Advanced metrics of extreme precipitation events Olga Zolina Meteorologisches Institut der Universität Bonn, Germany P.P.Shirshov Institute of Oceanology, Moscow, Russia  Complexity of extreme precipitation, definitions and uncertainties of metrics  Absolute extremes: use of raw data vs application of extreme value statistics  Relative extremeness: empirical and PDF-based indices  Problem of precipitation timing: duration of wet periods  Perspectives Outline: Workshop on extreme climate events, September 2010 Paris

Complexity of precipitation process implies the complexity in estimation of precipitation extremes Many more (compared to the other variables) metrics are needed to characterize it Methods for estimation of extremes need to account for clustering in space and in time Timing of the event is essential and should be accounted for both methods and data Precipitation is an event-like phenomenon, clustered in space and in time, it is not a classical scalar, like temperature or pressure 20 km Data requirements Method requirements Workshop on extreme climate events, September 2010 Paris

for treshhold=10 mm/day in Stenslese - 3 days in Bulken - 11 days 95%=12 mm/day 2 days 95%=22 mm/day 3 days How to define what is extreme precipitation: uncertanties of metrics Stensele, Sweeden Bulken, Norway JJA, days total=35.3mm intensity=3.9mm/day 4 days total=22.0mm intensity=5.5mm/day 95% for period =21.4 mm/day Bulken: 3 days, R95pTOT=33.021% Stensele: 0 days, R95pTOT=0% Workshop on extreme climate events, September 2010 Paris

Approaches for estimating precipitation extremes Absolute extremes Extremeness (relative extremes) Time- (area-) integrated extremes Raw data – based Intensities Maxima Peaks over threshold Contribution of the wettest days to totals from empirical distributions ETCCDI RxTOT index Wet spell durations and associated intensities & extremes PDF – based Percentiles of the theoretical PDFs IVD vs EVD Contribution of the wettest days to totals derived from theoretical PDFs Intensity-duration- frequency (IDF) distributions From engineering hydrology to climate Workshop on extreme climate events, September 2010 Paris

Absolute extremes: IVD vs EVD  PDF for the core (IVD, e.g. Gamma) may not capture the extremes accurately  EVD (e.g. GEV, GPD) may be strongly constrained by the threshold chosen and overestimate extremes  “fetishism of heavy tails” Is the concept “ absence of evidence is not evidence of absence ” always valid? Gamma GPD Maraun et al Cambridge Daily precipitation is a time-integrated value, not an elementary event,  difficulties in applying extreme statistics Overall record for Europe is 543 mm/day (Gard, France, ) Workshop on extreme climate events, September 2010 Paris maxima of daily precipitation 100-yr returns from GEV distribution Zolina 2010

Absolute precipitation extremes: observed changes in 95% percentile of precipitation % significant at 95% level JJA DJF Zolina et al. 2005, Geophys. Res. Lett. Zagreb 95 th percentile mean intensity  Changes in extremes differ from those in totals  Absolute extremes grow with seasonality in Western Europe Workshop on extreme climate events, September 2010 Paris

Seasonality in extreme precipitation trends over Germany significant at 95% level DJFJJA 95% significance level DJF Zolina et al. 2008, J. Geophys. Res. JJAYEAR Workshop on extreme climate events, September 2010 Paris

Approaches for estimating precipitation extremes Absolute extremes Extremeness (relative extremes) Time- (area-) integrated extremes Raw data – based Intensities Maxima Peaks over threshold Contribution of the wettest days to totals from empirical distributions ETCCDI RxTOT index Wet spell durations and associated intensities & extremes PDF – based Percentiles of the theoretical PDFs IVD vs EVD Contribution of the wettest days to totals derived from theoretical PDFs Intensity-duration- frequency (IDF) distributions From engineering hydrology to climate Workshop on extreme climate events, September 2010 Paris

Absolute extremes and relative extremeness Analyzing interannual changes, it is critical to know how much the fraction contributed by the uppermost wet days has changed Limitations of the empirical indices are associated with the finite number of wet days in sample (R95tot falls to zero)  Need to extent index of relative extremeness to the theoretical distributions Zolina et al. 2009, J. Hydrometeorol. R95pTOT Sodankyla, Finland, (26.65E, 67.37N) DJF Workshop on extreme climate events, September 2010 Paris

