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Unifying Dry Boundary Layers and Shallow Convection with ECMWF PBL Scheme

Explore the innovative ECMWF PBL scheme and its implications on stratocumulus, shallow convection, and mass flux diffusion. Discover the unified approach and key ingredients for transitioning between cloud types.

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Unifying Dry Boundary Layers and Shallow Convection with ECMWF PBL Scheme

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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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