1 / 14

Physical processes affecting stratocumulus

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.

oleg-morse
Télécharger la présentation

Physical processes affecting stratocumulus

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


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

  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 z above cloud base Stevens et al. 2003 QJ Lecture 15, Slide 2

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

  4. 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

  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 5

  6. 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. 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

  7. Sc physical processes: Turbulent 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 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 F- = flux -weF-+ Caldwell and Bretherton 2005 Lecture 15, Slide 7

  8. Profiles in a stratocumulus-capped mixed layer h+ qt+ W(z) E(z) B(z) T w z we ql W(zi) zi qv P zb FR hM qtM TMs W(0) qs hs Ts Fluxes State variables Lecture 15, Slide 8

  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: Lecture 15, Slide 9

  10. 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 ´ Tv 2m NT: Nicholls and Turton (1986) DL: Lilly (2002) LL: Lewellen&Lewellen (2003) 0 1  * 0.1 ´ Entrained fraction  Lecture 15, Slide 10

  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 11

  12. 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

  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 13

  14. 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

More Related