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Physical processes affecting stratocumulus. Siems et al. 1993. Profiles in a stratocumulus-capped mixed layer. ‘Well-mixed’: Moist-conserved variables s l = c p T + gz - Lq l , q t = q v + q l h = c p T + gz + Lq t are nearly uniform with height within the MBL. - PowerPoint PPT Presentation
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Lecture 15, Slide 2
Profiles in a stratocumulus-capped mixed layer
‘Well-mixed’: Moist-conserved variables
sl = cpT + gz - Lql,
qt = qv + ql
h = cpT + gz + Lqt
are nearly uniform with height within the MBL.
ql increases linearly with zabove cloud base
Stevens et al. 2003 QJ
Lecture 15, Slide 4
SCBL diurnal cycle in SE Pacific sonde time series
3-hourly sondes show:
1. Mixed-layer structure with strong sharp inversion
2. Regular night-time increase in inversion height, cloud thickness.
3. Decoupling measured by cloud base - LCL increases during daytime and during periods of drizzle on 19, 21 Oct. (local noon = 18 UTC)
(Bretherton et al. 2004)
Lecture 15, Slide 5
Sc physical processes: Radiation
Strong longwave cooling at cloud top destabilizes SCBL, creating turbulence
Shortwave heating in cloud cancels much of the longwave cooling during the day, weakening turbulence and favoring decoupling.
Subtropical CBL radiative energy loss is usually large compared to surface heat flux.
Net upward radiative flux
Diurnal cycle of net SCBL rad cooling
Lecture 15, Slide 6
Sc physical processes: Precipitation
Drizzle: Drops > 100 m radius, falling ~ 1 m s-1.
Sedimentation (in cloud only): Cloud droplets less than 20 m radius, falling a few cm s-1.
hourly cloud top
hourly LCL
hourly cloud base
Comstock et al. 2004
EPIC 8-mm vertically pointing ‘cloud radar’ observations of drizzling Sc
z
precip flux
1 mm/day
Lecture 15, Slide 7
Sc physical processes: Turbulent entrainment
• Driven by turbulence• Inhibited by a strong inversion • Must be measured indirectly
(flux-jump or budget residual methods).
• The 6-day diurnal cycle of entrainment rate from EPIC (right) was independently deduced from radiosondes and other ship-based observations based on SCBL mass (black), moisture (blue) and heat budgets (red). Typical magnitudes are small (5 mm/s) and measurement uncertainties are large.
Caldwell and Bretherton 2005
Entrainment zone
weF+
F-
flux -weF+
= flux -weF- + ′w ′Fe
Lecture 15, Slide 8
Profiles in a stratocumulus-capped mixed layer
z
zb
zi
qtM
qt+
hM
h+
qv
ql
w T
qs hs
TMs
Ts
W(z)
W(zi)
W(0)
we
P
ρ ′w ′qt
ρ ′w ′h
FR
E(z) B(z)
State variables Fluxes
Lecture 15, Slide 9
Parcel circuits in a Sc-capped mixed layer
• Note implied discontinuous increase in liquid water and buoyancy fluxes at cloud base turbulence driven from cloud, unlike dry CBL.
• Convective velocity w* ~ 1 m s-1:
w*3 =2.5 ′w ′b
0
zi
∫ dz
Lecture 15, Slide 10
Sc MLM entrainment closure
Evaporative enhancement: Less buoyant mixtures easier to entrain.
NT enhancement factor E = m/Tv
a2 = 15-60 A = 0.5 - 5 in typical Sc
Tv ´
ρ´
10
Entrained fraction
2m
Tv
NT: Nicholls and Turton (1986)DL: Lilly (2002)LL: Lewellen&Lewellen (2003)
Observational test with SE Pacific Sc diurnal cycle(Caldwell et al. 2005)
Nicholls-Turton (1986) entrainment closureFit to aircraft and lab obs and dry CBL
we =Aw*
3
zib, A=0.2(1+ a2E), b=gTv T0
* 0.1
Lecture 15, Slide 11
Eddy velocity vs. flux-partitioning closures
• Overall MLM evolution is not too sensitive to closure because the MLM adjusts we to maintain energy balance in which entrainment warming roughly balances total BL radiative cooling (which mainly just cares about whether the cloud fraction).
• Subcloud buoyancy fluxes are sensitive to the closure.
Lecture 15, Slide 12
MLM examplesSteady-state solutions: Higher SST, lower divergence promote
deeper mixed layer with thicker cloud.
Schubert et al. 1979a, JAS
Cloud top
Cloud base
SST = 16 C, D = 4x10-6 s-1
SST = 17 C, D = 3x10-6 s-1
Lecture 15, Slide 13
MLM response to a +2K SST jump
Two timescales:
Fast internal adjustment
tb = zi /CTV ~ 0.5 day
Slow inversion adjustment
ti = D-1 ~ 3 days
Schubert et al. 1979b JAS