation transcript:

Stratocumulus-topped Boundary Layer

Review of last lecture The turbulent closure problem: Number of unknowns > Number of equations Surface layer: related to gradient Mixed layer: Local theories (K-theory): < w’a’ >= - Ka dA/dz always down-gradient Non-local theories: organized eddies filling the entire BL, could be counter-gradient

Hadley circulation and cloud types Tropics L H Deep convection Trade wind cumulus Stratocumulus

Walker circulation and cloud types Deep convection Trade wind cumulus Stratocumulus

Stratocumulus clouds Stratocumulus, from the Latin stratus meaning ‘‘layer,’’ and cumulus meaning ‘‘heap,’’ is a genus of low clouds composed of an ensemble of individual convective elements that together assume a layered form. The cloud top pressure is defined to be between 680-1000 mb (surface-3 km) in the ISCCP cloud classification scheme

Video: Stratocumulus https://www.youtube.com/watch?v=4dBBTz8xKps

Satellite image of stratocumulus clouds

Global distribution of stratocumulus clouds Do we notice any patterns? From Wood (2012)

Thickness of stratocumulus clouds Generally less than 600 m From Wood (2012)

Why do clouds constitute a wildcard for climate change? Clouds are both good reflectors of solar radiation (cooling effect) and good absorbers of earth emitted longwave radiation (warming effect). The net effect (cooling or warming) depends on the type of cloud In a changing climate, increases in some types of clouds would promote warming, while increases in others would cause cooling Climate models have difficulties in simulating clouds Conclusion: Clouds cause the largest uncertainty in model simulations of future climate. Stronger warming effect Stronger cooling effect

Low cloud feedback is the leading component of the total cloud feedback From Soden (2011)

The model scatter in total cloud feedback is mainly caused by scatter in low cloud feedback From Soden (2011)

Recent field experiments

Video: VOCALS field experiment http://www.youtube.com/watch?v=r9Mwz0HNxaQ&list=PL28B8C70D55751A08&index=13

Vertical structure and formation mechanism of stratocumulus-topped boundary layer (STBL) Intense longwave radiative cooling at cloud top drives eddies in BL Eddies pick up moisture and maintain cloud Eddies also entrain warm, dry air from above the inversion Entrainment lifts the cloud, large-scale subsidence lowers it (From Lin et al. 2014)

Vertical structure of STBL Cloud top Cloud base Dots: observation; dotted line: expected for a well-mixed BL

Modeling of STBL Three generations of BL models Local (good for stable BL) Non-local forced by surface heating (good for convective BL without clouds) Non-local forced by surface heating and cloud-top cooling (good for STBL)

Simulation by IPCC AR5 models

Model simulated low cloud cover Observation

Model simulated cloud vertical structure Observation

Model simulated cloud feedback Observation

Summary Definition of stratocumulus clouds Global distribution Importance for global warming Vertical structure and formation mechanism of STBL Modeling of STBL： non-local forced by surface heating and cloud-top cooling

Works cited http://capita.wustl.edu/capita/datasets/modis/globfused/glob3d.html http://www.ssec.wisc.edu/sos/wvsst/wvsst.html http://www.arm.gov/news/facility/post/1025 http://apollo.lsc.vsc.edu/classes/met130/notes/chapter4/es_temp.html http://www.windows2universe.org/earth/Atmosphere/clouds/stratocumulus.html