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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 CO2 doubling (Randall et al. 1984).
• Coupled models have large biases in stratocumulus extend and SSTs.
DEMETER CGCM Surface Temperature Bias [K]
-6.0 -5.0 -4.0 -3.0 -2.0 -1.0 1.0 2.0 3.0 4.0 5.0 6.0
Surface Temperature [oC]
Forecast start dates: 05/1987-1996, FC period: months 4-6 (ASO), ens: 0-8
Bias: EXP(ECMWF_assim) regarding ERA-40 reanalysis
-6.0 -5.0 -4.0 -3.0 -2.0 -1.0 1.0 2.0 3.0 4.0 5.0 6.0
Surface Temperature [oC]
Forecast start dates: 05/1987-1995, FC period: months 4-6 (ASO), ens: 0-8
Bias: EXP(UKMO) regarding ERA-40 reanalysis
-6.0 -5.0 -4.0 -3.0 -2.0 -1.0 1.0 2.0 3.0 4.0 5.0 6.0
Surface Temperature [oC]
Forecast start dates: 05/1987-1992, FC period: months 4-6 (ASO), ens: 0-8
Bias: EXP(MPI) regarding ERA-40 reanalysis
-6.0 -5.0 -4.0 -3.0 -2.0 -1.0 1.0 2.0 3.0 4.0 5.0 6.0
Surface Temperature [oC]
Forecast start dates: 05/1987-1994, FC period: months 4-6 (ASO), ens: 0-8
Bias: EXP(CNRM) regarding ERA-40 reanalysis
-6.0 -5.0 -4.0 -3.0 -2.0 -1.0 1.0 2.0 3.0 4.0 5.0 6.0
Surface Temperature [oC]
Forecast start dates: 05/1987-1994, FC period: months 4-6 (ASO), ens: 0-8
Bias: EXP(LODYC) regarding ERA-40 reanalysis
UKMO
MPI
MeteoFrance
LODYC
4-6 month forecasts Aug/Sep/Oct 1987-1996
9 ensemble memberscomparison to ERA-40
www.ecmwf.int/research/demeter
ECMWF
EPIC: Peruvian stratocumulus – model comparison
EPIC obs
NCAR CAM2.0
GFDL AM2.10
ECMWF 23r4
NCEP
Bretherton et al, BAMS 2004
LWC [g/m3] qv [g/kg] [K]
Stratocumulus … from Satellite
MODIStrue color
(1540UTC, 20 Oct. 2001)
EPIC16-21Oct2001
Peru
SST & surface wind
Chile
Stratocumulus …Microphysics
LWP [g/m2] re [m] Nd [cm-3]
true colorMODIS
Stratocumulus … over OceansLos Lagos, Chile
Bernhard Mühr, www.wolkenatlas.deStratocumulus lenticularis
Stratocumulus … over Land
Stratocumulus stratiformis opacus cumulogenitusStratocumulus stratiformis translucidusBernhard Mühr, www.wolkenatlas.de
Yellowstone,USAGermany
Stratocumulus …Macroscales
visibleearth.nasa.gov
closed cellswith diameter:
10-15km
MISR sensoron 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 PacificArctic
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:
lp
l qTc
t v l, v sat t sat
t v t sat
q q q q q if q q
q q if q q
and total water:
Derive virt. potent. temp. flux {
3 ,37.0:15
'' '' ''
''61.0'' ''
21
21
Cat
cloudinqwww
cloudsubqwww
o
tlv
tlv
)61.01( lvv qq }:
Stratocumulus
Deardorff 1976, QJRMS, 102, 563-582.Stevens 2000, Geoph. Res. Letters, 27, 2557-2560.
00
ltqw ''
drizzle
lq
tq
lw ''vw '' tq
lLWR
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 4th point
sfc
v
entr
v swsw 2.0
14103 m
LWp
h
v
entr
v Fcdzswh
sw 5.01
5.00
shallow convection
dry diffusion
unification – an ED/MF approach
Combined mass flux/diffusion:
done
next
M1M
K
M2
Shallow cumulus Deep cumulusStratocumulusdry BL
zcb
zi
zi
KbotM
Ktop
KbotKbot
)( ,
iui
iMz
Kw
two box ED/MF decomposition
u
uuu e
eee
e
e
u
u aa )1(
a
)()1( uu
e
e
u
u wawawaw
M
M-fluxenv. fluxsub-core flux
zK
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 (wup= 0)
• mass flux:
• diffusion:• K-profile: surface and cloud top driven diffusion
• cloud top entrainment:
LWp
sfc
v
entr
v Fcswsw 2.0
• cloud cover:• total water variance equation
not yet
PBL
qt
z
uqttt
qt
h
w
z
w
z
qqw
t
2'22
2
zc
wu
1
Mz
M)(
Mass-flux K-diffusion
Results: Low cloud cover (new-old)
T511 time=10d
n=1402001 & 2004
old: CY28R4 new PBL
Results: EPIC column extracted from 3D forecasts
Winter Cloud Cover 36h forecast versus SYNOP observation(high pressure days over central Europe)
DJF2004/5
DJF2005/6
DJF2007/8
DJF2006/7
ED
MF
PB
L
M-O
diff
usi
on
VOCALS field experiment off Chile
GOES12 10.8µm ECMWF 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
sl
z
Stratus Cloud Amount vs. Stability
Klein & Hartmann 1993
Cloud Top Entrainment Instability (CTEI) - Theory
Randall 1980Deardorff 1980
0 tp qTch
p
sat
p
p
p
T
q
c
L
TL
c
LqgzTch
1
608.11
Randall - Deardorff criteria
Cloud Top Entrainment Instability – Observations
Duynkerke et al. 2003
CTEI criteria
vw ''
LWR
wqw '' lw ''
entrainmentrad. cooling
P
P
N
stratocumulus to trade cumulus
transition
criteria
ChileEPIC, Oct 2001
• static stability: θ700hPa- θsfc < 20K
• cloud top entrainment instability:
• buoyancy flux integral ratio:
N/P > 0.1
03.04.0 tl qLs
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