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Understanding the Sc to Cu Transition: Implications for Climate Models and Radiation Balance

This lecture explores the transition from stratocumulus (Sc) to cumulus (Cu) cloud formations and its significance within the Hadley circulation. The transition impacts global radiation balance and presents challenges for climate models due to the complex interactions between clouds and turbulence that require parameterization. The lecture delves into cloud-top entrainment instability, the role of buoyancy reversal, and the conditions under which stratocumulus clouds may persist despite instabilities. Insights into numerical simulations and observational studies are discussed to enhance understanding of this crucial atmospheric process.

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Understanding the Sc to Cu Transition: Implications for Climate Models and Radiation Balance

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  1. Randall 1980 Sc to Cu transition • A fundamental feature of the Hadley circulation. • Important to global radiation balance • A challenge for climate models, because it involves cloud-turbulence interaction that must be parameterized. Net cloud radiative effect Sc Cu MBL Lecture 7, Slide 1

  2. Mixed-layer models do not dissipate Sc downstream MLM run with July-mean SST and atmospheric conditions Wakefield and Schubert (1981) zi cloud thickness July-mean trajectory 400 800 600 Sc thickens downstream since inversion rises faster than cloud base  transition to Cu must result from breakdown of MLM MBL Lecture 7, Slide 2

  3. Cloud-top Entrainment Instability Randall (1980) and Deardorff (1980) suggested that Sc might be unstable if cloud-top entrainment could create negatively buoyant mixtures (‘buoyancy reversal’). Condition: 2 = h - cpTqt < 0 Issues: - Runaway instability or just entrainment enhancement? - Role of other processes (e.g. radiative cooling) Tv ´ Tv 1 0 2<0 ´ Entrained fraction  MBL Lecture 7, Slide 3

  4. CTEI hypothesis for Sc to Cu breakup Further downstream of Sc region, climatological 2 becomes negative  huge entrainment increase, Sc instability and breakup. Cu then develop in the entrainment-diluted boundary-layer. MBL Lecture 7, Slide 4

  5. Problem: Sc persist in presence of buoyancy reversal • Stricter CTEI criteria have been proposed (MacVean and Mason 1990) but do not match typical conditions of Sc breakup. Kuo and Schubert (1988) k = cpT/(L)  0.23 MBL Lecture 7, Slide 5

  6. LES of Sc to Cu transition • 2D, 4x3 km, x = 50 m, z = 25 m, 8 days • SST = 285 + 1.5 K d-1, D = 3x10-6 s-1, Vg = 7.1 ms-1 • Diurnally-averaged insolation for 30 N. Wyant et al. 1997 MBL Lecture 7, Slide 6

  7. Horizontal-mean statistics Sc Sc over Cu Cu MBL Lecture 7, Slide 7

  8. Sc breakup, decoupling and DIDECUPE DIDECUPE = Deepening-Induced Decoupling and Cumulus Penetrative Entrainment (Wyant et al. 1997) • Deeper Sc-capped boundary layers with weaker inversions over warmer water favor persistent decoupling. • Decoupling leads to development of a Cu layer, which takes over the entrainment, mixing in enough dry air to evaporate the Sc below the inversion. (Wyant et al. 1997) MBL Lecture 7, Slide 8

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