dargan m. w. frierson university of washington, department of atmospheric sciences collaborators:...
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DARGAN M. W. FRIERSONUNIVERSITY OF WASHINGTON, DEPARTMENT OF
ATMOSPHERIC SCIENCES
COLLABORATORS: MARSHALL STONER, DAEHYUN KIM, JIALIN LIN, IN-SIK KANG, MYONG-IN LEE, ADAM SOBEL, ERIC MALONEY, GILLES BELLON
Convectively Coupled Kelvin Waves and the MJO in a Hierarchy
of GCMs
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Outline
What sets speed/structure of convectively coupled equatorial waves? In a simplified GCM Modeling work with SNU group
What is required to generate a MJO-like structure? AM2 model work w/ Sobel, Maloney & Bellon Master’s thesis of Marshall Stoner
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Convectively Coupled Equatorial Waves
What sets speed? Moist 1st baroclinic mode? (gross moist stability:
Neelin, Emanuel, etc) Dry 2nd baroclinic mode? (Mapes, Majda, etc)
Observations show clear 2nd baroclinic structure (Kiladis et al 2009)
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CCKWs in a Simplified GCM
Convectively coupled Kelvin waves (CCKWs)dominate tropical variability in a simplified GCM Unfiltered Hovmoller diagram of
precipitation at the equator
In this model, gross moist stability controls the speed of these waves
Model of Frierson, Held & Zurita-Gotor (2006)Plot from Frierson (2007)
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Convectively coupled Kelvin waves
GMS reduction leads to slower convectively coupled waves:
GMS = 6.9 K GMS = 3.9 K GMS = 3.0 K
See Frierson (2007) for more detail
Ratio of grid-scale to convective (simplified Betts-Miller) precipitation sets the GMS
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Simplified Moist GCM CCKWs
These CCKWs are powered by evaporation-wind feedback Likely not true in reality in Indian Ocean…
Vertical structure is purely first-baroclinic mode Unrealistic…
Longitude
Composited pressure velocity
See Frierson (2007b) for more detail
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Equatorial Waves in a Full GCM
Experiments with SNU atmospheric GCM Run over observed SSTs, realistic geography Simplified Arakawa-Schubert (SAS) and Kuo
convection schemes Varying strength of convective trigger:
Tokioka entrainment limiter for SAS Higher Tokioka parameter => least entraining plumes
are eliminated Moisture threshold for Kuo
From always triggering convection to 95% RH required
See Lin, Lee, Kim, Kang and Fri. (2008, J Clim) & Fri. et al (submitted) for more
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Moist Static Energy
Vertical profile of MSE in the North West Pacific ITCZ for SAS simulations:
Higher entrainment => harder to warm upper troposphere Stronger trigger => more unstable
GMS also reduced
Tokioka values:
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Equatorial Waves in a Full GCM
Phase speeds in SAS simulations:
In Kuo simulations:
See Lin, Lee, Kim, Kang and Fri. (2008, J Clim) & Fri. et al (submitted) for more
• Wavespeed decreases with stronger moisture trigger• Simulated equivalent depths scale with gross moist stability
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CCKW Vertical Structures
In full GCM, many cases show 2nd baroclinic mode structures (unlike in simplified GCM)
Shallow -> deep -> stratiform
See Lin et al (2008) and Frierson et al (submitted) for more detail
Warm over cold temperature anomalies
Gradual moistening of boundary layer/midtroposphere
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CCKW Vertical Structures
Depends on convection scheme though!
Least inhibited SAS case => No tilt in omega (but OK temperature)
Most inhibited Kuo case => No tilt in omega, q (but OK temperature)
Kuo simulations never show tilted omega orhumidity.
Only mostinhibited case shows realistictemperatureperturbations
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Phase Speed Determination?
Estimated equivalent depths versus GMS:
1st baroclinic mode seems to explain phase speed Presence/absence of 2nd baroclinic mode doesn’t
appear to have effect
Circled cases have clear 2nd baroclinic structure
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Phase Speed Determination?
