Physical processes affecting stratocumulus

1
Physical processes affecting stratocumulus
Siems et al. 1993
2
Profiles in a stratocumulus-capped mixed layer

• Well-mixed Moist-conserved variables
• sl cpT gz - Lql,
• qtqv ql
• are nearly uniform with height within the MBL.

? ql increases linearly with z above cloud base
Stevens et al. 2003 QJ
3
Mixed layer capped by thick stratocumulus

• Aircraft profile off the California coast, 1976

moist adiabat
Albrecht et al. 1985
4
Decoupled SCBL - midday, North Atlantic.
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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)
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Radiation in SCBLs
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
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Precipitation
precip flux

• Drizzle Drops gt 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.

z
1 mm/day
EPIC 8-mm vertically pointing cloud radar
observations of drizzling Sc
hourly cloud top
hourly cloud base
hourly LCL
Comstock et al. 2004
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Entrainment
we
flux -weF
F

• 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 the
  radiosondes and other ship-based observations
  from SCBL mass (black), moisture (blue) and heat
  budgets (red). This gives an idea of typical
  magnitudes (5 mm/s) and uncertainties (large).

Entrainment zone
F-
flux -weF-
Caldwell and Bretherton 2005
9
Parcel circuits in a Sc-capped mixed layer

• Note implied discontinuous increase in liquid
  water and buoyancy fluxes at cloud base.