Distribution of fractional contribution (DFC) of daily precipitation to the total x i, i=1,...n is the daily precipitation, n is the number of wet days DFC for Gamma distribution - Gaussian hypergeometric function PDF: CDF: Zolina et al. 2009, J. Hydrometeorol. R95tt index instead of R95tot Workshop on extreme climate events, September 2010 Paris

Corellation R95tt vs R95tot Relative precipitation extremeness: PDF-based vs empirical index New index exhibits significant differences compared to the traditional index and may also show different variability patterns R95tt DJF R95tot DJF R95tot R95tt Sondakyla, Finland 26.65E, 67.37N, DJF Linear trend, % per dec Workshop on extreme climate events, September 2010 Paris

Approaches for estimating precipitation extremes Absolute extremes Extremeness (relative extremes) Time- (area-) integrated extremes Raw data – based Intensities Maxima Peaks over threshold Contribution of the wettest days to totals from empirical distributions ETCCDI RxTOT index Wet spell durations and associated intensities & extremes PDF – based Percentiles of the theoretical PDFs IVD vs EVD Contribution of the wettest days to totals derived from theoretical PDFs Intensity-duration- frequency (IDF) distributions From engineering hydrology to climate Workshop on extreme climate events, September 2010 Paris

Duration of precipitation: essential metric for estimation extremes  For design purposes a critical metric is the precipitation accumulated during consecutive days or over area  Time-integrated extremes may not correlate with the magnitude of extremes for single days IDF (Intensity-duration-frequency)-distributions : developed in engineering hydrology, however for minute- and hourly time scales only, not yet applied to climate studies Madsen, 2002, Wat. Res. Res. IDFs for Vietnam IDFs for Copenhagen WP=3 days Max = 27.1mm Total = 32.8mm WP=12 days Max = 6.2mm Total = 51.5mm Workshop on extreme climate events, September 2010 Paris

Changes in the duration of European wet periods normalized occurrence anomalies Zolina et al. 2010, Geophys. Res. Lett. Net effect of the number of wet days (Monte-Carlo simulation of the growing number of wet days, % per decade) 1%2%3% 0.17± ± ±0.25 It is not the effect of changing number of wet days!!! Linear trend in the WP duration: Workshop on extreme climate events, September 2010 Paris

How changing wet spells affect precipitation Linear trends in fractional contrbution of extremes to the total Short WPs (<2 days)Long WPs (>2 days) Workshop on extreme climate events, September 2010 Paris

Changes in the IDF distrbutions for daily preciptiation in Europe ( ) significant at 95% level Intensity-duration distribution for Central Germany Acc. precipitation Linear trends in time-integrated precipitation for all European stations (% per decade) All wet periodsExtremes (95 th percentile) Longer wet periods imply stronger extremes! Workshop on extreme climate events, September 2010 Paris

Conclusions and perspectives  Absolute extremes:  Existing methods are quite accurate, however more close look is needed on the approaches to estimation of very rare events  Absolute extremes show primarily growing intensity over Europe (up to 5% per decade) but for most regions spatial patterns are noisy and significance is low  There is a clear seasonality in long-term trends of Central European precipitation extremes: more extremes in winter and less in summer  Relative extremeness:  New R95tt index allows to overcome the problem of the finite number of wet days in the raw time series of daily precipitation  Compared to R95tot index, new R95tt index shows more homogeneous trend pattern with the trends being statistically significantly larger in the Central and Eastern Europe  Duration of wet spells:  During the last 60 years European wet spells have become longer by about 15-20%. Lengthening was not caused by the net effect of wet days  Extreme precipitation associated with longer wet spells have intensified by 12-18%, while extremes associated with short wet spells became weaker Workshop on extreme climate events, September 2010 Paris

Thank you Soja & Starkel, 2007, Geomorph. 4-month (1998) daily precipitation In Cherrapunji Extreme precipitation season (summer 1998) and catastrophic flood and land slide in Ladakh, Himalay Workshop on extreme climate events, September 2010 Paris