Tropical intraseasonal variability simulated

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Tropical intraseasonal variability simulated in
the NCEP Global Forecast System and Climate
Forecast System models
Kyong-Hwan Seo, Wanqiu Wang and Jae-K. E.
Schemm
Pusan National University Dept of
Atmospheric Sciences, Korea
CPC/NCEP/NOAA, USA
NOAAs 33rd Climate Diagnostics and Prediction
Workshop, Lincoln, Nebraska October 20-24, 2008
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Objectives
To investigate the capability for simulating the
tropical intraseasonal variability (focusing on
the MJO) in a series of atmosphere-ocean coupled
and uncoupled simulations using NCEP operational
general circulation models.
To evaluate the effects of the following factors
on the MJO simulation Air-sea coupling Model
horizontal resolution Deep convection
parameterization Basic state vertical shear
Basic state low-level westerlies SST Low-level
moisture convergence
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Models
NCEP Atmospheric Model GFS T62
(AMIP) NCEP Coupled model
CFS T62 (CMIP GFS T62 GFDL
MOM3) NCEP Coupled
high resolution run CFS T126 (SAS) NCEP Coupled
high resolution run with Relaxed Arakawa-Schubert
scheme
CFS T126 RAS
Simulation Period 15-20 years
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OBS
GFS T62
CFS T62
Power Spectra over equatorial Indian Ocean

• Red line calculated power spectra
• Blue line background red spectrum
• OBS pronounced 30-80 day signal
• GFS T62, CFS T62, CFS T126 less significant
• CFS T126RAS significant, vigorous power in
  30-80 day range

CFS T126
CFS T126RAS
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Leading 2 EOFs of combined OLR, U200 U850

• CFS T126RAS explained variance close to
  observation among simulations.
• Propagation is not revealed in this plot

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Regression against PC2
u850

• GFS, CFST62, CFS T126 propagation barrier
  problem over the Maritime continent
• Coherence in CFST62 is improved compared to
  GFST62
• CFS T126RAS shows a successful propagation
  across the Maritime continent
• Regressed dynamical variables are consistent
  with the observations

prate
-OLR
LHTFL
DSWRF
Tsfc
Sfc moist
converg.
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Factors for the improved MJO simulation

• Air-sea interaction
• Model horizontal resolution
• Basic state vertical shear
• Basic state low-level westerlies
• SST
• Deep convection parameterization
• Low-level moisture convergence
• Vertical profile of diabatic heating

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Factors for the MJO (3) Basic-state vertical
wind shear
Western Pacific

• Vertical easterly shear favors the eastward
  propagating waves (Zhang and Geller, 1994)
• CFS T126RAS shows the smallest easterly shear
• Background vertical wind shear is not the most
  important factor

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Factors for the MJO (4) Background low-level wind

• Easterly bias acts as a barrier to the eastward
  propagation of the MJO (Inness and Slingo 2003
  Flatau et al. 1997)
• CFS T126RAS shows the easterly bias over the
  Maritime continent and the western pacific
• This is not a major factor

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Factors for the MJO (5) SST

• A cold SST bias acts to suppress the development
  of the MJO convection (many references)
• CFS T126RAS shows an increased cold bias over
  the western Pacific Ocean
• This is not a major factor

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Factors for the MJO (6) Deep convecton
parameterization

• CFS T126RAS Active convective activity over the
  warm pools induce the enhanced lower-level
  circulation, which in turn helps maintain the MJO
  convection
• The positive feedback between the convection and
  circulation induces the continued eastward
  propagation across the Maritime Continent.
• This is a major factor

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Factors for the MJO (7) Low-level moisture
convergence

• Frictional wave-CISK mechanism is the main
  paradigm for the development and propagation of
  the MJO
• CFS T126RAS shows the strong surface layer
  moisture convergence both over the Indian Ocean
  and the western Pacific, which leads enhanced
  convection by 2-5 days, as similar as the
  observations
• -The phasing and magnitude of the lower-level
  moisture convergence are a key factor

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Global Circulation Response to the MJO
Convection OBS

• 200hPa Streamfunction regressed onto PC1 and PC2
• Half life cycle
• RED enhanced MJO convection
• Blue suppressed convection
• Tropics anticyclonic couplet at or west of
  enhanced convection tropical westerly anom east
  of enhanced convection Rossby-Kelvin wave
  response
• PNA-like response
• Continued influence to the Americas

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Global Circulation Response to the MJO CFS T126

• -RED enhanced MJO convection
• -Blue suppressed convection
• CFS T126 convection and streamfunction anomalies
  are weak
• No significant suppressed convection over the
  western Pacific at t6 t12 ? weaker
  circulation response

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Global Circulation Response to the MJO CFS
T126RAS

• RED enhanced MJO convection
• Blue suppressed convection
• CFS T126RAS stronger circulation response
• Similar pattern to the observation ? pattern
  correlation 0.84-0.91
• (vs 0.47-0.78 in CFS T126)

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Convectively Coupled Equatorial Waves OBS

• Observed equatorial OLR anomalies
• Antisymmetric component MRG and n0 EIG
  connected to each other
• Symmetric component n1 ER, n1 WIG/EIG, Kelvin
  and MJO
• Aligned along equivalent depth of 25m

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CFS T126

• Both coupled runs
• no MRG and n0 EIG signals shown
• n1 WIG, n1 EIG not generated
• -Kelvin wave is weaker than the observation
• n1 ER wave produced but with a slower phase
  speed bias
• Models have the equivalent depth of 12m
• Only significant isolated MJO signal appeared in
  CFS T126RAS

CFS T126RAS
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Summary

• MJO
• - The interactive air-sea coupling greatly
  improves the coherence between the convection,
  circulation and other surface fields.
• - CFS T126RAS produces statistically
  significant spectral peaks in the MJO spectral
  band and the strength of MJO convection and
  circulation is considerably improved.
• Most of all, the MJO convection signal is able to
  penetrate into the Maritime Continent and western
  Pacific.
• The proper and persistent interaction between the
  convection and circulation induces the continued
  eastward propagation across the Maritime
  Continent.
• The improved MJO simulation in CFS T126RAS
  improves the simulation of extratropical
  circulation anomalies
• Convectively Coupled Equatorial Waves
• ER wave and Kelvin wave are reproduced!
• No statistically significant peaks associated
  with MRG and EIG waves
• All models produce too excessive westward
  synoptic scale disturbances with periods of less
  than 5 days
• Model-generated eastward Kelvin waves are weaker
  than the observed.

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Discussions
Several limitations First, we are not able to
specifically determine which processes or
parameters of the RAS scheme are most important
for producing the enhanced MJO activity. For
instance, the questions which components are
responsible for the reduced autocorrelation seen
in CFS T126RAS and whether or not there is an
equivalent self-suppression process in this deep
convection scheme (see Lin et al. 2006) can not
be answered. Only the final consequences from
the interaction of convection and circulation are
seen. Second, the vertical diabatic heating
profiles associated with convective and
stratiform clouds are not available from the
current operational setting. In addition, the
causes of the excessive high-frequency
variability in the space-time spectral analysis
can not be addressed. Carefully designed
sensitivity test and more flexible implementation
of the operational model would be required to
resolve these issues. Nonetheless, this study
determined the plausible factors for the improved
simulation of the MJO.
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SAS vs RAS
SAS requires that a moist quasi-equilibrium
hypothesis is achieved for the cloud ensemble (or
the deepest single plume) at each integral. RAS
only relaxes the thermodynamic state toward
equilibrium rather than making instantaneous
adjustments to the equilibrium state as in SAS.
A brief description of these convection schemes
can be found in Das et al. (2002).