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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
Stratocumulus – Theory and ModelIrina Sandu

  • Introduction

  • Characterisation

  • Governing processes

  • Parameterization

  • Remaining Challenges

  • Summary


Boundary layer clouds over oceans

warm

EQ

cold

SST & surface wind

Shallow cumulus

stratocumulus


Stratocumulus why are they important
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)


Stratocumulus over land
Stratocumulus … over Land

Germany

Yellowstone,USA

Stratocumulus stratiformis translucidus

Stratocumulus stratiformis opacus cumulogenitus

Bernhard Mühr, www.wolkenatlas.de



Stratocumulus microscales or cloud microphysics
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


Characterisation of a stbl
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


Characterisation of a stbl1
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


Characterisation of a stbl2
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


Characterisation of a stbl3
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


Processing controlling cloud evolution
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)


Radiative transfer
Radiative transfer

Longwave radiation

TFT << Tcloud

cooling

Tcloud Tsea

warming

Strong downdrafts

At cloud top

Compensating

updrafts


Radiative transfer1
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


Cloud top entrainment
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


Cloud top entrainment1
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.


Surface fluxes
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
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 flux1
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


The diurnal cycle of a non precipitating stratocumulus
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)


The diurnal cycle of a non precipitating stratocumulus1
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)


Diurnal cycle during observed during fire i experiment
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)


Stratocumulus parameterization ingredients
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
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


Unification an ed mf approach
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


Parameterization choices

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


Results low cloud cover edmf old
Results: Low cloud cover (EDMF-old)

T511

time=10d

n=140

2001 & 2004

old: CY28R4

EDMF PBL



Vocals field experiment off chile
VOCALS field experiment off Chile

GOES12 10.8µm

ECMWF 10.8µm

12LT

0LT


Stratocumulus parameterization challenges
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


Stratocumulus to trade cumulus transition criteria
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


Summary
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



Simple conceptual model of the transition

(Bretherton et al., 1992)

…………….

Sandu, Stevens, Pincus, ACP, 2010

Sandu and Stevens, JAS, 2012


Main processes controlling the cloud evolution

rain

  • Inversion strength

  • Large-scale subsidence

  • Rain formation

  • Rain evaporation


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