Réf. Zolina & al. 2010 - A

Référence bibliographique complète

ZOLINA, O., SOMMER, C., GULEV, S. K., KOLLET, S. 2010. Changing structure of European precipitation: Longer wet periods leading to more abundant rainfalls, Geophysical Research Letters, Vol. 37, L06704. [Etude en ligne]

Abstract: Analysis of the duration of wet spells (consequent days with significant precipitation) in Europe and associated precipitation is performed over the period 1950–2008 using daily rain gauge data. During the last 60 years wet periods have become longer over most of Europe by about 15–20%. The lengthening of wet periods was not caused by an increase of the total number of wet days. Becoming longer, wet periods in Europe are now characterized by more abundant precipitation. Heavy precipitation events during the last two decades have become much more frequently associated with longer wet spells and intensified in comparison with 1950s and 1960s. The changes in the distribution of temporal characteristics of precipitation towards longer events and higher intensities should have a significant impact on the terrestrial hydrologic cycle including subsurface hydrodynamics, surface runoff and European flooding.

Mots-clés

 

 

Organismes / Contact

Meteorologisches Institut, Universitaet Bonn, Bonn, Germany
• P. P. Shirshov Institute of Oceanology, Moscow, Russia

 

(1) - Paramètre(s) atmosphérique(s) modifié(s)

(2) - Elément(s) du milieu impacté(s)

(3) - Type(s) d'aléa impacté(s)

(3) - Sous-type(s) d'aléa

 Precipitation (duration, intensity)

 Hydrology

 Floods

 

 

Pays / Zone

Massif / Secteur

Site(s) d'étude

Exposition

Altitude

Période(s) d'observation

Europe (western Europe and Russia)

 

 

 

 

 1950–2008

 

(1) - Modifications des paramètres atmosphériques

Reconstitutions

 

Observations

Review: European daily rain gauge observations and reanalyzes show upward tendencies in mean and heavy precipitation over the last decades [Klein Tank and Koennen, 2003, hereafter referred to as KTK03; Zolina et al., 2005; Groisman et al., 2005] and an increase in fractional contribution of heavy rainfalls to precipitation totals by 3–4% per decade [KTK03; Moberg et al., 2006; Zolina et al., 2009]. These tendencies were confirmed by regional studies [Frei and Schär, 2001; Zolina et al., 2008] and are consistent with the climate model projections [Semenov and Bengtsson, 2002; Emori and Brown, 2005; Scaife et al., 2008]. These analyses considered wet days (WD) by accounting for their daily sums while omitting the duration. However, the impact of precipitation on flooding is strongly related to the number of consecutive WDs with catastrophic floods being associated with anomalously long precipitation periods. Only few studies analyzing climate tendencies addressed precipitation duration. […] Ex.: Schmidli and Frei [2005] reported growing duration of wet spells over Swiss Alps during the 20th century.

In this study, the temporal evolution of the normalized occurrence anomalies of the contribution of WPs to the total number of WDs (WPn), derived from all European stations and smoothed with a 5-yr running mean. Over the past 60 years the fractional contribution of the long WPs to the total number of WDs gradually increased. Negative anomalies for long WPs during the 1950s and 1960s become primarily positive in the 1990s and 2000s while short WPs exhibit the opposite tendency. The negative linear trend for the number of short WPs and the remarkably positive trend for the occurrence of long WPs are both statistically significant.

The growing duration of WPs over the last 60 years constitutes a clear pan-European pattern. The linear trends in the WP duration over Central Western Europe, Scandinavia and European Russia are primarily positive, ranging from 2 to 4–5% per decade. Thus, the mean duration of WPs has increased in Western Europe by 0.6–0.7 days over 60 years and up to 0.5 days in European Russia. Negative trends of up to 3% per decade are observed in few locations in Southern, Western and Central Europe. The analysis clearly shows the opposite tendencies in the number of short and long spells, quantified respectively by the 50th and 90th percentile of the WP durations. The actual increase of the duration of the longest 10% of WPs over 60 yr period is about 1 day for the Western and Eastern Europe. […]

How the lengthening of WPs influenced the intensity of precipitation? Conventional analysis shows that rain intensity and totals during 1950–2008 increased from 1.5–2.5% per decade in Northern Western Europe to 4% per decade in Eastern Europe in agreement with Moberg et al. [2006] and Zolina et al. [2009]. The fractional contribution to precipitation totals from the relatively short WPs (<50th percentile, typically 1–2 days) decreased by 3–5% per decade with the strongest trend of −5% per decade in the European Russia. At the same time, the contribution to the totals from longer WPs (>50th percentile) increased almost everywhere from 1–3% per decade in Western Europe to 4–6% per decade in European Russia. Thus the contribution by short WPs to the precipitation total decreased during 60 years from 46–48% to about 28–33%, while the contribution from the longer WPs increased from 50% in the 1950s to roughly 70% in the 2000s.

