Stratocumulus – Theory and Model Martin Köhler (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. 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.

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 CO 2 doubling (Randall et al. 1984). Coupled models have large biases in stratocumulus extend and SSTs.

DEMETER CGCM Surface Temperature Bias [K] UKMO MPI Meteo France LODYC 4-6 month forecasts Aug/Sep/Oct ensemble members comparison to ERA-40 ECMWF

EPIC: Peruvian stratocumulus – model comparison EPIC obs NCAR CAM2.0 GFDL AM2.10 ECMWF 23r4 NCEP Bretherton et al, BAMS 2004 LWC [g/m 3 ] q v [g/kg] [K]

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

Stratocumulus …Microphysics LWP [g/m 2 ] r e [ m] N d [cm -3 ] true color MODIS

Stratocumulus … over Oceans Los Lagos, Chile Bernhard Mühr, Stratocumulus lenticularis

Stratocumulus … over Land Stratocumulus stratiformis opacus cumulogenitusStratocumulus stratiformis translucidus Bernhard Mühr, Yellowstone,USA Germany

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

Large-eddy Simulation of Stratocumulus Duynkerke et al. 2003

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

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

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

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 { }:

Stratocumulus Deardorff 1976, QJRMS, 102, Stevens 2000, Geoph. Res. Letters, 27, Radiative cooling Entrainment

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

Old ECMWF Stratocumulus Parameterization – a web 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 4 th point shallow convection dry diffusion

unification – an ED/MF approach Combined mass flux/diffusion: done next M1M1 M K M2M2 Shallow cumulusDeep cumulus Stratocumulus dry BL z cb zizi zizi K bot M K top K bot

two box ED/MF decomposition a M M-fluxenv. fluxsub-core flux Siebesma & Cuijpers, 1995

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

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

Results: Low cloud cover (new-old) T511 time=10d n= & 2004 old: CY28R4new PBL

Results: EPIC column extracted from 3D forecasts

Winter Cloud Cover 36h forecast versus SYNOP observation (high pressure days over central Europe) DJF 2004/5 DJF 2005/6 DJF 2007/8 DJF 2006/7 EDMF PBL M-O diffusion

VOCALS field experiment off Chile GOES µmECMWF 10.8µm 12LT 0LT

Stratocumulus Parameterization Challenges 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) shall. conv. well mixed slsl z

Stratus Cloud Amount vs. Stability Klein & Hartmann 1993

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

Cloud Top Entrainment Instability – Observations Duynkerke et al CTEI criteria

entrainment rad. cooling P P N stratocumulus to trade cumulus transition criteria Chile EPIC, Oct 2001 static stability: θ 700hPa - θ sfc < 20K cloud top entrainment instability: buoyancy flux integral ratio: N/P > 0.1

Stratocumulus & Aerosols Why?

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

preVOCA: VOCALS at Oct 2006 – Low Cloud