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Large-Eddy Simulation of a stratocumulus to cumulus transition as observed during the First Lagrangian of ASTEX

Large-Eddy Simulation of a stratocumulus to cumulus transition as observed during the First Lagrangian of ASTEX. cumulus penetrating stratocumulus, Bretagne, France. View on Delft, Johannes Vermeer, 1661. Stephan de Roode and Johan van der Dussen Delft University of Technology , Netherlands .

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Large-Eddy Simulation of a stratocumulus to cumulus transition as observed during the First Lagrangian of ASTEX

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  1. Large-Eddy Simulation of a stratocumulus to cumulus transition as observed during the First Lagrangian of ASTEX cumulus penetrating stratocumulus, Bretagne, France View on Delft, Johannes Vermeer, 1661 Stephan de Roode and Johan van der Dussen Delft University of Technology, Netherlands

  2. ASTEX First Lagrangian12-14 June 1992 Contents ASTEX First Lagrangian - observations - previous numerical studies Large-Eddy Simulation results - mean state - turbulence

  3. Observed stratocumulus to cumulus transition Bretherton and Pincus, 1995 Bretherton et al, 1995 Duynkerke et al, 1995 De Roode and Duynkerke, 1997

  4. Observed stratocumulus to cumulus transition Bretherton and Pincus, 1995 Bretherton et al, 1995 Duynkerke et al, 1995 De Roode and Duynkerke, 1997 GCSS case, 1995 EUCREM/GCSS, Duynkerke et al, 1999 like GCSS ATEX case, Stevens et al, 2001 Study of ASTEX First Lagrangian wtih SCM and 2D models by Bretherton et al, 1999: "there are substantial quantitative differences in the cloud cover and liquid water path between models."

  5. Satellite images Flights 1 and 5

  6. Inversion jumps ASTEX transition to cumulus, k increases EUROCS FIRE case Lock (2009)

  7. Entrainment rates too large? Dales paper to be submitted by Heus et al.

  8. Set-up of simulation Initialization Take vertical profiles from GCSS ASTEX A209 intercomparison case Time varying forcing SST and subsidence rate from Bretherton et al. (1999) Geostrophic winds from observations Dutch Atmospheric Large-Eddy Simulation (Dales) model (Heus et al, 2009) 1283 grid points (6.4x6.4x2.5 km3) on 32 processors Monotone advection ("kappa") scheme for passive scalars Time step Dt = 0.5 s (Runge-Kutta) Bulk microphysics (Khairoutdinov and Kogan 2000) Longwave radiation: "GCSS WG1" parameterization Shortwave radiation: "EUROCS FIRE" delta-Eddington, Independent Pixel Approximation

  9. Simulated mean cloud-top height

  10. Simulated cloud boundaries mean cloud top mean cloud base lowest cloud base

  11. Entrainment rate Including microphysics (rain and cloud droplet sedimentation) decreases entrainment rate

  12. Cloud liquid water path evolution

  13. Mean state Hour 9 Mean state Hour 19

  14. Buoyancy flux Large buoyancy fluxes during Flight 4

  15. Vertical velocity variance decoupled boundary layer (double peak in w'2)

  16. Drizzle and total humidity flux

  17. Summary Strong points + aircraft observations of turbulence and entrainment rate + critical regime with regard to cloud fraction + passes through two previous intercomparison cases (revisiting Flights 2 and 3) Weak points - quite large uncertainty in total humidity fluxes Status of the case - refine forcing of free atmosphere (longwave radiative cooling, geostrophic forcing) Research questions: Study and provide detailed 3D LES fields on a large horizontal domain + connection to radiation community (McICA) + length scale analysis (NWP horizontal resolution -> 1 km)

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