London 2 May 2008 Extreme (European) Windstorms and Expected Changes in a Warmer Climate Lennart Bengtsson Professor ESSC, University of Reading Max Planck Institute for Meteorology, Hamburg Thanks to Kevin Hodges, ESSC and colleagues in Hamburg

London 2 May 2008 Hurricane Katrina August 2005 ECMWF operational analyses, 850 hPa vorticity

London 2 May 2008 Before Katrina… David & Kimberly King Waveland, MS

London 2 May 2008 …After Katrina David & Kimberly King Waveland, MS

London 2 May 2008 Extra-tropical cyclones Have European wind storms became stronger? What is the physical mechanisms for extra-tropical cyclones? How well can we reproduce extreme storms with global models? Extreme storms in a warmer climate. What can be expected? What can we learn from model experiments?

London 2 May 2008 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

London 2 May 2008, 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

London 2 May 2008 Have European wind storms became stronger? Not really, but there are considerable changes from year to year and between different periods lasting several decades. Our interpretation is that these changes are chaotic and not generally predictable beyond a week or two. Increase in damages is mainly related to increased exposure and higher level of reporting.

London 2 May 2008 What is the physical mechanisms for extra-tropical cyclones? Extra-tropical storms are driven by temperature differences whereby available potential energy in converted to kinetic energy. This has been known since the beginning of the last century (Max Margules) This is the reason why the most devastating wind storms occur during winter Release of latent heat in precipitation is only of minor importance in difference to tropical cyclones

London 2 May 2008 How well can we reproduce extreme storms with global models? As I will show the latest generation of GCMs is doing this very well These GCMs have very much in common with models for weather prediction The reason is that extra-tropical cyclones normally cover vast areas and can be well resolved with grid resolution of some 50 km Special problems are related to high level of turbulence which is local and difficult to predict. However, while intense wind gusts cannot be well predicted they are generally well associated with specific regions of the cyclones.

London 2 May 2008 How well can we reproduce extreme storms with global models? We use results from the high -resolution global model developed by the Max Planck Institute for Meteorology in Hamburg. Preliminary indications are that other state of the art GCM such as the Hadley Centre, UK and GFDL, Princeton, USA give similar results We compare results for two 32 year periods; ( to be called 20C) and ( to be called 21C). For the future simulation we have used IPCC scenario A1B We first investigate the 100 most intense cyclones at20C

London 2 May 2008 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 determine extreme winds at the 99 and 99.9 percentiles We also use surface pressure minima and surface pressure tendencies(deepening rates)

London 2 May 2008 Selection of storm tracks Level 850 hPa Lifetime ≥ 48 hours Intensity in vorticity ≥10 -5 s -1 Movements ≥1000km

London 2 May 2008 Storm tracks (DJF) over 32 years with maximum wind speed of 50m/s. Left (20C), right (C21), Scenario A1B. Model ECHAM5 (T213) 20C21C

London 2 May 2008 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

London 2 May 2008 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. Evolution of central pressure, vorticity, wind speed and precipitation

London 2 May 2008 Composite vortex (100 most intense) at maximum intensification left pressure and wind, right pressure and precipitation 20C, DJF. Movement of cyclone is to the right. mm/hour

London 2 May 2008 Composite vortex (100 most intense) at maximum precipitation left pressure and wind, right pressure and precipitation. 15 hrs later 20C, DJF Movement of cyclone is to the right

London 2 May 2008 Composite vortex (100 most intense) at maximum intensity left pressure and wind, right pressure and precipitation. 15 hrs later 20C, DJF Movement of cyclone is to the right

London 2 May 2008 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. Evolution of central pressure, vorticity, wind speed and precipitation

London 2 May 2008 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

London 2 May 2008 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.

London 2 May 2008 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

London 2 May 2008 Number of events as a function of maximum wind speed

London 2 May 2008 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 NHAtl.Atl/EuPac.ArcticNEurSEur DJF20C DJF21C MAM20C MAM21C JJA20C 56(15)16(12) 16 39(3) 000 JJA21C 77(14)17(10) 17 59(4) 000 SON20C SON21C

London 2 May 2008 Wind in a grid point space DJF

London 2 May 2008 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

London 2 May 2008 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 , scenario A1B

London 2 May 2008 Change in wind speed maximum at the 99 percentiles. Calculated from all gridpoints every 6 hours, DJF

London 2 May 2008 Change in wind speed maximum at the 99.9 percentiles. Calculated from all gridpoints every 6 hours, DJF

London 2 May 2008 Precipitation in a grid point space 6 hourly DJF

London 2 May 2008 Hourly precipitation intensity at 20C (DJF), 99 percentile

London 2 May 2008 Percentage change in hourly precipitation intensity between 21C and 20C (DJF), 99 percentile

London 2 May 2008 Hourly precipitation intensity at 20C (DJF), 99.9 percentile

London 2 May 2008 Percentage change in hourly precipitation intensity between 21C and 20C (DJF), 99.9 percentile

London 2 May 2008 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

London 2 May 2008 Conclusions 4. 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. 5. The distribution of storm as a function of maximum wind speed is similar to ERA-40 but wind speeds are systematically stronger in ECHAM5 6. 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 7. 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 that is on the same time scale as that of geostrophic adjustment.

London 2 May 2008 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.

London 2 May 2008 END