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UW PBL/ShCu schemes in CAM3

UW PBL/ShCu schemes in CAM3. Grenier-Bretherton (2001 MWR) moist TKE/explicit entrainment PBL scheme, modified so TKE diagnostic. Bretherton-McCaa - Grenier (2004 MWR) shallow cu scheme: - Buoyancy sorting bulk entraining-detraining plume - Cu-base mass flux

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UW PBL/ShCu schemes in CAM3

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  1. UW PBL/ShCu schemes in CAM3 • Grenier-Bretherton (2001 MWR) moist TKE/explicit entrainment PBL scheme, modified so TKE diagnostic. • Bretherton-McCaa-Grenier (2004 MWR) shallow cu scheme: • - Buoyancy sorting bulk entraining-detraining plume • - Cu-base mass flux • - Plume momentum equation for overshooting entrainment • Updates since Breckenridge: • - Fixed climate-changing bug in Zhu-Bretherton code. • - Many minor technical improvements. • - Runs stably in FV and CAPT modes. • - ShCu scheme also being tested in GFDL AM2. • cam3_1_brnchT_apep_dani_IC Chris Bretherton and Sungsu Park, U. Washington

  2. What’s up? • Last June, we appeared to have a viable CAM3-UW with cloud climatology similar to CAM3. • But we found physical and numerical ShCu problems.

  3. The bug • ShCu formulated with moist conserved variables, but must detrain liquid water to stratiform cloud scheme. There is currently no right way to do this! • Ping chose to remove all Cu condensate after each timestep (defensible), but computed removal rate assuming entire column filled with Cu updraft…a huge overestimate. Much of this removed condensate was rained out before detrainment, resulting in detrained air being erroneously dry and inhibiting cloud cover. • Ping later realized that one should normalize the removal rate by the Cu cloud fraction, but didn’t do extensive simulations with this fix.

  4. Effect on cloud climatology • Correcting bug increases low cloud cover by 5-10%, and increases net TOA radiation by 8 W m-2. -DSWCF from bug fix

  5. Sungsu’s contribution • We went back to Jim McCaa’s version of ShCu code and spent many months getting back global TOA radiation balance with numerically and physically consistent code. • In the process, we have technically improved the ShCu code in many ways and improved its numerical stability. • More defensible condensate detrainment. • More stable implicit implementation of CIN closure. • Simple microphysics (precipitate all updraft condensate exceeding a threshold qcmax = 0.5 g kg-1). • Raised low cloud RHcrit from 0.9 to 0.95 over ocn. • Raised top of ‘low cloud’ from 750 hPa to 600 hPa. • ShCu called for all Cu depths after Z-M. • We also switched our Linux simulations from Portland to Lahey compilers, which decreased LWP by ~30% into good agreement with IBM simulations.

  6. CAM3-UW Climatology (L30, T42 and FV 2x2.5) • WGNE diagnostics: www.atmos.washington.edu/~sungsu • Net TOA radiative flux: -0.1/0.5 W m-2 (FV/T42)…but LWCF and SWCF individually 5 W m-2 too strong from hi-lat cloud excess (also SAT bias). CAM3.0 FV CAM3-UW T42 • FV simulation has a superior low cloud climatology in low latitudes

  7. TOA Net CRF CAM3-UW FV • CAM3-UW FV simulation is particularly nice again T42

  8. Tropical precipitation CAM3-UW-FV • Behavior little changed from standard CAM3 CAM3 CAM3-UW

  9. Latitude-height structure CAM3-UW-T42 CAM3.0 Little change in bias patterns

  10. +2K Cloud Feedbacks/Climate Sensitivity • l = DTs/DG = 0.51 K/(W m-2), 5% less than CAM3 • Weak negative CRF sensitivity: DLWCF/DTs = -0.12 W m-2 K-1 DSWCF/DTs = -0.23 W m-2 K-1

  11. EUROCS project • JJA 1998 • GCSS intercomparison • JJA 1998/2003 • Observations • ISCCP data • SSM/I product • TOVS atmosphere • GPCP precipitation • AIRS data • Reanalyses • NCEP/ERA40 CAM3-UW in CAPT (forecast mode) • Cecile Hannay (NCAR Clouds CPT liaison) with CAPT. • JJA 1998, GCSS NE Pacific cross-section.

  12. Daily T42L30 CAM3 forecasts initialized with ERA40. 1-day error relative to ERA40 CAM3 and CAM3-UW both quickly lower inversion height

  13. CAM3-UW No Z-M No ZM • ShCu scheme run as only cumulus scheme. • Net TOA = -5 W m-2 without tuning. • No double-ITCZ bias or Red Sea monsoon, but moves LHF, precip from land to warm ocn. • Worsens midlat LWP overestimate. • Needs defensible ice microphysics. With ZM

  14. No ZM Low clouds • Without ZM, subtropical Sc to Cu transitions are more realistic. With ZM

  15. Upcoming research • The UW schemes integrate well into CAM3, but we need to test in slab-ocean and fully coupled mode. • More inspired use of CAPT framework. • Diagnose high-latitude PBL/cloud issues. • Excess PBL cloud (common with CAM3) • Corresponding hi-lat warm winter/cold summer bias. • Integration of shallow and deep convection. • Thanks to Phil, Dani, Cecile, Rich for ongoing help.

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