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Hurricane type vortices in ECHAM5. How will they change in a warmer climate? Lennart Bengtsson with Kevin Hodges, ESSC Monika Esch and co-workers at MPI Tropical cyclones in a future climate what could be expected?

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hurricane type vortices in echam5 how will they change in a warmer climate

Hurricane type vortices in ECHAM5. How will they change in a warmer climate?

Lennart Bengtsson

with

Kevin Hodges, ESSC

Monika Esch and co-workers at MPI

MPI for Meteorology

Hamburg

tropical cyclones in a future climate what could be expected
Tropical cyclones in a future climatewhat could be expected?
  • Higher SST and higher atmospheric moisture would generally favor more intense storms ( e.g. Emanuel 1988, 1999)
  • This is supported by modeling results by Knutson and Tuleya (2004) driving an limited area model with CMIP2+ boundary data ( 9 different models).
  • Increasing vertical wind-shear and reduced relative humidity would counteract this tendency. Such influences occur in the tropical N. Atlantic during El Nino.
  • How will the number of storms change? What are the general conditions controlling the number of tropical storms?
  • What are the critical conditions in modeling tropical storms? Are results from large scale models with limited resolution credible?

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slide3

After Emanuel

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modeling approach
Modeling approach
  • Simulation of tropical cyclones per se in a global GCM
  • Using limited area models at high resolution
  • Identifying climate predictors in a GCM

(SST, vorticity, static stability, relative humidity, vertical wind-shear)

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simulation of tropical cyclones per se in a global gcm
Simulation of tropical cyclones per se in a global GCM
  • Disadvantage: Difficulties to resolve the intense features of a tropical storm
  • Examples of studies:

Bengtsson et al. 1995, 1996, Tellus

Sugi et al. 2002, JMS, Japan

McDonald et al. 2005, Clim.Dyn.

Chauvin et al. 2006, Clim Dyn.

Oouchi et al. 2006, JMS, Japan

Yoshimori et al. 2006, JMS, Japan

MPI for Meteorology

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simulation of hurricane type vortices with a gcm
Simulation of hurricane type vortices with a GCM
  • Typical criteria:
  • An identifiable vortex
  • A minimum in the surface pressure
  • Surface wind speed above a given value
  • A warm core vortex ( reduced circulation with height)

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slide7

Effect of 2xCO2 From Bengtsson et al., 1996 (Tellus) ( number of cyclones /basin)

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using limited area models at high resolution
Using limited area models at high resolution
  • Disadvantages:

Generation of storms, Large scale influences difficult to handle

  • Examples of studies:

Knutson et al. 1998, Science

Knutson and Tuleya, 1999, Clim. Dyn.

Knutson and Tuleya, 2004, J Clim.

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slide9
Impact of CO2-induced warming on simulated hurricane intensityKnutson and Tuleya (2004, J of Climate)
  • They used a high resolution limited area model driven by the SST and moisture of 9 CGCM from the CMIP 2+ project.
  • CMIP2 uses 1%yr-1 increase over an 80-year period implying an increase by a factor of 2.2.
  • Model calculations are undertaken in NW Pacific-, NE Pacific- and Atlantic basin
  • Four different convective schemes are tested (no significant differences)
  • RESULTS:
  • Max. surface wind speed increases by 6%
  • Min. central pressure by 14%
  • Max. precipitation by 24%
  • Hurricane increase by a factor of 1/2 in the Simpson-Saffir scale

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identifying climate predictors in a gcm
Identifying climate predictors in a GCM
  • Disadvantage:

Lack of proper understanding, ad hoc selection of predictors, overestimation of the effect of SST

  • Examples of studies

Gray, 1979 Met. over the Tropical oceans, RMetSoc

Royer et al. 1998, Clim. Change

Chauvin et al. 2006, Clim. Dyn.

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objectives of the present study hurricane type vortices htv in echam5
Objectives of the present studyHurricane type vortices (HTV) in ECHAM5

.

How do the HTVs respond to anthropogenic climate change and how does this depend on resolution?

What changes occur in intensity, life time and power dissipation index?

What possible mechanisms control the change in HTV? What are the dominant factors?

