Advanced metrics of extreme precipitation events

1. Workshop on extreme climate events, September 2010 Paris Advanced metrics of extreme precipitation events Olga Zolina Meteorologisches Institut der Universität Bonn, Germany P.P.Shirshov Institute of Oceanology, Moscow, Russia Outline: • 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

2. Workshop on extreme climate events, September 2010 Paris Complexity of precipitation process implies the complexity in estimation of precipitation extremes Precipitation is an event-like phenomenon, clustered in space and in time, it is not a classical scalar, like temperature or pressure 20 km 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 20 km Data requirements Method requirements

3. JJA, 1982 Stensele, Sweeden Bulken, Norway 9 days total=35.3mm intensity=3.9mm/day for treshhold=10 mm/day in Stenslese - 3 days in Bulken - 11 days 4 days total=22.0mm intensity=5.5mm/day Workshop on extreme climate events, September 2010 Paris How to define what is extreme precipitation: uncertanties of metrics 95% for 1950-2007 period =21.4 mm/day Bulken: 3 days, R95pTOT=33.021% Stensele: 0 days, R95pTOT=0% 95%=22 mm/day 3 days 95%=12 mm/day 2 days

4. Workshop on extreme climate events, September 2010 Paris Approaches for estimating precipitation extremes

5. Cambridge GPD Gamma Maraun et al. 2010 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? 1898-2009 maxima of daily precipitation 100-yr returns from GEV distribution 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, 08.09.2002) Zolina 2010

6. -10--6 -6--4 -4--2 -2-0 0-2 2-4 4-6 6-10 % Workshop on extreme climate events, September 2010 Paris Absolute precipitation extremes: observed changes in 95% percentile of precipitation Zagreb 1951-2000 DJF 95th percentile mean intensity 1951-2000 JJA • Changes in extremes differ from those in totals • Absolute extremes grow with seasonality in Western Europe significant at 95% level Zolina et al. 2005, Geophys. Res. Lett.

7. 5 5 4 4 3 3 ) % ) ( 2 2 % d ( n d e 1 1 r n t e r r 0 0 a t e r n a -1 -1 i e L n i L -2 -2 -3 -3 -4 -4 -5 -5 significant at 95% level 10 20 30 40 50 60 70 80 90 100 10 20 30 40 50 60 70 80 90 100 P r e c i p i t a t i o n c l a s s e s ( % ) P r e c i p i t a t i o n c l a s s e s ( % ) Workshop on extreme climate events, September 2010 Paris Seasonality in extreme precipitation trends over Germany 1950-2004 DJF JJA YEAR DJF JJA 95% significance level Zolina et al. 2008, J. Geophys. Res.

8. Workshop on extreme climate events, September 2010 Paris Approaches for estimating precipitation extremes

9. 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 R95pTOTSodankyla, Finland, (26.65E, 67.37N) DJF 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.

10. Distribution of fractional contribution (DFC) of daily precipitation to the total Workshop on extreme climate events, September 2010 Paris xi, i=1, ...n is the daily precipitation, nis the number of wet days DFC for Gamma distribution PDF: CDF: - Gaussian hypergeometric function R95tt index instead ofR95tot Zolina et al. 2009, J. Hydrometeorol.

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

12. Workshop on extreme climate events, September 2010 Paris Approaches for estimating precipitation extremes

13. 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 IDFs for Vietnam WP=3 days Max = 27.1mm Total = 32.8mm IDFs for Copenhagen WP=12 days Max = 6.2mm Total = 51.5mm Madsen, 2002, Wat. Res. Res. IDF (Intensity-duration-frequency)-distributions: developed in engineering hydrology, however for minute- and hourly time scales only, not yet applied to climate studies

14. Workshop on extreme climate events, September 2010 Paris Changes in the duration of European wet periods normalized occurrence anomalies It is not the effect of changing number of wet days!!! Linear trend in the WP duration: 1950-2008 Zolina et al. 2010, Geophys. Res. Lett.

15. 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)

16. Workshop on extreme climate events, September 2010 Paris Changes in the IDF distrbutions for daily preciptiation in Europe (1950-2009) Linear trends in time-integrated precipitation for all European stations (% per decade) Intensity-duration distribution for Central Germany All wet periods Extremes (95th percentile) Acc. precipitation significant at 95% level Longer wet periods imply stronger extremes!

17. 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

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