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Cécile Hannay, Jeff Kiehl, Ping Zhu, and Jim Hack

Cloud diagnostics in subsidence regions: comparison of the NCAR and GFDL models versus observations and reanalysis. Cécile Hannay, Jeff Kiehl, Ping Zhu, and Jim Hack National Center for Atmospheric Research, Boulder. GCSS Meeting, Athens, May 16-20, 2005. Motivation.

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Cécile Hannay, Jeff Kiehl, Ping Zhu, and Jim Hack

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  1. Cloud diagnostics in subsidence regions: comparison of the NCAR and GFDL models versus observations and reanalysis Cécile Hannay, Jeff Kiehl, Ping Zhu, and Jim Hack National Center for Atmospheric Research, Boulder GCSS Meeting, Athens, May 16-20, 2005

  2. Motivation • NCAR and GFDL models: change in low-level clouds • for a 2xCO2 scenario was different in the 2 models • large impact on climate sensitivity • Here: we compare present day climate runs of the 2 models • versus observations and reanalysis in stratocumulus regions. • To better understand: • what determine the cloud properties in these regions ? how well the models represent these stratocumulus zones ?

  3. Climatological datasets • NCAR model (CAM3): AMIP run, T85 resolution , 1979-1999 • GFDL model (AM2p12): AMIP run, 2x2.5 resolution, 1983-1998 • Observations and reanalysis: • ISCCP, stratocumulus clouds, 1983-1999 • SSM/I, liquid water path, 1987-2000 • ERBE, shortwave cloud forcing, 1985-1989 • HadISST: sea surface temperature, 1979-1999 • ECMWF reanalysis (ERA40): T, w, surface fluxes, 1979-1999

  4. Eastern Pacific Stratocumulus: models versus ISCCP NCAR ISCCP GFDL • NCAR model: - clouds too close from the coast • - over-predicts clouds in East Equatorial zone • GFDL model: - underestimates stratocumulus • - clouds too far from the coast

  5. NCAR GFDL North Equatorial South Error on SWCF: models versus ERBE • SWCF: Error up to 30 W/m2 locally. • when coupling with ocean model: important to produce the correct amount of stratocumulus at the right location

  6. Processes influencing stratocumulus

  7. North Equatorial South Annual cycles in observations and reanalysis Domain-averaged monthly means Stratocumulus (%)ISCCP SST (K)HadISST Static stability (K)ERA40 W500 (mb/day)ERA40 Buoyancy flux (W/m2)ERA40

  8. Static stability = consistent predictor of cloud • NCAR diagnoses cloud fraction using ”Klein Line” about 6% increase in cloud per degree of static stability (Klein and Hartmann, 1993) • Relationship does not hold for smaller domain and timescale averaging • warmer climate: increase in static stability and therefore in stratocumulus but also changes in other quantities (i.e. subsidence, buoyancy flux) => no guarantee that change in stratocumulus will follow Klein Line

  9. Cloud fraction scheme • NCAR: diagnostic cloud fraction (empirical “Klein line”) • GFDL: prognostic cloud fraction (modified Tiedtke, 1993)

  10. Jan Apr Jul Oct Jan Apr Jul Oct Jan Apr Jul Oct GCM results North Equatorial South Stratocumulus (%)––––– ISCCP––––– NCAR ––––– GFDL Static stability (K) ––––– ERA40––––– NCAR ––––– GFDL W500 (mb/day) ––––– ERA40––––– NCAR ––––– GFDL Buoyancy flux (W/m2) ––––– ERA40––––– NCAR ––––– GFDL

  11. GCM results North Equatorial South Stratocumulus (%)––––– ISCCP––––– NCAR ––––– GFDL SWCF (W/m2) ––––– ERBE––––– NCAR ––––– GFDL LWP (g/m2) ––––– SSMI––––– NCAR ––––– GFDL

  12. Conclusion • Analysis of stratocumulus annual cycle in 3 regions • The relationships between stratocumulus and other quantities are different in the North from the Equatorial/South regions • Static stability is the only consistent predictor for the 3 regions - However: correlation does not holdat smaller domain or timescale - warmer climate: no guarantee that change in stratocumulus will follow Klein Line • Both models have problems. At this stage, it is hard to tell which model predicts the change in stratocumulus clouds more accurately.

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