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Understanding Stratocumulus Clouds: Theory, Modeling, and Challenges

Delve into the complexities of stratocumulus clouds, from definition to parameterization, through visual examples and physical perspectives. Explore the motivation behind studying these clouds and the global implications they have on climate. Discover the importance of effective parameterization and the ongoing challenges faced in modeling these unique cloud formations.

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Understanding Stratocumulus Clouds: Theory, Modeling, and Challenges

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  1. Stratocumulus – Theory and ModelMartin Köhler(room 108) • Definition • Phenomenology • Theory • Parameterization • Remaining Challenges • Summary

  2. Stratocumulus - Definition PBL clouds – visual definition • Fog: Result of moist air in contact with cold surface. • Stratocumulus: Horizontally variable; near PBL top. • Stratus: Horizontally uniform; near PBL top. • Nimbostratus: Precipitating; deep. Stratocumulus-Topped Boundary Layer (STBL) physical definition Mixed layer PBL with cloud near top.

  3. Stratocumulus - Motivation • Cloud top albedo is 50-80% (in contrast to 7 % at ocean surface). • A 4% increase in global stratocumulus extend would offset 2-3K • global warming from CO2 doubling (Randall et al. 1984). • Coupled models have large biases in stratocumulus extend and SSTs.

  4. DEMETER CGCM Surface Temperature Bias [K] ECMWF UKMO MPI Meteo France 4-6 month forecasts Aug/Sep/Oct 1987-1996 9 ensemble members comparison to ERA-40 www.ecmwf.int/research/demeter LODYC

  5. EPIC: Peruvian stratocumulus – model comparison EPIC obs NCAR CAM2.0 GFDL AM2.10 ECMWF 23r4 NCEP qv[g/kg] q[K] LWC[g/m3] Bretherton et al, BAMS 2004

  6. Stratocumulus … from Satellite Peru EPIC 16-21Oct2001 Chile SST & surface wind MODIS true color (1540UTC, 20 Oct. 2001)

  7. Stratocumulus …Microphysics re [mm] LWP [g/m2] Nd [cm-3] MODIS true color

  8. Stratocumulus … over Oceans Los Lagos, Chile Stratocumulus lenticularis Bernhard Mühr, www.wolkenatlas.de

  9. Stratocumulus … over Land Germany Yellowstone,USA Stratocumulus stratiformis translucidus Stratocumulus stratiformis opacus cumulogenitus Bernhard Mühr, www.wolkenatlas.de

  10. Stratocumulus …Macroscales closed cells with diameter: 10-15km MISR sensor on Terra satellite visibleearth.nasa.gov

  11. Large-eddy Simulation of Stratocumulus Duynkerke et al. 2003

  12. stratocumulus … LES: DYCOMS II cloud vertical velocity simulation by Bjorn Stevens (1995) (dx=35m, dz=5m)

  13. Annual Stratus Cloud Amount Californian Arctic North Atlantic North Pacific Canarian China Australian Namibian Peruvian Circumpolar Ocean Surface based observations (mean=29%) Klein & Hartmann 1993

  14. Physical processes in marine stratocumulus • PBL: turbulent fluxes • cloud: condensation • aerosols: drizzle • radiation: destabilization • LS dynamics: subsidence • cloud top entrainment • decoupling Nieuwstadt & Duynkerke 1996

  15. Mixed layer perspective of stratocumulus Consider variables that are conserved for moist processes e.g. liquid water potential temperature: and total water: }: Derive virt. potent. temp. flux {

  16. Stratocumulus Entrainment Radiative cooling Deardorff 1976, QJRMS, 102, 563-582. Stevens 2000, Geoph. Res. Letters, 27, 2557-2560.

  17. PBL budget considerations: IMET-stratocumulus Heat budget: +40 +10 -80 +20 +10 [W/m2] Moisture budget: -3.0 +3.3 -0.3 [mm/day]

  18. Stratocumulus parameterization - Ingredients • Strong mixing • K-diffusion • mass-flux • well-mixed assumption • Cloud scheme • diagnostic • prognostic • Cloud top entrainment • Function of cloud top radiative cooling and surface flux • Radiation interaction • Transition to trade cumulus • high/low cloud fraction

  19. Old ECMWF Stratocumulus Parameterization – a web shallow convection • dry diffusion • variable: dry static energy and WV • PBL top entrainment: • shallow convection • closure: moist static energy equilibrium in sub-cloud layer • updraft entrainment/detrainment: • cloud creation: detrainment of cloud volume and cloud water • cloud • supersaturation removal into cloud • cloud top entrainment: • radiation • resolution: every 3 hours and every 4th point dry diffusion

  20. Schematic: Old and New PBL

  21. unification – an M/K approach zi K next done zcb zi M2 M1 M M Ktop Kbot Kbot Kbot Stratocumulus dry BL Shallow cumulus Deep cumulus Combined mass flux/diffusion:

  22. two box M/K decomposition a M sub-core flux env. flux M-flux Siebesma & Cuijpers, 1995

  23. BOMEX LES decomposition M-flux sub-core flux total flux env. flux M-flux covers 80% of flux Siebesma & Cuijpers, 1995

  24. updaft model: entrainment: , τ=500s, c=0.55 detrainment: 3·10-4 m-1 in cloud parcel determines PBL depth (wup= 0) mass flux: parameterization choices • cloud cover: • total water variance equation not yet Mass-flux K-diffusion • diffusion: • surface and cloud top driven diffusion • cloud top entrainment: cloud variability

  25. Results: Low cloud cover (new-old) T511 time=10d n=140 2001 & 2004 old: CY28R4 new PBL

  26. Results: EPIC column extracted from 3D forecasts

  27. Stratocumulus Parameterization Challenges z shall. conv. well mixed sl • cloud top entrainment • numerics • drizzle • droplets larger near PBL top critical cloud depth of ~300m • role of aerosols? • cloud regime (stratocumulus/trade cumulus) • CTEI • open/closed cells • decoupling • (due to solar and terrestrial heating/cooling • cloud cut of from moisture supply from below • thinning of cloud)

  28. Stratus Cloud Amount vs. Stability Klein & Hartmann 1993

  29. Cloud Top Entrainment Instability (CTEI) - Theory Randall - Deardorff criteria Randall 1980 Deardorff 1980

  30. Cloud Top Entrainment Instability – Observations CTEI criteria Duynkerke et al. 2003

  31. stratocumulus to trade cumulus transitioncriteria entrainment rad. cooling P N P • cloud thickness: • static stability: θ700hPa- θsfc < 20K • cloud top entrainment instability: EPIC, Oct 2001 Chile • buoyancy flux integral ratio: N/P > 0.1

  32. Stratocumulus & Aerosols Why?

  33. Summary • Stratocumulus: important • climate • land temperature • Stratocumulus: simple • horizontally uniform (cloud fraction ~100%) • vertically uniform (well-mixed) • Stratocumulus: difficult to parameterize • multiple processes • multiple scales

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