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
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
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
Contents
Determining the degree of decoupling
Moisture and heat fluxes in the decoupled subcloud layer
Evolution of the subcloud layer state
Entrainment scaling
Stratocumulus cloud layer rises with timeSlow increase in cumulus cloud base height
Total water content for ASTEX
- binned mean + (all data) mean horizontal legs
Bulk structure of a decoupled boundary layer
Park et al., 2004, Wood and Bretherton 2004
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αq =qT z i
−( ) − qT,0
qT z i+
( ) − qT,0
αq = 0 if vertically well mixed
Quantification of decoupling from observations
Wood and Bretherton 2004
larger αq for deeper boundary layers
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αq =qT z i
−( ) − qT,0
qT z i+
( ) − qT,0
Decoupling parameter αq consistent between LES models
Example: Turbulent fluxes in ASTEX from DALES at t=23 hr
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w'w' m2s−2( )
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w'θv ' mKs-1( )
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ρcpw'qT ' Wm-2( )
Next step: quantify buoyancy and total specific humidity flux at the cumulus base height
Buoyancy flux ratio rv becomes similar to CBL
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rθv=
w'θv 'cu base
w'θv 'sfc
CBL value -0.2
Moisture flux ratio rq exhibits clear diurnal cycleMean value rq slightly smaller than unity
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rq =w'qT '
cu base
w'qT 'sfc
Virtual potential temperature evolution in the subcloud layer
h
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w'θv '0 = cDUML θv,0 − θv,ML( )
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w'θv 'h = rθvw'θv '0
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∂v,ML
∂t= −
∂w'θv '
∂z=
1 − rθv
h
⎛
⎝ ⎜
⎞
⎠ ⎟cDUML θv,0 − θv,ML( )
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w'θv ' flux profile
Virtual potential temperature evolution in the subcloud layer
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∂v,ML
∂t=
θv,0 − θv,ML
τθv
h
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w'θv ' flux profile
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τv=
h
cDUML 1 − rθv( )
If h = 500 m, rv = -0.2, cD=0.001, UML = 10 m/s then
τv ≈ 0.5 day
Assume SST increases linearly with time (and likewise v,0)
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v,0 t( ) = θv,00 + γtTime-dependent BC
Solution
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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
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
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τq =h
cDUML 1 − rq v( )
If rqv = 1, τq ∞ (then steady-state qv,ML)
Surface forcing obeys Clausius-Clapeyron
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qs,0 t( ) = qs,0 t =0e t / τ cc
Clausius-Clapeyron time scale:
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τcc =Rv T0 t =0( )
2
L vγ≈ 5 days
Solution
Time-dependent BC
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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
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
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)
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∂VL,CLD
∂t=
∂θVL
∂t
⎛
⎝ ⎜
⎞
⎠ ⎟subcloud
≈w eΔθVL
z i − h−
1
ρcp
ΔF
z i − h
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∂VL
∂t= −
∂w'θ'VL
∂z−
1
ρcp
∂F
∂z , θVL = θL 1+ 0.61qv − qL( )
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w e =z i − h
ΔθVL
∂θVL
∂t
⎛
⎝ ⎜
⎞
⎠ ⎟subcloud
+1
ρcp
ΔF
ΔθVL
Nighttime quasi-steady state entrainment value is rather close to the actual entrainment rate in solid stratocumulus
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
To do
Scaling behavior of drizzle in LES runs (how does it depend on the LWP?)