ABSTRACT Simulating surface wind over complex terrain is a challenge in regional climate modellin... more ABSTRACT Simulating surface wind over complex terrain is a challenge in regional climate modelling. Therefore, this study aims at identifying a setup of the WRF model that minimizes systematic errors of surface winds in hindcast simulations. Major factors of the model configuration are tested to find a suitable setup: the horizontal resolution, the PBL parameterization scheme and the way WRF is nested to the driving dataset. Hence, a number of sensitivity simulations at a spatial resolution of 2 km are carried out and compared to observations. Given the importance of wind storms, the analysis is based on case studies of 24 historical wind storms that caused great economic damage in Switzerland. Each of these events is downscaled using eight different model setups, but sharing the same driving dataset. The results show that the unresolved topography leads to a general overestimation of wind speed in WRF. However, this bias can be substantially reduced by using a PBL scheme that explicitly considers the effects of non-resolved topography, which also improves the spatial structure of wind speed over Switzerland. The wind direction, although generally well reproduced, is not very sensitive to the PBL scheme. Further sensitivity tests include four types of nesting methods: nesting only at the boundaries of the outermost domain, analysis and spectral nudging, and the so-called re-forecast method, where the simulation is frequently restarted. These simulations show that restricting the freedom of the model to develop large-scale disturbances slightly increases the temporal agreement with the observations, at the same time that it further reduces the overestimation of wind speed, especially for maximum wind peaks. The model skill is also evaluated in the outermost domains, where the resolution is coarser. The results demonstrate the important role of horizontal resolution, where the step from 6 to 2 km significantly improves model performance. In summary, the combination of a grid size of 2 km, the non-local PBL scheme modified to explicitly account for non-resolved orography, as well as analysis or spectral nudging, is a superior combination when dynamical downscaling is aimed at reproducing real wind fields.
ABSTRACT Cyclones, which develop over the western Mediterranean and move northeastward are a majo... more ABSTRACT Cyclones, which develop over the western Mediterranean and move northeastward are a major source of extreme weather and known to be responsible for heavy precipitation over Central Europe and the Alps. As the relevant processes triggering these so-called Vb-events and their impact on extreme precipitation are not yet fully understood, this study focusses on gaining insight into the dynamics of past events. For this, a cyclone detection and tracking tool is applied to the ERA-Interim reanalysis (1979–2013) to identify prominent Vb-situations. Precipitation in the ERA-Interim and the E-OBS datasets is used to evaluate case-to-case precipitation amounts and to assess consistency between the two datasets. Both datasets exhibit high variability in precipitation amounts among different Vb-events. While only 23 % of all Vb-events are associated with extreme precipitation, around 15 % of all extreme precipitation days (99 percentile) over the Alpine region are induced by Vb-events, although Vb-cyclones are rare events (2.3 per year). To obtain a better understanding of the variability within Vb-events, the analysis of the 10 heaviest and lowest precipitation Vb-events reveals noticeable differences in the state of the atmosphere. These differences are most pronounced in the geopotential height and potential vorticity field, indicating a much stronger cyclone for heavy precipitation events. The related differences in wind direction are responsible for the moisture transport around the Alps and the orographical lifting along the Alps. These effects are the main reasons for a disastrous outcome of Vb-events, and consequently are absent in the Vb-events associated with low precipitation. Hence, our results point out that heavy precipitation related to Vb-events is mainly related to large-scale dynamics rather than to thermodynamic processes.
