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Sensitivity to the PBL and convective schemes in forecasts with CAM along the Pacific Cross-section Cécile Hannay, Jeff Kiehl, Dave Williamson, Jerry Olson,

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Presentation on theme: "Sensitivity to the PBL and convective schemes in forecasts with CAM along the Pacific Cross-section Cécile Hannay, Jeff Kiehl, Dave Williamson, Jerry Olson,"— Presentation transcript:

1 Sensitivity to the PBL and convective schemes in forecasts with CAM along the Pacific Cross-section Cécile Hannay, Jeff Kiehl, Dave Williamson, Jerry Olson, Jim Hack, Richard Neale and Chris Bretherton* National Center for Atmospheric Research, Boulder *University of Washington, Seattle Joint GCSS-GPCI/BLCL-RICO Workshop, NASA/GISS, 18-21 September 2006

2 Motivation Using forecast runs to test new parameterizations during the model development ? Is the GCSS-Pacific Cross-Section a good candidate to do this ?

3 Outline Models: PBL and convective schemes Cross-section: climate runs versus observations. Forecast runs settings Forecast errors along the cross-section Examples: 3 cloud regimes – ITCZ region – Trade-Cumulus – Stratocumulus Conclusion

4 Models: PBL and convective schemes CAM - Boundary layer: Holtslag-Boville (1993) - Shallow convection: Hack (1993) - Deep convection: Zhang-McFarlane (1995) CAM-UW (Chris Bretherton) - Turbulence scheme: Grenier-Bretherton (2001) includes explicit entrainment at the top of the PBL - Shallow convection: cloud-base mass flux based on surface TKE and convection inhibition near cloud base CAM-dilute (Richard Neale) - Deep convection: parcels are diluted by environment air

5 Observations along the cross-section (JJA 1998) SWCFLWCF LWP Low cloud Mid/high cloud Precipitation CERES SSM/I ISCCP, D2GPCPISCCP, D2

6 Model versus observations SWCFLWCF LWP Low cloud Mid/high cloud Precipitation CERES SSM/I ISCCP, D2 GPCP ISCCP, D2 --- Obs --- CAM --- CAM-UW --- CAM-dilute

7 Forecast run specification Strategy If the model is initialized realistically, we assume the error comes from the parameterizations deficiencies. Advantages Full feedback  SCM Deterministic  statistical Look at process level Limitations Accuracy of the atmospheric state ? Initialize realistically ERA40 reanalysis CAM 5-day forecast Starting daily at 00 UT Observations ERA40

8 Forecast errors and climate errors (CAM-ERA40) Cloud regimes => range of error structures Climate bias appears very quickly in CAM Climate error ~ Forecast error at day 5 Forecast T error (K), day 1Forecast T error (K), day 5Climate T error (K), JJA1998 Forecast q error (g/kg), day 1Forecast q error (g/kg), day 5Climate q error (g/kg), JJA1998

9 Forecast temperature errors at day 5 CAM-UW Some improvement in the cumulus region CAM-dilute Reduces T bias near ITCZ Error increases above 300 mb and in the lower troposphere. Changes in regions where the deep convection is not active CAM CAM-UW CAM-dilute

10 Select a range of cloud regimes and forecast errors 3 locations ITCZStratocumulusTrade cumulus Forecast T error at day 5, CAM

11 ITCZ regime: forecast T error (JJA 1998) CAM CAM-dilute ITCZ region: very sensitive to the deep convective scheme

12 Total tendencyAdvective tendencyPhysics tendency ITCZ regime: Temperature equation --- CAM --- CAM-dilute

13 Select a range of cloud regimes and forecast errors 3 locations ITCZStratocumulusTrade cumulus Forecast T error at day 5, CAM

14 Stratocumulus: moisture and PBL (JJA 1998) PBL heightSpecific humidity CAM CAM-UW Stronger daily cyclePBL collapses day 0 day 1 day 2 day 5

15 Stratocumulus: timeseries of T and q error T CAM -T ERA40 q CAM -q ERA40

16 Stratocumulus: q equation (single forecast) CAM CAM-UW q Advective tendency Physics tendency

17 Stratocumulus regime (Physics terms) CAM CAM-UW PBL tendency Shallow tendency Prognostics cloud water tendency

18 Conclusion CAM forecasts allows for diagnosing model errors in the different cloud regimes. Climate bias appears very quickly in CAM – Where deep convection is active, error is set within 1 day – 5-day errors are comparable to the mean climate errors. New schemes: CAM-UW and CAM-dilute -CAM-dilute: improves the warm bias in upper troposphere, but cold bias increases in lower troposphere and near top of the model. -CAM-UW: does not change the error structure but CAM-UW operates very differently than CAM at the process level. Difficult to decide what is causing the errors in such a coupled system => need observations. => Comparison along the A-train

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21 Observations along the cross-section SWCFLWCF LWP Low cloud Mid/high cloud Precipitation CERES SSM/I ISCCP, D1GPCPISCCP, D1

22 Model versus observations SWCFLWCF LWP Low cloud Mid/high cloud Precipitation CERES SSM/I ISCCP, D1 GPCP ISCCP, D1 --- Obs --- CAM --- CAM-UW --- CAM-dilute

23 Cumulus regime: Forecast q errors CAM CAM-UW

24 Cumulus regime: moisture budget terms 2 PBL/ShCu schemes operate in very different way.

25 ITCZ regime: Precipitation (JJA 1998) - GPCP Dataset Daily precipitation - CAM Loses water very quickly during day 1. - CAM-dilute Precipitation increases during day 1.

26 ITCZ regime: Temperature equation

27 Stratocumulus regime (Q, CLOUD, CLDLIQ)

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