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Sc to Cu transition

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.

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Sc to Cu transition

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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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