1. Lecture 15, Slide 1 Physical processes affecting stratocumulus Siems et al. 1993

2. Lecture 15, Slide 2 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.  q l increases linearly with z above cloud base Stevens et al. 2003 QJ

3. Lecture 15, Slide 3 Decoupled SCBL - midday, North Atlantic.

4. 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)

5. 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

6. 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

7. 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 wewe F+F+ F-F- flux -w e F + = flux -w e F - +

8. Lecture 15, Slide 8 Profiles in a stratocumulus-capped mixed layer z zbzb zizi q tM qt+qt+ hMhM h+h+ qvqv qlql w T qsqs hshs T Ms TsTs W(z)W(z) W(zi)W(zi) W(0) wewe P FRFR E(z)E(z) B(z)B(z) State variables Fluxes

9. 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 :

10. Lecture 15, Slide 10 Sc MLM entrainment closure Evaporative enhancement: Less buoyant mixtures easier to entrain. NT enhancement factor E =  m /  T v a 2 = 15-60  A = 0.5 - 5 in typical Sc T v ´ ´´ 1 0 Entrained fraction  2m2m Tv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 closure Fit to aircraft and lab obs and dry CBL  *  0.1

11. 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.

12. Lecture 15, Slide 12 MLM examples Steady-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

13. Lecture 15, Slide 13 MLM response to a +2K SST jump Two timescales: Fast internal adjustment t b = z i /C T V ~ 0.5 day Slow inversion adjustment t i = D -1 ~ 3 days Schubert et al. 1979b JAS

14. 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