Transcript Figures

Physical processes affecting stratocumulus
Siems et al. 1993
Lecture 15, Slide 1
Profiles in a stratocumulus-capped mixed layer
‘Well-mixed’: Moistconserved variables
sl = cpT + gz - Lql,
qt = q v + q l
h = cpT + gz + Lqt
are nearly uniform with
height within the MBL.
 ql increases
linearly with z
above cloud base
Stevens et al. 2003 QJ
Lecture 15, Slide 2
Decoupled SCBL - midday, North Atlantic.
Lecture 15, Slide 3
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 4
Sc physical processes: Radiation
Net upward radiative flux
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.
Diurnal cycle of net SCBL rad cooling
Lecture 15, Slide 5
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.
precip flux
z
1 mm/day
EPIC 8-mm vertically pointing ‘cloud radar’ observations of drizzling Sc
hourly cloud base
hourly cloud top
hourly LCL
Comstock et al. 2004
Lecture 15, Slide 6
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.
F+
we
flux -weF+
Entrainment zone
F-
= flux -weF- + wF 
Caldwell and Bretherton 2005
Lecture 15, Slide 7
e
Profiles in a stratocumulus-capped mixed layer
h+
qt+
z
W(z)
T
w
we
ql
zi
qv
E(z) B(z)
W(zi)
 w h 
P
 w qt
zb
FR
qtM
hM
TMs
qs hs
Ts
W(0)
Fluxes
State variables
Lecture 15, Slide 8
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:
zi
w  2.5  wbdz
3
*
0
Lecture 15, Slide 9
Sc MLM entrainment closure
Nicholls-Turton (1986) entrainment closure
Fit to aircraft and lab obs and dry CBL
w*3
we  A
, A  0.2(1  a2 E), b  g Tv T0
zi b
Observational test with
SE Pacific Sc diurnal cycle
(Caldwell et al. 2005)
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 ´
Tv
2m
0
´
 *  0.1
1
Entrained fraction 
Lecture 15, Slide 10
NT: Nicholls and Turton (1986)
DL: Lilly (2002)
LL: Lewellen&Lewellen (2003)
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 11
MLM examples
Steady-state solutions: Higher SST, lower divergence promote
deeper mixed layer with thicker cloud.
Cloud top
SST = 16 C, D = 4x10-6 s-1
Cloud base
SST = 17 C, D = 3x10-6 s-1
Schubert et al. 1979a, JAS
Lecture 15, Slide 12
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
Lecture 15, Slide 13
Schubert et al. 1979b JAS
MLM diurnal cycle
MLM prediction: cloud
thickens during the day
because of decreased
entrainment, opposite to
observations. MLM breaks
down during day and in
deeper or drizzly BLs due to
BL decoupling (next lecture)
Lecture 15, Slide 14
Schubert 1976 JAS