Atmospheric Chemistry and Physics Discussions, 2015
ABSTRACT After strong volcanic eruptions stratospheric ozone changes are modulated by heterogeneo... more ABSTRACT After strong volcanic eruptions stratospheric ozone changes are modulated by heterogeneous chemical reactions (HET) and dynamical perturbations related to the radiative heating in the lower stratosphere (RAD). Here, we assess the relative importance of both processes as well as the effect of the resulting ozone changes on the dynamics using ensemble simulations with the atmosphere–ocean–chemistry–climate model (AOCCM) SOCOL-MPIOM forced by eruptions with different strength. The simulations are performed under present day and preindustrial conditions to investigate changes in the response behaviour. The results show that the HET effect is only relevant under present day conditions and causes a pronounced global reduction of column ozone. These ozone changes further lead to a slight weakening of the Northern Hemisphere (NH) polar vortex during mid-winter. Independent from the climate state the RAD mechanism changes the column ozone pattern with negative anomalies in the tropics and positive anomalies in the mid-latitudes. The influence of the climate state on the RAD mechanism significantly differs in the polar latitudes, where an amplified ozone depletion during the winter months is simulated under present day conditions. This is in contrast to the preindustrial state showing a positive column ozone response also in the polar area. The dynamical response of the stratosphere is clearly dominated by the RAD mechanism showing an intensification of the NH polar vortex in winter. Still under present day conditions ozone changes due to the RAD mechanism slightly reduce the response of the polar vortex after the eruption.
The applicability of three different cyclone detection and tracking schemes is investigated in EC... more The applicability of three different cyclone detection and tracking schemes is investigated in ECMWF (ERA-40). Cyclone climatologies and cyclone characteristics are intercompared within the three methods. The comparison of the methods is based on the ERA-40 data set and shows a generally good correspondence between different schemes. However, trends in cyclone characteristics, like the number of cyclones, are sensitive to
The unstable midlatitude ocean-atmosphere coupling motivates the definition of two decadal regime... more The unstable midlatitude ocean-atmosphere coupling motivates the definition of two decadal regimes with distinct implications for the North Atlantic cyclone variability. Phases with low (high) decadal variability of the North Atlantic Oscillation, which are connected with an annular (sectoral) spatial scale of the geopotential height teleconnection pattern, are identified as a hemispheric (regional) regime. In the hemispheric regime during a
The newly developed atmosphere-ocean-chemistry-climate model SOCOL-MPIOM is presented by demonstr... more The newly developed atmosphere-ocean-chemistry-climate model SOCOL-MPIOM is presented by demonstrating the influence of the interactive chemistry module on the climate state and the variability. Therefore, we compare pre-industrial control simulations with (CHEM) and without (NOCHEM) interactive chemistry. In general, the influence of 5 the chemistry on the mean state and the variability is small and mainly restricted to the stratosphere and mesosphere. The largest differences are found for the atmospheric dynamics in the polar regions, with slightly stronger northern and southern winter polar vortices in CHEM. The strengthening of the vortex is related to larger stratospheric temperature gradients, which are attributed to a parametrization of the absorption of ozone 10 and oxygen in the Lyman-alpha, Schumann-Runge, Hartley, and Higgins bands. This effect is parametrized in the version with interactive chemistry only. A second reason for the temperature differences between CHEM and NOCHEM is related to diurnal variations in the ozone concentrations in the higher atmosphere, which are missing in NOCHEM. Furthermore, stratospheric water vapour concentrations differ substantially 15 between the two experiments, but their effect on the temperatures is small. In both setups, the simulated intensity and variability of the northern polar vortex is inside the range of present day observations. Sudden stratospheric warming events are well reproduced in terms of their frequency, but the distribution amongst the winter months is too uniform.
