The influence of “fixing” the Southern Ocean shortwave ......1. We “fixed” the Southern...
Transcript of The influence of “fixing” the Southern Ocean shortwave ......1. We “fixed” the Southern...
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The influence of “fixing” the Southern Ocean shortwave radiation model bias
on global energy budgets and circulation patterns
Jennifer Kay, Vineel Yettella (CU-Boulder) Brian Medeiros, Cecile Hannay (NCAR)
Peter Caldwell (LLNL)
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Excessive Absorbed Shortwave Radiation over Southern Ocean
(e.g., CESM-CAM5 below)
Kay et al. 2014 Figure 1
Today’s talk: 1) How do we “fix” this excessive ASR bias? 2) What are the climate impacts of “fixing it”?
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Cloud phase biases in atmosphere-only CAM5 runs (using simulator-enabled comparisons with CALIPSO)
Over the Southern Ocean: Not enough Liquid, Too much Ice
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CAM5’s shallow convection scheme controls cloud condensate phase via a
temperature ramp…
Let’s add more supercooled cloud liquid in shallow convective clouds (“experiment”)… We’re off to races!!
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Encouraging results in “experiment” with more supercooled cloud liquid
Present-day atmosphere-only
CAM5 runs: experiment - control
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Increasing supercooled liquid in shallow convective clouds dramatically reduces the
CAM5 Southern Ocean absorbed shortwave bias
Figure 4 - Kay et al. submitted
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Global Cooling and Happy Polar Bears
Runaway global cooling and sea ice expansion with increased supercooled liquid in shallow convective clouds…
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PROGRESS: We “fixed” both Southern Ocean and Tropical shortwave radiation biases!
Figure 5 - Kay et al. submitted
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Hypothesis Verified: Increasing supercooled liquid in CAM5 clouds “fixes” the excessive absorbed shortwave
radiation over the Southern Ocean.
We “fixed” it!!
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What are the climate impacts of “fixing” the excessive Southern Ocean absorbed shortwave bias?
Cooler Southern Ocean?
Reduced tropical rainfall biases?
Jet changes?
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Sea Surface Temperatures: Cooling in the Southern Ocean,
Warming Elsewhere
Fully coupled 1850 runs SST difference (deg C): tuned experiment - control
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Stronger meridional temperature gradients and stronger jet (SH, especially in DJF)
Fully coupled 1850 runs: tuned experiment - control
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Does reducing the Southern Ocean
shortwave radiation bias also
reduce tropical precipitation
biases (i.e., the double ITCZ)?
Hwang and Frierson 2013
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Influence of the Southern
Ocean shortwave
radiation on tropical
precipitation (a la Hwang
and Frierson)
Not overwhelming
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In the fully coupled tuned experiment, cross-equatorial heat transport changes in the
ocean (!), not the atmosphere.
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Original cartoon
With Ocean cartoon
Kay et al. (submitted)
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1. We “fixed” the Southern Ocean shortwave radiation bias by increasing supercooled liquid in shallow convective clouds.
2. First order climate impact = Global cooling, Snowball Earth. 3. Tuned experiment with reduced shortwave bias: brighter/cooler Southern
Ocean, dimmer/warmer Tropics, more poleward heat transport, stronger SH jet, and small tropical rainfall changes.
4. Dynamic ocean heat transport is crucial when assessing the influence of interhemispheric temperature gradients on climate.
Summary: Climate impacts of “fixing”
Southern Ocean cloud/radiation biases
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Kay, J. E., Yettella, V.,. Medeiros, B. Hannay, C. and P. Caldwell (submitted May 2015), Global climate impacts of reduced Southern Ocean shortwave radiation bias in the Community Earth System Model, J. Climate Kay, J. E., Bourdages, L., Miller, N., Morrison, A., Yettella, V., Eaton, B. and H. Chepfer (in prep) Evaluation of cloud phase in the Community Atmosphere Model version 5 using spaceborne lidar observations, JAMES
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Tuned experiment is balanced and has reduced shortwave cloud
radiative effect RMSE
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Large Southern Ocean Shortwave Radiation Climate Model Biases
Trenberth and Fasullo 2010 SOCRATES Planning Team
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CAM5 Cloud Liquid Tendencies
Figure 2 - Kay et al. submitted
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CESM-CAM5 Large Ensemble Model Components
Kay et al. 2015 BAMS
Let’s use a global fully coupled climate model
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Reduced Southern Ocean biases in “tuned experiment”
Fully coupled 1850 runs: tuned experiment-control
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Small precipitation changes with reduced shortwave bias
Why can’t we reproduce the Hwang and Frierson (2013) extratropical-tropical teleconnection?
Solid=control Dash=tuned experiment
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Cross-equatorial atmospheric heat transport is what
generates tropical precipitation asymmetry.
Hwang and Frierson 2013
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Hypothesis: Southern Ocean clouds in climate models (including CESM-CAM5) contain
insufficient amounts of supercooled liquid water.
How can we reduce CAM5’s Southern Ocean cloud phase biases?
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Hwang and Frierson 2013
mechanism does not work in a fully
coupled model where ocean heat
transport can adjust.
Hwang and Frierson 2013
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Atmospheric subsidence
difference (tuned experiment -
control)
Large tropical circulation response only occurs
when ocean heat transport cannot change.
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Largest Northward ITCZ shift with fixed ocean heat transport.
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Let’s examine atmospheric heat transport change in slab and fully coupled experiments…