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Stratocumulus – Theory and Model Martin K ö hler (room 108)

Stratocumulus – Theory and Model Martin K ö hler (room 108). Definition Phenomenology Theory Parameterization Remaining Challenges Summary. Stratocumulus - Definition. PBL clouds – visual definition Fog: Result of moist air in contact with cold surface.

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Stratocumulus – Theory and Model Martin K ö hler (room 108)

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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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