The aim of this work is to elucidate the impact of changes in solar irradiance and energetic part... more The aim of this work is to elucidate the impact of changes in solar irradiance and energetic particles versus volcanic eruptions on tropospheric global climate during the Dalton Minimum (DM, AD 1780-1840. Separate variations in the (i) solar irradiance in the UV-C with wavelengths λ < 250 nm, (ii) irradiance at wavelengths λ > 250 nm, (iii) in energetic particle spectrum, and (iv) volcanic aerosol forcing were analyzed separately, and (v) in combination, by means of small ensemble calculations using a coupled atmosphere-ocean chemistry-climate model. Global and hemispheric mean surface temperatures show a significant dependence on solar irradiance at λ > 250 nm. Also, powerful volcanic eruptions in 1809, 1815, 1831 and 1835 significantly decreased global mean temperature by up to 0.5 K for 2-3 years after the eruption. However, while the volcanic effect is clearly discernible in the Southern Hemispheric mean temperature, it is less significant in the Northern Hemisphere, partly because the two largest volcanic eruptions occurred in the SH tropics and during seasons when the aerosols were mainly transported southward, partly because of the higher northern internal variability. In the simulation including all forcings, temperatures are in reasonable agreement with the tree ring-based temperature anomalies of the Northern Hemisphere. Interestingly, the model suggests that solar irradiance changes at λ < 250 nm and in energetic particle spectra have only an insignificant impact on the climate during the Dalton Minimum. This downscales the importance of top-down processes (stemming from changes at λ < 250 nm) relative to bottom-up processes (from λ > 250 nm). Reduction of irradiance at λ > 250 nm leads to a significant (up to 2 %) decrease in the ocean heat content (OHC) between 0 and 300 m in depth, whereas the changes in irradiance at λ < 250 nm or in energetic particles have virtually no effect. Also, volcanic aerosol yields a very strong response, reducing the OHC of the upper ocean by up to 1.5 %. In the simulation with all forcings, the OHC of the uppermost levels recovers after 8-15 years after volcanic eruption, while the solar signal and the different volcanic eruptions dominate the OHC changes in the deeper ocean and prevent its recovery during the DM. Finally, the simulations suggest that the volcanic eruptions during the DM had a significant impact on the precipitation patterns caused by a widening of the Hadley cell and a shift in the intertropical convergence zone. www.clim-past.net/10/921/2014/ Clim. Past, 10, 921-938, 2014 www.clim-past.net/10/921/2014/ Clim. Past, 10, 921-938, 2014 Clim. Past, 10, 921-938, 2014 www.clim-past.net/10/921/2014/ J. G. Anet et al.: Sun and volcanoes: effect on the troposphere in the DM 935 efficient and appealing visualization of the model data. E. Rozanov, A.
To increase the sparse knowledge of long-term Southern Hemisphere (SH) climate variability we ass... more To increase the sparse knowledge of long-term Southern Hemisphere (SH) climate variability we assess an ensemble of 4 transient simulations over the last 500 yr performed with a state-of-the-art atmosphere ocean general circulation model. The model is forced with reconstructions of solar irradiance, greenhouse gas (GHG) and volcanic aerosol concentrations. A 1990 control simulation shows that the model is able
The resumption process of the North Atlantic meridional overturning circulation (MOC) is investig... more The resumption process of the North Atlantic meridional overturning circulation (MOC) is investigated in a series of freshwater hosing experiments using a comprehensive coupled climate model. Four different freshwater perturbations are applied to the North Atlantic between 50 � N and 70 � N leading to a substantial weakening of MOC and an expansion of winter sea ice cover over
Medieval Climate Anomaly (MCA) terrestrial proxy records (speleothem, NW Scotland; tree rings, Mo... more Medieval Climate Anomaly (MCA) terrestrial proxy records (speleothem, NW Scotland; tree rings, Morocco) from close to the centres of action of the winter North Atlantic Oscillation (NAO) indicate that the MCA was characterized by a pervasive positive phase of the NAO(1). Spatial gradients across Europe in this MCA synoptic climatological interpretation were analyzed using the Proxy Surrogate Reconstruction approach based
One of the most prominent features of mid-latitude atmospheric variability is blocking. Blocking ... more One of the most prominent features of mid-latitude atmospheric variability is blocking. Blocking events are anticyclones with an equivalent barotropic signature and persistent in time. The longer than synoptic day-to-day time scales has led to intrinsic interest during the last decades aiming to expand the predictability beyond classical numerical weather prediction. The study aims to present the extreme behavior of blocking and to investigate relations to other extreme events, like cold and dry spells. To assess the behavior of blocking events we developed a new event-based method. The method identifies 500-hPa geopotential height maxima and tracks these relative maxima with a next-neighborhood search in time. This new method agrees with the reference grid-point based method in the deduced climatological pattern of blocking frequency. Applying the method to ERA-40 data in winter for the Atlantic-European region we found a trend towards a reduction of blocking episodes. The mean sur...
