22 nov. 2007 essctc in a warmer climate? lennart bengtsson how might tropical cyclones change in a...

22 Nov. 2007 ESSC TC in a warmer climate?Lennart Bengtsson

How might Tropical Cyclones change in a Warmer Climate?

Professor Lennart BengtssonESSCMany thanks to Kevin Hodges andto colleagues in UK, Germany and Japan


Tropical Cyclones in a warmer climate

• Societal damages are to a large extent related to extreme weather mostly in association with tropical and extra-tropical cyclones

• The cost for hurricane Katrina is estimated to some 100 G$

• There are many examples of great loss of life due to extreme tropical cyclones.

• Damages are mostly related to high winds, flooding due to the high precipitation and in coastal areas to high sea-level and waves.

• The question whether cyclones may intensify in a future climate is consequently an issue of primary importance for society.

• This is further enhanced by the ongoing increased exposure to extreme weather independent of climate change.


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.

• Some GCM indicate reduced number of cyclones in a warmer climate

• 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?



• Present observational evidence

• What can we expect based on theory?What can we expect based on theory?

• Modeling approachModeling approach

• Model experimentModel experiment

• ResultsResults

• Concluding remarksConcluding remarks


What is a tropical cyclone?

• It is a warm core vortex developing over tropical oceans. The maximum intensity occurs around 900hPa.

• Tropical cyclones are classified according to maximum intensity• Tropical depression <17m/s• Tropical storm 17-33 m/s• Hurricane/Typhoon in categories 1 to 5• 1: 33-42, 2: 43-49, 3: 50-58, 4: 59-69, 5: >69

• Actual Archive of TC, see: • http://weather.unisys.com/hurricane/


Are there observational evidence that tropical cyclones are becoming more

frequent, 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.

• Some recent studies have used PDI ( time integral of max. wind cube) which is overly sensitive to observational accuracy

• Most researchers agree that there is no global increase in tropical cyclones, but there are different views concerning the trend in the Atlantic.


Is the number of North Atlantic tropical cyclones significantly underestimated prior to the availability of satellite observations?

by Edmund Chang and Yanjuan GuoGRL, doi:10.1029/2007GL030169, 2007

(table LB in a comment to GRL)

A:1920-1965 B:1966-2005 Ratio B/A

All 347 444 1.28

US (landfall) 136 130 0.96

CC (landfall) 109 95 0.87

NS(within 300km from land)

56 125 2.23

OO (over ocean) 46 94 2.04


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”

• 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




• What can we expect based on theory?

• Modeling approach

• Model experiment

• Results

• Concluding remarks


After Emanuel The Carnot cycle concept


What happens to the hydrological cycle in a warmer climate?See e.g. Held and Soden, 2006, J. of Clim.

• Observations and models show that water vapor follows temperature according to Clausius- Clapeyron expression.

• The increase in precipitation is much slower.• This increases the residence time of water in the atmosphere.• This reduces the large scale vertical mass flux• This slows down the large scale circulation


Clausius - Clapeyron expression

• A typical value of alfa in the lower troposphere is

0.07/K or 7% increase in the saturation water vapor for each 1K in temperature


Globally integrated water vapor 1979-2005From Held and Soden, 2006

DT +0.45K

Des +3%

Full line GFDL model

Dashed line measurements


Semenov and Bengtsson, 2002Secular trends in daily precipitation characteristics

Clim.Dyn. 123-140

+30%


Top, IPCC models: massflux, water vapor and precipitation Below, GFDl model

P = Mq ( From Held and Soden (2006))


Energy balance for upward and downward motion

Follows Lq Qc = P







• Results



Modeling approach

• Direct simulation of tropical cyclones 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)


Simulation of tropical cyclones with a GCM

• Typical criteria:

• An identifiable vortex ( often at 850 hPa)

• A minimum in the surface pressure

• Surface wind speed above a given value (> 18m/s)

• A warm core vortex ( reduced circulation with height)


Direct simulation of tropical cyclones 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


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.


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


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.







• Results



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 TCs• 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


Objectives of the modeling studyTropical cyclones (TC) in ECHAM5 in the

Northern Hemisphere(Using high global resolution)

How do the TCs 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 TC? What are the dominant factors?


