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Stratocumulus – Theory and Model Irina Sandu

Stratocumulus – Theory and Model Irina Sandu. Introduction Characterisation Governing processes Parameterization Remaining Challenges Summary. Boundary layer clouds over oceans. warm. EQ. cold. SST & surface wind. Shallow cumulus. stratocumulus.

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Stratocumulus – Theory and Model Irina Sandu

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  1. Stratocumulus – Theory and ModelIrina Sandu • Introduction • Characterisation • Governing processes • Parameterization • Remaining Challenges • Summary

  2. Boundary layer clouds over oceans warm EQ cold SST & surface wind Shallow cumulus stratocumulus

  3. Stratocumulus – Why are they important? • Cover in (annual) mean 29% of the planet (Klein and Hartmann, 1993) • 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 extent and SSTs. Wood, 2012,based on Han and Waren, 2007 Stratocumulus cloud cover (annual mean)

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

  5. Stratocumulus …Macroscales Wood, 2012

  6. Stratocumulus ….Microscales or Cloud microphysics Drizzle drops ~ 40 µm <D < 100-500 µm Precipitation embryos: D ~ 40 µm CCN : D ~ 0.01-10 µm Cloud droplets : ~ 1 µm < D < ~ 40 µm Entrainement of warm and dry air Mixing n(D) Cloud droplet sedimentation D 40 µm Collection Condensation n(D) ECS n(D) D 40 µm D 40 µm Drizzle sedimentation CCN activation n(D) w Drizzle evaporation D 40 µm

  7. Characterisation of a STBL hSTBL (m) LWP (kg m-2) (Liquid Water Path) rt (g kg-1) rsat θl(K) (g kg-1) Subsidence 0.2g/kg STBL 20g/kg

  8. Characterisation of a STBL Subsidence hSTBL (m) STBL 0.2g/kg LWP (kg m-2) 20g/kg (Liquid Water Path) rt (g kg-1) rsat θl(K) (g kg-1) Such a cloudy system is extremely sensitive to thermodynamical conditions

  9. Characterisation of a STBL Subsidence hSTBL (m) STBL LWP (kg m-2) (Liquid Water Path) rt (g kg-1) rsat θl(K) (g kg-1) Such a cloudy system is extremely sensitive to thermodynamical conditions

  10. Characterisation of a STBL Subsidence hSTBL (m) STBL LWP (kg m-2) 0.6K (Liquid Water Path) rt (kg kg-1) rsat θl(K) (kg kg-1) Such a cloudy system is extremely sensitive to thermodynamical conditions

  11. Processing controlling cloud evolution LW radiative cooling Warm, dry,subsiding free-troposphere l qt ql Entrainment warming, drying ql,adiabatic SW absorption drizzle Surface heat and moisture fluxes Courtesy of Bjorn Stevens (data from DYCOMS-II)

  12. Radiative transfer Longwave radiation TFT << Tcloud cooling Tcloud Tsea warming Strong downdrafts At cloud top Compensating updrafts

  13. Radiative transfer v decoupling Shortwave radiation • partially compensates LW cooling • Stabilises the cloud layer • Slight inversion at the cloud base • The cloud water content diminishes

  14. Cloud top entrainment Cloud top entrainment LW cooling turbulence • Entrained air cools • Cloudy air warms, a part of liquid water evaporates • LWC at cloud top inferior to adiabatic case • Growth of the STBL • Warming and drying of the STBL

  15. Cloud top entrainment More entrainment Stronger heating Positive feedback = Colder than cloudy air Rapid cloud dissipation Cloud top entrainment instability • growth of the STBL, warming and heating, partially compensates the radiative cooling, modifies cloud droplet distribution.

  16. Surface fluxes • Sensible heat flux (H) • Important for maintaining turbulence in the under cloud layer • Latent heat flux • Vapour supply for the cloud layer • Role in the cloud break up (transition to shallow cumulus)

  17. Precipitation flux • Even if weak (1mm/day) important for STBL dynamics and energetics • Precipitation flux ~ 30 W/m2 (same as latent flux!) • Latent heat released during drizzle formation acts to weaken the vertical movements

  18. Precipitation flux v stable Scenario I LE LE v stable Scenario II decoupling Cumulus unstable • Under cloud evaporation affects the dynamics of the boundary layer drizzle

  19. The diurnal cycle of a non-precipitating stratocumulus Night-time LW radiative cooling The cloud deepens Well-mixed Warm, dry,subsiding free-troposphere l qt ql Entrainment warming, drying ql,adiabatic Surface heat and moisture fluxes Courtesy of Bjorn Stevens (data from DYCOMS-II)

  20. The diurnal cycle of a non-precipitating stratocumulus The cloud thins Decoupled Daytime Warm, dry, subsiding free-troposphere l qt ql Entrainment warming, drying LW radiative cooling ql,adiabatic SW absorption Stabilisation of the subcloud layer Surface heat and moisture fluxes Courtesy of Bjorn Stevens (data from DYCOMS-II)

  21. Diurnal cycle during observed during FIRE-I experiment Cloud top LWP Cloud base 8 LT 12 LT 16 LT 20LT 0LT Hignett, 1991 (data from FIRE-I)

  22. Stratocumulus parameterization - Ingredients • Strong mixing • Cloud top driven • Surface driven • Cloud top entrainment • function of cloud top radiative cooling and surface flux • Radiation interaction • Drizzle • Transition to trade cumulus • high/low cloud fraction

  23. 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 formation: 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

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

  25. updraft 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: • K-profile to represent the surface driven diffusion • Ktop ~ FLW to represent the cloud top driven diffusion • cloud top entrainment: cloud variability

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

  27. Results: EPIC column extracted from 3D forecasts

  28. VOCALS field experiment off Chile GOES12 10.8µm ECMWF 10.8µm 12LT 0LT

  29. Stratocumulus Parameterization Challenges z shall. conv. well mixed sl • cloud top entrainment • numerics • the scheme is active only if the boundary layer is unstable • drizzle • amount/evaporation • cloud regime (stratocumulus/trade cumulus) • open/closed cells • decoupling • interaction between solar warming and drizzle evaporative cooling

  30. stratocumulus to trade cumulus transition criteria entrainment rad. cooling P N P • EIS (Wood and Bretherton, 2006) • static stability: θ700hPa- θsfc < 14K • buoyancy flux integral ratio: N/P > 0.1 EPIC, Oct 2001

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

  32. The stratocumulus to cumulus transition NE Pacific warm EQ cold

  33. Simple conceptual model of the transition (Bretherton et al., 1992) ……………. Sandu, Stevens, Pincus, ACP, 2010 Sandu and Stevens, JAS, 2012

  34. Main processes controlling the cloud evolution rain • Inversion strength • Large-scale subsidence • Rain formation • Rain evaporation

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