Gerd Tetzlaff

No upward trends in the occurrence of extreme floods in central Europe

Extreme river floods have been a substantial natural hazard in Europe over the past centuries, and radiative effects of recent anthropogenic changes in atmospheric composition are expected to cause climate changes, especially enhancement of the hydrological cycle, leading to an increased flood risk. For the past few decades, however, observations from Europe do not show a clear increase in flood occurrence rate. Here we present longer-term records of winter and summer floods in two of the largest rivers in central Europe, the Elbe and Oder rivers. For the past 80 to 150 yr, we find a decrease in winter flood occurrence in both rivers, while summer floods show no trend, consistent with trends in extreme precipitation occurrence. The reduction in winter flood occurrence can partly be attributed to fewer events of strong freezing—following such events, breaking river ice at the end of the winter may function as a water barrier and enhance floods severely. Additionally, we detect significant long-term changes in flood occurrence rates in the sixteenth to nineteenth centuries, and conclude that reductions in river length, construction of reservoirs and deforestation have had minor effects on flood frequency.

Extreme floods in central Europe over the past 500 years: Role of cyclone pathway ''Zugstrasse Vb''

[1] Anthropogenically induced climate change has been hypothesized to add to the risk of extreme river floods because a warmer atmosphere can carry more water. In the case of the central European rivers Elbe and Oder, another possibility that has been considered is a more frequent occurrence of a weather situation of the type ‘‘Zugstrasse Vb,’’ where a low-pressure system travels from the Adriatic region northeastward, carrying moist air and bringing orographic rainfall in the mountainous catchment areas (Erzgebirge, Sudeten, and Beskids). Analysis of long, homogeneous records of past floods allows us to test such ideas. M. Mudelsee and co-workers recently presented flood records for the middle parts of the Elbe and Oder, which go continuously back to A.D. 1021 and A.D. 1269, respectively. Here we review the reconstruction and assess the data quality of the records, which are based on combining documentary data from the interval up to 1850 and measurements thereafter, finding both the Elbe and Oder records to provide reliable information on heavy floods at least since A.D. 1500. We explain that the statistical method of kernel occurrence rate estimation can overcome deficiencies of techniques previously used to investigate trends in the occurrence of climatic extremes, because it (1) allows nonmonotonic trends, (2) imposes no parametric restrictions, and (3) provides confidence bands, which are essential for evaluating whether observed trends are real or came by chance into the data. We further give a hypothesis test that can be used to evaluate monotonic trends. On the basis of these data and methods, we find for both the Elbe and Oder rivers (1) significant downward trends in winter flood risk during the twentieth century, (2) no significant trends in summer flood risk in the twentieth century, and (3) significant variations in flood risk during past centuries, with notable differences between the Elbe and Oder. The observed trends are shown to be both robust against data uncertainties and only slightly sensitive to land use changes or river engineering, lending support for climatic influences on flood occurrence rate. In the case of winter floods, regional warming during the twentieth century has likely reduced winter flood risk via a reduced rate of strong river freezing (breaking ice at the end of winter may function as a water barrier and enhance a high water stage severely). In the case of summer floods, correlation analysis shows a significant, but weak, relation between flood occurrence and meridional airflow, compatible with a ‘‘Zugstrasse Vb’’ weather situation. The weakness of this relation, together with the uncertainty about whether this weather situation became more frequent, explains the absence of trends in summer flood risk for the Elbe and Oder in the twentieth century. We finally draw conclusions about flood disaster management and modeling of flood occurrence under a changed climate. INDEX TERMS: 1610 Global Change: Atmosphere (0315, 0325); 1620 Global Change: Climate dynamics (3309); 1655 Global Change: Water cycles (1836); 1821 Hydrology: Floods; KEYWORDS: climate change, extreme events, flood risk

Global and Seasonal Variations of Stratospheric Gravity Wave Activity Deduced from the CHAMP/GPS Satellite

Abstract Global analyses of gravity wave (GW) activity in the stratosphere are presented using radio occultation data from the Challenging Minisatellite Payload (CHAMP) satellite. Temperature profiles obtained from CHAMP/ GPS radio occultations are first compared with ground-based instruments. In general, good agreement is found between these different techniques. Monthly mean values of potential energy Ep, being a measure of GW activity, which is estimated with radiosonde observations, are compared with CHAMP/GPS data and it is found that radiosonde-observed Ep values are slightly higher than those estimated with radio occultations. Strong diurnal variation of GW activity has been found. From the global morphology of GW activity, large Ep values are noticed, besides at tropical latitudes, even at midlatitudes during winter, but not during equinoxes. This suggests that wave activity at stratospheric heights is not only modulated due to orography (mountain/lee waves) but mainly depends on seasonal variatio...

Trends in flood risk of the River Werra (Germany) over the past 500 years

Abstract A record of floods from 1500 to 2003 of the River Werra (Germany) is presented. The reconstruction is based on combining documentary and instrumental data. Because both data types have overlapping time intervals, it was possible to apply similar thresholds for flood definition and obtain a rather homogenous flood series. The kernel method yielded estimates of time-dependent flood risk. Bootstrap confidence bands helped to assess the significance of trends. The following was found: (a) the overall risk of floods in winter (November—April) is approximately 3.5 times higher than the summer flood risk; (b) winter flood risk peaked at around 1760 and 1860—it increases again during the past decades; and (c) summer flood risk peaked at around 1760—it shows a long-term decrease from then on. These trends for the Werra contrast with those of nearby River Elbe, reflecting the high spatial variability of orographic rainfall.

