Extra tropical cyclones in a warmer climate will they be more intense
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Extra-tropical cyclones in a warmer climate. Will they be more intense?. Professor Lennart Bengtsson ESSC Many thanks to Kevin Hodges, Noel Keenlyside and MPI modeling team. Extra-tropical cyclones.

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Extra tropical cyclones in a warmer climate will they be more intense

Extra-tropical cyclones in a warmer climate.Will they be more intense?

Professor Lennart Bengtsson

ESSC

Many thanks to Kevin Hodges, Noel Keenlyside and MPI modeling team

ETC in a warmer climate?

Lennart Bengtsson


Extra tropical cyclones

Extra-tropical cyclones

  • Societal damages due to mid-latitude cyclones are generally related to high winds and flooding.

  • According to Munich Re (2002) damages due to wind storms since 1950 were 324 G$ and insured losses 106 G$

  • Damages due to tropical cyclones dominate but extreme winter storms in Europe may cause annual damages of several GEuro.

  • The question whether cyclones may intensify in a future climate is consequently an issue of primary importance for society. There is evidence form both theory and model experiments that this may happen for tropical storms but will it also occur for extra-tropical storms?

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

ETC in a warmer climate?

Lennart Bengtsson


Extra tropical cyclones in a warmer climate will they be more intense1

Extra-tropical cyclones in a warmer climate.Will they be more intense?

  • We have addressed the following scientific objectives

  • How well does the ECHAM5 T213 model represents the dynamics and energetics of intense extra-tropical cyclones?

  • How is maximum wind speed and precipitation related to the life cycle of the cyclone?

  • What is the importance of resolution?

  • What changes might occur in a warmer climate?

ETC in a warmer climate?

Lennart Bengtsson


Background

Background

  • Are there any physical reasons that extra-tropical cyclones might become more intense in a warmer climate?

  • Do we have any evidence that extra-tropical cyclones have become more intense?

  • Are present GCM able to represent intense extra-tropical cyclones?

  • What are the evidence from climate change experiments?

ETC in a warmer climate?

Lennart Bengtsson


Do we have any evidence that extra tropical cyclones have become more intense

Do we have any evidence that extra-tropical cyclones have become more intense?

  • Extreme storms are rare and require long and reliable observational records. Indications are that several decades of homogeneous data are needed.

  • There are still general problems to detect extreme storms in sufficient details as observational records are insufficient, although the situation today is significantly better than in the past.

  • For this reason trend calculations must be critically assessed.

ETC in a warmer climate?

Lennart Bengtsson


Do we have any evidence that extra tropical cyclones have become more intense1

Do we have any evidence that extra-tropical cyclones have become more intense?

  • Several interesting studies have been published most are limited to Northern and Western Europe.

  • WASA group (1998)

  • Alexandersson et al. (2000)

  • Weisse et al, (2005)

  • Here are some findings from Weisse et al.(ibid)

  • A general increase in extreme cyclones (10m wind) from 1958 until 1990, therefter a weakening.

  • The pattern follows variations in the large scale atmospheric circulation (e.g. NAO)

  • There is no robust trend indicating an increase of extreme winds

ETC in a warmer climate?

Lennart Bengtsson


Extra tropical cyclones in a warmer climate will they be more intense

Longer term records using geostrophic winds indicate that extreme winds in Northwestern Europe were as intense in the end of the 19th century as in the end of the last century

,

IPCC, 2007

ETC in a warmer climate?

Lennart Bengtsson


Extra tropical cyclones in a warmer climate will they be more intense

Do we have any physical reasons why extra-tropical cyclones might become more intense in a warmer climate?

  • As extra-tropical storms depends on available potential energy (proportional to temperature variance in the low and middle troposphere). Changes here may effect the number and intensity of the storms.?

  • Release of latent heat is also important so more water vapor in the atmosphere may be important. Yes

  • Tropical storms move into the extra-tropics and may also contribute.Yes

  • Upper level cyclogenesis related to sharp gradients in potential vorticity may also contribute to low level intensification.?

ETC in a warmer climate?

Lennart Bengtsson


5 year global temperature change in the last two decades

5 year global temperature changein the last two decades

ETC in a warmer climate?

Lennart Bengtsson


Extra tropical cyclones in a warmer climate will they be more intense

Change in temperature at different levels (21C - 20C). Black 1000 hPa, red 850 hPa, green 600 hPa, blue 500 hPa, yellow 400 hPa, cyan 300hPa, magenta 200hPa. Left DJF, right JJA.

ECHAM 5, A1B, DT in 110 years

ETC in a warmer climate?