Hurricane Katrina August 2005ECMWF operational analyses, 850 hPa vorticity


Katrina vorticity at different levels


Selection of TC 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 a TC


Selection of criteria for selecting tropical storms (TC)

(using ECMWF operational analyses)

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


Objectives of the present studyTropical cyclones 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, and 2080-2100• We have also used three AMIP2 runs (20 years) with

T63 and T159 as a validation study







• Results



Structure of modeled tropical cyclones

This shows the averaged structure of the 100 most intense storms at the time when the

reach their maximum intensity


Tangential (left) and Radial winds (right) for the T63 resolution. Negative values inflow. Average of 100 tropical cyclones. At the time of maximum wind. Radius 5 degrees.

South


Tangential (left) and Radial winds (right) for the T213 resolution. Negative values inflow. Average of 100 tropical cyclones. Radius 5 degrees.

The flow is predominantly inward to the rear and left of the storm and outward to the front and right (Frank 1977 MWR)

Observations:


Tangential (left) and Radial winds (right) for the T319 resolution. Negative values inflow. Average of 100 tropical cyclones. Radius 5 degrees.


Hurricane Mitch, Oct -Nov. 1998Kepert 2006 JAS


Hurricane Mitch: Tangential winds (left) and Radial winds (right) at 500 m. Units: 5 m/s

Kepert 2006 JAS

outflow

West


Tangential wind cross sectionTemperature anomaly

T63 resolution


Tangential wind cross sectionTemperature anomaly

T213 resolution


Comparison with observations from the Tropical Warning Centers

and with ERA-40 re-analyses


Super Typhoon 21 (1991) in ERA-40 (left)and selected similar storm in ECHAM5 (right)

Intensity (vorticity at 850 hPa)


Hurricane genesis (a) observed, (b) ERA-40 and (c ) ECHAM5

T159


Hurricane track density(a) observed, (b) ERA-40 and (c ) ECHAM5

T159


Hurricane track density (Atlantic)(d) observed, (e) ERA-40 and (f ) ECHAM5

T159


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)


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


SST difference (C 21-C 20)


TCs at T63 resolution C19 (black), C20 (red) and C21(blue)


Hurricane genesis T213 From top C21-C20, C21 and C20


Hurricane track density, T213From top C21-C20, C21 and C20


Number of TCs/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


Accumulated precipitation ( in mm and for an area with a radius of 5 degrees) along the track of the TCs for

C20 and C21(T213)TCs reaching >33ms-1

C21

C20

Total increase 30%







• Results



Climate change and the water cycle

• Atmosphere appears to conserve relative humidity. This means that atmospheric water vapor follows Clausius- Clapeyron relation. (Held and Soden, 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%.


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.


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 TCs reaching 33ms-1 PDI (power dissipation index) increases by 16%


Why do we see a reduced number of tropical cyclones in a warmer climate?

• We suggest the following mechanism

• There is a reduction in the large-scale tropical circulation at climate warming due to increased static stability and a rapid increase in the amount of water vapor in the atmosphere.

• This generally leads to less favorable conditions to generate organized onset vortices which are seen as key atmospheric conditions for the generation of tropical storms.


Why do we see an intensification of the tropical storms in a warmer climate?

• We suggest the following process

• Due to chance favorable conditions are likely to evolve

• In such situations the mechanisms proposed by Emanuel may be applied, that means that the maximum wind speed is proportional to the sqr. of the available latent energy provided to the storm

• To demonstrate this with a GCM requires sufficient resolution ( say 50-100 km or less)


What is the effect of even higher resolution?

20-year simulation at T319 resolution


T319 Intensity changes 20C to 21C


Tropical cyclones in different regions, T319 resolution


Tropical cyclones in different regions, T213 resolution


Conclusions from the T319 resolution(40 km)

• Results confirm the finding from the T213 experiment

• The number of tropical cylones are reduced by some 10% at the end of the 21st century

• The stronger storms are becoming more intense (Cyclones with wind speeds >50m/s increase

from 12 to 17 per year compared to the end of the 20th century)

Intensification occurs in all tropical storm regions


END

22 nov. 2007 essctc in a warmer climate? lennart bengtsson how might tropical cyclones change in a...

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