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Revisiting Swiss temperature trends 1959-2008*

Revisiting Swiss temperature trends 1959-2008* Paulo Ceppi, Simon C. Scherrer , Andreas M. Fischer, Christof Appenzeller Federal of Office of Meteorology and Climatology MeteoSwiss 16 July 2010 11 th International Meeting on Statistical Climatology, Edinburgh UK

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Revisiting Swiss temperature trends 1959-2008*

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  1. Revisiting Swiss temperature trends 1959-2008* Paulo Ceppi, Simon C. Scherrer, Andreas M. Fischer, Christof Appenzeller Federal of Office of Meteorology and Climatology MeteoSwiss 16 July 2010 11th International Meeting on Statistical Climatology, Edinburgh UK * submitted to the International Journal of Climatology

  2. MotivationTemperature trends: The hallmark of climate change new Swiss grid  local climate change = large scale + local processes IPCC 2007, WG1 • seasonal differences • different processes on different spatial scales

  3. Outline • Geographical setting, input data and gridding • What are the linear temperature trends (yearly / seasonal)? • Can regional climate models explain the observed trends? • What is the trend contribution of … large scale circulation? … local factors? • Conclusions

  4. Geographical setting and input data • 91 homogeneous station series, 1959-2008 • station altitude range: 203 to 3580 m asl • ~2 km x 2 km gridded data set Edinburgh Switzerland

  5. Monthly anomaly griddingclimatological distance λ • spatial interpolation of altitude corrected residuals • influence of topography determined independently every month λ = 0.05 λ = 0.1 λ = 0.01 summit setting valley setting C. Frei, MeteoSwiss

  6. Gridded temperature anomaliesExample December 2009 topography good representation of local/altitude effects! C. Frei, MeteoSwiss

  7. Yearly temperature trends1959-2008 (°C/10yrs) average OLS trend: +0.35°C/10yrs • stronger trends than in global mean (+0.13°C/10yrs) • highly significant trends everywhere (p < 0.0005) • small spatial variability • no altitude dependence

  8. Seasonal temperature trends1959-2008 (°C/10yrs) °C/10yrs NH-land MAM DJF JJA SON density trend [°C/10yrs] • trends positive, (almost) everywhere significant • large seasonal differences of the trends [°C/10yrs] • autumn weak (+0.02-0.38), summer strong (+0.34-0.62) • clearly larger than NH-land (exception autumn) • altitude depences in autumn (stronger for low altitudes)

  9. Can climate models reproduce warming?Northern Switzerland but: models are NOT forced with observed circulation!  Can observed circulation changes explain the differences? anomaly [°C]wrt 1961-1990 ENSEMBLES models (smoothed) 20yr Gaussian smoothing observed change (smoothed)

  10. Different warming by circulation changes? Linear regression model Quantify effect of circulationon T via regression model: observed temperatures atmospheric circulation “modell error”other effects • principal components (PCs) of geopotential height at 500 hPa over N-Atlantic/Europe • model calibration: 1959-83 / validation: 1984-2008 • choice of PCs by „stepwise selection“

  11. Circulation effects on temperatureSwiss mean 1959-2008 bright: calibration dark: validation observed / modelled temperatures trend lines 1959-2008 0.35 0.13 observed / modelled trend (°C/10yrs) 0.39 0.21 0.39 0.08 0.46 0.07 0.17 0.02

  12. Summary temperature trend analysisSwitzerland 1959-2008  Can regional climate models account for the residual (circulation corrected) trends?

  13. Summary temperature trend analysisSwitzerland 1959-2008

  14. Altitude dependencegridded temperature trends 1959-2008 °C/10yrs 0.6 0.4 altitude [m asl] negative anomaly 0.2 positive anomaly 0 -0.2 trend (°C/10yrs)

  15. Swiss temperature trend anomalies1959-2008 [°C/10yrs] month-to-month evolution more trend than all altitude average 3000 2500 2000 altitude [m asl] 1500 1000 changes in fog/visibility? 500 Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec less trend than all altitude average month of the year

  16. Swiss temperature trend anomalies1959-2008 [°C/10yrs]incl. mean temperature evolution [°C] more trend than all altitude average 3000 2500 2000 altitude [m asl] 1500 1000 500 Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec less trend than all altitude average month of the year

  17. Swiss temperature trend anomalies1959-2008 [°C/10yrs] … and mean snow pack [cm] more trend than all altitude average 3000 snow-albedo effect? 2500 100 2000 30 altitude [m asl] 100 1500 5 30 1000 5 500 less trend than all altitude average Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec month of the year

  18. Conclusions Swiss temperature trends stronger than NH-land trend (1.6) Large seasonal differences, altitude dependence in autumn ~50% of the trend in winter due to changes in circulation – much less in other seasons Regional climate models underestimate circulation corrected trends in spring & summer Local processes important for local trends, especially in autumn (fog), spring (snow-albedo effect) and summer

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