In the next step the authors estimated the impact of increasing WP duration onto the changes in the intensity of heavy precipitation associated with different WPs. Intense events were quantified as those exceeding the 95th percentile estimated from the raw daily time series for which the 95th percentile threshold was computed from 60-year records. Results obtained using the Gamma distribution or the Extreme Value Distribution are very similar to those based on the raw data. The fractional contribution of heavy precipitation associated with different WPs to the precipitation total was computed as the contribution of the 5% of the most intense precipitation events [KTK03]. For annual series these estimates are very close to those obtained using the theoretically Distribution of Fractional Contribution [Zolina et al., 2009].

On average, the number of heavy precipitation events (exceeding 95th percentile) peaks for 2–3 day WPs. Over Scandinavia this peak is shifted to 3–4 days and in European Russia the highest occurrence of intense rainfalls is associated with WPs of 1–2 days duration. The normalized fraction anomalies of heavy precipitation daily events occurring during WPs of different length were computed. During 1950–2008 the occurrence of the association of heavy rainfall with longer WPs has increased by 3–4% per decade while the occurrence of heavy precipitation during shorter WPs (1–2 days) has decreased by 3% per decade. The percentage of heavy rainfalls associated with WPs shorter than 3 days decreased from 60% in the 1950s–1960s to 45% in the 1990s–2000s, while the occurrence of intense precipitation associated with longer WPs increased from 40% to 55%. Furthermore heavy rainfalls associated with longer WPs intensified over Europe with upward trends of 2–3% per decade in Western Europe and >5% per decade in European Russia, implying actual changes from 4 to 9 mm/day over the 60-year period. Alternatively, the intensity of heavy precipitation events associated with shorter WPs either do not show any trend or locally decrease, e.g. in Central Europe, by 2–4% per decade.

The influence of the increasing duration of European WPs over the last 60 years is summarized in 2-dimensional duration-intensity distribution. The occurrence of WPs longer than 2–3 days with high mean daily precipitation intensities have clearly increased, while short and moderately long WPs with precipitation <4 mm/day have decreased in frequency. Very similarly to the tendencies in the mean intensity, the intensity of heavy precipitation events (exceeding the 95th percentile and contributing 25 to 40% to the total) occurring during long wet spells increased by 6–8% per decade, while heavy rainfalls occurring during short WPs weakened by 4–6% per decade.

Summary and Conclusions: This analysis shows that European precipitation has not only increased and become more extreme during the last 60 years but also its structure has changed: short rain events have been regrouped into prolonged wet spells. Heavy precipitation events associated with longer WPs have intensified by about 12–18% during the last 60 years, while heavy rainfalls associated with short WPs became less intense. The lengthening of the European wet spells combined with an increased occurrence of associated heavy precipitation hints at an increasing role of moisture advection by cyclones in forming extremes with a pace exceeding that implied by local temperature changes [Trenberth et al., 2003; Allan and Soden, 2008]. Direct association of cyclone activity with the duration of WPs requires analysis for individual seasons which is difficult due to the limited (to provide the robust statistics) number of WPs per season or month. In this respect an accurate derivation of the theoretical PDFs for the WP duration and associated intensity will provide good prospect for the further analysis. In the future the role of changing duration of WPs in forming precipitation trends, including the intensity and occurrence of heavy and extreme rainfalls should be investigated in climate model simulations.

Modélisations

 

Hypothèses

 

 

Informations complémentaires (données utilisées, méthode, scénarios, etc.)

In this work the authors quantified changes in the duration of precipitation events during 1950–2008 using daily data from nearly 700 European rain gauges and associated changes in the intensities of mean and heavy precipitation with the duration of events. This allows to analyse the alternation of wet periods with different durations and to quantify changes in the structure of heavy precipitation on synoptic scale, thus, providing new metrics of precipitation variability useful for hydroclimate applications.

Daily rain gauge observations were taken from the recently updated European Climate Assessment (ECA) dataset. Details of the ECA composition are given by Klok and Klein Tank [2009]. Over European Russia, Belorussia and Ukraine for the 1990s and 2000s 57 ECA stations contain artifacts due to incorrect record decoding. These were substituted by the original data from the Russian Hydrometerological Service (RHS) collection and 32 new RHS stations were added. From the resulting 1558 stations 699 records which miss less than 10% of daily values in the annual records were selected. They cover the period 1950–2008 with the highest density in the Western Europe and Scandinavia.

Besides the traditionally used in precipitation analysis WDs the authors introduce wet spells or wet periods (WPs). For each year WPs were quantified as consecutive days with significant precipitation (>1 mm/day). This threshold excludes very light precipitation and accounts for the limited accuracy of rain gauges [KTK03]. This threshold was used for identification of dry day spells [Groisman and Knight 2008]. For the WPs the authors analyzed their durations and associated intensities.

Significance of linear trends was estimated using the Student t-test together with the non-parametric Mann-Kendall test and was also analyzed with respect to the confidence intervals of the trend significance (the reliability ratio). Only trends satisfying all three criteria at 95% level were considered to be significant.

