stephan de roode and johan van der dussen delft university of technology , netherlands

The EUCLIPSE/GCSS model intercomparison study of a

stratocumulus to cumulus cloud transition as observed during

ASTEX

Stephan de Roode and Johan van der Dussen

Delft University of Technology, Netherlands

http://www.euclipse.nl/

Current boundary-layer cloud model intercomparison

studies for large-eddy simulation and single-column

models

CGILS (steady state solutions of stratocumulus and cumulus)

- aim: understand cloud-climate feedback for SST=+2K

Lagrangian cloud transitions (SST increases along the trajectory)

- ASTEX (aircraft) and composite cases (cloud fraction from satellite)

- aim: understand and validate modeling results of cloud regime transition

This talk:

- Subsidence, entrainment, and cloud layer depth evolution

ASTEX transition: Large-scale divergence

difficult to quantify from the ECMWF model

€

∂z inv

∂t= wsubs

−Div⋅zinv

{ +went

Large-scale divergence: is it important for the stratocumulus-to-cumulus transition?

Mean state evolution: liquid water potential

temperature

285 290 295 300 305 3100

1000

2000

3000

θL ( )K

285 290 295 300 305 310

θL ( )K

290 295 300 305 310

θL ( )K

290 295 300 305 310 315

θL ( )K

t = 3 hr t = 8 hr t = 20 hr t = 36 hr

Divergence from ERA-40 (changing sign)

Divergence constant (5×10-6 s-1)

t > 20 hr : boundary height differs considerably

ASTEX Lagrangian: Dutch Atmospheric Large-Eddy Simulation

ResultsDivergence from ERA-40 (changing sign)

Divergence constant (5×10-6 s-1)

cloud break up for case with larger divergence

Entrainment of dry and warm air from above the inversion:

cloud top height (ztop) rises

cloud base height (zbase) rises

Cloud deepening:

€

∂zcld

∂t=

∂z top −zbase

∂t> 0

(g/kg)

z (m

) qsat

cloud base

qT

cloud top

Cloud layer depth evolution

€

zcld = fct qT ,qsat T( )( )

(g/kg)

z (m

)

qsat

cloud base rises

qT

cloud top rises

Specific humidityExample: entrainment drying

€


(g/kg)

z (m

) qsat

lower cloud base height

qT

Example: longwave radiative cooling,saturation specific humidity decreases

Randall: in some cases cloud layer depth can increase by entrainment only

This presentation: consider all heat and moisture fluxes

€


Cloud base and top height evolution for a well-mixed cloud layer

€

∂zcld

∂t= we +w −

weΔqT +w' qT 'z=zb−ΔSqT

−L vqs

RvT2weΔθL +w' θL 'z=zb

−ΔFrad / ρcp( )[ ]

z i −zb( )∂qs

∂z

⎛ ⎝ ⎜

⎞ ⎠ ⎟

Rad cooling –Cloud thickening

Cloud deepening by entrainment

Moistening –Cloud thickening

Drizzle –Cloud thinning

Entrainment drying –Cloud thinning

Entrainment warming –Cloud thinning

Subsidence - Cloud thinning

€

∂z i

∂t= we +w

€

zcld = z top −zbase

Cloud base and top height evolution for ASTEX as a function of the entrainment rate

cp<w' L'>zbase = 11 W/m2 , Lv<w'qT'>zbase = 60 W/m2

L = 5 K , qT = -1.1 g/kg LW = 74 W/m2

Div = 5 x 10-6 s-1

zbase = 300 m , ztop= 600 m

-150

-100

-50

0

50

100

150

0 0.5 1 1.5 2

d/dt ztop

(m/h)

entrainment rate (cm/s)



L = 5 K , qT = -1.1 g/kg LW = 74 W/m2

Div = 5 x 10-6 s-1


-150

-100

-50

0

50

100

150

0 0.5 1 1.5 2

d/dt ztop

d/dt zbase

(m/h)



-150

-100

-50

0

50

100

150

0 0.5 1 1.5 2

d/dt ztop

d/dt zbase

d/dt zcld

(m/h)


cloud layer thickness growths for went < 1.9 cm/s

Example: DYCOMS II RF01


L = 8.5 K , qT = -7.5 g/kg LW = 74 W/m2

Div = 5 x 10-6 s-1


For went > 0.4 cm/s,

cloud layer rapidly thins

-300

-200

-100

0

100

200

300

0 0.5 1 1.5 2

d/dt zbase

d/dt ztop

d/dt zcld

(m/h)


Steady-state cloud layer depth: equilibrium entrainment

rates


LW = 70 W/m2

Div = 5 x 10-6 s-1

zbase = 300 m , zi= 650 m

equilibrium entrainment rate

ASTEX

DYCOMS II RF01


rates


LW = 70 W/m2

Div = 5 x 10-6 s-1

zbase = 300 m , zi= 650 m


ASTEX

DYCOMS II RF01

entrainment rate (Nicholls and Turton, 1986)


rates


LW = 70 W/m2

Div = 5 x 10-6 s-1

zbase = 500 m , zi= 800 m


ASTEX

DYCOMS II RF01Cloud top height decreases(entrainment smaller than subsidence)

Conclusions

Large-scale divergence is important for the evolution of the stratocumulus cloud deck

In a large part of the regime where the cloud layer depth is in a steady state the cloud

top increases with time

Acknowledgements

The EUCLIPSE project is an international effort, funded under Framework Program 7 of the European Union

NWO-NCF has sponsored the use of the Dutch Supercomputer "Huygens"

Irina Sandu (MPI), Chris Bretherton (UW), and Adrian Lock (UKMO) are thanked for their help setting up

the ASTEX intercomparison case

stephan de roode and johan van der dussen delft university of technology , netherlands

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