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lameubiquityMechanics

Feb 21, 2014 (3 years and 5 months ago)

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Stratocumulus


Theory and Model


Irina Sandu and Martin Kohler



Motivation




Characterisation




Governing processes




Parameterization




Remaining Challenges




Summary

Stratocumulus
-

Motivation



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 CO
2

doubling (Randall et al. 1984).




Coupled models have large biases in stratocumulus extent and SSTs.

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%)

Stratocumulus over ocean

MODIS

true color

(1540UTC, 20 Oct. 2001)

EPIC

16
-
21Oct2001

Peru

SST & surface wind

Chile

Stratocumulus … over Land

Stratocumulus stratiformis opacus cumulogenitus

Stratocumulus stratiformis translucidus

Bernhard M
ü
hr,
www.wolkenatlas.de

Yellowstone,USA

Germany

Stratocumulus …Macroscales

visibleearth.nasa.gov

closed cells

with diameter:

10
-
15km





MISR sensor

on Terra satellite



Condensation

w

Drizzle
evaporation

CCN activation

Cloud droplet
sedimentation

D

40 µm

D

40 µm

n(D)

n(D)

D
dt
dD
1

Mixing

D

n(D)

Drizzle
sedimentation

Collection

D

40 µm

n(D)

ECS

40 µm

Stratocumulus ….Microscales or Cloud microphysics

Drizzle drops


~ 40 µm <D < 100
-
500 µm

Precipitation embryos:
D ~ 40 µm


Cloud droplets :

~ 1 µm < D < ~ 40 µm

CCN :

D ~ 0.01
-
10 µm

Entrainement of
warm and dry air

Subsidence

θ
l

(K)

r
t

(g kg
-
1
)

STBL

h
STBL

(m)

LWP

(kg m
-
2
)

(
Liquid Water Path
)

r
sat


(g kg
-
1
)

Characterisation of a STBL

0.2g/kg

20g/kg

Subsidence

θ
l

(K)

r
t

(g kg
-
1
)

h
STBL

(m)

STBL

LWP

(kg m
-
2
)

(
Liquid Water Path
)

Such a cloudy system is extremely sensitive to thermodynamical conditions

r
sat


(g kg
-
1
)

Characterisation of a STBL

0.2g/kg

20g/kg

Subsidence

θ
l

(K)

r
t

(g kg
-
1
)

h
STBL

(m)

STBL

LWP

(kg m
-
2
)

(
Liquid Water Path
)

r
sat


(g kg
-
1
)

Such a cloudy system is extremely sensitive to thermodynamical conditions

Characterisation of a STBL

Subsidence

θ
l

(K)

r
t

(kg kg
-
1
)

h
STBL

(m)

STBL

LWP

(kg m
-
2
)

(
Liquid Water Path
)

r
sat


(kg kg
-
1
)

Characterisation of a STBL

Such a cloudy system is extremely sensitive to thermodynamical conditions

0.6K

1D representation of the STBL (I)


Adiabatic model : no exchange with the exterior

L

T
q
Conserved
variables

0

c
N
Droplet
concentration

c
q
Cloud water
content

0

h
C
LWC
w

)
1
(
T
C
r
L
p
c
v
l




c
v
t
q
q
q


1D representation of the STBL (II)


Non adiabatic system

LE

H

LW

LW

SW

Cloud top
entrainment



Influenced by:




radiation




entrainment




surface fluxes




drizzle

drizzle

Radiative transfer


LW radiation


T
FT

<< T
cloud

Strong cooling

T
cloud



T
sea


Slight warming

Strong
downdrafts

Compensating

updrafts


SW radiation




partially compensates LW


cooling




Stabilises the cloud layer




Slight inversion at the cloud


base




The cloud water content


diminishes


v

decoupling



0



v
w

Cloud top entrainment


Entrained air cools



Cloudy air warms, a part of
liquid water evaporates




Cloud top
entrainment

LW cooling

turbulence




LWC at cloud top inferior to adiabatic case




Growth of the STBL




Warming and drying of the STBL


Cloud top entrainment

Colder than
cloudy air

More entrainment

Stronger heating

Rapid cloud dissipation

Cloud top entrainment instability

Positive feedback

=



growth of the STBL, warming and heating, partially compensates the
radiative cooling, modifies cloud droplet distribution.

