Post on 17-Dec-2015
25 October 2006 MPI for MeteorologyHamburg
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
25 October 2006 MPI for MeteorologyHamburg
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?
25 October 2006 MPI for MeteorologyHamburg
After Emanuel
25 October 2006 MPI for MeteorologyHamburg
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)
25 October 2006 MPI for MeteorologyHamburg
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
25 October 2006 MPI for MeteorologyHamburg
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)
25 October 2006 MPI for MeteorologyHamburg
Effect of 2xCO2 From Bengtsson et al., 1996 (Tellus) ( number of cyclones /basin)
25 October 2006 MPI for MeteorologyHamburg
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.
25 October 2006 MPI for MeteorologyHamburg
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
25 October 2006 MPI for MeteorologyHamburg
Intensification of hurricanes at 2xCO2
Knutson and Tuleya (2004)
25 October 2006 MPI for MeteorologyHamburg
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.
25 October 2006 MPI for MeteorologyHamburg
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?
25 October 2006 MPI for MeteorologyHamburg
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
25 October 2006 MPI for MeteorologyHamburg
Hurricane Katrina August 2005ECMWF operational analyses, 850 hPa vorticity
25 October 2006 MPI for MeteorologyHamburg
Katrina vorticity at different levels
25 October 2006 MPI for MeteorologyHamburg
Katrina850 hPa vorticity at different resolution
25 October 2006 MPI for MeteorologyHamburg
KatrinaWind speed max. and surface pressure
min.
25 October 2006 MPI for MeteorologyHamburg
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
25 October 2006 MPI for MeteorologyHamburg
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
25 October 2006 MPI for MeteorologyHamburg
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
25 October 2006 MPI for MeteorologyHamburg
Comparison with observations from the Tropical Warning Centers
and with ERA-40 re-analyses
25 October 2006 MPI for MeteorologyHamburg
Super Typhoon 21 (1991) in ERA-40 (left)and selected similar storm in ECHAM5 (right)
Intensity (vorticity at 850 hPa)
25 October 2006 MPI for MeteorologyHamburg
Lifetime of HTVs in days
25 October 2006 MPI for MeteorologyHamburg
Hurricane genesis (a) observed, (b) ERA-40 and (c ) ECHAM5 T159
25 October 2006 MPI for MeteorologyHamburg
Hurricane track density(a) observed, (b) ERA-40 and (c ) ECHAM5 T159
25 October 2006 MPI for MeteorologyHamburg
Hurricane track density (Atlantic)(d) observed, (e) ERA-40 and (f ) ECHAM5 T159
25 October 2006 MPI for MeteorologyHamburg
Hurricane type vortices (21 years)ECHAM, ERA-40, JRA 25 and Observed
>18ms-1 >33ms-1 >50ms-1
ECHAM5 T159 2017 731 72
ERA40 1447 297 1
JRA25 1234 323 2
Observed 1236 724 346
All (6, 6, 4) >2x10-4 >5x10-4 >1x10-3
ECHAM5 T159 2073 1848 599 24
ERA40 1747 1557 40 0
JRA25 1306 933 7 0
25 October 2006 MPI for MeteorologyHamburg
ECHAM 5 max. wind speed in different regions
25 October 2006 MPI for MeteorologyHamburg
ERA-40 max. wind speed in different regions
25 October 2006 MPI for MeteorologyHamburg
“Observed” max. wind speed in different regions
25 October 2006 MPI for MeteorologyHamburg
Annual variability of HTVsObserved, ERA-40 and ECHAM5- T159
25 October 2006 MPI for MeteorologyHamburg
Interannual variability of HTVs > 33ms-1
Observed, ERA-40 and ECHAM5- T159
25 October 2006 MPI for MeteorologyHamburg
Active Atlantic years - non active years (left)Active West-Pacific years - less active (right)
From above: SST, 850 hPa Vel. Pot., wind shear 250-850 pPa
25 October 2006 MPI for MeteorologyHamburg
Courtesy
J. O’Brien
25 October 2006 MPI for MeteorologyHamburg
The same ERA-40 responseActive Atlantic (left), Active West Pacific (right)
25 October 2006 MPI for MeteorologyHamburg
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
25 October 2006 MPI for MeteorologyHamburg
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
25 October 2006 MPI for MeteorologyHamburg
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
25 October 2006 MPI for MeteorologyHamburg
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)
25 October 2006 MPI for MeteorologyHamburg
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
25 October 2006 MPI for MeteorologyHamburg
SST difference (C 21-C 20)
25 October 2006 MPI for MeteorologyHamburg
HTVs at T63 resolution C19 (black), C20 (red) and C21(blue)
25 October 2006 MPI for MeteorologyHamburg
Number of HTVs (T63) for C 20 and C21 for wind speed and vorticity
T63 >18ms-1 >33ms-1 >50ms-1
20C (1961-1990) 34 6.1 0.1
21C (2071-2100) 26 6.4 0.1
T63 All (6, 6, 4) >2x10-4 s-1 >5x10-4 s-1 >1x10-3 s-1
20C (1961-1990) 35 5.5 0 0
21C (2071-2100) 27 5.6 0 0
25 October 2006 MPI for MeteorologyHamburg
Hurricane genesis T63From top C21-C20, C21 and C20
25 October 2006 MPI for MeteorologyHamburg
Hurricane track density T63From top C21-C20, C21 and C20
25 October 2006 MPI for MeteorologyHamburg
Changes of HTVs in four hurricane regions (T63)
25 October 2006 MPI for MeteorologyHamburg
Number of HTVs/year (T213) for C 20 and C21 for wind speed and vorticity
T213 All (6, 6, 4) >2x10-4 s-1 >5x10-4 s-1 >1x10-3 s-1
20C (1961-1990) 104 97 40 6.0
21C (2071-2100) 94 90 49 9.8
T213 >18ms-1 >33ms-1 >50ms-1
20C (1961-1990) 100 33 3.7
21C (2071-2100) 92 36 4.9
25 October 2006 MPI for MeteorologyHamburg
Hurricane genesis T213 From top C21-C20, C21 and C20
25 October 2006 MPI for MeteorologyHamburg
Hurricane track density, T213From top C21-C20, C21 and C20
25 October 2006 MPI for MeteorologyHamburg
Changes of HTVs in four hurricane regions (T213)
25 October 2006 MPI for MeteorologyHamburg
Change in HTV lifetime (T213)
25 October 2006 MPI for MeteorologyHamburg
Change in min. surface pressure (T213)
25 October 2006 MPI for MeteorologyHamburg
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
C21
C20
Total increase 30%
25 October 2006 MPI for MeteorologyHamburg
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%.
25 October 2006 MPI for MeteorologyHamburg
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.
25 October 2006 MPI for MeteorologyHamburg
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%
25 October 2006 MPI for MeteorologyHamburg
What is the effect of even higher resolution?
25 October 2006 MPI for MeteorologyHamburg
HTVs at C20 at four different resolutionsNumber/year as a function of vorticity
25 October 2006 MPI for MeteorologyHamburg
END