9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Extra-tropical cyclones in...

9 July 2008, COLA, Washington

ETC in a warmer climate?Lennart Bengtsson

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

Professor Lennart BengtssonESSCMany 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. Probably

• 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

QuickTime™ and aTIFF (Uncompressed) decompressor

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)

Track density Genesis



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 calculate total precipitation in a circular area within 5° of the centre. ( ca 106 km2)

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



Example of an extreme extra-tropical cyclone and its evolution



Structure and evolution of extra-tropical cyclones

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



ECHAM 5

T213

Cyclones in different stages of development

Max. deepening

Max. precipitation

Max. intensity



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



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



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 agrees better with the model results)



ERA-40

Cyclones in different stages of development



Development of an intense extra-tropical cyclone ( composite of the 100 most intense storms (DJF) at 20C. (ECHAM5/T213) (left) and ERA 40(right).Time units

are in 6 hours centered at the time of minimum pressure. Evolution of central pressure, vorticity, wind speed and precipitation



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 due to higher resolution



Vertical tilt of the composite cyclone 36 hours before and after maximum intensity, ECHAM5 (top),

ERA-40 (below)



Comparison between ERA 40 and ECMWF Interim reanalysis (composite of 50

cyclones)



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 along storm tracks

+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.



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



Lifecycle composites of the 100 most intense storms in T42 ξ850 for the NH, DJF for T213 ECHAM5 in 21C and 20C (transparent colour). Parameters shown are MSLP

(hPa) ( in black), ξ850 (10-5s-1), (in red) 925hPa winds (m s-1) (in green) and area averaged total precipitation (mm hr-1) ( in blue). Time step is 6 hours.



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



Minimum surface pressure at NH and SH. 20C (full) and 21C (dashed)



Maximum wind speed at NH and SH. 20C (full) and 21C (dashed)



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. Storm tracks generated between 20 and 30N are excluded.

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

DJF 20C 546 277 277 256 15 14 1

DJF 21C 517 251 254 255 16 22 1

MAM 20C 208 105 106 96 9 2 0

MAM 21C 175 76 77 95 3 1 0

JJA 20C 16 13 13 3 0 0 0

JJA 21C 17 10 10 7 0 0 0

SON 20C 272 141 144 122 7 12 0

SON 21C 249 125 126 122 3 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 1498 257 304 70

DJF 21C 1369 299 270 52

MAM 20C 840 115 98 29

MAM 21C 808 141 77 29

JJA 20C 350 26 15 0

JJA 21C 336 34 13 0

SON 20C 979 145 168 22

SON 21C 949 136 197 21



Extreme winds, 99.5 percentiles both

hemispheres DJF and JJA



Precipitation all seasons both hemispheres,

20C (full), 21C (dashed)



Percentage increase i 6hr prec from 20C to 21C

NH extra- tropics

North Atlantic

North

Europe

South Europe

Mean DJF 8.0 4.0 15.4 -19.9

Mean JJA 8.0 1.9 -3.5 -47.1

99th percentiles

DJF 15.8 13.9 19.4 -6.0

99th

percentilesJJA 17.9 11.1 6.6 -42.3

Max. DJF 29.4 30.2 20.3 12.6

Max. JJA 37.1 29.9 32.4 4.1



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 cyclones 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.



Can we have sufficient confidence in one particular

climate model?• The overall response to warmer climate is a marked increase in the

column water vapour. This increase follows the Clausius-Clapeyron relation is common with other GCMs. Similarly, the much slower increase in total precipitation is also common with other GCMs ( Held and Soden, 2006). However also in agreement with other studies, extreme precipitation does increase more rapidly. This is physically credible as convective weather systems are expected to more effectively draw on the higher level of water vapour.

• We believe we can have confidence in the change in maximum wind speed. To predict maximum wind speed in a local environment exposed to coastal and sharp orographic obstacles is not feasible with a model of the kind used here, but the model should be capable of providing reliable predictors for such predictions. We see no reasons that such predictors will change in a warmer climate and consequently even local extremes are not likely to change either. Our confidence in the model is based on its ability to practically perfectly reproduce the high frequency change of surface pressure in the composite cyclone. Furthermore, we have no indications of any systematic increase in available potential energy in the warmer climate nor in a higher level of baroclinicity. We expect that other state of the art GCMs will behave similarly.



Is the integration sufficiently long?

• 32 years is likely to be too short for regional assessment of extreme conditions in extra-tropical cyclones ( Weisse et al., 2005). However, based on the assessment of three different 30-year integrations with the same model at lower resolution ( Bengtsson et al., 2006) suggest that overall hemispheric statistics should be robust.



Is the resolution and the physical parameterization

adequate?

• Here we are on less safe ground and it could well be the case that future ultra-high resolution using, say, non-hydrostatic equations will give a different result. However, extra-tropical cyclones are well described even by the quasi-geostrophic equations, so we do not expect that the larger cyclones will be very much different. Additional short- and medium-range prediction with similar models are accurate and forecast errors are more related to errors in the initial state. In areas, where extreme statistics is available, the results agree with observations as well as with that of limited area models.



. How confident can we be that the feedback from the release

of latent heat is insignificant in generating stronger winds?

• The fact that extra-tropical cyclones are most intense during the cold season and in situations with strong frontal gradients and a well developed jet stream with strong upper air winds strongly suggest that these conditions, that also follow from theoretical considerations, are the main drivers of extreme extra-tropical cyclones. In difference to tropical cyclones where the release of latent heat is quasi-symmetrically organized around the cyclone the extra-tropical cyclones are different in this respect. Moreover, the evolution of an extra-tropical cyclone is characterized by a rapid transient process with a fast built up of frontal precipitation and an equally fast collapse of organized precipitation as the cyclone occludes and rapidly weakens.

• In order to better explore the influence of latent heat release we ordered the 100 most intense cyclones based upon precipitation intensity instead of wind speed maximum ( not shown). These cyclones have weaker maximum winds but the interesting result was that there was no increase in extreme winds but rather a slight decrease at 21C.

• However, there are reports of intense small-scale extra-tropical cyclones and polar lows which may have features more in common with tropical cyclones, where an enhancement by latent heat cannot be excluded. We intend to investigate this in a future study.



Summary

• 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

9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Extra-tropical cyclones in...

Documents

Transcript of 9 July 2008, COLA, Washington ETC in a warmer climate? Lennart Bengtsson Extra-tropical cyclones in...