Synoptic conditions of extreme windstorms over Switzerland in a changing climate

This paper reports on a method using composites for studying synoptic conditions of a series of windstorm events selected on the basis of maximum wind speeds in Switzerland. The composite storm-averaged conditions indicate how flow fields, as well as related surface conditions, are organised so as to produce high wind speeds near the surface. On average, high winds in Switzerland, mainly generated by transient synoptic-scale eddies, are characterised by a minimum in the mean sea level pressure field over southern Norway, anticyclonic conditions south of 35°N and a steep pressure gradient over continental western Europe. The geopotential aloft has a predominant zonal structure, producing high winds between 45°N and 50°N over the eastern Atlantic and further inland; the jet stream has its maximum speed at 50°N over the Celtic Sea and Brittany at 250hPa. Close to the surface, large temperature contrasts between the warm waters of the Atlantic Ocean and Mediterranean Sea and the cooler continent are diagnosed. The results thus obtained differ to those produced by other methods based on the analysis of deep cyclones or of strong vorticity in the northern North Atlantic Ocean basin. Differences of the composite mean synoptic conditions for current (1961-1990) and future climate (2071-2100) as simulated by the Global Climate Model HadAM3H in the context of the EU PRUDENCE project indicate that windstorms in a warmer world are generated by a subtle modification of the atmospheric baroclinicity, especially over the ocean and where greater ocean-continent temperature contrasts are simulated during winters. However, there are no signs of reduced storm activity as the climate warms by the end of the twenty-first centur

Development of a model-based high-resolution extreme surface wind climatology for Switzerland

An innovative methodology aimed at establishing a numerical model-based high-resolution climatology of extreme winds over Switzerland is described, that makes use of the Canadian Regional Climate Model where a new windgust parameterization has been implemented. Self-nesting procedures allow windstorms to be studied at resolution as high as 2-km. The analysis of ten major windstorms concludes that the average spatial pattern and magnitude of the simulated windspeeds are well captured, and the areas that experienced extreme winds correspond well with observations and to the location where forest damage was reported following the last two of these storms. This climatology would eventually serve to form risk assessment maps based on the exceedance of windspeed thresholds. There is, however, a need for further investigations to encompass the full range of potential extreme wind cases. The ultimate goal of this methodology is to assess the change in the behaviour of extreme winds for a climate forced by enhanced greenhouse gas concentrations, and the impact of future windstorms over the Alpine region at high resolutio

Rejet de questionnaires présumés avoir des erreurs de réponse dans les sondages avec panel Web

Cette thèse tire son origine de l’appréhension des praticiens de la recherche commerciale envers la professionnalisation des répondants résultant de l’utilisation des panels Web. Plusieurs praticiens estiment que les répondants complétant un grand nombre de sondages le feraient par l’appât du gain résultant des incitatifs et seraient moins vigilants en répondant aux sondages. Les praticiens déploient à ces fins des techniques de détection d’erreurs de réponse pour identifier des répondants inattentifs. La thèse porte sur l’évaluation de trois techniques de détection d’erreurs de réponse. Les résultats révèlent que les techniques de détection d’erreurs de réponse reposant sur des manifestations de l’heuristique du seuil de la réponse satisfaisante comme les schèmes linéaires et un court temps d’achèvement souffrent d’erreurs de type 1 et de type 2. À terme, ces erreurs peuvent influencer les résultats des sondages. L’analyse révèle également que le nombre de sondages complétés par les panélistes n’est pas un bon indicateur d’erreurs de réponse en utilisant les tests de la logique. Ainsi, les panélistes complétant un grand nombre de sondages n’effectuent pas significativement plus d’erreurs de réponse. Cela invalide en quelque sorte les appréhensions envers la professionnalisation des panélistes.Abstract : This thesis draws its inspiration from the apprehensions of research practitioners towards respondent professionalization resulting from the use of web panels. Many practitioners assume that panellists who complete a large number of surveys do so to maximise their gains resulting from the provided incentives. These respondents are then assumed to be less vigilant in answering the said surveys. As a result, research firms deploy response error detection techniques to identify inattentive respondents. The thesis assesses three commonly deployed response error detection techniques. The results show that techniques relying on satisficing manifestations such as straightlining and speeding tend to suffer from type 1 and type 2 errors. These errors can alter the results of the survey and thus affect de validity of the findings. The findings also reveal that the number of surveys completed by panellists is not a good predictor of response errors as measured by a logic test. As a case in point, panellists who completed many surveys are not more prone to show response errors. The latter somewhat invalidates the earlier mentioned apprehensions towards panellists

Numerical investigations of extreme winds over Switzerland during 1990-2010 winter storms with the Canadian Regional Climate Model