 

(2) - Effets du changement climatique sur le milieu naturel

Reconstitutions

 

Observations

 

Modélisations

 

Hypothèses

 

 

Sensibilité du milieu à des paramètres climatiques

Informations complémentaires (données utilisées, méthode, scénarios, etc.)

 

 

 

(3) - Effets du changement climatique sur l'aléa

Reconstitutions

 

Observations

 

Modélisations

 

Hypothèses

The changes in the distribution of temporal characteristics of precipitation towards longer events and higher intensities should have a significant impact on the terrestrial hydrologic cycle including subsurface hydrodynamics, surface runoff and European flooding.

 

Paramètre de l'aléa

Sensibilité des paramètres de l'aléa à des paramètres climatiques

Informations complémentaires (données utilisées, méthode, scénarios, etc.)

 

The impact of precipitation on flooding is strongly related to the number of consecutive WDs with catastrophic floods being associated with anomalously long precipitation periods.

 

 

(4) - Remarques générales

 From the hydrologic perspective, it is important to account for the potential impacts of the changed precipitation regime on water resources and flooding. While there is no significant trend in flood frequencies for the past 80–150 years in Europe [Mudelsee et al., 2003], a clustering of floods has occurred in the last decades [Bunde et al., 2005]. Changing character of WPs may significantly alter change in the frequency and strength of floods. Lengthening of WPs will also enhance groundwater recharge and soil moisture storage [Maxwell and Kollet, 2008]. However, it is not easy to evaluate the transient hydrodynamic response at the catchment scale without integrated hydrologic simulations, accurately reproducing the variability across all spatiotemporal scales from days to decades and from meters to kilometers.

 

(5) - Syntèses et préconisations

 

Références citées :

Allan, R. P., and B. J. Soden (2008), Atmospheric warming and the amplification of precipitation extremes, Science, 321(5895).

Bunde, A., et al. (2005), Longterm memory: A natural mechanism for the clustering of extreme events and anomalous residual times in climate records, Phys. Rev. Lett., 94(4), 048701.

Emori, S., and S. J. Brown (2005), Dynamic and thermodynamic changes in mean and extreme precipitation under changed climate, Geophys. Res. Lett., 32, L17706.

Frei, C., and C. Schär (2001), Detection probability of trends in rare events: Theory and application to heavy precipitation in the Alpine region, J. Clim., 14, 1568–1584.

Groisman, P. Y., and R. W. Knight (2008), Prolonged dry episodes over the conterminous United States: New tendencies emerging during the last 40 years, J. Clim., 21, 1850–1862.

Groisman, P. Y., et al. (2005), Trends in intense precipitation in the climate record, J. Clim., 18(9), 1326–1350.

Klein Tank, A. M. G., and G. P. Koennen (2003), Trends in indices of daily temperature and precipitation extremes in Europe, 1946–99, J. Clim., 16, 3665–3680.

Klok, E. J., and A. M. G. Klein Tank (2009), Undated and extended European data set of daily climate observations, Int. J. Climatol., 29, 1182–1191.

Maxwell, R. M., and S. J. Kollet (2008), Interdependence of groundwater dynamics and landenergy feedbacks under climate change, Nat. Geosci., 1, 665–669.

Moberg, A., et al. (2006), Indices for daily temperature and precipitation extremes in Europe analyzed for the period 1901–2000, J. Geophys. Res., 111, D22106.

Mudelsee, M., et al. (2003), No upward trends in the occurrence of extreme floods in central Europe, Nature, 425(6954), 166–169.

Scaife, A. A., C. K. Folland, L. V. Alexander, A. Moberg, and J. R. Knight (2008), European climate extremes and the North Atlantic Oscillation, J. Clim., 21, 72–83.

Schmidli, J., and C. Frei (2005), Trends of heavy precipitation and wet and dry spells in Switzerland during the 20th century, Int. J. Climatol., 25, 753–771.

Semenov, V. A., and L. Bengtsson (2002), Secular trends in daily precipitation characteristics: greenhouse gas simulation with a coupled AOGCM, Clim. Dyn., 19, 123–140.

Trenberth, K. E., et al. (2003), The changing character of precipitation, Bull. Am. Meteorol. Soc., 84, 1205–1217.

Zolina, O., C. Simmer, A. Kapala, and S. K. Gulev (2005), On the robustness of the estimates of centennialscale variability in heavy precipitation from station data over Europe, Geophys. Res. Lett., 32, L14707.

Zolina, O., C. Simmer, A. Kapala, S. Bachner, S. K. Gulev, and H. Maechel (2008), Seasonally dependent changes of precipitation extremes over Germany since 1950 from a very dense observational network, J. Geophys. Res., 113, D06110.

Zolina, O., C. Simmer, K. Belyaev, A. Kapala, and S. K. Gulev (2009), Improving estimates of heavy and extreme precipitation using daily records from European rain gauges, J. Hydrometeorol., 10, 701–716.