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Stephan de Roode , Irina Sandu, Johan van der Dussen, Pier Siebesma, Bjorn Stevens,

LES results of the EUCLIPSE Lagrangian transitions: Entrainment, decoupling and low cloud transitions. Stephan de Roode , Irina Sandu, Johan van der Dussen, Pier Siebesma, Bjorn Stevens, Andy Ackerman, Peter Blossey, and Adrian Lock. Entrainment.

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Stephan de Roode , Irina Sandu, Johan van der Dussen, Pier Siebesma, Bjorn Stevens,

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  1. LES results of the EUCLIPSE Lagrangian transitions: Entrainment, decoupling and low cloud transitions Stephan de Roode, Irina Sandu, Johan van der Dussen, Pier Siebesma, Bjorn Stevens, Andy Ackerman, Peter Blossey, and Adrian Lock

  2. Entrainment Schematic of cumulus penetrating stratocumulus by Bjorn Stevens (based on ATEX intercomparison) Cloud layer  Boundary layer is decoupled  Subcloud layer dynamics scale like dry convective boundary layer

  3.  Determining the degree of decoupling  Moisture and heat fluxes in the decoupled subcloud layer  Evolution of the subcloud layer state  Entrainment scaling Contents

  4. Stratocumulus cloud layer rises with timeSlow increase in cumulus cloud base height

  5. Total water content for ASTEX DHARMA UKMOSAM UCLADALES - binned mean + s (all data) mean horizontal legs

  6. Bulk structure of a decoupled boundary layer Park et al., 2004, Wood and Bretherton 2004 aq = 0 if vertically well mixed

  7. Wood and Bretherton 2004 Quantification of decoupling from observations larger aq for deeper boundary layers

  8. Decoupling parameter aq consistent between LES models

  9. Example: Turbulent fluxes in ASTEX from DALES at t=23 hr Next step: quantifybuoyancy and totalspecifichumidity flux at the cumulus base height

  10. Buoyancy flux ratio rqv becomes similar to CBL CBL value -0.2

  11. Moisture flux ratio rq exhibits clear diurnal cycleMean value rq slightly smaller than unity

  12. Virtual potential temperature evolution in the subcloud layer h

  13. Virtual potential temperature evolution in the subcloud layer h If h = 500 m, rqv = -0.2, cD=0.001, UML = 10 m/s then tqv ≈ 0.5 day

  14. Time-dependent BC Assume SST increases linearly with time (and likewise qv,0) Assume constant subcloud layer height h Solution

  15. Similar approach for the subcloud humidity Time scale: If rqv = 1, tq∞ (then steady-state qv,ML)

  16. Time-dependent BC Surface forcing obeys Clausius-Clapeyron Clausius-Clapeyron time scale: Solution As tq > 0 , subcloud humidity tendency typically smaller than surface saturation value

  17. Decoupled subcloud layer: qv,ML increases in accord with changes in the SST Subconclusion Question: whataboutcloudlayer? Net cooling due to longwave radiative loss -> will lead to a destabilization Idea: entrainment is "needed" to prevent destabilization

  18. What is the entrainment rate needed to give a quasi-steady state(this means qLV tendency in the cloud is similar to the subcloud layer)

  19. Nighttime quasi-steady state entrainment value is rather close to the actual entrainment rate in solid stratocumulus

  20. Dynamics in a decoupled subcloud layer similar to the dry CBL Decoupling makes it easier to understand subcloud layer dynamics -> subcloud temperature dictated by time-varying SST Humidity flux exhibits a distinct diurnal cycle -> Max during the night, nearly all evaporated moisture in transported to the stratocumulus During night-time quasi-steady state boundary layer -> entrainment rate controled by SST tendency Conclusions

  21. Scaling behavior of drizzle in LES runs (how does it depend on the LWP?) To do

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