Lennart Bengtsson


Globally integrated water vapor 1979 2005 from held and soden 2006

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

DT +0.45K

Des+3%

Full line GFDL model

Dashed line measurements

ETC in a warmer climate?

Lennart Bengtsson


Clausius clapeyron expression

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

ETC in a warmer climate?

Lennart Bengtsson


Semenov and bengtsson 2002 secular trends in daily precipitation characteristics clim dyn 123 140

Semenov and Bengtsson, 2002Secular trends in daily precipitation characteristicsClim.Dyn. 123-140

+30%

ETC in a warmer climate?

Lennart Bengtsson


What happens to the hydrological cycle in a warmer climate see e g held and soden 2006 j of clim

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

ETC in a warmer climate?

Lennart Bengtsson


Extra tropical cyclones in a warmer climate will they be more intense

Top, IPCC models: massflux, water vapor and precipitation Below, GFDl modelP = Mq ( From Held and Soden (2006))

ETC in a warmer climate?

Lennart Bengtsson


What is new and typical of this study

What is new and typical of this study?

  • We explore a global climate simulation using a higher horizontal resolution than previously used in similar studies.

  • We use the ECHAM5 model at T213 resolution (ca 60km) and investigate 32 years from the 20th century, 1959-1990 (20C) and 32 years from the 21st century, 2069-2100 (21C). SST data are taken from a T63 coupled model.

  • We are making use of the A1B scenario

  • We explore transient storm track in a Lagrangian sense using data for every 6 hr.

ETC in a warmer climate?

Lennart Bengtsson


Selection of storm tracks

Selection of storm tracks

  • Level 850 hPa

  • Lifetime ≥ 48 hours

  • Intensity in vorticity ≥10-5s-1

  • Movements ≥1000km

ETC in a warmer climate?

Lennart Bengtsson


Storm track density and storm track genesis at 20c djf t213

Storm track density and storm track genesis at 20C DJF (T213)

ETC in a warmer climate?

Lennart Bengtsson


Extra tropical cyclones in a warmer climate will they be more intense

Storm tracks (DJF) over 32 years with maximum wind speed of 50m/s. Left 1959-1990 (20C), right 2069-2100 (C21), Scenario A1B. Model ECHAM5 (T213)

20C

21C

ETC in a warmer climate?

Lennart Bengtsson


Structure and evolution of extra tropical cyclones

Structure and evolution of extra-tropical cyclones

  • We explore here the 100 most intense cyclones in terms of maximum wind speed.

ETC in a warmer climate?

Lennart Bengtsson


Identification of extreme extra tropical events

Identification of extreme extra-tropical events

  • We identify cyclones by searching for maximum of 850 hPa vorticity using data for every 6 hrs.

  • We search for the maximum wind within a radius of 5° of the vorticity centre. Wind speed is determined at 925 hPa

  • We use maximum wind of 25m/s, 35m/s and 45 m/s. This corresponds broadly to 8Bf, 10Bf and 12 Bf at 10 m, respectively

  • We also use surface pressure minima and surface pressure tendencies(deepening rates)

ETC in a warmer climate?

Lennart Bengtsson


Extra tropical cyclones in a warmer climate will they be more intense

Development of an intense extra-tropical cyclone ( composite of the 100 most intense storms) (DJF) at 20C. Time units are in 6 hours centered at the time of minimum pressure. Evolution of central pressure, vorticity, wind speed and precipitation

30 hours

ETC in a warmer climate?

Lennart Bengtsson


Extra tropical cyclones in a warmer climate will they be more intense

Composite vortex (100 most intense) at maximum intensificationleft pressure and wind, right pressure and precipitation20C, DJF. Movement of cyclone is to the right.

mm/hour

ETC in a warmer climate?

Lennart Bengtsson


Extra tropical cyclones in a warmer climate will they be more intense

Temperature(500-850 hPa) at maximum deepening rate

ETC in a warmer climate?

Lennart Bengtsson


Vortex vertical structure wind field at maximum intensification

Vortex vertical structureWind field at maximum intensification

ETC in a warmer climate?

Lennart Bengtsson


Extra tropical cyclones in a warmer climate will they be more intense

Composite vortex (100 most intense) at maximum precipitationleft pressure and wind, right pressure and precipitation. 15 hrs later20C, DJF Movement of cyclone is to the right

ETC in a warmer climate?

Lennart Bengtsson


Extra tropical cyclones in a warmer climate will they be more intense

Composite vortex (100 most intense) at maximum intensityleft pressure and wind, right pressure and precipitation. 15 hrs later20C, DJF Movement of cyclone is to the right

ETC in a warmer climate?

