atlantic hurricanes and climate change: modeling studies hurricane katrina, aug. 2005 gfdl model...
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![Page 1: Atlantic Hurricanes and Climate Change: Modeling Studies Hurricane Katrina, Aug. 2005 GFDL model simulation of Atlantic hurricane activity Tom Knutson](https://reader036.vdocuments.us/reader036/viewer/2022062304/56649d545503460f94a305c6/html5/thumbnails/1.jpg)
Atlantic Hurricanes and Climate Change: Modeling Studies
Hurricane Katrina, Aug. 2005
GFDL model simulation of Atlantic hurricane activity
Tom Knutson
Geophysical Fluid Dynamics Lab/NOAAPrinceton, New Jersey
http://www.gfdl.noaa.gov/~tk
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Collaborators:
Joe SirutisIsaac HeldGabe VecchiBob TuleyaMorris BenderSteve GarnerMing ZhaoS.-J. Lin
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Source:Vecchi et al. Science (2008)
Projection 1: Absolute SST
• ~300% projected increase in Power Dissipation
• Indirect attribution: CO2 SST Hurricanes
Projection 2: Relative SST
• Projected change: sign uncertain, +/- 80%
• No Attribution
Statistical projections of 21st century Atlantic hurricane activity have a large dependence on the predictor used.
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Zetac Regional Model reproduces the interannual variability and trend of Atlantic hurricane counts (1980-2006)
18-km grid model nudged toward large-scale (wave 0-2) NCEP Reanalyses
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Source: Knutson et al., 2007, Bull. Amer. Meteor. Soc.
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Projected Atlantic region climate changes: 18-Model CMIP3 ensemble
Higher shear
Higher potential intensity
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1) Decreased frequency of tropical storms (-27%) and hurricanes (-18%).
3) Caveat: this model does not simulate hurricanes as strong as those observed.
The model provides projections of Atlantic hurricane and tropical storm frequency changes for late 21st century, downscaled from a multi-model ensemble climate change (IPCC A1B scenario):
Storm Intensities (Normalized by frequency)
4) A more consistent intensity increase is apparent after adjusting for decreased frequency
2) Increased frequency and intensity of the strongest hurricanes(5 12)
Source: Knutson et al., 2008, Nature Geoscience.
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Zetac Model Downscaling:
• 9 individual CMIP3 models now completed
• (plus the 18-model ensemble mean)
• A consistent decrease in tropical storms becomes a mixture of decreases and increases for major (Cat 3) hurricanes (in terms of central pressure)
Provisional Results: Do not quote or cite.
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Tropical Cyclones Frequency Projections (Late 21st century) - Summary
Blue = decrease
Red = increase
Source: Knutson et al. 2010
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The new model simulates increased hurricane rainfall rates in the warmer climate (late 21st century, A1B scenario) …consistent with previous studies…
Present Climate Warm Climate
Warm Climate – Present Climate
Rainfall Rates (mm/day)
Avg. Rainfall Rate Increases: 50 km radius: +37%100 km radius: +23%150 km radius: +17%400 km radius: +10%
Average Warming: 1.72oC
Source: Knutson et al., 2008, Nature Geoscience.
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A further downscaling step to 9-km triply nested GFDL hurricane modelMorris Bender, et al., Science, Feb. 2010.
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18km grid Zetac regional climate model
9 km GFDL hurricane model
observed
Simulated distributions of maximum wind speeds(Atlantic tropical storms and hurricanes, 1980-2006)
Down-scaled GFDL hurricane prediction model produced much more realistic distribution of maximum wind speeds compared to Zetac.
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20 30 40 50 60 70 80Maximum Wind Speed (m/s)
No
rmalized
occu
rrences
Source: Bender, et al., Science, 2010.
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Distributions (CDF’s) of Atlantic tropical cyclone intensities (1980-2006).
Red: 1980-1994 (inactive)Blue: 1995-2006 (active)
Fraction of storms above indicated intensity
Fraction of storms above indicated intensity
Source: Bender et al., Science, 2010.
GFDL Hurricane Model intensity distribution is also shifted to higher intensities in active years, but the difference is smaller than observed.
