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Hierarchy of models for studying the climatology and structure of tropical cyclones
Isaac Held – DOE Climate Modeling PI Meeting, May 2014
Princeton U. researchers supported in part by DOE/Regional and Global Climate Modeling:
Wenyu Zhou, Andrew Ballinger, Tim Merlis (thanks) [See also poster on tropical upper tropospheric temperature trends
Stephan Fueglistaler, Tom Flannaghan ]
Aqua-planet fixed–SST and slab ocean
Rotating radiative-convective equilibrium in on doubly-periodic f-plane
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Claim:
we are entering the golden age of simulation of tropical cyclones in global models
{analogous to the state of simulation of
baroclinic eddies in the early days of atmospheric GCMs}
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Starting point is 50km (25 km) GFDL HiRAM model with realistic boundary conditions
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Model captures the seasonal cycle of hurricane frequency over various ocean basins
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Model captures ENSO effect on hurricane genesis frequency
El-Nino years minus La-Nina years (observation)
El-Nino years minus La-Nina years (C180HiRAM)
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Model simulates interannual variability and recent trends in frequency of Atlantic tropical cyclones
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Number of North Atlan0c tropical cyclones in each year Red: observa0ons Blue: model ensemble mean Shading: model spread
3-member ensemble CMIP5 50km time-slice model Using observed ocean temperatures and sea ice
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TCs
Hurricanes
Hurricanes/TCs
Effect of change in convection scheme on TCs in HiRAM
Inhibiting parameterized convection =>
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Sensitivity of global mean frequency to ”divergence damping” in dynamical core
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Aqua-planet with zonally symmetric climate and imposed fluxes that move ITCZ into Northern Hemisphere – Tim Merlis
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Tim Merlis, Princeton 2012
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Tim Merlis, Princeton 2012
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Typically, number of TCs decreases with global warming with realistic boundary conditions But in the aqua-planet configuration, the number increases because ITCZ moves poleward Understanding this result has 3 distinct parts -- how does the ITCZ move with warming -- how does the TC number change with ITCZ latitude -- how does the TC number change with warming with fixed ITCZ latitude
Tim Merlis, et al – GRL 2013
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As tropical SST distribution is flattened, number of TCs decreases but intensity increases
(ACE increases)
Why?
Andrew Ballinger et al, to be submitted
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297K 305K 301K
Near surface wind speed
End state of homogeneous model of tropics is closely-packed hurricanes!
Rotating radiative convective equilibrium on doubly-periodic f-plane
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Lots of interesting parameter dependencies
Example: Radius of maximum winds increases with increase in
environmental rotation rate despite decrease in the size of storms
Wenyu Zhou et al, J. Atmos. Sci 2014
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Plausibly realistic TC statistics in global models just the starting point
Idealized models retaining GCM physics/numerics but with simplified boundary conditions/geometry
are valuable for
1) developing an understanding of tropical storm statistics, and
2) understanding differences between global models
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HadISST
Hurrell/CMIP5