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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. Extra-tropical cyclones.

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

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  1. 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

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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.? ETC in a warmer climate? Lennart Bengtsson

  9. 5 year global temperature changein the last two decades ETC in a warmer climate? Lennart Bengtsson

  10. 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

  11. 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

  12. 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

  13. Semenov and Bengtsson, 2002Secular trends in daily precipitation characteristicsClim.Dyn. 123-140 +30% ETC in a warmer climate? Lennart Bengtsson

  14. 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

  15. Top, IPCC models: massflux, water vapor and precipitation Below, GFDl modelP = Mq ( From Held and Soden (2006)) ETC in a warmer climate? Lennart Bengtsson

  16. 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

  17. 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

  18. Storm track density and storm track genesis at 20C DJF (T213) Track density Genesis ETC in a warmer climate? Lennart Bengtsson

  19. 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) ETC in a warmer climate? Lennart Bengtsson

  20. Example of an extreme extra-tropical cyclone and its evolution ETC in a warmer climate? Lennart Bengtsson

  21. 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

  22. Max. deepening ECHAM 5 T213 Cyclones in different stages of development Max. precipitation Max. intensity ETC in a warmer climate? Lennart Bengtsson

  23. 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

  24. Vertical tilt of the composite cyclone 36 hours before and after maximum intensity ETC in a warmer climate? Lennart Bengtsson

  25. 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) ETC in a warmer climate? Lennart Bengtsson

  26. ERA-40 Cyclones in different stages of development ETC in a warmer climate? Lennart Bengtsson

  27. 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 ETC in a warmer climate? Lennart Bengtsson

  28. 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 ETC in a warmer climate? Lennart Bengtsson

  29. Vertical tilt of the composite cyclone 36 hours before and after maximum intensity, ECHAM5 (top), ERA-40 (below) ETC in a warmer climate? Lennart Bengtsson

  30. Comparison between ERA 40 and ECMWF Interim reanalysis (composite of 50 cyclones) ETC in a warmer climate? Lennart Bengtsson

  31. 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

  32. Number of cyclones at &63 and T213 as a function of vorticity at T42 resolution ETC in a warmer climate? Lennart Bengtsson

  33. Intense cyclones at T63 and T213 resolution mm/1hr ETC in a warmer climate? Lennart Bengtsson

  34. 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

  35. 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

  36. Extremes of winds at Ekofisk (North Sea) at 925 hPa90, 95, 99 and 99,5 percentiles ETC in a warmer climate? Lennart Bengtsson

  37. 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% ETC in a warmer climate? Lennart Bengtsson

  38. 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

  39. 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

  40. 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

  41. 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. ETC in a warmer climate? Lennart Bengtsson

  42. Windin a grid point space DJF ETC in a warmer climate? Lennart Bengtsson

  43. 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

  44. 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

  45. Change in wind speed maximum at the 99 percentiles. Calculated from all gridpoints every 6 hours, DJF ETC in a warmer climate? Lennart Bengtsson

  46. 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

  47. Precipitation in a grid point space6 hourly DJF ETC in a warmer climate? Lennart Bengtsson

  48. Hourly precipitation intensity at 20C (DJF), 99 percentile ETC in a warmer climate? Lennart Bengtsson

  49. Percentage change in hourly precipitation intensity between 21C and 20C (DJF), 99 percentile ETC in a warmer climate? Lennart Bengtsson

  50. Hourly precipitation intensity at 20C (DJF), 99.9 percentile ETC in a warmer climate? Lennart Bengtsson

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