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tracking methods and vortex identification
Tracking methods and vortex identification
  • Tracks are followed from its generation (6x10-6s-1) until it disappears as an extra-tropical cyclones north of 60N
  • We calculate the total life-time of the HTVs
  • We are able to identify the transition from a tropical to an extra-tropical vortex
  • Alternatively we use the wind speed at 925 hPa as a selection criteria for intense storms
  • We have also calculated the potential dissipation index, PDI. See Emanuel, Nature, 2005

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hurricane katrina august 2005 ecmwf operational analyses 850 hpa vorticity
Hurricane Katrina August 2005ECMWF operational analyses, 850 hPa vorticity

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katrina vorticity at different levels
Katrina vorticity at different levels

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selection of htv indicators
Selection of HTV indicators
  • We use criteria for minimum vorticity at 850 hPa (V), minimum vertical vorticity gradient (G) between 850 and 250 hPa, and number of time steps of 6 hrs (T) when these conditions are fulfilled.
  • (V, G, T) = (6, 6, 4) vorticity at 850 hPa = 6x10-5s-1

vorticity 850- 250 hPa = 6x10-5s-1 conditions fulfilled at least 24 hrs

(6, 6, 4) is defined as an HTV

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selection of criteria for selecting hurricane type vortices htv

All Tropical Storms

Hurricanes, Typhoons, Cyclones

>33ms-1

(6, 6, 4)

(10, 6, 4)

(12, 6, 4)

2003

75

33

71

48

39

2004

72

36

79

52

41

2005

80

38

83

62

48

Selection of criteria for selecting hurricane type vortices (HTV)

MPI for Meteorology

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objectives of the present study hurricane type vortices htv in echam520
Objectives of the present studyHurricane type vortices (HTV) in ECHAM5
  • We have used scenario A1B and studied the periods 1861-1890, 1961-1990 and 2071-2100
  • We have explored the coupled T63 run (3 runs) for all periods and
  • T213 time - slice 1961-1990 and 2071-2100
  • T319 time - slice 1971-1990, (2081-2100 to be done)
  • We have also used AMIP2 runs (20 years) with T63 and T159 as a validation study

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comparison with observations from the tropical warning centers and with era 40 re analyses
Comparison with observations from the Tropical Warning Centers and with ERA-40 re-analyses

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slide22
Super Typhoon 21 (1991) in ERA-40 (left)and selected similar storm in ECHAM5 (right)Intensity (vorticity at 850 hPa)

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lifetime of htvs in days
Lifetime of HTVs in days

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hurricane track density a observed b era 40 and c echam5 t159
Hurricane track density(a) observed, (b) ERA-40 and (c ) ECHAM5 T159

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hurricane track density atlantic d observed e era 40 and f echam5 t159
Hurricane track density (Atlantic)(d) observed, (e) ERA-40 and (f ) ECHAM5 T159

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hurricane type vortices 21 years echam era 40 jra 25 and observed

All (6, 6, 4)

>18ms-1

>33ms-1

>2x10-4

>50ms-1

>5x10-4

>1x10-3

ECHAM5 T159

2073

1848

599

24

ECHAM5 T159

2017

731

72

ERA40

1747

1557

40

0

ERA40

1447

297

1

JRA25

1306

933

7

0

JRA25

1234

323

2

Observed

1236

724

346

Hurricane type vortices (21 years)ECHAM, ERA-40, JRA 25 and Observed

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slide33

Active Atlantic years - non active years (left)Active West-Pacific years - less active (right)From above: SST, 850 hPa Vel. Pot., windshear 250-850 pPa

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slide34

Courtesy

J. O’Brien

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the same era 40 response active atlantic left active west pacific right
The same ERA-40 responseActive Atlantic (left), Active West Pacific (right)

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are there observational evidence that hurricanes are becoming more intense
Are there observational evidence that hurricanes are becoming more intense?

Why is it so difficult?

  • Longer term records are needed due to internal variability
  • There have been large changes in the observing systems making it easier to detect more tropical cyclones in later years.
  • Recent papers have used PDI ( time integral of max. wind cube) which is overly sensitive to observational accuracy
  • Model studies (e.g. Knutson and Tuleya, 2004) indicate small changes in intensity as of now which are hardly detectable

MPI for Meteorology

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are there observational evidence that hurricanes are becoming more intense37
Are there observational evidence that hurricanes are becoming more intense?
  • YES
  • Webster et al. (2005), Science, Emanuel (2005), Nature, Sriver and Huber (2006), GRL
  • NO
  • Chan (2006), Science, Klotzbach ( 2006), GRL, Landsea et al. (2006), Science

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slide38
There are recent claims that there is an increase in hurricane intensity ( e.g. Emanuel (2005), Webster et al. (2005)
  • Are these findings credible?
  • They are generally not supported by operational meteorologists
  • According to Knutson and Tuleya (2004) any changes are probably undetectable “for decades to come”
  • Results from this study and some additional work may presumably reduce the likelihood of detection further
  • There are structural problems in the detection of trends
  • Changes in observing systems
  • Difficulties to separate a genuine change in storms from societal causes behind the huge increase in damages and damage cost

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what may happen in a warmer climate

What may happen in a warmer climate?

