stephan de roode , irina sandu, johan van der dussen, pier siebesma, bjorn stevens,
DESCRIPTION
LES results of the EUCLIPSE Lagrangian transitions: Entrainment, decoupling and low cloud transitions. Stephan de Roode , Irina Sandu, Johan van der Dussen, Pier Siebesma, Bjorn Stevens, Andy Ackerman, Peter Blossey, and Adrian Lock. Entrainment. - PowerPoint PPT PresentationTRANSCRIPT
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LES results of the EUCLIPSE Lagrangian transitions:
Entrainment, decoupling and low cloud transitions
Stephan de Roode, Irina Sandu, Johan van der Dussen,
Pier Siebesma, Bjorn Stevens,
Andy Ackerman, Peter Blossey, and Adrian Lock
![Page 2: Stephan de Roode , Irina Sandu, Johan van der Dussen, Pier Siebesma, Bjorn Stevens,](https://reader030.vdocuments.site/reader030/viewer/2022032612/568133ef550346895d9ae080/html5/thumbnails/2.jpg)
Schematic of cumulus penetrating stratocumulus
by Bjorn Stevens (based on ATEX intercomparison)
Cloud layer
Entrainment
Boundary layer is decoupled
Subcloud layer dynamics scale like dry convective boundary layer
![Page 3: Stephan de Roode , Irina Sandu, Johan van der Dussen, Pier Siebesma, Bjorn Stevens,](https://reader030.vdocuments.site/reader030/viewer/2022032612/568133ef550346895d9ae080/html5/thumbnails/3.jpg)
Contents
Determining the degree of decoupling
Moisture and heat fluxes in the decoupled subcloud layer
Evolution of the subcloud layer state
Entrainment scaling
![Page 4: Stephan de Roode , Irina Sandu, Johan van der Dussen, Pier Siebesma, Bjorn Stevens,](https://reader030.vdocuments.site/reader030/viewer/2022032612/568133ef550346895d9ae080/html5/thumbnails/4.jpg)
Stratocumulus cloud layer rises with timeSlow increase in cumulus cloud base height
![Page 5: Stephan de Roode , Irina Sandu, Johan van der Dussen, Pier Siebesma, Bjorn Stevens,](https://reader030.vdocuments.site/reader030/viewer/2022032612/568133ef550346895d9ae080/html5/thumbnails/5.jpg)
Total water content for ASTEX
- binned mean + (all data) mean horizontal legs
![Page 6: Stephan de Roode , Irina Sandu, Johan van der Dussen, Pier Siebesma, Bjorn Stevens,](https://reader030.vdocuments.site/reader030/viewer/2022032612/568133ef550346895d9ae080/html5/thumbnails/6.jpg)
Bulk structure of a decoupled boundary layer
Park et al., 2004, Wood and Bretherton 2004
€
αq =qT z i
−( ) − qT,0
qT z i+
( ) − qT,0
αq = 0 if vertically well mixed
![Page 7: Stephan de Roode , Irina Sandu, Johan van der Dussen, Pier Siebesma, Bjorn Stevens,](https://reader030.vdocuments.site/reader030/viewer/2022032612/568133ef550346895d9ae080/html5/thumbnails/7.jpg)
Quantification of decoupling from observations
Wood and Bretherton 2004
larger αq for deeper boundary layers
€
αq =qT z i
−( ) − qT,0
qT z i+
( ) − qT,0
![Page 8: Stephan de Roode , Irina Sandu, Johan van der Dussen, Pier Siebesma, Bjorn Stevens,](https://reader030.vdocuments.site/reader030/viewer/2022032612/568133ef550346895d9ae080/html5/thumbnails/8.jpg)
Decoupling parameter αq consistent between LES models
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Example: Turbulent fluxes in ASTEX from DALES at t=23 hr
€
w'w' m2s−2( )
€
w'θv ' mKs-1( )
€
ρcpw'qT ' Wm-2( )
Next step: quantify buoyancy and total specific humidity flux at the cumulus base height
![Page 10: Stephan de Roode , Irina Sandu, Johan van der Dussen, Pier Siebesma, Bjorn Stevens,](https://reader030.vdocuments.site/reader030/viewer/2022032612/568133ef550346895d9ae080/html5/thumbnails/10.jpg)
Buoyancy flux ratio rv becomes similar to CBL
€
rθv=
w'θv 'cu base
w'θv 'sfc
CBL value -0.2
