DARGAN M. W. FRIERSON UNIVERSITY OF WASHINGTON, DEPARTMENT OF ATMOSPHERIC SCIENCES
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Transcript of DARGAN M. W. FRIERSON UNIVERSITY OF WASHINGTON, DEPARTMENT OF ATMOSPHERIC SCIENCES
DARGAN M. W. FRIERSONUNIVERSITY OF WASHINGTON, DEPARTMENT OF
ATMOSPHERIC SCIENCES
COLLABORATORS: SARAH KANG, ISAAC HELD, MING ZHAO, JIALIN LIN, IN-SIK KANG, DAEHYUN KIM, MYONG-IN LEE, ADAM SOBEL, ERIC MALONEY,
GILLES BELLON
Experiments with a Hierarchy of GCMs: ITCZ Response to High Latitude Forcing, and Tropical
Variability
Modeling Philosophy
Models aren’t reality… They can only tell us so much about the real
atmosphere
A major advantage of using models is ability to play around with parameters Turning feedbacks on/off Modifying/simplifying boundary conditions Changing physical parameterizations
Comprehensive and Simplified GCMs
With comprehensive models, not always straight-forward to perform such experiments Can affect many aspects of model (e.g., convection scheme
affects clouds, etc) Can cause fidelity of simulated climate to decrease Requires careful experimental design
Simplified GCMs are useful for aiding the above Here we’ll discuss:
Moist GCM with highly simplified physics (Frierson 2005) No cloud- or water vapor-radiative feedbacks Simplified Betts-Miller convection scheme
Aquaplanet full GCM simulations Realistic geography full GCM simulations
Outline
ITCZ response to extratropical forcing With Sarah Kang & Isaac Held
Convectively coupled Kelvin waves With Jialin Lin, In-Sik Kang, Daehyun Kim & Myong-In
Lee
MJO With Adam Sobel, Eric Maloney & Gilles Bellon
ITCZ Location
Pioneering work by Chiang, Biasutti and Battisti (2004) and Chiang and Bitz (2005): Showed strong sensitivity of ITCZ to high latitude sea
ice and land ice in LGM simulation using CCSM
Moistening
Drying
Southward displacement of ITCZ occurs in LGM climate
Paleoclimate data is consistent with such a shift
From Chiang and Bitz (2005)
Extratropical Influences on ITCZ
Sarah Kang’s thesis work (2009): Effect of high latitude forcing on ITCZ
location/structure/intensity Simplified moist GCM and aquaplanet full GCM
(AM2) runs w/ idealized forcing:
NH cooling
SH warming
From Kang, Held, Fri., & Zhao (2008, J Clim) and Kang, Fri. & Held (in press, JAS)
Forcing
Think glaciers + sea ice in NH, plus warming in SH (to keep global mean temperature the same)
ITCZ Changes
In both models, ITCZ precipitation shifts towards warmed hemisphere
Tropical precip in full GCM
• Response is sensitive to parameters which affect cloud feedbacks
• Response is significantly larger in full GCM as compared with simplifiedGCM
From Kang, Held, Fri., & Zhao (2008, J Clim) and Kang, Fri. & Held (in press, JAS)
Mechanism for ITCZ Response
We argue energy flux is of key importance
8
Anomalous energy flux into cooled region
Change in MSE flux in simplified GCM
Less flux into warmed region
Mechanism for ITCZ Response
ITCZ latitude ~ “Energy flux equator”
8 Define “energy flux equator” as zero crossing of energy flux
Shifted into SH in perturbed case
In tropics, mean circulation does most of the flux => v=0 there =>ITCZ is nearby
Change in MSE flux in simplified GCM
ITCZ location (-) is approximatelysame as energy flux equator (--)for full GCM
Mechanism for Energy Flux Change
Eddies modify fluxes in midlatitudes Quasi-diffusively: they can be well-approximated with
a moist energy balance model
Anomalous Hadley circulation modifies fluxes in tropics
See Kang, Held, Fri., & Zhao (2008, J Clim) & Kang, Fri. & Held (in press, JAS) for more
Role of Cloud-Radiative Forcing
Differences in cloud-radiative forcing (CRF) affect ITCZ as follows: CRF = extra forcing at certain latitude bands Forcing is again propagated away byeddies quasi-diffusively Changes in energy flux equator then result in changes in ITCZ location Result in massive differences in ITCZ shift for same
forcing!
Similar mechanism seen in energy fluxes in IPCC model simulations of global warming Current work of my grad student Ting Hwang
Role of “Gross Moist Stability”
In idealized model, we can take the energy flux argument one step further
Can predict mass flux response (and hence precip response), with the “gross moist stability” of the tropics:
Changes in parameters of simplified Betts-Miller scheme can change (as shown in Frierson 2007a, JAS) Larger GMS when convection can easily reach high levels Smaller GMS when there’s an abrupt trigger for convection
Role of “Gross Moist Stability”
For identical forcing and identical energy flux response, the precip response can be significantly different
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See Kang et al 2008; also Frierson 2007a
Tropical Variability in Simplified GCM
Convectively coupled Kelvin waves dominate tropical variability in the idealized GCM
Unfiltered Hovmoller diagram of precipitation at the equator
Does gross moist stability control the speed of these waves (as in simple theories)?
From Frierson (2007b, JAS)
Convectively coupled Kelvin waves
GMS reduction also leads to slower convectively coupled waves:
GMS = 7 K GMS = 4.5 K GMS = 2.5 K
See Frierson (2007b) for more detail
Wavespeed can be tuned to essentially any value in this model
Idealized Moist GCM Kelvin Waves
Kelvin waves 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
Equatorial Waves in a Full GCM
Experiments with SNU atmospheric GCM Run over observed SSTs, realistic geography Simplified Arakawa-Schubert convection scheme Varying strength of convective trigger
See Lin, Lee, Kim, Kang and Frierson (2008, J Clim) & Fri. et al (in prep) for more
• Wavespeed decreases with stronger moisture trigger• Due to smaller GMS, as in simplified GCM
Moist Static Energy
Vertical profile of MSE in the North West Pacific ITCZ:
MSE clearly reduced at higher levels (more unstable)
GMS also reduced
Vertical structures
In full GCM, the waves show realistic vertical phase tilts (unlike in simplified GCM)
Shallow -> deep -> stratiform
See Lin et al (2008) and Frierson et al (in prep) for more detail
Warm over cold temperature anomalies
Gradual moistening of boundary layer/midtroposphere
MJO in Realistic 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 (2008, Nature Geoscience; 2009, J. Adv. Modeling Earth Systems)
Obs (NCEP) Modified GFDL model Unmodified GFDL model
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 (2008, Nature Geoscience; 2009, J. Adv. Modeling Earth Systems)
Evap-Wind Feedback in Modeled MJO
MJO greatly weakened when evaporation-wind feedback (EWF) is turned off!
With EWF Without EWF
See SMBF (2008, Nature Geoscience; 2009, J. Adv. Modeling Earth Systems)
Conclusions
ITCZ is affected by high latitude forcing by following processes: Energy fluxes: “energy flux equator” Cloud-radiative forcing Gross moist stability
Convectively coupled waves in simple and full GCM are affected by “gross moist stability” Full GCM shows second baroclinic mode
characteristics
Simulated MJO in full GCM extremely sensitive to evaporation-wind feedback