A component time-series model for SO2 data: Forecasting, interpretation and modification

Abstract A time-series forecasting method is developed to enable advance warning of smog in winter. A component model for the time series of SO 2 concentration essentially using a recursive Kalman algorithm is constructed on the basis of spectral analysis. It is found that the smog episodes with low frequencies and time-dependent power spectra are solely represented by the trend component. This component is therefore investigated in the phase space, where it exhibits a typical trajectory. For forecasting, one part of the data is used to establish the parameters and another part is used to test the extrapolation. The extrapolation and interpolation behaviour of the Kalman filter used is investigated. The trend component is found not to agree with the behaviour of the trajectory in the phase space. A modified method is proposed to extrapolate the time-dependent spectrum of the trend component, namely local harmonic approximation. This method is tested and compared with linear extrapolation and is found to provide a generalisation, producing closer correspondence between the concentration values predicted and those actually observed.

Numerical solution of the stochastic collection equation—comparison of the Linear Discrete Method with other methods

Abstract The Linear Discrete Method is used to solve the Stochastic Collection Equation (SCE) numerically. Comparisons are made with the Method of Moments, the Berry–Reinhardt model and the Linear Flux Method. Simulations for all numerical methods are shown for the kernel after Golovin [Bull. Acad. Sci. USSR, Geophys. Ser. 5 (1963) 783] and are compared with the analytical solution for two different initial distributions. BRM seems to give the best results and LDM gives good results, too. LFM overestimates the drop growth for the right tail of the distribution and MOM does the same but over the entire drop spectrum. For the hydrodynamic kernel after Long [J. Atmos. Sci. 31 (1974) 1040], simulations are presented using the four numerical methods (LDM, MOM, BRM, LFM). Especially for high resolutions, the solutions of LDM and LFM approach each other very closely. In addition, LDM simulations using the hydrodynamic kernel after Bohm [Atmos. Res. 52 (1999) 167] are presented, which show good correspondence between low- and high-resolution results. Computation efficiency is especially important when numerical schemes are to be included in larger models. Therefore, the computation times of the four methods were compared for the cases with the Golovin kernel. The result is that LDM is the fastest method by far, needing less time than other methods by a factor of 2–7, depending on the case and the bin resolution. For high resolutions, MOM is the slowest. For the lowest resolution, this holds for LFM.

Application of a spectral correction method for measurements of covariances with fast-response sensors in the atmospheric boundary layer up to a height of 130 m and testing of the corrections

Measurement of meteorological quantities with fast-response sensors by means of the eddycorrelation technique allows calculation of turbulent transport without too severe demands with regard to stationarity and homogeneity. However, the measurement system causes frequency-dependent errors due to measuring height, averaging interval, sampling rate and mean wind speed. A correction method (Moore, 1986) was applied to measurements taken on a 130 m tower located at the German North Sea coast. The errors for 10-min averages, obtained at a height of 10 m were between 20 and 30%, mainly caused by the inertia of the sensors, increasing to between 30 and 60% at 130 m height due to a loss of low-frequency parts of the spectra because of the averaging interval. The corrections to the obtained data were validated by comparing the cospectra calculated from the data with the spectra used for the computation of the correction factors.

Estimates of the information provided by GPS slant data observed in Germany regarding tomographic applications

[1] The observation of GPS slant delays from ground GPS networks can be used to reconstruct spatially resolved humidity fields in the troposphere by means of tomographic techniques. Tomography is always related to the solution of inverse problems which are very sensitive to the quality of the input data. Prior to a tomographic reconstruction, it is therefore necessary to quantify the information provided by a given set of GPS slant delay data. This work describes the properties and the information content of more than two million GPS slant delays taken in March 2006 by a continuously operating German GPS network. The temporal and spatial distribution of the slant paths in the atmosphere and their angular distribution in the local system of the GPS station is given. These distributions depend on the satellite orbits and show some characteristic pattern. The available information is estimated by investigating the distribution of intersection points between the slant paths. From these data it is possible to identify regions that are well covered by GPS slant paths and to evaluate the applicability of the existing German GPS stations for continuous atmosphere sounding.

Observations of area averaged near-surface wind- and temperature-fields in real terrain using acoustic travel time tomography

Any physical description setting for the distribution of momentum, energy and matter within the turbulent boundary layer of the atmosphere is usually made according to models based on the assumption of horizontal homogeneity. However, the earths surface not being homogeneous, the consequence of this approach is that these physical models cannot be transferred nor applied under non-homogeneous conditions. The conventional methods used for observing micro-meteorology data do not appear to be adequate in interpreting data observed on heterogeneous surfaces. In this connection, the method of acoustic travel-time tomography is here introduced. This method uses the variability of the speed of sound according to meteorological quantities such as wind and temperature in order to observe synchronous data of wind- and air-temperature fields under natural conditions in a given area. This allows to shun a main requirement of conventional micro-meteorological experiments - the horizontally homogeneous conditions.

On Modeling Dry Deposition of Long-Lived and Chemically Reactive Species over Heterogeneous Terrain

An explicit multi-layer subgrid-scheme was developed for a meso-γ/β-scale model to consider subgrid-scale surface heterogeneity, dry deposition, biogenic and anthropogenic emission of trace gases. Since dry deposition measurements of highly reactive trace species are scarce we try to evaluate this scheme by heuristic principles. The results of simulations conducted for a 5 × 5 km2 resolution with and without this scheme are evaluated by using results of a model run with 1 × 1 km2 resolution, which is taken as a ‘grand thruth’ and which has the same resolution as the subgrid. The explict multi-layer subgrid scheme provides a similar distribution of dry deposition fluxes as the much more computationally expensive simulation with the 1 × 1 km2 resolution.