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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.

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physical processes affecting stratocumulus
Physical processes affecting stratocumulus

Siems et al. 1993

Lecture 15, Slide 1

profiles in a stratocumulus capped mixed layer
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 z

above cloud base

Stevens et al. 2003 QJ

Lecture 15, Slide 2

scbl diurnal cycle in se pacific sonde time series
SCBL diurnal cycle in SE Pacific sonde time series

3-hourly sondes show:

  • Mixed-layer structure with strong sharp inversion
  • Regular night-time increase in inversion height, cloud thickness.
  • 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
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 5

sc physical processes precipitation
Sc physical processes: Precipitation

precip flux

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.


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
Sc physical processes: Turbulent entrainment


flux -weF+


  • 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.

Entrainment zone


= flux -weF-+

Caldwell and Bretherton 2005

Lecture 15, Slide 7

profiles in a stratocumulus capped mixed layer1
Profiles in a stratocumulus-capped mixed layer

























State variables

Lecture 15, Slide 8

parcel circuits in a sc capped mixed layer
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:

Lecture 15, Slide 9

sc mlm entrainment closure
Sc MLM entrainment closure

Nicholls-Turton (1986) entrainment closure

Fit to aircraft and lab obs and dry CBL

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 ´



NT: Nicholls and Turton (1986)

DL: Lilly (2002)

LL: Lewellen&Lewellen (2003)



 * 0.1


Entrained fraction 

Lecture 15, Slide 10

eddy velocity vs flux partitioning closures
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
MLM examples

Steady-state solutions: Higher SST, lower divergence promote deeper mixed layer with thicker cloud.

SST = 16 C, D = 4x10-6 s-1

Cloud top

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
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

Lecture 15, Slide 13

mlm diurnal cycle
MLM diurnal cycle

Schubert 1976 JAS

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