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

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

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

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."

Satellite images Flights 1 and 5

Inversion jumps transition to cumulus,  increases EUROCS FIRE case ASTEX Lock (2009)

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

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) grid points (6.4x6.4x2.5 km 3 ) on 32 processors Monotone advection ("kappa") scheme for passive scalars Time step  t = 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

Simulated mean cloud-top height

Simulated cloud boundaries mean cloud top mean cloud base lowest cloud base

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

Cloud liquid water path evolution

Mean state Hour 9 Mean state Hour 19

Buoyancy flux Large buoyancy fluxes during Flight 4

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

Drizzle and total humidity flux

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)