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From trade cumuli to deep moist convection

From trade cumuli to deep moist convection. Chris Bretherton and Zhiming Kuang University of Washington (with inspiration from GCSS WG1). Some features of trade cumulus convection. Shallow Cu are always highly entraining.

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From trade cumuli to deep moist convection

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  1. From trade cumuli to deep moist convection Chris Bretherton and Zhiming Kuang University of Washington (with inspiration from GCSS WG1)

  2. Some features of trade cumulus convection • Shallow Cu are always highly entraining. • Convective overshoot and penetrative entrainment are important in the long-term evolution of the Cu layer. • Their typical buoyancy excess is much less than undilute. • Their cloud base properties are connected to the statistics of the subcloud layer turbulence – the Cu are the vertical extensions of the moistest, strongest subcloud updrafts. These points apply to most deep convection, but are often overlooked in parameterizations.

  3. BOMEX LES(Siebesma and Cuijpers 1995) (dq/dt)adv = - 1 g/kg-d Wmin = -0.65 cm/s Qr = -2 K/d

  4. BOMEX GCSS results (Siebesma et al. 2003) Overshooting Cu ‘Bottom-heavy’ cloud population ‘Core’ lateral entrainment rate 1-2 km-1

  5. Clouds are barely buoyant 2-3s cloud base humidity, w excess

  6. Feedbacks in shallow (and deep?) oceanic Cu Cloudbase CIN is the rapid mass flux ‘valve’ - Weaker CIN More mass flux More compensating Cu-layer warming Penetrative entrainment slowly adjusts Cu buoyancy - More Cu buoyancy Stronger updraft More penetrative entrainment Stronger ql stratification (Bretherton et al. 2004, MWR)

  7. Oceanic shallow-to-deep CRM Setup: BOMEX with enhanced SHF, LHF (inadvertently doubled Qrad) CSU CRM, Lx = Ly = 12.8 km, Lz = 28 km. Dx = Dy = 100 m, Dz = 50 m below 12 km.

  8. Results 143 hr 48 hr

  9. Cloud fields (qc>0.1 g/kg) 48 hr 143 hr

  10. In-cloud buoyancy is remarkably small Hor avg Cloud W>1 m/s Adiabatic

  11. A plume-ensemble view of entrainment In-cloud PDF, hours 50-52 • All updrafts are diluted. • e = 0.5-1/km • in vigorous updrafts. • Most cloud is much more dilute 0.25/km 0.5/km Mass-flux PDF, hours 50-52 1/km 2/km 4/km 8 1 W<0 2 4

  12. Later and deeper… Cloud PDF,158-160 hrs • Qualitatively similar, but • core updraft e ~ 0.5/km. 0.25/km 1 0.5 2 e = 4 Mass-flux PDF,158-160 hrs 8 4 2 1 W<0

  13. Buoyancy and Vertical Velocity qv, 50-52 hrs cloud • Even a little less dilution makes a very strong updraft! clear w, 50-52 hrs

  14. Conclusions • Both for shallow and deep convection, all cumuli are substantially entrainment-diluted such that their mean buoyancy is nearly zero, even in vigorous updrafts. • Below the LFC, the Cu mass flux is dominated by updrafts with a narrow range of entrainment rates, suggesting a single ensemble plume may suffice. • Penetrative entrainment dominated by stronger, less dilute updrafts- requires consideration of plume ensemble. • Cloud base updraft properties represent moist tail of PBL air; mass flux is regulated via CIN. • Over the long term, penetrative entrainment regulates mean sounding to keep typical cloud buoyancy small.

  15. qv, 158-160 hrs w, 158-160 hrs

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