A dual mass flux framework for boundary layer convection

1. A dual mass flux framework for boundary layer convection Explicit representation of cloud base coupling mechanisms Roel Neggers, Martin Köhler, Anton Beljaars

2. Contents The coupling between cumulus clouds and the mixed layer Mass flux feedbacks The dual mass flux scheme The eddy diffusivity mass flux (EDMF) approach An extension for shallow cumulus Introducing more complexity New parameterizations SCM evaluation Idealized prototype cases Daily evaluation against ARM and CloudNet data Interactive with the larger scale dynamics

3. Cumulus mass flux at cloud base Definition Closures staying close to thís structure: I. Using a constant pre-factor Grant, 2001: “Cloud-base fluxes in the cumulus-capped boundary layer”. Quart. J. Roy. Met. Soc., 127, 407-422. II. Retaining the area fraction in the cumulus mass flux Neggers,Siebesma, Lenderink and Holtslag, 2004: “An Evaluation of Mass Flux Closures for Diurnal Cycles of Shallow Cumulus”. MWR, 132, 2525-2538. Bretherton, McCaa and Grenier, 2004: “A new parameterization for shallow cumulus convection and its application to marine subtropical cloud-topped boundary layers. Part I: Description and 1D results”. Mon. Wea. Rev., 132, 864-882.

4. A *SIMPLE* equilibrium model including a mass flux closure with flexible fraction Neggers, Stevens and Neelin, 2006: “A Simple Equilibrium Model for Shallow Cumulus Topped Mixed Layers”. Special Issue of the Theor. Comp. Fluid-Dynamics, in press, September 2006. humidity heat mass

5. Variance scaling at mixed layer top Neggers, Stevens and Neelin, 2006: “Variance Scaling in Shallow Cumulus Topped Mixed Layers”. Submitted to QJRMS, August 2006.

6. Equilibrium model solution BOMEX forcings and boundary conditions

7. Feedbacks between cumulus mass flux and humidity at mixed-layer top

8. Equilibrium model solution on ERA40 forcings and boundary conditions 50 years of ship based synops observations (Norris, J.Clim. 1998)

9. Moist updrafts Dry updrafts h a2up a1up Flexible area partitioning K diffusion z0 1 - aup aup The dual mass flux scheme – main EDMF upgrades Eddy diffusivity - mass flux (EDMF) approach I. Multiple updrafts II. Flexible updraft area fractions – LCL coupling III. Updraft initialization IV. Updraft budgets V. Mass transport VI. Diffusion – an entrainment efficiency closure for cumulus VII. Extension of EDMF into statistical cloud modelling (a bimodal PDF)

10. W E M2 LCL LCL E E LCL h M2 M1 h M1 Stratocumulus Transition Shallow cumulus Dry convection a1 a2 I. Multiple updrafts with flexible area fractions Moist and dry updrafts can coexist at any time Flexible updraft area fractions facilitate representation of gradual regime transitions

11. 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 II. 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

12. Variance scaling at mixed layer top This gives an implicit solution for the area fraction that carries mass flux: shape parameter of Beta function Neggers, Stevens and Neelin, 2006: “Variance Scaling in Shallow Cumulus Topped Mixed Layers”. Submitted to QJRMS, August 2006.

14. SCM evaluation I. For idealized prototype cases (LES) II. Daily evaluation against ARM and CloudNet data III. Interactive with the large scale dynamics

15. SCM results for prototype cases l qt and qs cloud fraction condensate

16. RICO marine steady state shallow cumulus BOMEX – interactive radiation

17. cy30r1 DualM ARM SGP transient continental shallow cumulus 21 June 1997

18. 2 cy30r1 [hr] cy30r1 DualM

19. Developing suppressed cumulus… the mass flux valve mechanism at work Moist updraft mass flux [m/s]

20. Daily evaluation against ARM and CloudNet data At multiple sites around the world where relevant observational data is available Setup: - initialize with ECMWF analysis state at midnight (local time) - 2 day time integration - relaxation towards forecast state (on a 6 hour timescale) - LS advective forcings are also applied

21. Results cy30r1 DualM Chilbolton June 7, 2006 A diurnal cycle of shallow cumulus

22. cy30r1 DualM Cabauw 24 July 2006 Shallow cumulus cy30r1 DualM A too fast hydrological cycle

23. ARM SGP transient continental shallow and deep cumulus July 1999 comparison to ARSCL (radar,lidar)

24. Preliminary IFS results Climate run Shallow cu mass flux [m s-1]

25. Summary An extension for EDMF is formulated that allows representation of shallow cumulus convectionKey component is the explicit representation of the coupling of shallow cumulus clouds to the mixed layerThis is done through flexible mass flux area fractions, which introduces cloud base valve-mechanismsThe DualM scheme brings significant improvements for shallow cumulus concerning i) mixed layer stability ii) structure of mean, turbulent and cloudy state iii) mixed layer depth and 2m humidityFurther evaluation against ARM and CloudNet data illustrates that these results are representative of nature and robust for different large-scale scenarios. Outlook: Implementation and testing in IFS Implementation completed Evaluation in full progress