Physical processes affecting stratocumulus

Lecture 15, Slide 1

Physical processes affecting stratocumulusSiems et al. 1993

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 3

Decoupled SCBL - midday, North Atlantic.

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

Lecture 15, Slide 14

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)

Schubert 1976 JAS