A B S T R A C T Atmospheric blocking is an important source of low-frequency variability. We appl... more A B S T R A C T Atmospheric blocking is an important source of low-frequency variability. We apply a blocking detection and tracking method to ERA-40 data for the Atlantic-European region to assess linkages to extreme events, that is, cold and dry spells in the extended winter season (November-April). The method is feature-oriented, identifies 500-hPa geopotential height maxima, and connects them with a next-neighbourhood search in time. The analysis reveals a statistically significant decrease of number of blocking events over the period of ERA-40. Winters with an increased number of blocking events are associated with negative temperature anomalies over Central to Eastern Europe and dryer conditions, whereas Southern Europe experiences warmer and wetter conditions during such episodes. Using extreme value statistics we show evidence that cold spells, and to some extent dry spells, are strongly associated with extremes in blocking frequency. The number of cold spell days increases with the duration of blocking events indicating that cold spells need time to establish during blocking events, thus return periods of cold spells are longer than those for blocking events. This behaviour is not found for the relation of dry spells with blocking events. Still, blocking episodes have a higher occurrence of dry spells than climatology.
The influence of climate change on storm surges including increased mean sea level change and the... more The influence of climate change on storm surges including increased mean sea level change and the associated insurable losses are assessed for the North Sea basin. In doing so, the newly developed approach couples a dynamical storm surge model with a loss model. The key element of the approach is the generation of a probabilistic storm surge event set. Together with parametrizations of the inland propagation and the coastal protection failure probability this enables the estimation of annual expected losses. The sensitivity to the parametrizations is rather weak except when the assumption of high level of increased mean sea level change is made. Applying this approach to future scenarios shows a substantial increase of insurable losses with respect to the present day. Superimposing different mean sea level changes shows a nonlinear behavior at the country level, as the future storm surge changes are higher for Germany and Denmark. Thus, the study exhibits the necessity to assess the socio-economic impacts of coastal floods by combining the expected sea level rise with storm surge projections.
ABSTRACT Simulating surface wind over complex terrain is a challenge in regional climate modellin... more ABSTRACT Simulating surface wind over complex terrain is a challenge in regional climate modelling. Therefore, this study aims at identifying a setup of the WRF model that minimizes systematic errors of surface winds in hindcast simulations. Major factors of the model configuration are tested to find a suitable setup: the horizontal resolution, the PBL parameterization scheme and the way WRF is nested to the driving dataset. Hence, a number of sensitivity simulations at a spatial resolution of 2 km are carried out and compared to observations. Given the importance of wind storms, the analysis is based on case studies of 24 historical wind storms that caused great economic damage in Switzerland. Each of these events is downscaled using eight different model setups, but sharing the same driving dataset. The results show that the unresolved topography leads to a general overestimation of wind speed in WRF. However, this bias can be substantially reduced by using a PBL scheme that explicitly considers the effects of non-resolved topography, which also improves the spatial structure of wind speed over Switzerland. The wind direction, although generally well reproduced, is not very sensitive to the PBL scheme. Further sensitivity tests include four types of nesting methods: nesting only at the boundaries of the outermost domain, analysis and spectral nudging, and the so-called re-forecast method, where the simulation is frequently restarted. These simulations show that restricting the freedom of the model to develop large-scale disturbances slightly increases the temporal agreement with the observations, at the same time that it further reduces the overestimation of wind speed, especially for maximum wind peaks. The model skill is also evaluated in the outermost domains, where the resolution is coarser. The results demonstrate the important role of horizontal resolution, where the step from 6 to 2 km significantly improves model performance. In summary, the combination of a grid size of 2 km, the non-local PBL scheme modified to explicitly account for non-resolved orography, as well as analysis or spectral nudging, is a superior combination when dynamical downscaling is aimed at reproducing real wind fields.
