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approach: moist combined M ass-flux/ K -diffusion ( MK )

The new ECMWF PBL scheme and beyond unifying dry BLs, stratocumulus and shallow convection Martin K öhler & Roel Neggers. approach: moist combined M ass-flux/ K -diffusion ( MK ) stratocumulus: single M -flux, bottom and top K

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approach: moist combined M ass-flux/ K -diffusion ( MK )

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  1. The new ECMWF PBL scheme and beyondunifying dry BLs, stratocumulus and shallow convectionMartin Köhler & Roel Neggers • approach: moist combined Mass-flux/K-diffusion (MK) • stratocumulus: single M-flux, bottom and top K • shallow convection: dry/cloudy M-flux, bottom K thanks to Pier Siebesma, Joao Teixeira, Adrian Tompkins, Anton Beljaars GCSS all 5-2005 new ECMWF PBL scheme Köhler & Neggers 1

  2. current PBL parameterization frameworks moist mass-flux convection dry diffusion PBL • ECMWF (2004): (also NCAR, NCEP, Arpege, DWD, …) • Teixeira & Cheinet (2003): diffusion only • Siebesma, Neggers, Cheinet (1998-2004): mass flux only cloud base (figures: Cheinet, 2004) GCSS all 5-2005 new ECMWF PBL scheme Köhler & Neggers 2

  3. unification – an MK approach zi K zcb zi M2 M1 M M Kbot Kbot Kbot Ktop Stratocumulus dry BL Shallow cumulus Deep cumulus • key ingredients: • moist conserved variables • combined Mass-flux/K-diffusion solver • cloud variability • transition between stratocumulus and shallow convection GCSS all 5-2005 new ECMWF PBL scheme Köhler & Neggers 3

  4. stratocumulus … LES: DYCOMS II vertical velocity&cloud simulation by Bjorn Stevens (dx=35m, dz=5m) GCSS all 5-2005 new ECMWF PBL scheme Köhler & Neggers 4

  5. shallow convection … LES: BOMEX w [m/s] vertical velocity&cloud water (dx=dy=dz=40m) GCSS all 5-2005 new ECMWF PBL scheme Köhler & Neggers 6

  6. shallow convection … aircraft observations: Florida SCMS GCSS all 5-2005 new ECMWF PBL scheme Köhler & Neggers 7

  7. two box M/K decomposition(Siebesma and Cuijpers, 1995) w x 0 e u e a M sub-core flux env. flux M-flux GCSS all 5-2005 new ECMWF PBL scheme Köhler & Neggers 8

  8. BOMEX LES decomposition M-flux sub-core flux total flux env. flux M-flux covers 80% of flux Siebesma & Cuijpers, 1995 GCSS all 5-2005 new ECMWF PBL scheme Köhler & Neggers 9

  9. M-flux and updraft properties: initialization PDF (Holtslag & Moeng, 1991) parcel excess: (assumes correlation of w, T, q) M-flux: K reduction due to M-term: a=5% wsfc 0 GCSS all 5-2005 new ECMWF PBL scheme Köhler & Neggers 10

  10. updaft model: entrainment: , τ=500s, c=0.55 detrainment: 3·10-4 m-1 in cloud parcel determines PBL depth (wup= 0) mass flux: parameterization choices • cloud cover: • total water variance equation not yet Mass-flux K-diffusion • diffusion: • surface and cloud top driven diffusion • cloud top entrainment: cloud variability GCSS all 5-2005 new ECMWF PBL scheme Köhler & Neggers 11

  11. Results: Low cloud cover (new-old) T511 time=10d n=140 2001 & 2004 old: CY28R4 new PBL GCSS all 5-2005 new ECMWF PBL scheme Köhler & Neggers 12

  12. Results: EPIC column extracted from 3D forecasts old PBL new MK PBL EPIC obs 700 750 800 850 900 950 1000 Liquid Water Path [g/m2] WV Mixing Ratio [g/kg] 300 45 250 200 Model Levels Pressure [hPa] 150 50 100 50 55 0 60 6 10 4 8 0 2 16 17 18 19 20 21 22 Local Time [October 2001 days] GCSS all 5-2005 new ECMWF PBL scheme Köhler & Neggers 13

  13. unification with shallow convection zi K zcb zi M2 M1 M M Kbot Kbot Ktop Kbot Stratocumulus dry BL Shallow cumulus Deep cumulus • key extension: • multiple updrafts: experimental next done GCSS all 5-2005 new ECMWF PBL scheme Köhler & Neggers 14

  14. A statistical mass flux framework for organized updrafts PDF of {w, qt ,ql } Top % of updrafts that are explicitly modelled Dry updraft K diffusion Moist updraft GCSS all 5-2005 new ECMWF PBL scheme Köhler & Neggers 15

  15. Moist Dry BOMEX: SCM vs. LES updraft w [m/s] updraft ε [1/m] LES dry updraft LES moist updraft SCM dry updraft SCM moist updraft SCM dry updraft SCM moist updraft GCSS all 5-2005 new ECMWF PBL scheme Köhler & Neggers 16

  16. moist updraft qt excess dry updraft qt excess [g/kg] [g/kg] GCSS all 5-2005 new ECMWF PBL scheme Köhler & Neggers 17

  17. M1 +M2 +K mass flux [m/s] diffusion coef. [m2/s] w’qt’ [m/s] K GCSS all 5-2005 new ECMWF PBL scheme Köhler & Neggers 18

  18. cloud fraction [%] Mean state properties CY28R4 Added vertical transport by the dry updraft is necessary to maintain humidity in the top half of the subcloud layer MK PBL normalized qt /qt0 CY28R4 Skewness can be used to create a decreasing cloud fraction with height GCSS all 5-2005 new ECMWF PBL scheme Köhler & Neggers 19

  19. Concluding Thoughts • moist combined Mass-flux/K-diffusion scheme successful in dry BL and stratocumulus • now being extended for shallow convection stratocumulus: • K-diffusion appropriate to produce well-mixed layer • but: M-flux adds potential of counter gradient fluxes • downdrafts initiated by CTEI might further improve fluxes shallow convection • M-flux appropriate for moist convection (80%) • 2 intrinsic discretizations: • horizontal: cloudy updraft vs. environment • vertical: below vs. above cloud base • natural framework: moist M1 and dry M2 & K GCSS all 5-2005 new ECMWF PBL scheme Köhler & Neggers 20

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