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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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/


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

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

ASTEX transition: Large-scale divergence difficult to quantify from the ECMWF model

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


Mean state evolution liquid water potential temperature

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


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

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


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

Entrainment of dry and warm air from above the inversion:

 cloud top height (ztop) rises

 cloud base height (zbase) rises

Cloud deepening:


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

Cloud layer depth evolution

qT

cloud top

qsat

z (m)

cloud base

(g/kg)


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

Specific humidity

Example: entrainment drying

qT

qsat

cloud top rises

z (m)

cloud base rises

(g/kg)


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

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)


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

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


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

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


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

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


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

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


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

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

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 rates1

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 rates2

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

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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