The EUCLIPSE/GCSS model intercomparison study of a stratocumulus to cumulus cloud transition as obse...
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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/.

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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 DSST=+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

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


Divergence from ERA-40 (changing sign)

Divergence constant (5×10-6 s-1)

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

Mean state evolution: liquid water potential temperature

 t > 20 hr : boundary height differs considerably


ASTEX Lagrangian: Dutch Atmospheric Large-Eddy Simulation Results

Divergence from ERA-40 (changing sign)

Divergence constant (5×10-6 s-1)

we (cm/s)

LWP (g/m2)

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:


Cloud layer depth evolution

qT

cloud top

qsat

z (m)

cloud base

(g/kg)


Specific humidity

Example: entrainment drying

qT

qsat

cloud top rises

z (m)

cloud base rises

(g/kg)


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

qT

qsat

z (m)

lower cloud base height

(g/kg)


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

Entrainment warming –

Cloud thinning

Rad cooling –

Cloud thickening

Entrainment drying –

Cloud thinning

Moistening –

Cloud thickening

Cloud deepening

by entrainment

Drizzle –

Cloud thinning

Subsidence -

Cloud thinning


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

rcp<w'qL'>zbase= 11 W/m2, rLv<w'qT'>zbase = 60 W/m2

DqL= 5 K, DqT = -1.1 g/kg

DLW= 74 W/m2

Div = 5 x 10-6 s-1

zbase = 300 m, ztop= 600 m


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

rcp<w'qL'>zbase= 11 W/m2, rLv<w'qT'>zbase = 60 W/m2

DqL= 5 K, DqT = -1.1 g/kg

DLW= 74 W/m2

Div = 5 x 10-6 s-1

zbase = 300 m, ztop= 600 m


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

cloud layer thickness growths for went < 1.9 cm/s


Example: DYCOMS II RF01

For went > 0.4 cm/s,

cloud layer rapidly thins

rcp<w'qL'>zbase= 11 W/m2, rLv<w'qT'>zbase = 60 W/m2

DqL= 8.5 K, DqT = -7.5 g/kg

DLW= 74 W/m2

Div = 5 x 10-6 s-1

zbase = 500 m, ztop= 800 m


Steady-state cloud layer depth: equilibrium entrainment rates

equilibrium entrainment rate

ASTEX

DYCOMS II RF01

rcp<w'qL'>zbase= 11 W/m2, rLv<w'qT'>zbase = 80 W/m2

DLW= 70 W/m2

Div = 5 x 10-6 s-1

zbase = 300 m, zi= 650 m


Steady-state cloud layer depth: equilibrium entrainment rates

equilibrium entrainment rate

entrainment rate (Nicholls and Turton, 1986)

ASTEX

DYCOMS II RF01

rcp<w'qL'>zbase= 11 W/m2, rLv<w'qT'>zbase = 80 W/m2

DLW= 70 W/m2

Div = 5 x 10-6 s-1

zbase = 300 m, zi= 650 m


Steady-state cloud layer depth: equilibrium entrainment rates

equilibrium entrainment rate

ASTEX

DYCOMS II RF01

Cloud top height decreases

(entrainment smaller than subsidence)

rcp<w'qL'>zbase= 11 W/m2, rLv<w'qT'>zbase = 80 W/m2

DLW= 70 W/m2

Div = 5 x 10-6 s-1

zbase = 500 m, zi= 800 m


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


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