ABSTRACT Cyclones, which develop over the western Mediterranean and move northeastward are a majo... more ABSTRACT Cyclones, which develop over the western Mediterranean and move northeastward are a major source of extreme weather and known to be responsible for heavy precipitation over Central Europe and the Alps. As the relevant processes triggering these so-called Vb-events and their impact on extreme precipitation are not yet fully understood, this study focusses on gaining insight into the dynamics of past events. For this, a cyclone detection and tracking tool is applied to the ERA-Interim reanalysis (1979–2013) to identify prominent Vb-situations. Precipitation in the ERA-Interim and the E-OBS datasets is used to evaluate case-to-case precipitation amounts and to assess consistency between the two datasets. Both datasets exhibit high variability in precipitation amounts among different Vb-events. While only 23 % of all Vb-events are associated with extreme precipitation, around 15 % of all extreme precipitation days (99 percentile) over the Alpine region are induced by Vb-events, although Vb-cyclones are rare events (2.3 per year). To obtain a better understanding of the variability within Vb-events, the analysis of the 10 heaviest and lowest precipitation Vb-events reveals noticeable differences in the state of the atmosphere. These differences are most pronounced in the geopotential height and potential vorticity field, indicating a much stronger cyclone for heavy precipitation events. The related differences in wind direction are responsible for the moisture transport around the Alps and the orographical lifting along the Alps. These effects are the main reasons for a disastrous outcome of Vb-events, and consequently are absent in the Vb-events associated with low precipitation. Hence, our results point out that heavy precipitation related to Vb-events is mainly related to large-scale dynamics rather than to thermodynamic processes.
Atmospheric Chemistry and Physics Discussions, 2015
ABSTRACT After strong volcanic eruptions stratospheric ozone changes are modulated by heterogeneo... more ABSTRACT After strong volcanic eruptions stratospheric ozone changes are modulated by heterogeneous chemical reactions (HET) and dynamical perturbations related to the radiative heating in the lower stratosphere (RAD). Here, we assess the relative importance of both processes as well as the effect of the resulting ozone changes on the dynamics using ensemble simulations with the atmosphere–ocean–chemistry–climate model (AOCCM) SOCOL-MPIOM forced by eruptions with different strength. The simulations are performed under present day and preindustrial conditions to investigate changes in the response behaviour. The results show that the HET effect is only relevant under present day conditions and causes a pronounced global reduction of column ozone. These ozone changes further lead to a slight weakening of the Northern Hemisphere (NH) polar vortex during mid-winter. Independent from the climate state the RAD mechanism changes the column ozone pattern with negative anomalies in the tropics and positive anomalies in the mid-latitudes. The influence of the climate state on the RAD mechanism significantly differs in the polar latitudes, where an amplified ozone depletion during the winter months is simulated under present day conditions. This is in contrast to the preindustrial state showing a positive column ozone response also in the polar area. The dynamical response of the stratosphere is clearly dominated by the RAD mechanism showing an intensification of the NH polar vortex in winter. Still under present day conditions ozone changes due to the RAD mechanism slightly reduce the response of the polar vortex after the eruption.
The applicability of three different cyclone detection and tracking schemes is investigated in EC... more The applicability of three different cyclone detection and tracking schemes is investigated in ECMWF (ERA-40). Cyclone climatologies and cyclone characteristics are intercompared within the three methods. The comparison of the methods is based on the ERA-40 data set and shows a generally good correspondence between different schemes. However, trends in cyclone characteristics, like the number of cyclones, are sensitive to
The unstable midlatitude ocean-atmosphere coupling motivates the definition of two decadal regime... more The unstable midlatitude ocean-atmosphere coupling motivates the definition of two decadal regimes with distinct implications for the North Atlantic cyclone variability. Phases with low (high) decadal variability of the North Atlantic Oscillation, which are connected with an annular (sectoral) spatial scale of the geopotential height teleconnection pattern, are identified as a hemispheric (regional) regime. In the hemispheric regime during a
The newly developed atmosphere-ocean-chemistry-climate model SOCOL-MPIOM is presented by demonstr... more The newly developed atmosphere-ocean-chemistry-climate model SOCOL-MPIOM is presented by demonstrating the influence of the interactive chemistry module on the climate state and the variability. Therefore, we compare pre-industrial control simulations with (CHEM) and without (NOCHEM) interactive chemistry. In general, the influence of 5 the chemistry on the mean state and the variability is small and mainly restricted to the stratosphere and mesosphere. The largest differences are found for the atmospheric dynamics in the polar regions, with slightly stronger northern and southern winter polar vortices in CHEM. The strengthening of the vortex is related to larger stratospheric temperature gradients, which are attributed to a parametrization of the absorption of ozone 10 and oxygen in the Lyman-alpha, Schumann-Runge, Hartley, and Higgins bands. This effect is parametrized in the version with interactive chemistry only. A second reason for the temperature differences between CHEM and NOCHEM is related to diurnal variations in the ozone concentrations in the higher atmosphere, which are missing in NOCHEM. Furthermore, stratospheric water vapour concentrations differ substantially 15 between the two experiments, but their effect on the temperatures is small. In both setups, the simulated intensity and variability of the northern polar vortex is inside the range of present day observations. Sudden stratospheric warming events are well reproduced in terms of their frequency, but the distribution amongst the winter months is too uniform.
