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

Motivation

• Using forecast runs to test new parameterizations during

the model development ?

• Is the GCSS-Pacific Cross-Section a good candidate to

do this ?

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

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

Observations along the cross-section (JJA 1998)

SWCF LWCF LWP

Low cloud Mid/high cloud Precipitation

CERES

CERES SSM/I

ISCCP, D2 GPCPISCCP, D2

Model versus observations

SWCF LWCF LWP

Low cloud Mid/high cloud Precipitation

CERES

CERES SSM/I

ISCCP, D2

GPCP

ISCCP, D2

--- Obs--- CAM --- CAM-UW--- CAM-dilute

Forecast run specification

• Strategy If the model is initialized realistically, we assume the error comes from the parameterizations deficiencies.

• Advantages Full feedback SCMDeterministic statistical Look at process level

• LimitationsAccuracy of the atmospheric state ?

Initialize realistically ERA40 reanalysis

CAM

5-day forecastStarting daily at 00 UT

Observations ERA40

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 1 Forecast T error (K), day 5 Climate T error (K), JJA1998

Forecast q error (g/kg), day 1 Forecast q error (g/kg), day 5 Climate q error (g/kg), JJA1998

Forecast temperature errors at day 5

CAM-UWSome improvement in the cumulus region

CAM-diluteReduces T bias near ITCZError increases above 300 mb and in the lower troposphere.Changes in regions where the deep convection is not active

CAM CAM-UW CAM-dilute

Select a range of cloud regimes and forecast errors

3 locations

ITC

Z

Stratocum

ulus

Trade cum

ulus

Forecast T error at day 5, CAM

ITCZ regime: forecast T error (JJA 1998)

CAM CAM-dilute

ITCZ region: very sensitive to the deep convective scheme

Total tendency Advective tendency Physics tendency

€

∂T∂t

= −V • ∇T −ω(∂T

∂p−RT

pc p)+Qphysics

ITCZ regime: Temperature equation

Select a range of cloud regimes and forecast errors

3 locations

ITC

Z

Stratocum

ulus

Trade cum

ulus

Forecast T error at day 5, CAM

Stratocumulus: moisture and PBL (JJA 1998)

PBL heightSpecific humidity

CAM

CAM-UW

Stronger daily cyclePBL collapses

day 0day 1 day 2day 5

Stratocumulus: timeseries of T and q error

TCAM-TERA40

qCAM-qERA40

Stratocumulus: q equation (single forecast)

CAM

CAM-UW

q Advective tendency

Physics tendency

Stratocumulus regime (Physics terms)

CAM

CAM-UW

PBL tendency

Shallow tendency

Prognostics cloud water tendency

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

Observations along the cross-section

SWCF LWCF LWP

Low cloud Mid/high cloud Precipitation

CERES

CERES SSM/I

ISCCP, D1 GPCPISCCP, D1

Cumulus regime: Forecast q errors

CAM CAM-UW

Cumulus regime: moisture budget terms

2 PBL/ShCu schemes operate in very different way.

ITCZ regime: Precipitation (JJA 1998)

- GPCP DatasetDaily precipitation

- CAMLoses water very quickly during day 1.

- CAM-dilutePrecipitation increases during day 1.

ITCZ regime: Temperature equation

Stratocumulus regime (Q, CLOUD, CLDLIQ)