ALBERT

Beschreibung: This work presents a new bias-correction method for precipitation over complex terrain that explicitly considers orographic characteristics. This consideration offers a good alternative to the standard empirical quantile mapping (EQM) method during colder climate states in which the orography strongly deviates from the present-day state, e.g. during glacial conditions such as the Last Glacial Maximum (LGM). Such a method is needed in the event that absolute precipitation fields are used, e.g. as input for glacier modelling or to assess potential human occupation and according migration routes in past climate states. The new bias correction and its performance are presented for Switzerland using regional climate model simulations at 2 km resolution driven by global climate model outputs obtained under perpetual 1990 and LGM conditions. Comparing the present-day regional climate model simulation with observations, we find a strong seasonality and, especially during colder months, a height dependence of the bias in precipitation. Thus, we suggest a three-step correction method consisting of (i) a separation into different orographic characteristics, (ii) correction of very low intensity precipitation, and (iii) the application of an EQM, which is applied to each month separately. We find that separating the orography into 400 m height intervals provides the overall most reasonable correction of the biases in precipitation. The new method is able to fully correct the seasonal precipitation bias induced by the global climate model. At the same time, some regional biases remain, in particular positive biases over high elevated areas in winter and negative biases in deep valleys and Ticino in winter and summer. A rigorous temporal and spatial cross-validation with independent data exhibits robust results. The new bias-correction method certainly leaves some drawbacks under present-day conditions. However, the application to the LGM demonstrates that it is a more appropriate correction compared to the standard EQM under highly different climate conditions as the latter imprints present-day orographic features into the LGM climate.

Beschreibung: Extratropical cyclones of type Vb, which develop over the western Mediterranean and move northeastward, are major natural hazards that are responsible for heavy precipitation over central Europe. To gain further understanding in the governing processes of these Vb cyclones, the study explores the role of soil moisture and sea surface temperature (SST) and their contribution to the atmospheric moisture content. Thereby, recent Vb events identified in the ERA-Interim reanalysis are dynamically downscaled with the Weather Research and Forecasting (WRF) model. Results indicate that a mean high-impact summer Vb event is mostly sensitive to an increase in the Mediterranean SSTs and rather insensitive to Atlantic SSTs and soil moisture changes. Hence, an increase of +5 K in Mediterranean SSTs leads to an average increase of 24 % in precipitation over central Europe. This increase in precipitation is mainly induced by larger mean upward moisture flux over the Mediterranean with increasing Mediterranean SSTs. This further invokes an increase in latent energy release, which leads to an increase in atmospheric instability, i.e. in convective available potential energy. Both the increased availability of atmospheric moisture and the increased instability of the atmosphere, which is able to remove extra moisture from the atmosphere due to convective processes, are responsible for the strong increase in precipitation over the entire region influenced by Vb events. Precipitation patterns further indicate that a strong increase in precipitation is found at the eastern coast of the Adriatic Sea for increased Mediterranean SSTs. This premature loss in atmospheric moisture leads to a significant decrease in atmospheric moisture transport to central Europe and the northeastern flanks of the Alpine mountain chain. This leads to a reduction in precipitation in this high-impact region of the Vb event for an increase in Mediterranean SSTs of +5 K. Furthermore, the intensity of the Vb cyclones, measured as a gradient in the 850 hPa geopotential height field around the cyclone centre, indicates that an upper bound for intensity might be reached for the most intense Vb event.