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)







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

Precipitation flux


drizzle

Cumulus

LE

LE


v

stable


v

unstable

stable

decoupling


Under cloud evaporation affects the dynamics of the boundary layer

The real STBL (3D)

LE

H

LW

LW

SW

Cloud top
entrainment

1D

3D

Large scale
advection

Large scale
subsidence

Horizontal
advection

drizzle

What else

drizzle

21
/33



l

q
t

Courtesy of Bjorn Stevens (data from DYCOMS
-
II)

Warm, dry,subsiding free
-
troposphere

Entrainment warming, drying

Surface heat and
moisture fluxes

q
l

q
l,adiabatic

Night
-
time

Well
-
mixed

The cloud
deepens

LW radiative
cooling

The diurnal cycle of a non
-
precipitating stratocumulus

22
/33



l

q
t

Courtesy of Bjorn Stevens (data from DYCOMS
-
II)

Warm, dry, subsiding free
-
troposphere

Entrainment warming, drying

Surface heat and
moisture fluxes

q
l

q
l,adiabatic

LW radiative
cooling

Daytime

SW
absorption

The cloud thins

Decoupled

Stabilisation of the
subcloud layer

The diurnal cycle of a non
-
precipitating stratocumulus

Diurnal cycle during observed during FIRE
-
I experiment

Hignett, 1991 (data from FIRE
-
I)

Cloud top

Cloud base


8 LT 12 LT 16 LT 20LT 0LT



LWP

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

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 formation: 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

sfc
v
entr
v
s
w
s
w






2
.
0
1
4
10
3





m


LW
p
h
v
entr
v
F
c
dz
s
w
h
s
w









5
.
0
1
5
.
0
0
shallow

convection

dry diffusion

unification


an ED/MF approach

Combined mass flux/diffusion:


done

next

M
1

M

K

M
2

Shallow cumulus

Deep cumulus

Stratocumulus

dry BL

z
cb

z
i

z
i

K
bot

M

K
top

K
bot

K
bot

)
(
,













i
u
i
i
M
z
K
w
K
entr

cloud variability

parameterization choices


updraft 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 to represent the surface
driven diffusion


Ktop ~

F
LW
to represent the cloud top
driven diffusion



cloud top entrainment:




LW
p
sfc
v
entr
v
F
c
s
w
s
w









2
.
0


cloud cover:


total water variance equation








not yet



PBL
qt
z
u
qt
t
t
qt
h
w
z
w
z
q
q
w
t
2
'
2
2
2
















z
c
w
u




1
M
z
M
)
(






Mass
-
flux

K
-
diffusion

Results
:

Low cloud cover (new
-
old)

T511

time=10d

n=140

2001 & 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)

DJF

2004/5

DJF

2005/6

DJF

2007/8

DJF

2006/7

EDMF PBL

M
-
O diffusion

VOCALS field experiment off Chile

GOES12 10.8
µ
m

ECMWF 10.8
µ
m

12LT

0LT

Stratocumulus Parameterization Challenges



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


shall.

conv.

well

mixed

s
l

z

v
w
'
'

LW
R
w
q
w
'
'
l
w
'
'

entrainment

rad. cooling

P

P

N

stratocumulus

to
trade cumulus

transition criteria

EPIC, Oct 2001



EIS (Wood and Bretherton, 2006)



static stability:

θ
700hPa
-

θ
sfc
< 14K




buoyancy flux integral ratio:

N
/
P
> 0.1

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