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Stephan de Roode and Johan van der Dussen Delft University of Technology , Netherlands

The EUCLIPSE/GCSS model intercomparison study of a stratocumulus to cumulus cloud transition as observed during ASTEX. Stephan de Roode and Johan van der Dussen Delft University of Technology , Netherlands http://www.euclipse.nl/.

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Stephan de Roode and Johan van der Dussen Delft University of Technology , Netherlands

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  1. The EUCLIPSE/GCSS model intercomparison study of a stratocumulus to cumulus cloud transition as observed during ASTEX Stephan de Roode and Johan van der Dussen Delft University of Technology, Netherlands http://www.euclipse.nl/

  2. Current boundary-layer cloud model intercomparison studies for large-eddy simulation and single-column models CGILS(steady state solutions of stratocumulus and cumulus) - aim: understand cloud-climate feedback for DSST=+2K Lagrangian cloud transitions (SST increases along the trajectory) - ASTEX (aircraft) and composite cases (cloud fraction from satellite) - aim: understand and validate modeling results of cloud regime transition This talk: - Subsidence, entrainment, and cloud layer depth evolution

  3. ASTEX transition: Large-scale divergence difficult to quantify from the ECMWF model Large-scale divergence: is it important for the stratocumulus-to-cumulus transition?

  4. Divergence from ERA-40 (changing sign) Divergence constant (5×10-6 s-1) t = 3 hr t = 8 hr t = 20 hr t = 36 hr Mean state evolution: liquid water potential temperature  t > 20 hr : boundary height differs considerably

  5. ASTEX Lagrangian: Dutch Atmospheric Large-Eddy Simulation Results Divergence from ERA-40 (changing sign) Divergence constant (5×10-6 s-1) we (cm/s) LWP (g/m2) cloud break up for case with larger divergence

  6. Entrainment of dry and warm air from above the inversion:  cloud top height (ztop) rises  cloud base height (zbase) rises Cloud deepening:

  7. Cloud layer depth evolution qT cloud top qsat z (m) cloud base (g/kg)

  8. Specific humidity Example: entrainment drying qT qsat cloud top rises z (m) cloud base rises (g/kg)

  9. Example: longwave radiative cooling, saturation specific humidity decreases  Randall: in some cases cloud layer depth can increase by entrainment only  This presentation: consider all heat and moisture fluxes qT qsat z (m) lower cloud base height (g/kg)

  10. Cloud base and top height evolution for a well-mixed cloud layer Entrainment warming – Cloud thinning Rad cooling – Cloud thickening Entrainment drying – Cloud thinning Moistening – Cloud thickening Cloud deepening by entrainment Drizzle – Cloud thinning Subsidence - Cloud thinning

  11. Cloud base and top height evolution for ASTEX as a function of the entrainment rate rcp<w'qL'>zbase = 11 W/m2 , rLv<w'qT'>zbase = 60 W/m2 DqL = 5 K , DqT = -1.1 g/kg DLW = 74 W/m2 Div = 5 x 10-6 s-1 zbase = 300 m , ztop= 600 m

  12. Cloud base and top height evolution for ASTEX as a function of the entrainment rate rcp<w'qL'>zbase = 11 W/m2 , rLv<w'qT'>zbase = 60 W/m2 DqL = 5 K , DqT = -1.1 g/kg DLW = 74 W/m2 Div = 5 x 10-6 s-1 zbase = 300 m , ztop= 600 m

  13. Cloud base and top height evolution for ASTEX as a function of the entrainment rate cloud layer thickness growths for went < 1.9 cm/s

  14. Example: DYCOMS II RF01 For went > 0.4 cm/s, cloud layer rapidly thins rcp<w'qL'>zbase = 11 W/m2 , rLv<w'qT'>zbase = 60 W/m2 DqL = 8.5 K , DqT = -7.5 g/kg DLW = 74 W/m2 Div = 5 x 10-6 s-1 zbase = 500 m , ztop= 800 m

  15. Steady-state cloud layer depth: equilibrium entrainment rates equilibrium entrainment rate ASTEX DYCOMS II RF01 rcp<w'qL'>zbase = 11 W/m2 , rLv<w'qT'>zbase = 80 W/m2 DLW = 70 W/m2 Div = 5 x 10-6 s-1 zbase = 300 m , zi= 650 m

  16. Steady-state cloud layer depth: equilibrium entrainment rates equilibrium entrainment rate entrainment rate (Nicholls and Turton, 1986) ASTEX DYCOMS II RF01 rcp<w'qL'>zbase = 11 W/m2 , rLv<w'qT'>zbase = 80 W/m2 DLW = 70 W/m2 Div = 5 x 10-6 s-1 zbase = 300 m , zi= 650 m

  17. Steady-state cloud layer depth: equilibrium entrainment rates equilibrium entrainment rate ASTEX DYCOMS II RF01 Cloud top height decreases (entrainment smaller than subsidence) rcp<w'qL'>zbase = 11 W/m2 , rLv<w'qT'>zbase = 80 W/m2 DLW = 70 W/m2 Div = 5 x 10-6 s-1 zbase = 500 m , zi= 800 m

  18. Conclusions  Large-scale divergence is important for the evolution of the stratocumulus cloud deck  In a large part of the regime where the cloud layer depth is in a steady state the cloud top increases with time Acknowledgements The EUCLIPSE project is an international effort, funded under Framework Program 7 of the European Union  NWO-NCF has sponsored the use of the Dutch Supercomputer "Huygens"  Irina Sandu (MPI), Chris Bretherton (UW), and Adrian Lock (UKMO) are thanked for their help setting up the ASTEX intercomparison case

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