Beschreibung: Extra-tropical cyclones of type Vb, which develop over the western Mediterranean and move northeastward, are major natural hazards being responsible for heavy precipitation over Central Europe. To gain further understanding in the governing processes of these Vb cyclones the stu-dy explores the role of soil moisture and sea surface temperature (SST) and their contribution to the atmospheric moisture content. Thereby, recent Vb events identified in the ERA-Interim reanalysis are dynamically downscaled with the Weather Research and Forecasting model (WRF). Results indicate that a mean Vb event is mostly sensitive to an increase in the Mediterranean SSTs, e.g., an increase of +5 K leads to an average increase of 24 % in precipitation over Central Europe. This increase in precipitation is mainly induced by larger mean upward moisture flux over the Mediterranean with increasing Mediterranean SSTs. This further invokes an increase in latent energy release, which leads to an increase in atmospheric instability, i.e., in convective available potential energy. Both, the increased availability of atmospheric moisture and the increased instability of the atmosphere, which is able to remove extra moisture from the atmosphere due to convective processes, are responsible for the strong increase in precipitation over the entire region influenced by Vb events. Precipitation patterns further indicate that a strong increase in precipitation is found at the eastern coast of the Adriatic Sea for increased Mediterranean SSTs. This premature loss in atmospheric moisture leads to a significant decrease in atmospheric moisture transport to Central Europe and the northeastern flanks of the Alpine mountain chain. This leads to a reduction in precipitation in this high-impact region of the Vb event for an increase in Mediterranean SSTs of +5 K. Furthermore, the intensity of the Vb cyclones, measured as a gradient in the 850-hPa geopotential height field around the cyclone centre, indicates that an upper bound for intensity might be reached for the most intense Vb event. This fact indicates that strong cyclones are more strongly steered by the present atmospheric conditions.

Beschreibung: In June 1876, June 1910, and August 2005, northern Switzerland was severely impacted by heavy precipitation and extreme floods. Although occurring in different centuries, all three events featured very similar precipitation patterns and an extratropical storm following a cyclonic, so-called Vb (five b of the van Bebber trajectories) trajectory around the Alps. Going back in time from the recent to the historical cases, we explore the potential of dynamical downscaling of a global reanalysis product from a grid size of 220 to 3 km. We investigate sensitivities of the simulated precipitation amounts to a set of differing configurations in the regional weather model. The best-performing model configuration in the evaluation, featuring a 1 d initialization period, is then applied to assess the sensitivity of simulated precipitation totals to cyclonic moisture flux along the downscaling steps. The analyses show that cyclone fields (closed pressure contours) and tracks (minimum pressure trajectories) are well defined in the reanalysis ensemble for the 2005 and 1910 cases, while deviations from the ensemble mean increase for the 1876 case. In the downscaled ensemble, the accuracy of simulated precipitation totals is closely linked to the exact trajectory and stalling position of the cyclone, with slight shifts producing erroneous precipitation, e.g., due to a break-up of the vortex if simulated too close to the Alpine topography. Simulated precipitation totals only reach the observed ones if the simulation includes continuous moisture fluxes of 〉200 kg m−1 s−1 from northerly directions and high contributions of (embedded) convection. Misplacement of the vortex and concurrent uncertainties in simulating convection, in particular for the 1876 case, point to limitations of downscaling from coarse input for such complex weather situations and for the more distant past. On the upside, single (contrasting) members of the historical cases are well capable of illustrating variants of Vb cyclone dynamics and features along the downscaling steps.

Beschreibung: Extratropical cyclones in winter and their characteristics are investigated in depth for the Atlantic European region, as they are responsible for a significant part of the rainfall and extreme wind and/or precipitation-induced hazards. Here, we use a seamless transient simulation with a state-of-the-art fully-coupled Earth System Model from 850 to 2100CE as basis for the analysis. The RCP8.5 scenario is applied in the 21st century. During the Common Era, cyclone characteristics show pronounced variations on interannual and decadal time scales, but no external forcing imprint is found prior to 1850. Thus, variations of extratropical cyclone characteristics are mainly caused by internal variability of the coupled climate system. When anthropogenic forcing becomes dominant in the 20th century, a decrease of the cyclone occurrences mainly over the Mediterranean and a strong increase of extreme cyclone-related precipitation become detectable. The latter is due to thermodynamics as it follows the Clausius-Clapeyron relation. An important finding, though, is that the relation between temperature and extreme cyclone-related precipitation is not always controlled by the Clausius-Clapeyron relation, which suggests that dynamical processes can play an important role in generating extreme cyclone-related precipitation – for example in the absence of anomalously warm background conditions. Thus, the importance of dynamical processes, even on decadal time scales, might explain the conundrum that proxy records suggest enhanced occurrence of precipitation extremes during rather cold periods in the past.