2nd baroclinic mode and cloud-radiative forcing effects on GMS
Stratiform phase =>higher GMS Shallow phase =>
lower GMSCRF changes have small effecteverywhere
Mode structure effect on GMSaverages to zero, and are small near center of the wave
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Open Questions
Reasons for second baroclinic mode structure And why seen in some fields more easily than others?
Applicability to other models? Need for thorough comparisons of composites
Relation to changes in mean precipitation?
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MJO in GCMs
Work with Sobel, Maloney, & Bellon using GFDL AM2 model w/ realistic geography
First crank up Tokioka “entrainment limiter” to get a better MJO simulation:
See SMBF (Nature Geoscience 2008; J. Adv. Modeling Earth Systems in press)
Obs (NCEP) Modified GFDL model Unmodified GFDL model
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MJO in GFDL AM2 Model
Ratio of variance in eastward/westward intraseasonal bands: 2.6 for modified GFDL model Less than the observed value of 3.5, but larger than
nearly all models in Zhang et al (2006) comparison
Higher entrainment in convection scheme => more sensitivity to midtropospheric moisture
Next test role of evaporation-wind feedbacks in driving the modeled MJO Set windspeed dependence in drag law formulation to
globally averaged constant value
See SMBF (Nature Geoscience 2008; J. Adv. Modeling Earth Systems in press)
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Evap-Wind Feedback in Modeled MJO
MJO greatly weakened when evaporation-wind feedback (EWF) is turned off!
With EWF Without EWF
See SMBF (Nature Geoscience 2008; J. Adv. Modeling Earth Systems 2009)
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MJO in Aquaplanet AM2
What is required to have a MJO-like structure in a model? Land-sea contrast? Zonal asymmetry/Walker cell? Evaporation-wind feedback?
Experiments with Neale & Hoskins aquaplanet AMIP boundary conditions “QOBS” & “Flat” GFDL AM2 model with Tokioka modification
M.S. thesis work of Marshall Stoner (2010)
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Zonally Symmetric Results
Log(variance) spectra: QOBS (left) and “Flat” (right)
Enhanced power in eastward intraseasonal bandConnected to moist Kelvin wave?
More clear dominance of east over westLess connected to Kelvin wave?
M.S. thesis work of Marshall Stoner (2010)
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Intraseasonal Composites
Composites of structure:
When WISHE is suppressed, QOBS ISV (left) remains, while Flat ISV (right) disappears
Connected to midlatitude wave trains, smaller scale
More similar to observed MJO?
QOBS Flat
M.S. thesis work of Marshall Stoner (2010)
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Mean States
Mean states (solid = QOBS, dashed = flat):
Flat has weaker easterlies, and a double ITCZStandard WISHE likely drives the waves
M.S. thesis work of Marshall Stoner (2010)
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How about Flat + a Walker cell?
Surface winds
Now mean westerlies over much of the tropics
Will WISHE still be important? (standard theory assumes mean easterlies)
M.S. thesis work of Marshall Stoner (2010)
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Walker Cell Case
MJO-like variability still exists (although weaker) Again it disappears if WISHE is suppressed
Surface winds
Log(variance)
Variance avoidssurface westerlyregion?
M.S. thesis work of Marshall Stoner (2010)
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WISHEful Thinking
Evaporation composites for Flat (zonally symmetric) and Flat + Walker
Flat
Flat + Walker cell
Both essentially have evaporation leading the wave
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Open Questions
What sets scale, speed of the MJO-like phenomenon? Related to Kelvin wave at all, or a moisture mode? Advection of dry air by WWBs & Rossby cyclones appears to
be important in setting speed as well as WISHE
Comparisons with other models (including CRMs) Similar mechanisms acting? (mechanism denial
experiments in a range of models) Compare composites as well as spectra
Understanding of how/when different mechanisms can power waves can help our interpretation of observations
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Conclusions
Convectively coupled waves in simple and full GCM are affected by “gross moist stability” Full GCM shows second baroclinic mode
characteristics
MJO-like structures can exist in aquaplanet model Zonally symmetric or with Walker cell More realistic ISV is powered by WISHE in mostly
traditional manner