1. Stratocumulus-topped Boundary Layer

2. 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

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

4. Walker circulation and cloud types
Deep convection
Trade wind cumulus
Stratocumulus

5. 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 mb (surface-3 km) in the ISCCP cloud classification scheme

6. Video: Stratocumulus

7. Satellite image of stratocumulus clouds

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

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

10. 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

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

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

13. Recent field experiments

14. Video: VOCALS field experiment

15. 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)

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

17. 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)

18. Simulation by IPCC AR5 models

19. Model simulated low cloud cover
Observation

20. Model simulated cloud vertical structure
Observation

21. Model simulated cloud feedback
Observation

22. 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

23. Works cited