Beschreibung: In June 1876, June 1910 and August 2005, northern Switzerland was severely impacted by heavy precipitation and extreme floods. Although occurring in three different centuries, all three events featured very similar precipitation patterns and an extra-tropical storm following a cyclonic, so called Vb trajectory around the Alps. Going back in time from the recent to the historical cases, we explore the potential of dynamical downscaling a global reanalysis product from a grid size of 220 km to 3 km. We use the full, 56-member ensemble provided in the reanalysis and a regional weather model to investigate sensitivities of the simulated precipitation amounts to a set of differing model configurations. These setups are evaluated by combining spatial verification metrics, inter-subjective visual inspection and an objective similarity measure. The best-performing model setup, featuring a 1-day initialization period and moderate spectral nudging, is then applied to assess the sensitivity of simulated precipitation totals to cyclonic moisture flux along the downscaling steps. The analyses show that cyclone fields and tracks are well defined in the reanalysis ensemble for the 2005 and 1910 cases, while deviations increase for the 1876 case. In the downscaled ensemble, the accuracy of simulated precipitation totals is closely linked to the exact trajectory of the cyclone, with slight shifts producing erroneous precipitation, e.g., due to a break-up of the vortex if simulated too close to the Alpine topography. To reproduce the extreme events, continuous moisture fluxes of 〉 200 kg m−1s−1 from accurate directions are required. Misplacements of the vortex, in particular for the 1876 case, point to limitations of downscaling from coarse input for such complex weather situations and for the more distant past. On the upside, a well-reasoned selection of reanalysis members for downscaling may be adequate in cases where the driving large-scale features in the atmosphere are well known.

Beschreibung: Extratropical cyclones in winter and their characteristics are investigated in depth for the Atlantic European region, as they are responsible for a significant part of the rainfall and extreme wind and/or precipitation-induced hazards. The analysis is based on a seamless transient simulation with a state-of-the-art fully coupled Earth system model from 850 to 2100 CE. The Representative Concentration Pathway 8.5 (RCP8.5) scenario is used in the 21st century. During the Common Era, cyclone characteristics show pronounced variations on interannual and decadal timescales, but no external forcing imprint is found prior to 1850. Thus, variations of extratropical cyclone characteristics are mainly caused by internal variability of the coupled climate system. When anthropogenic forcing becomes dominant in the 20th century, a decrease of the cyclone occurrences mainly over the Mediterranean and a strong increase of extreme cyclone-related precipitation become detectable. The latter is due to thermodynamics as it follows the Clausius–Clapeyron relation. An important finding, though, is that the relation between temperature and extreme cyclone-related precipitation is not always controlled by the Clausius–Clapeyron relation, which suggests that dynamical processes can play an important role in generating extreme cyclone-related precipitation – for example, in the absence of anomalously warm background conditions. Thus, the importance of dynamical processes, even on decadal timescales, might explain the conundrum that proxy records suggest enhanced occurrence of precipitation extremes during rather cold periods in the past.

Beschreibung: This work presents a new bias-correction method for precipitation that considers orographic characteristics, which makes it flexible to be used under highly different climate conditions, e.g., glacial conditions. The new bias-correction and its performance are presented for Switzerland using a regional climate simulation under perpetual 1990 conditions at 2-km resolution driven by a simulation performed with a global climate model. Comparing the regional simulations with observations, we find a strong seasonal and height dependence of the bias in precipitation commonly observed in regional climate modelling over complex terrain. Thus, we suggest a 3-step correction method consisting of (i) a separation into different orographic characteristics, (ii) correction of low intensity precipitation, and finally (iii) the application of empirical quantile mapping, which is applied to each month separately. Testing different orographic characteristics shows that separating in 400-m height-intervals provides the overall most reasonable correction of the biases in precipitation and additionally at the lowest computational costs. The seasonal precipitation bias induced by the global climate model is fully corrected, whereas some regional biases remain, in particular positive biases in winter over mountains and negative biases in winter and summer in deep valleys and Ticino. The biases over mountains are difficult to judge, as observations over complex terrain are afflicted with uncertainties, which may be more than 30 % above 1500 m a.s.l. A rigorous cross validation, which trains the correction method with independent observations from Germany, Austria and France, exhibits a similar performance compared to just using Switzerland as training and verification region. This illustrates the robustness of the new method. Thus, the new bias-correction provides a flexible tool which is suitable in studies where orography strongly changes, e.g., during glacial times.