30 april 2008 mpi, hamburg etc in a warmer climate? lennart bengtsson extra-tropical cyclones in a...

30 April 2008 MPI, Hamburg

ETC in a warmer climate?Lennart Bengtsson

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

• 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.



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?



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?



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.



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



,

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



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.?



5 year global temperature changein the last two decades

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are needed to see this picture.



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



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

DT +0.45K

Des +3%

Full line GFDL model

Dashed line measurements



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



Semenov and Bengtsson, 2002Secular trends in daily precipitation characteristics

Clim.Dyn. 123-140

+30%



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



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

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



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.



Selection of storm tracks

• Level 850 hPa• Lifetime ≥ 48 hours• Intensity in vorticity ≥10-5s-1

• Movements ≥1000km



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



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



Structure and evolution of extra-tropical cyclones

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



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)



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



Composite vortex (100 most intense) at maximum intensificationleft pressure and wind, right pressure and precipitation

20C, DJF. Movement of cyclone is to the right.

mm/hour



Temperature(500-850 hPa) at maximum deepening rate



Vortex vertical structureWind field at maximum intensification



Composite vortex (100 most intense) at maximum precipitationleft pressure and wind, right pressure and precipitation. 15 hrs later

20C, DJF Movement of cyclone is to the right



Composite vortex (100 most intense) at maximum intensityleft pressure and wind, right pressure and precipitation. 15 hrs later

20C, DJF Movement of cyclone is to the right



Vortex vertical structureWind field at maximum intensity



Temperature(500-850 hPa) at minimum pressure



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



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



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)



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



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



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




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



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



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



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



Intense cyclones at T63 and T213 resolution

mm/1hr



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



Extremes of winds at Ekofisk (North Sea) at 925 hPa

90, 95, 99 and 99,5 percentiles



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%



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



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.



Number of extreme storms in the Atlantic and the Pacific from ERA-40 and ECHAM5

Reanalysis data are for 1979-2001. ECHAM5 has been normalized to 22 years

Area/Model( total nr. of storms ca. 16000)

NH

25N-90N

Atlantic Pacific

ERA-40

>45 m/s

110 72 32

ECHAM5

>45 m/s

394 197 188

ECHAM5

>50 m/s

52 32



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




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



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



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.



Windin a grid point space

DJF



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



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



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



Precipitation in a grid point space6 hourly

DJF



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



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



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



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

99.9 percentile



Other seasons



Number of events as a function of maximum wind speed

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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



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.

>45m/s NH Atl. Atl/Eu Pac. Arctic NEur SEur

DJF 20C 573 286 286 274 13 14 1

DJF 21C 549 256 259 276 13 16 0

MAM 20C 220 106 107 106 7 1 0

MAM 21C 190 77 78 108 3 0 0

JJA 20C 56(15) 16(12) 16 39(3) 0 0 0

JJA 21C 77(14) 17(10) 17 59(4) 0 0 0

SON 20C 410 171 175 221 6 10 0

SON 21C 384 171 173 206 2 7 1



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.

>35m/s Atl/Europ Arctic NEuro SEuro

DJF 20C 1490 191 166 53

DJF 21C 1359 217 147 39

MAM 20C 855 87 60 30

MAM 21C 803 109 36 29

JJA 20C 370 21 10 1

JJA 21C 346 30 13 0

SON 20C 1037 107 101 16

SON 21C 1016 88 108 21



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



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.



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 of present climate.

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



END

30 april 2008 mpi, hamburg etc in a warmer climate? lennart bengtsson extra-tropical cyclones in a...

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