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Experiences with a statistical cloud scheme in combination with Kain-Fritsch convection

Experiences with a statistical cloud scheme in combination with Kain-Fritsch convection. Wim de Rooy KNMI. Pier Siebesma Geert Lenderink Sander Tijm. q sat(T). q t. (T,q t ). T. Statistical cloud scheme. Determine cloud cover and liquid water using the sub-grid variability :.

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Experiences with a statistical cloud scheme in combination with Kain-Fritsch convection

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  1. Experiences with a statistical cloud scheme in combination with Kain-Fritsch convection Wim de Rooy KNMI Pier Siebesma Geert Lenderink Sander Tijm

  2. qsat(T) . qt (T,qt) T Statistical cloud scheme • Determine cloud cover and liquid water using the sub-grid variability:

  3. Go over to one variable: sqt-qsat(p,T) For a certain PDF, cloud cover and liquid water can be written as a function of just one variable: Main Problem: How to determine s? (for simplicity: )

  4. How to parameterize variance?Link it to convection/turbulence schemes using a variance budget: Production Dissipation convection turbulence Variance due to convection Variance due to turbulence Lenderink& Siebesma 2000

  5. Very simple: • = (0.02qs)2 (Lenderink) • Simple: • lturb=40 (Siebesma) • lturb=0.2z for z<900 and 180 for z> 900m (Chaboureau & Bechtold) Variance due to/coupled with turbulence: • Less simple (stability dependent): • lturb from moist CBR (implementation Colin Jones) Even without convective or turbulent activity (free atmosphere) some variance is needed!

  6. Project: Statistical cloud scheme • (from simple to complex) • First 1D tests cases (start with BOMEX): • Use cloud cover (ac) diagnostically • Use ac and ql prognostically • Subsequently 3D • Test cases • Long time verification

  7. Problems with Hirlam KF convection: • Intermittent behavior • Negative buoyant cloud (TV_UPDRAFT < TV_ENVIRONMENT) • Artificially looking closure for shallow convection • Mass flux does not decrease (enough) with height • Results for Bomex are disappointing • Fundamental problems? (Sander Jonker) • Code is complex, hard to understand (and slow)

  8. q profile BOMEX with standard Hirlam KF

  9. Some of the adaptations to KF: • Fractional entrainment/detrainment according to Siebesma (2003) • Vertical velocity equation (Gregory, 2001) • More simple and physically appealing closure (Grant, 2001) •  Good results with convection and statistical cloud scheme for Bomex

  10. q profile BOMEX with adaptations

  11. Variance BOMEX Hirlam (40 lvls) with modified KF LES

  12. Cloud cover Bomex LES Hirlam with modified KF

  13. Our conclusion: It is not appealing to build • further developments on the current Hirlam KF code. • Two alternatives: • KF from Meso-NH • Latest ECMWF IFS convection code • Peter Bechtold coded 1 and currently works on 2. • His advice: Use 2 (performance, speed, use of AROME is • compatible with 2, synergy, etc.) • => Implement 2 in Hirlam(1D)

  14. Provisional results: Hirlam 1d with new ECMWF convection, old Sundqvist condensation and moist CBR (Colin Jones)

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