![Page 11: Stephan de Roode , Irina Sandu, Johan van der Dussen, Pier Siebesma, Bjorn Stevens,](https://reader030.vdocuments.site/reader030/viewer/2022032612/568133ef550346895d9ae080/html5/thumbnails/11.jpg)
Moisture flux ratio rq exhibits clear diurnal cycleMean value rq slightly smaller than unity
€
rq =w'qT '
cu base
w'qT 'sfc
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Virtual potential temperature evolution in the subcloud layer
h
€
w'θv '0 = cDUML θv,0 − θv,ML( )
€
w'θv 'h = rθvw'θv '0
€
∂v,ML
∂t= −
∂w'θv '
∂z=
1 − rθv
h
⎛
⎝ ⎜
⎞
⎠ ⎟cDUML θv,0 − θv,ML( )
€
w'θv ' flux profile
![Page 13: Stephan de Roode , Irina Sandu, Johan van der Dussen, Pier Siebesma, Bjorn Stevens,](https://reader030.vdocuments.site/reader030/viewer/2022032612/568133ef550346895d9ae080/html5/thumbnails/13.jpg)
Virtual potential temperature evolution in the subcloud layer
€
∂v,ML
∂t=
θv,0 − θv,ML
τθv
h
€
w'θv ' flux profile
€
τv=
h
cDUML 1 − rθv( )
If h = 500 m, rv = -0.2, cD=0.001, UML = 10 m/s then
τv ≈ 0.5 day
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Assume SST increases linearly with time (and likewise v,0)
€
v,0 t( ) = θv,00 + γtTime-dependent BC
Solution
€
v,ML t( ) = γtlinear term
{ + θv,00 − γτθv+ γτθv
+ θv,ML t =0− θv,00( )e
−t / τθv
"memory term"1 2 4 4 4 4 4 3 4 4 4 4 4
Assume constant subcloud layer height h
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Similar approach for the subcloud humidity
Time scale:€
∂qv,ML
∂t=
1 − rq v
h
⎛
⎝ ⎜
⎞
⎠ ⎟cDUML qs,0 − qv,ML( ) =
qs,0 − qv,ML
τq
€
τq =h
cDUML 1 − rq v( )
If rqv = 1, τq ∞ (then steady-state qv,ML)
![Page 16: Stephan de Roode , Irina Sandu, Johan van der Dussen, Pier Siebesma, Bjorn Stevens,](https://reader030.vdocuments.site/reader030/viewer/2022032612/568133ef550346895d9ae080/html5/thumbnails/16.jpg)
Surface forcing obeys Clausius-Clapeyron
€
qs,0 t( ) = qs,0 t =0e t / τ cc
Clausius-Clapeyron time scale:
€
τcc =Rv T0 t =0( )
2
L vγ≈ 5 days
Solution
Time-dependent BC
€
qv,ML t( ) =qs,0 t( )
1+τq
τcc
+ qv,ML t =0−
qs,0 t =0
1+τq
τcc
⎛
⎝
⎜ ⎜ ⎜ ⎜
⎞
⎠
⎟ ⎟ ⎟ ⎟
e−t / τq
"memory term"1 2 4 4 4 4 3 4 4 4 4
As τq > 0 , subcloud humidity tendency typically smaller than surface saturation value
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Subconclusion
Decoupled subcloud layer:
v,ML increases in accord with changes in the SST
Question: what about cloud layer?
Net cooling due to longwave radiative loss
->
will lead to a destabilization
Idea: entrainment is "needed" to prevent destabilization
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What is the entrainment rate needed to give a quasi-steady state
(this means LV tendency in the cloud is similar to the subcloud layer)
€
∂VL,CLD
∂t=
∂θVL
∂t
⎛
⎝ ⎜
⎞
⎠ ⎟subcloud
≈w eΔθVL
z i − h−
1
ρcp
ΔF
z i − h
€
∂VL
∂t= −
∂w'θ'VL
∂z−
1
ρcp
∂F
∂z , θVL = θL 1+ 0.61qv − qL( )
€
w e =z i − h
ΔθVL
∂θVL
∂t
⎛
⎝ ⎜
⎞
⎠ ⎟subcloud
+1
ρcp
ΔF
ΔθVL
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Nighttime quasi-steady state entrainment value is rather close to the actual entrainment rate in solid stratocumulus
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Conclusions
Dynamics in a decoupled subcloud layer similar to the dry CBL
Decoupling makes it easier to understand subcloud layer dynamics
-> subcloud temperature dictated by time-varying SST
Humidity flux exhibits a distinct diurnal cycle
-> Max during the night, nearly all evaporated moisture in transported to the stratocumulus
During night-time quasi-steady state boundary layer
-> entrainment rate controled by SST tendency
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To do
Scaling behavior of drizzle in LES runs (how does it depend on the LWP?)