Geen diatitel

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Identifying natural hazardsin climate databases
Albert Klein Tank KNMI, the Netherlands 19
September 2002 acknowledgementsLisa Alexander
(Met Office, UK) Janet Wijngaard, Aryan van
Engelen Günther Können (KNMI) 36
ECA-participants (Europe Middle East)
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European Study http//www.knmi.nl
/samenw/eca
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River Rhine flooding 1995 precipitation data GPCC
(GPCC, 1995)
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Recent Elbe flooding 2002 precipitation data
(GPCC, 1995)
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Studying trends what types of extremes?

• Trends in extreme events characterised by the
  size of their societal or economic impacts
• Trends in very rare extreme events analysed by
  the parameters of extreme value distributions
• Trends in observational series of phenomena with
  a daily time scale using indices of extremes

YES
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Motivation for choice of extremes

• The statistical detection probability of trends
  depends on the return period of the extreme event
  and the length of the observational series
• For extremes in daily series of e.g. temperature
  and precipitation having typical length 50 yrs,
  the optimal return period is 10-30 days rather
  than 10-30 years

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example 80 detection prob. (5-level)
T
(see also Frei Schär, J.Climate, 2001)
N
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Approach

• Using derived climate indices as proposed by the
  joint CCL/CLIVAR Working Group on Climate Change
  Detection (Peterson et al., WMO-TD No. 1071,
  2001)
• Focus on counts of days crossing a threshold
  either absolute/fixed thresholds or
  percentile/variable thresholds relative to local
  climate
• Standardisation enables comparisons between
  results obtained in different parts of the
  world(e.g. Frich et al., Clim. Res. 2002 also
  in IPCC-TAR)

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Temperature indices I

• Example of frost days as an extreme index based
  on an absolute temperature threshold

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Frich et al. (Clim.Res., 2002) in IPCC-TAR
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Temperature indices II

• Example of winter warm spells and cold spells
  as an extreme index based on counts of events
  over a (seasonally varying) percentile threshold

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IPCC-TAR (Ch.2, Folland and Karl)
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Precipitation indices I

• Example of R95tot extreme index for the
  precipitation fraction due to very wet days

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Easterling et al. (BAMS, 2000) in IPCC-TAR see
also Groisman et al. (Clim.Change, 1999)
Linear trends in rainy season over 50 years
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Frich et al. (Clim.Res., 2002) in IPCC-TAR
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• the end...