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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/. - PowerPoint PPT Presentation
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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 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
€
zcld = fct qT ,qsat T( )( )
(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
€
zcld = fct qT ,qsat T( )( )
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)
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
d/dt zbase
(m/h)
entrainment rate (cm/s)
Cloud base and top height evolution for ASTEX as a function of the entrainment rate
-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)
entrainment rate (cm/s)
cloud layer thickness growths for went < 1.9 cm/s
Example: DYCOMS II RF01
cp<w' L'>zbase = 11 W/m2 , Lv<w'qT'>zbase = 60 W/m2
L = 8.5 K , qT = -7.5 g/kg LW = 74 W/m2
Div = 5 x 10-6 s-1
zbase = 500 m , ztop= 800 m
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)
entrainment rate (cm/s)
Steady-state cloud layer depth: equilibrium entrainment
rates
cp<w' L'>zbase = 11 W/m2 , Lv<w'qT'>zbase = 80 W/m2
LW = 70 W/m2
Div = 5 x 10-6 s-1
zbase = 300 m , zi= 650 m
equilibrium entrainment rate
ASTEX
DYCOMS II RF01
Steady-state cloud layer depth: equilibrium entrainment
rates
cp<w' L'>zbase = 11 W/m2 , Lv<w'qT'>zbase = 80 W/m2
LW = 70 W/m2
Div = 5 x 10-6 s-1
zbase = 300 m , zi= 650 m
equilibrium entrainment rate
ASTEX
DYCOMS II RF01
entrainment rate (Nicholls and Turton, 1986)
Steady-state cloud layer depth: equilibrium entrainment
rates
cp<w' L'>zbase = 11 W/m2 , Lv<w'qT'>zbase = 80 W/m2
LW = 70 W/m2
Div = 5 x 10-6 s-1
zbase = 500 m , zi= 800 m
equilibrium entrainment rate
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
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