TY - JOUR A1 - Bürger, Gerd T1 - A conundrum of trends BT - comment on a paper by Lischeid et al. (2021) JF - Journal of hydrology N2 - This comment is meant to reiterate two warnings: One applies to the uncritical use of ready-made (openly available) program packages, and one to the estimation of trends in serially correlated time series. Both warnings apply to the recent publication of Lischeid et al. about lake-level trends in Germany. KW - Linear trends KW - Autocorrelation KW - Pre-whitening Y1 - 2022 U6 - https://doi.org/10.1016/j.jhydrol.2022.127745 SN - 0022-1694 SN - 1879-2707 VL - 609 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Bürger, Gerd T1 - A counterexample to decomposing climate shifts and trends by weather types JF - International Journal of Climatology N2 - The literature contains a sizable number of publications where weather types are used to decompose climate shifts or trends into contributions of frequency and mean of those types. They are all based on the product rule, that is, a transformation of a product of sums into a sum of products, the latter providing the decomposition. While there is nothing to argue about the transformation itself, its interpretation as a climate shift or trend decomposition is bound to fail. While the case of a climate shift may be viewed as an incomplete description of a more complex behaviour, trend decomposition indeed produces bogus trends, as demonstrated by a synthetic counterexample with well-defined trends in type frequency and mean. Consequently, decompositions based on that transformation, be it for climate shifts or trends, must not be used. KW - analysis KW - climate KW - statistical methods Y1 - 2018 U6 - https://doi.org/10.1002/joc.5519 SN - 0899-8418 SN - 1097-0088 VL - 38 IS - 9 SP - 3732 EP - 3735 PB - Wiley CY - Hoboken ER - TY - JOUR A1 - Bürger, Gerd T1 - A seamless filter for daily to seasonal forecasts, with applications to Iran and Brazil JF - Quarterly Journal of the Royal Meteorological Society N2 - A digital filter is introduced which treats the problem of predictability versus time averaging in a continuous, seamless manner. This seamless filter (SF) is characterized by a unique smoothing rule that determines the strength of smoothing in dependence on lead time. The rule needs to be specified beforehand, either by expert knowledge or by user demand. As a result, skill curves are obtained that allow a predictability assessment across a whole range of time-scales, from daily to seasonal, in a uniform manner. The SF is applied to downscaled SEAS5 ensemble forecasts for two focus regions in or near the tropical belt, the river basins of the Karun in Iran and the Sao Francisco in Brazil. Both are characterized by strong seasonality and semi-aridity, so that predictability across various time-scales is in high demand. Among other things, it is found that from the start of the water year (autumn), areal precipitation is predictable with good skill for the Karun basin two and a half months ahead; for the Sao Francisco it is only one month, longer-term prediction skill is just above the critical level. KW - climate drift KW - ensemble prediction KW - seamless prediction KW - seasonal forecast skill Y1 - 2019 VL - 146 IS - 726 PB - WILEY-VCH CY - Weinheim ER - TY - JOUR A1 - Murawski, Aline A1 - Bürger, Gerd A1 - Vorogushyn, Sergiy A1 - Merz, Bruno T1 - Can local climate variability be explained by weather patterns? A multi-station evaluation for the Rhine basin JF - Hydrology and earth system sciences : HESS N2 - To understand past flood changes in the Rhine catchment and in particular the role of anthropogenic climate change in extreme flows, an attribution study relying on a proper GCM (general circulation model) downscaling is needed. A downscaling based on conditioning a stochastic weather generator on weather patterns is a promising approach. This approach assumes a strong link between weather patterns and local climate, and sufficient GCM skill in reproducing weather pattern climatology. These presuppositions are unprecedentedly evaluated here using 111 years of daily climate data from 490 stations in the Rhine basin and comprehensively testing the number of classification parameters and GCM weather pattern characteristics. A classification based on a combination of mean sea level pressure, temperature, and humidity from the ERA20C reanalysis of atmospheric fields over central Europe with 40 weather types was found to be the most appropriate for stratifying six local climate variables. The corresponding skill is quite diverse though, ranging from good for radiation to poor for precipitation. Especially for the latter it was apparent that pressure fields alone cannot sufficiently stratify local