Lennart Bengtsson


Vortex vertical structure wind field at maximum intensity

Vortex vertical structureWind field at maximum intensity

ETC in a warmer climate?

Lennart Bengtsson


Extra tropical cyclones in a warmer climate will they be more intense

Temperature(500-850 hPa) at minimum pressure

ETC in a warmer climate?

Lennart Bengtsson


Vertical tilt of the composite cyclone 36 hours before and after maximum intensity

Vertical tilt of the composite cyclone 36 hours before and after maximum intensity

ETC in a warmer climate?

Lennart Bengtsson


Changes in eddy available potential energy pe and kinetic energy ke units in mj m 2 and 6hours

Changes in eddy available potential energy (PE) and kinetic energy (KE). Units in MJ/m2 and 6hours

ETC in a warmer climate?

Lennart Bengtsson


Comparison with era 40

Comparison with ERA-40

  • The number of cyclones and their relative intensity is virtual identical to model results

  • ERA-40 underestimates precipitation ( initialization problems)

  • ERA-40 underestimate wind speed maximum and vorticity ( mainly resolution reasons, interimanalysis agreees better with the model results)

ETC in a warmer climate?

Lennart Bengtsson


Number of cyclones at t63 and t213 as a function of vorticity at t42 resolution

Number of cyclones at T63 and T213 as a function of vorticity at T42 resolution

ETC in a warmer climate?

Lennart Bengtsson


Extra tropical cyclones in a warmer climate will they be more intense

Development of an intense extra-tropical cyclone ( composite of the 100 most intense storms (DJF) at 20C. From ERA-040. Time units are in 6 hours centered at the time of minimum pressure. Evolution of central pressure, vorticity, wind speed and precipitation

ETC in a warmer climate?

Lennart Bengtsson


Extra tropical cyclones in a warmer climate will they be more intense

Development of an intense extra-tropical cyclone ( composite of the 100 most intense storms (DJF) at 20C. Time units are in 6 hours centered at the time of minimum pressure. Evolution of central pressure, vorticity, wind speed and precipitation

ETC in a warmer climate?

Lennart Bengtsson


Extra tropical cyclones in a warmer climate will they be more intense

Number of storm tracks for a given maximum wind speed. ERA-40 for three different periods and ECHAM5. The higher maximum wind speeds in ECHAM5 are likely to be due to the higher resolution

ETC in a warmer climate?

Lennart Bengtsson


Cases of extreme10 m wind red and at 925 hpa for era 40 black and echam5 blue

Cases of extreme10 m wind (red) and at 925 hPa for ERA-40 (black) and ECHAM5(blue)

ETC in a warmer climate?

Lennart Bengtsson


Effect of resolution

Effect of resolution

  • We compare T213 with T63

  • Using the same resolution (T42) the number and relative distribution of cyclones are the same)

  • At full resolution intensities are underestimated

ETC in a warmer climate?

Lennart Bengtsson


Number of cyclones at 63 and t213 as a function of vorticity at t42 resolution

Number of cyclones at &63 and T213 as a function of vorticity at T42 resolution

ETC in a warmer climate?

Lennart Bengtsson


Intense cyclones at t63 and t213 resolution

Intense cyclones at T63 and T213 resolution

mm/1hr

ETC in a warmer climate?

Lennart Bengtsson


Extra tropical cyclones in a warmer climate will they be more intense

Wind speeds at 925 hPa (ca 400m above the surface) at the 99.9 percentile, ECHAM5 model at T213 resolution(60 km) For the period 1960-1990

ETC in a warmer climate?

Lennart Bengtsson


Extra tropical cyclones in a warmer climate will they be more intense

Wind speeds at 925 hPa (ca 400m above the surface) at the 99.9 percentile, ECHAM5 model at T63 resolution(250 km) For the period 1960-1990

ETC in a warmer climate?

Lennart Bengtsson


Extremes of winds at ekofisk north sea at 925 hpa 90 95 99 and 99 5 percentiles

Extremes of winds at Ekofisk (North Sea) at 925 hPa90, 95, 99 and 99,5 percentiles

ETC in a warmer climate?

Lennart Bengtsson


Extra tropical cyclones in a warmer climate will they be more intense

Extra-tropical cyclones in a warmer climate( C21: 2070-2100. A1B)Main investigation is for NH winter (DJF)

  • Structure and distribution of cyclones virtually identical

  • No significant increase in wind speed maximum, vorticity or minimum pressure

  • Significant increase in precipitation

  • (i) global precipitation + 6%

  • (ii) accumulated precipitation +11%

  • (iii) maximum precipitation > 30%

ETC in a warmer climate?