The aim of this work is to elucidate the impact of changes in solar irradiance and energetic part... more The aim of this work is to elucidate the impact of changes in solar irradiance and energetic particles versus volcanic eruptions on tropospheric global climate during the Dalton Minimum (DM, AD 1780-1840. Separate variations in the (i) solar irradiance in the UV-C with wavelengths λ < 250 nm, (ii) irradiance at wavelengths λ > 250 nm, (iii) in energetic particle spectrum, and (iv) volcanic aerosol forcing were analyzed separately, and (v) in combination, by means of small ensemble calculations using a coupled atmosphere-ocean chemistry-climate model. Global and hemispheric mean surface temperatures show a significant dependence on solar irradiance at λ > 250 nm. Also, powerful volcanic eruptions in 1809, 1815, 1831 and 1835 significantly decreased global mean temperature by up to 0.5 K for 2-3 years after the eruption. However, while the volcanic effect is clearly discernible in the Southern Hemispheric mean temperature, it is less significant in the Northern Hemisphere, partly because the two largest volcanic eruptions occurred in the SH tropics and during seasons when the aerosols were mainly transported southward, partly because of the higher northern internal variability. In the simulation including all forcings, temperatures are in reasonable agreement with the tree ring-based temperature anomalies of the Northern Hemisphere. Interestingly, the model suggests that solar irradiance changes at λ < 250 nm and in energetic particle spectra have only an insignificant impact on the climate during the Dalton Minimum. This downscales the importance of top-down processes (stemming from changes at λ < 250 nm) relative to bottom-up processes (from λ > 250 nm). Reduction of irradiance at λ > 250 nm leads to a significant (up to 2 %) decrease in the ocean heat content (OHC) between 0 and 300 m in depth, whereas the changes in irradiance at λ < 250 nm or in energetic particles have virtually no effect. Also, volcanic aerosol yields a very strong response, reducing the OHC of the upper ocean by up to 1.5 %. In the simulation with all forcings, the OHC of the uppermost levels recovers after 8-15 years after volcanic eruption, while the solar signal and the different volcanic eruptions dominate the OHC changes in the deeper ocean and prevent its recovery during the DM. Finally, the simulations suggest that the volcanic eruptions during the DM had a significant impact on the precipitation patterns caused by a widening of the Hadley cell and a shift in the intertropical convergence zone. www.clim-past.net/10/921/2014/ Clim. Past, 10, 921-938, 2014 www.clim-past.net/10/921/2014/ Clim. Past, 10, 921-938, 2014 Clim. Past, 10, 921-938, 2014 www.clim-past.net/10/921/2014/ J. G. Anet et al.: Sun and volcanoes: effect on the troposphere in the DM 935 efficient and appealing visualization of the model data. E. Rozanov, A.
