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A dual mass flux scheme for shallow cumulus convection Results for GCSS RICO

Explore an upgraded EDMF for shallow cumulus convection & transitions, with detailed results and benefits. Implementing flexible updraft area fractions facilitates gradual regime transitions in representing cumulus clouds.

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A dual mass flux scheme for shallow cumulus convection Results for GCSS RICO

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  1. A dual mass flux scheme for shallow cumulus convection Results for GCSS RICO Roel Neggers, Martin Köhler, Anton Beljaars

  2. Contents The DualM scheme An extension to EDMF for shallow cumulus convection A *VERY* short overview (more on Thursday) RICO results Mean state Equilibration Updraft profiles & fluxes Variances Cloudy state & precipitation Experiments with an entrainment efficiency closure for cumulus

  3. The current PBL scheme (cy30r1) Eddy diffusivity - mass flux (EDMF) decomposition for turbulent transport Siebesma et al. (Submitted to JAS) Currently applied to dry PBL and stratocumulus topped PBL onlyGoal: upgrade EDMF to enable representation of i) shallow cumulus convection ii) transitions What is needed? dry updraft moist updraft K K

  4. Moist updrafts Dry updrafts h a2up a1up Flexible area partitioning K diffusion z0 1 - aup aup The dual mass flux scheme – main EDMF upgrades I. Multiple updrafts N=2: dry updrafts stopping at cloud base, moist updrafts continuing as cumulus clouds. II. Flexible updraft area fractions Closure as a function of model state Facilitates representation of i) gradual regime transitions, ii) coupling of cumuli to mixed layer III. Extension of EDMF partitioning into the statistical cloud scheme A matching bimodal PDF (passive clouds and updraft clouds) Consistent treatment of PBL transport and cloudiness IV. Top-entrainment efficiency closures for cumulus Explicit treatment of transport across strong unresolved gradients ( cu inversion, transition layer) V. … and many more …

  5. W E M2 LCL LCL E E LCL h M2 M1 h M1 Stratocumulus Shallow cumulus Dry convection a1 a2 Moist and dry updrafts can co-exist at any time Flexible updraft area fractions facilitate representation of gradual regime transitions

  6. III. Vertical integration of updraft budgets then reconstructs the joint-PDF I. First guess of cloud core fraction at cloud base, using local variance scaling II. These fractions are subsequently used in surface initialization of updrafts Closure of the flexible updraft area fractionsExplicit representation of the coupling between cumulus updrafts and mixed-layer Knowledge of how many updrafts manage to reach LFC is `back-stitched` into the partitioning and initialization of the updrafts This carries information on cloud base transition layer properties

  7. RICO results – mean state

  8. RICO results – cloudy state DualM cy30r1

  9. RICO results - equilibration Area fraction is retained in the updraft mass flux The flexibility of a and its closure introduce a valve mechanism in M, always acting to equilibrate the mixed layer budgets

  10. RICO results - updrafts Lateral entrainment closure makes differently initialized updrafts diverge with height.

  11. RICO results - fluxes LES

  12. RICO results – variance I Used in statistical cloud scheme

  13. RICO results – variance II Extension of EDMF into the modelling of clouds – a bimodal distribution qt Updraft PDF Diffusive PDF l Relations between moments Lewellen & Yoh (JAS,1993)

  14. RICO results – variance II Closure of updraft variance

  15. RICO results – bimodal clouds

  16. Updraft clouds Passive clouds

  17. RICO results - precipitation

  18. RICO results - precipitation How important is it? RICO BOMEX

  19. RICO results – entrainment efficiency experiments All flux into cu inversion is explicitly modelled through the K-component of EDMF Wyant et al. (1997) RICO ATEX

  20. Summary A shallow cumulus extension is formulated for EDMFMore complexity is added to the system of equations, in various new ways: - multiple transporting updrafts - flexible updraft area fractions - EDMF partitioning is extended into the PBL cloud scheme (a bimodal PDF)Assuming flexible updraft area fractions facilitates representation of gradual regime transitions Closure of the updraft area fractions allows explicit representation of the coupling between cumuli and the mixed layer, introducing a valve mechanism in the cumulus mass fluxThe mean, turbulent and cloud structure of the RICO short composite case is reproducedPrecipitation appears to be an important player in the SCM thermodynamic budgetsA more detailed discussion of model physics on Thursday…

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