We have used the AIB scenario

And the coupled MPI model at T63 resolution used in the IPCC 4th assessment

Higher resolution experiments use the transient SST from T63 (time - window)

We study C20 (1961- 1990)

C21 ( 2071-2100)

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what is a1b
What is A1B?
  • Middle of the line scenario
  • Carbon emission peaking in the 2050s (16 Gt/year)
  • CO2 reaching 450 ppm. in 2030
  • CO2 reaching 700 ppm. in 2100
  • SO2 peaking in 2020 then coming done to 20% thereof in 2100

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sst difference c 21 c 20
SST difference (C 21-C 20)

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number of htvs t63 for c 20 and c21 for wind speed and vorticity

T63

T63

All (6, 6, 4)

>18ms-1

>2x10-4 s-1

>33ms-1

>5x10-4 s-1

>50ms-1

>1x10-3 s-1

20C (1961-1990)

20C (1961-1990)

35

34

5.5

6.1

0

0.1

0

21C (2071-2100)

21C (2071-2100)

27

26

5.6

6.4

0.1

0

0

Number of HTVs (T63) for C 20 and C21 for wind speed and vorticity

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number of htvs year t213 for c 20 and c21 for wind speed and vorticity

T213

T213

All (6, 6, 4)

>18ms-1

>2x10-4 s-1

>33ms-1

>5x10-4 s-1

>50ms-1

>1x10-3 s-1

20C (1961-1990)

20C (1961-1990)

104

100

97

33

40

3.7

6.0

21C (2071-2100)

21C (2071-2100)

94

92

90

36

4.9

49

9.8

Number of HTVs/year (T213) for C 20 and C21 for wind speed and vorticity

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change in htv lifetime t213
Change in HTV lifetime (T213)

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change in min surface pressure t213
Change in min. surface pressure (T213)

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slide53

Accumulated precipitation ( in mm and for an area with a radius of 5 degrees) along the track of the HTV for C20 and C21(T213)HTVs reaching >33ms-1

C20

C21

Total increase 30%

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climate change and the water cycle
Climate change and the water cycle
  • Atmosphere appears to conserve relative humidity. This means that atmospheric water vapor follows Clausius- Clapeyron relation. (Soden and Held, 2006, J. Clim.)
  • We see an increase of 27% in atmospheric water vapor at C21 compared to C20
  • Precipitation must be balanced by evaporation. Evaporation is driven by the surface energy balance which increases slower than atmospheric water vapor. In fact it can even diminish at a high aerosol concentration (ECHAM4).
  • Precipitation increases by 6% both globally and in the tropics
  • This means that the residence time of water in the atmosphere increases from 8.7 to 10.3 days or by 16%.

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result from present study reduced number of hurricanes
Result from present study:Reduced number of hurricanes
  • There is s reduction in the number of tropical cyclones in agreement with most previous studies.
  • We suggest this is due to a weakening of the tropical circulation. This can best be seen as a slowing down of the hydrological cycle by some 16%
  • The radiative cooling of the tropics increases due to more water vapor in the upper troposphere and dynamical cooling due to increased static stability. This processes can compensate warming from release of latent heat (6%) with a less active tropical circulation.

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result from present study more intense hurricanes
Result from present study:More intense hurricanes
  • Given favorable atmospheric condition we suggest the ideas put forward by Emanuel and Holland comparing a hurricane with a Carnot cycle can be applied.
  • This will provide an energy input broadly proportional to the specific humidity at a higher temperature (following SST)
  • The intensification of the tropical cyclones depends on sufficient model resolution to accurately describe the convergence of momentum which generate the very high wind speeds at the core of the vortex.
  • For HTVs reaching 33ms-1 PDI (power dissipation index) increase by 16%

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htvs at c20 at four different resolutions number year as a function of vorticity
HTVs at C20 at four different resolutionsNumber/year as a function of vorticity

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slide59

END

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