Lennart Bengtsson


Extra tropical cyclones in a warmer climate will they be more intense

Storm tracks (DJF) over 32 years with maximum wind speed of 50m/s. Left 1959-1990 (20C), right 2069-2100 (C21), Scenario A1B. Model ECHAM5 (T213)

20C

21C

ETC in a warmer climate?

Lennart Bengtsson


Extra tropical cyclones in a warmer climate will they be more intense

Storm tracks (DJF) over 32 years with maximum wind speed > 45m/s. Left 1959-1990 (20C), right 2069-2100 (C21), Scenario A1B. Model ECHAM5 (T213). Colored points indicate centre of cyclone at the time of maximum wind speed within 5 degrees from the centre.

ETC in a warmer climate?

Lennart Bengtsson


Extra tropical cyclones in a warmer climate will they be more intense

Number of extreme storms in the Atlantic and the Pacific from ERA-40 and ECHAM5Reanalysis data are for 1979-2001. ECHAM5 has been normalized to 22 years

ETC in a warmer climate?

Lennart Bengtsson


Extra tropical cyclones in a warmer climate will they be more intense

Development of an intense extra-tropical cyclone ( composite of the 100 most intense storms (DJF) at 21C. Time units are in 6 hours centered at the time of minimum pressure. Evolution of central pressure, vorticity, wind speed and precipitation

ETC in a warmer climate?

Lennart Bengtsson


Extra tropical cyclones in a warmer climate will they be more intense

Development of an intense extra-tropical cyclone ( composite of the 100 most intense storms (DJF). Time units are in 6 hours centered at the time of minimum pressure. Full lines 20C, dashed lines 21C

Surface pressure in hPa

ETC in a warmer climate?

Lennart Bengtsson


Extra tropical cyclones in a warmer climate will they be more intense

Development of an intense extra-tropical cyclone ( composite of the 100 most intense storms (DJF), showing maximum wind speed. Time units are in 6 hours centered at the time of minimum pressure. Full lines 20C, dashed lines 21C

Maximum wind speed within 5 degrees from the centre of the cyclone

Maximum wind speed within 5 degrees from the centre

ETC in a warmer climate?

Lennart Bengtsson


Extra tropical cyclones in a warmer climate will they be more intense

Development of an intense extra-tropical cyclone ( composite of the 100 most intense storms (DJF). Time units are in 6 hours centered at the time of minimum pressure. Full lines 20C, dashed lines 21C

Precipitation intensity in mm/hr averaged over a circular geodetic area with 5 degree radius following the storm.

ETC in a warmer climate?

Lennart Bengtsson


Wind in a grid point space

Windin a grid point space

DJF

ETC in a warmer climate?

Lennart Bengtsson


Extra tropical cyclones in a warmer climate will they be more intense

Wind speeds at 925 hPa (ca 400m above the surface) at the 99.9 percentile, ECHAM5 model at T213 resolution(60 km) For the period 1960-1990

ETC in a warmer climate?

Lennart Bengtsson


Extra tropical cyclones in a warmer climate will they be more intense

Wind speeds at 925 hPa (ca 400m above the surface) at the 99.9 percentile, ECHAM5 model at T213 resolution(60 km) For the period 2070-2100, scenario A1B

ETC in a warmer climate?

Lennart Bengtsson


Change in wind speed maximum at the 99 percentiles calculated from all gridpoints every 6 hours djf

Change in wind speed maximum at the 99 percentiles. Calculated from all gridpoints every 6 hours, DJF

ETC in a warmer climate?

Lennart Bengtsson


Extra tropical cyclones in a warmer climate will they be more intense

Change in wind speed maximum at the 99.9 percentiles. Calculated from all gridpoints every 6 hours, DJF

ETC in a warmer climate?

Lennart Bengtsson


Precipitation in a grid point space 6 hourly

Precipitation in a grid point space6 hourly

DJF

ETC in a warmer climate?

Lennart Bengtsson


Hourly precipitation intensity at 20c djf 99 percentile

Hourly precipitation intensity at 20C (DJF), 99 percentile

ETC in a warmer climate?

Lennart Bengtsson


Percentage change in hourly precipitation intensity between 21c and 20c djf 99 percentile

Percentage change in hourly precipitation intensity between 21C and 20C (DJF), 99 percentile

ETC in a warmer climate?