To increase the sparse knowledge of long-term Southern Hemisphere (SH) climate variability we ass... more To increase the sparse knowledge of long-term Southern Hemisphere (SH) climate variability we assess an ensemble of 4 transient simulations over the last 500 yr performed with a state-of-the-art atmosphere ocean general circulation model. The model is forced with reconstructions of solar irradiance, greenhouse gas (GHG) and volcanic aerosol concentrations. A 1990 control simulation shows that the model is able
The resumption process of the North Atlantic meridional overturning circulation (MOC) is investig... more The resumption process of the North Atlantic meridional overturning circulation (MOC) is investigated in a series of freshwater hosing experiments using a comprehensive coupled climate model. Four different freshwater perturbations are applied to the North Atlantic between 50 � N and 70 � N leading to a substantial weakening of MOC and an expansion of winter sea ice cover over
Medieval Climate Anomaly (MCA) terrestrial proxy records (speleothem, NW Scotland; tree rings, Mo... more Medieval Climate Anomaly (MCA) terrestrial proxy records (speleothem, NW Scotland; tree rings, Morocco) from close to the centres of action of the winter North Atlantic Oscillation (NAO) indicate that the MCA was characterized by a pervasive positive phase of the NAO(1). Spatial gradients across Europe in this MCA synoptic climatological interpretation were analyzed using the Proxy Surrogate Reconstruction approach based
One of the most prominent features of mid-latitude atmospheric variability is blocking. Blocking ... more One of the most prominent features of mid-latitude atmospheric variability is blocking. Blocking events are anticyclones with an equivalent barotropic signature and persistent in time. The longer than synoptic day-to-day time scales has led to intrinsic interest during the last decades aiming to expand the predictability beyond classical numerical weather prediction. The study aims to present the extreme behavior of blocking and to investigate relations to other extreme events, like cold and dry spells. To assess the behavior of blocking events we developed a new event-based method. The method identifies 500-hPa geopotential height maxima and tracks these relative maxima with a next-neighborhood search in time. This new method agrees with the reference grid-point based method in the deduced climatological pattern of blocking frequency. Applying the method to ERA-40 data in winter for the Atlantic-European region we found a trend towards a reduction of blocking episodes. The mean sur...
A B S T R A C T Atmospheric blocking is an important source of low-frequency variability. We appl... more A B S T R A C T Atmospheric blocking is an important source of low-frequency variability. We apply a blocking detection and tracking method to ERA-40 data for the Atlantic-European region to assess linkages to extreme events, that is, cold and dry spells in the extended winter season (November-April). The method is feature-oriented, identifies 500-hPa geopotential height maxima, and connects them with a next-neighbourhood search in time. The analysis reveals a statistically significant decrease of number of blocking events over the period of ERA-40. Winters with an increased number of blocking events are associated with negative temperature anomalies over Central to Eastern Europe and dryer conditions, whereas Southern Europe experiences warmer and wetter conditions during such episodes. Using extreme value statistics we show evidence that cold spells, and to some extent dry spells, are strongly associated with extremes in blocking frequency. The number of cold spell days increases with the duration of blocking events indicating that cold spells need time to establish during blocking events, thus return periods of cold spells are longer than those for blocking events. This behaviour is not found for the relation of dry spells with blocking events. Still, blocking episodes have a higher occurrence of dry spells than climatology.
The influence of climate change on storm surges including increased mean sea level change and the... more The influence of climate change on storm surges including increased mean sea level change and the associated insurable losses are assessed for the North Sea basin. In doing so, the newly developed approach couples a dynamical storm surge model with a loss model. The key element of the approach is the generation of a probabilistic storm surge event set. Together with parametrizations of the inland propagation and the coastal protection failure probability this enables the estimation of annual expected losses. The sensitivity to the parametrizations is rather weak except when the assumption of high level of increased mean sea level change is made. Applying this approach to future scenarios shows a substantial increase of insurable losses with respect to the present day. Superimposing different mean sea level changes shows a nonlinear behavior at the country level, as the future storm surge changes are higher for Germany and Denmark. Thus, the study exhibits the necessity to assess the socio-economic impacts of coastal floods by combining the expected sea level rise with storm surge projections.
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