Cécile Hannay, Jeff Kiehl, Dave Williamson, Jerry Olson,
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Transcript of Cécile Hannay, Jeff Kiehl, Dave Williamson, Jerry Olson,
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)