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Active era
Inactive era
Active era
Inactive era
Maximum Wind Speed (m/s)
Observed intensities
Simulated intensities
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12 In a warmer climate (late 21st century A1B scenario) the hurricane model simulates an expanded distribution of Atlantic hurricane intensities.
The strongest hurricanes increase in number for the downscaled ensemble-mean climate warming…
…and increase for 3 of 4 individual climate models tested.
Control
Source: Bender et al., Science, 2010.
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Late 21st Century Climate Warming Projection-- Average of 18 CMIP3 Models
(27 Simulated Hurricane Seasons) Source: Bender et al., Science, 2010
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Control Climate (Odd Years Only)
GFDL-CM2.1
MRI-CGCM
MPI-ECHAM5
UKMO-HADCM3
Degrees Longitude East
Deg
rees Latitu
de
NWS VERSION (GFDL)
Tracks of Storms that Reached Category 4 or 5 Intensity
Degrees Longitude East Degrees Longitude East
Deg
rees Latitu
de
Late 21st Century Warmed Climate Projection based on 4 Individual CMIP3 Climate Models
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SUMMARY OF PROJECTED CHANGE
• Colored bars show changes for the18 model CMIP3 ensemble (27 seasons); dots show range of changes across 4 individual CMIP models (13 seasons).
Cat 4+5 frequency: 81% increase, or 10% per decade
Source: Bender et al., Science, 2010.
Estimated net impact of these changes on damage potential:
+28%
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Emergence Time Scale: If the observed Cat 4+5 data since 1944 represents the noise (e.g. through bootstrap resampling), how long would it take for a trend of ~10% per decade in Cat 4+5 frequency to emerge from noise? Answer: ~60 yr (by then 95% of cases are positive)
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Source: Bender et al., Science, 2010.
Instead, assume residuals from a 4th order polynomial: 55 yr
Instead, resample chunks of length 3-7 yr: 65-70 yr
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GFDL HIRAM 50km grid global model: Simulated vs Observed Tropical Storm Tracks (1981-2005)
Source: Zhao et al. J. Climate (2009)
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HIRAM 50 km grid model TC correlations
for several basins
North Atlantic
East Pacific
West Pacific
red: observationsblue: HiRAM ensemble meanshading: model uncertainty
corr=0.83
corr=0.62
corr=0.52
Hurricane counts for each basinare normalized by atime-independent multiplicative factor
Correlation for the SouthPacific is ~0.3 and insignificantfor the Indian Ocean
Source: Zhao, Held, Lin, and Vecchi (J. Climate, 2009)
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Projected Changes in Regional Hurricane Activity
GFDL 50-km HIRAM, using four projections of late 21st Century SSTs.
Red/yellow = increaseBlue/green = decrease
• Regional increases/decreases much larger than global-mean.
• Pattern depends on details of SST change.
Source: Zhao, Held, Lin and Vecchi (J. Climate, 2009)
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Unit: Number per year
18-model CMIP3 Ensemble GFDL CM2.1
HadCM3 ECHAM5
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Global Model Tropical Cyclone Climate Change Experiments: Use A1B Scenario late 21st century projected
SST changes from several CMIP3 models
GFDL CM2.1 HadCM3
ECHAM5 CMIP3 18-model Ensemble
Source: Zhao, Held, Lin, and Vecchi (J. Climate, in press)
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Unit: Deg C
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Conclusions:
i) GFDL model late 21st century (ensemble) projections suggest a decrease in the number of hurricanes in the Atlantic (-24% to -32%), but nearly a doubling in the frequency of very intense (Cat 4-5) hurricanes by 2100. Preciptation rates in hurricanes are also projected to increase (~20% within 100km of storm center).
ii) Based on present understanding, we would not expect a 10%/decade increase in Cat 4-5 frequency, if it occurred, to be detectable for a number of decades.
iii) Not all individual climate models (when downscaled) produce an increase in Cat 4-5 frequency as was simulated for the 18-model ensemble climate change signal.
iv) Remaining caveats include model limitations for the projected SST patterns (e.g., clouds; indirect aerosols); limitations of intense hurricane simulations; and observed data concerns (i.e., cat 4-5 record).
v) Future work: expanding TC downscaling work to other basins; statistical approaches to downscaling are also being pursued (Villarini/Smith et al.; Held and Zhao)
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