variability. To test the skill of the latest generation of GCMs from the CMIP5 ensemble in reproducing the frequency, seasonality, and persistence of the derived weather patterns, output from 15 GCMs is evaluated. Most GCMs are able to capture these characteristics well, but some models showed consistent deviations in all three evaluation criteria and should be excluded from further attribution analysis. Y1 - 2016 U6 - https://doi.org/10.5194/hess-20-4283-2016 SN - 1027-5606 SN - 1607-7938 VL - 20 SP - 4283 EP - 4306 PB - Copernicus CY - Göttingen ER - TY - JOUR A1 - Didovets, Iulii A1 - Krysanova, Valentina A1 - Bürger, Gerd A1 - Snizhko, Sergiy A1 - Balabukh, Vira A1 - Bronstert, Axel T1 - Climate change impact on regional floods in the Carpathian region JF - Journal of hydrology : Regional studies N2 - Study region: Tisza and Prut catchments, originating on the slopes of the Carpathian mountains. Study focus: The study reported here investigates (i) climate change impacts on flood risk in the region, and (ii) uncertainty related to hydrological modelling, downscaling techniques and climate projections. The climate projections used in the study were derived from five GCMs, downscaled either dynamically with RCMs or with the statistical downscaling model XDS. The resulting climate change scenarios were applied to drive the eco-hydrological model SWIM, which was calibrated and validated for the catchments in advance using observed climate and hydrological data. The changes in the 30-year flood hazards and 98 and 95 percentiles of discharge were evaluated for the far future period (2071-2100) in comparison with the reference period (1981-2010). New hydrological insights for the region: The majority of model outputs under RCP 4.5 show a small to strong increase of the 30-year flood level in the Tisza ranging from 4.5% to 62%, and moderate increase in the Prut ranging from 11% to 22%. The impact results under RCP 8.5 are more uncertain with changes in both directions due to high uncertainties in GCM-RCM climate projections, downscaling methods and the low density of available climate stations. KW - Climate change impact KW - Floods KW - Hydrological modelling KW - SWIM KW - Tisza KW - Prut KW - Carpathians KW - Ukraine Y1 - 2019 U6 - https://doi.org/10.1016/j.ejrh.2019.01.002 SN - 2214-5818 VL - 22 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Menzel, Lucas A1 - Niehoff, Daniel A1 - Bürger, Gerd A1 - Bronstert, Axel T1 - Climate change impacts on river flooding : a modelling study of three meso-scale catchments Y1 - 2002 ER - TY - INPR A1 - Bürger, Gerd T1 - Comment on "Bias correction, quantile mapping, and downscaling: revisiting the inflation issue" T2 - Journal of climate N2 - In a recent paper, Maraun describes the adverse effects of quantile mapping on downscaling. He argues that when large-scale GCM variables are rescaled directly to small-scale fields or even station data, genuine small-scale covariability is lost and replaced by uniform variability inherited from the larger scales. This leads to a misrepresentation mainly of areal means and long-term trends. This comment acknowledges the former point, although the argument is relatively old, but disagrees with the latter, showing that grid-size long-term trends can be different from local trends. Finally, because it is partly incorrectly addressed, some clarification is added regarding the inflation issue, stressing that neither randomization nor inflation is free of unverified assumptions. KW - Climate change KW - Statistics KW - Climate variability Y1 - 2014 U6 - https://doi.org/10.1175/JCLI-D-13-00184.1 SN - 0894-8755 SN - 1520-0442 VL - 27 IS - 4 SP - 1819 EP - 1820 PB - American Meteorological Soc. CY - Boston ER - TY - JOUR A1 - Murawski, Aline A1 - Vorogushyn, Sergiy A1 - Bürger, Gerd A1 - Gerlitz, Lars A1 - Merz, Bruno T1 - Do changing weather types explain observed climatic trends in the rhine basin? BT - an analysis of within- and between-type changes JF - Journal of geophysical of geophysical research-atmosheres N2 - For attributing hydrological changes to anthropogenic climate change, catchment models are driven by climate model output. A widespread approach to bridge the spatial gap between global climate and hydrological catchment models is to use a weather generator conditioned on weather patterns (WPs). This approach assumes that changes in local climate are characterized by between-type changes of patterns. In this study we test this assumption by analyzing a previously developed WP classification for the Rhine basin, which is based on dynamic and thermodynamic variables. We quantify changes in pattern characteristics and associated climatic properties. The amount of between- and within-type changes