Lennart Bengtsson


Hourly precipitation intensity at 20c djf 99 9 percentile

Hourly precipitation intensity at 20C (DJF), 99.9 percentile

ETC in a warmer climate?

Lennart Bengtsson


Percentage change in hourly precipitation intensity between 21c and 20c djf 99 9 percentile

Percentage change in hourly precipitation intensity between 21C and 20C (DJF), 99.9 percentile

ETC in a warmer climate?

Lennart Bengtsson


Other seasons

Other seasons

ETC in a warmer climate?

Lennart Bengtsson


Number of events as a function of maximum wind speed

Number of events as a function of maximum wind speed

ETC in a warmer climate?

Lennart Bengtsson


Areas investigated

Areas investigated

  • NH:- (0, 360), (25.0, 90.0)N

  • Atl.:-(60W, 0), (25.0, 70.0)N

  • AtlEuro:-(60W, 40E), (25.0, 70.0)N

  • Pac.:-(120E, 120W), (25.0, 70.0)N

  • Arctic:-(0, 360), (70.0, 90.0)N

  • NEuro:-(10W, 40E), (47.5, 70.0)N

  • SEuro:-(10W, 40E), 30.0, 47.5)N

ETC in a warmer climate?

Lennart Bengtsson


Extra tropical cyclones in a warmer climate will they be more intense

Number of extreme cyclones at 20C and 21 C at different seasons. Winds are at 925 hPa. Winds>45 m/s at 925 hPa corresponds broadly to > 12 Bf at 10m above the surface. Red color indicate where there are more events at 21C. Figures within brackets exclude storm tracks which are generated between 20 and 30N.

ETC in a warmer climate?

Lennart Bengtsson


Extra tropical cyclones in a warmer climate will they be more intense

Number of extreme cyclones at 20C and 21 C at different seasons. Winds are at 925 hPa. Winds>35 m/s at 925 hPa corresponds broadly to > 10 Bf at 10m above the surface. Red color indicate where there are more events at 21C.

ETC in a warmer climate?

Lennart Bengtsson


Conclusions

Conclusions

  • 1. There is an overall reduction in the number of extra-tropical storms. This covers virtually all areas and all seasons. For cyclones reaching a maximum wind speed of 25m/s or higher at 925 hPa or 8 Bf in our scaling, the total reduction is around 5%. The same proportional reduction occurs if we consider cyclones with wind speeds above 45m/s or 12 Bf.

  • 2. The largest reduction in the most intense cyclones (>12Bf) occurs during DJF and MAM. During JJA there is an increase in 21C. This increase in intensity is related to more powerful tropical cyclones that enter mid latitude regions. This mainly occurs in the Pacific Ocean.

  • 3. Using surface pressure below a given limit as a proxy for wind speed is misleading. The minimum surface pressure of the most intense cyclones is actually lower in 21C but maximum wind speed and vorticity is slightly lower than at 20C

ETC in a warmer climate?

Lennart Bengtsson


Conclusions1

Conclusions

  • 4. There is a slight poleward movements of the cyclones having the effect the number of cyclones in Northern Europe and Arctic is practically unchanged. For the same reason the number of cyclones in the Mediterranean region is proportionally more reduced.

  • 5. There is an increase in the number of intense cyclones in the Arctic (ca 10%) but no clear tendency over Northern Europe. In order to get a representative number this is based on storms >35m/s.

  • 6. The distribution of storm as a function of maximum wind speed is similar to ERA-40 but wind speeds are systematically stronger in ECHAM5

  • 7. There is slight regional intensification stronger wind speeds at the higher percentiles) over part of eastern Atlantic and western Europe as obtained from the set of grid point data. We suggest that this may be related to the strengthening of the SST gradient between 40 and 50N south of Greenland

  • 8. We see no indication of any effect from the higher level of latent heat at 21C. Generally release of latent hear has little effect on extra-tropical cyclones because the way precipitation is organized around frontal surfaces, the rapidity of the dynamical processes which is on the same time scale as that of geostrophic adjustment.

ETC in a warmer climate?

Lennart Bengtsson


Conclusions2

Conclusions

  • Accumulated precipitation around extra tropical cyclones increase by some 11%.

  • Extreme precipitation ( accumulated over 6 hours) increases by more than 30% in some areas in the storm track region by more than 50%.

  • Extreme precipitation in a warmer climate will clearly fall outside the range ofpresent climate.

  • Extreme winds are likely to fall within the range of the present climate.

ETC in a warmer climate?

Lennart Bengtsson


Extra tropical cyclones in a warmer climate will they be more intense

END

ETC in a warmer climate?

Lennart Bengtsson


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