is investigated by comparing observed trends to trends resulting solely from WP occurrence. To overcome uncertainties in trend detection resulting from the selected time period, all possible periods in 1901-2010 with a minimum length of 31 years are analyzed. Increasing frequency is found for some patterns associated with high precipitation, although the trend sign highly depends on the considered period. Trends and interannual variations of WP frequencies are related to the long-term variability of large-scale circulation modes. Long-term WP internal warming is evident for summer patterns and enhanced warming for spring/autumn patterns since the 1970s. Observed trends in temperature and partly in precipitation are mainly associated with frequency changes of specific WPs, but some amount of within-type changes remains. The classification can be used for downscaling of past changes considering this limitation, but the inclusion of thermodynamic variables into the classification impedes the downscaling of future climate projections. KW - attribution KW - weather pattern KW - trend analysis KW - downscaling KW - hypothetical trend Y1 - 2018 U6 - https://doi.org/10.1002/2017JD026654 SN - 2169-897X SN - 2169-8996 VL - 123 IS - 3 SP - 1562 EP - 1584 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Bürger, Gerd A1 - Sobie, S. R. A1 - Cannon, A. J. A1 - Werner, A. T. A1 - Murdock, T. Q. T1 - Downscaling extremes an intercomparison of multiple methods for future climate JF - Journal of climate N2 - This study follows up on a previous downscaling intercomparison for present climate. Using a larger set of eight methods the authors downscale atmospheric fields representing present (1981-2000) and future (2046-65) conditions, as simulated by six global climate models following three emission scenarios. Local extremes were studied at 20 locations in British Columbia as measured by the same set of 27 indices, ClimDEX, as in the precursor study. Present and future simulations give 2 x 3 x 6 x 8 x 20 x 27 = 155 520 index climatologies whose analysis in terms of mean change and variation is the purpose of this study. The mean change generally reinforces what is to be expected in a warmer climate: that extreme cold events become less frequent and extreme warm events become more frequent, and that there are signs of more frequent precipitation extremes. There is considerable variation, however, about this tendency, caused by the influence of scenario, climate model, downscaling method, and location. This is analyzed using standard statistical techniques such as analysis of variance and multidimensional scaling, along with an assessment of the influence of each modeling component on the overall variation of the simulated change. It is found that downscaling generally has the strongest influence, followed by climate model; location and scenario have only a minor influence. The influence of downscaling could be traced back in part to various issues related to the methods, such as the quality of simulated variability or the dependence on predictors. Using only methods validated in the precursor study considerably reduced the influence of downscaling, underpinning the general need for method verification. Y1 - 2013 U6 - https://doi.org/10.1175/JCLI-D-12-00249.1 SN - 0894-8755 VL - 26 IS - 10 SP - 3429 EP - 3449 PB - American Meteorological Soc. CY - Boston ER - TY - JOUR A1 - Köhn-Reich, Lisei A1 - Bürger, Gerd T1 - Dynamical prediction of Indian monsoon BT - past and present skill JF - International Journal of Climatology N2 - Ongoing development of dynamical atmosphere-ocean general circulation models keep expectations high regarding seasonal predictions of Indian monsoon rainfall. This study compares past and present skill of four currently operating forecasting systems, CFSv2 from NCEP, ENSEMBLES, System 4 and the newest SEAS5 from ECMWF, by analysing correlations of respective hindcasts with observed all-India summer rainfall. For the common time period 1982-2005, only ENSEMBLES and CFSv2 give significantly skilful forecasts. It is shown that skill is highly dependent on the chosen time period. Especially the intense El Nino of 1997 seems to degrade the predictions, most notably for SEAS4 and SEAS5 which seem to be linked to El Nino too strongly. We show that by discarding that year, a regime shift in the 1990s is no longer visible. Overall, we observe a convergence of skill towards the present with correlations of about 0.4 for CFSv2 and of 0.6 for System 4 and SEAS5. KW - correlation skill KW - dynamical seasonal prediction KW - Indian summer monsoon Y1 - 2019 U6 - https://doi.org/10.1002/joc.6039 SN - 0899-8418 SN - 1097-0088 VL - 39 IS - 8 SP - 3574 EP - 3581 PB - Wiley CY - Hoboken ER -