This study reports on the ability of the Canadian Regional Climate Model to simulate the surface wind gusts of 24 severe mid-latitude storms in Switzerland during the period 1990-2010. A multiple self-nesting approach is used, reaching a final 2-km grid which is centred over Switzerland, a country characterised by complex topography. A physically-based wind gust parameterization scheme is applied to simulate local surface gusts. Model performance is evaluated by comparing simulated wind speeds to time series at weather stations. While a number of simulated variables are reproduced in a realistic manner, the surface wind gusts show differences when compared to observed values. Results indicate that the performance of this parameterization scheme not only depends on the accuracy of the simulated planetary boundary layer, the vertical temperature, wind speed and atmospheric humidity profiles, but also on the accuracy of the reproduction of the surface fields such as temperature and moistur

Drivers of phytoplankton responses to summer wind events in a stratified lake: A modeling study

Extreme wind events affect lake phytoplankton by deepening the mixed layer and increasing internal nutrient loading. Both increases and decreases in phytoplankton concentration after strong wind events have been observed, but the precise mechanisms driving these responses remain poorly understood or quantified. We coupled a one-dimensional physical model to a biogeochemical model to investigate the factors regulating short-term phytoplankton responses to summer wind events, now and under expected warmer future conditions. We simulated physical, chemical, and biological dynamics in Lake Erken, Sweden, and found that strong wind could increase or decrease the phytoplankton concentration in the euphotic zone 1 week after the event, depending on antecedent lake physical and chemical conditions. Wind had little effect on phytoplankton concentration if the mixed layer was deep prior to wind exposure. Higher incoming shortwave radiation and hypolimnetic nutrient concentration boosted phytoplankton concentration, whereas higher surface water temperatures decreased concentrations after wind events. Medium-intensity wind events resulted in more phytoplankton than high-intensity wind. Simulations under a future climate scenario did not show marked differences in the way wind events affect phytoplankton concentration. These findings help to better understand how wind impacts vary as a function of local environmental conditions and how climate warming and changing extreme weather dynamics will affect lake ecosystems

Future extreme events in European climate: an exploration of regional climate model projections

This paper presents an overview of changes in the extreme events that are most likely to affect Europe in forthcoming decades. A variety of diagnostic methods are used to determine how heat waves, heavy precipitation, drought, wind storms, and storm surges change between present (1961-90) and future (2071-2100) climate on the basis of regional climate model simulations produced by the PRUDENCE project. A summary of the main results follows. Heat waves - Regional surface warming causes the frequency, intensity and duration of heat waves to increase over Europe. By the end of the twenty first century, countries in central Europe will experience the same number of hot days as are currently experienced in southern Europe. The intensity of extreme temperatures increases more rapidly than the intensity of more moderate temperatures over the continental interior due to increases in temperature variability. Precipitation - Heavy winter precipitation increases in central and northern Europe and decreases in the south; heavy summer precipitation increases in north-eastern Europe and decreases in the south. Mediterranean droughts start earlier in the year and last longer. Winter storms - Extreme wind speeds increase between 45°N and 55°N, except over and south of the Alps, and become more north-westerly than cuurently. These changes are associated with reductions in mean sea-level pressure, leading to more North Sea storms and a corresponding increase in storm surges along coastal regions of Holland, Germany and Denmark, in particular. These results are found to depend to different degrees on model formulation. While the responses of heat waves are robust to model formulation, the magnitudes of changes in precipitation and wind speed are sensitive to the choice of regional model, and the detailed patterns of these changes are sensitive to the choice of the driving global model. In the case of precipitation, variation between models can exceed both internal variability and variability between different emissions scenario

Snow pack in the Swiss Alps under changing climatic conditions: an empirical approach for climate impacts studies

In many instances, snow cover and duration are a major controlling factor on a range of environmental systems in mountain regions. When assessing the impacts of climatic change on mountain ecosystems and river basins whose origin lie in the Alps, one of the key controls on such systems will reside in changes in snow amount and duration. At present, regional climate models or statistical downscaling techniques, which are the principal methods applied to the derivation of climatic variables in a future, changing climate, do not provide adequate information at the scales required for investigations in which snow is playing a major role. A study has thus been undertaken on the behavior of snow in the Swiss Alps, in particular the duration of the seasonal snow-pack, on the basis of observational data from a number of Swiss climatological stations. It is seen that there is a distinct link between snow-cover duration and height (i.e., temperature), and that this link has a specific "signature" according to the type of winter. Milder winters are associated with higher precipitation levels than colder winters, but with more solid precipitation at elevations exceeding 1,700-2,000 m above sea-level, and more liquid precipitation below. These results can be combined within a single diagram, linking winter minimum temperature, winter precipitation, and snow-cover duration. The resulting contour surfaces can then be used to assess the manner in which the length of the snow-season may change according to specified shifts in temperature and precipitation. While the technique is clearly empirical, it can be combined with regional climate model information to provide a useful estimate of the length of the snow season with snow cover, for various climate-impacts studies

Application of a new wind gust parameterization: multiscale case studies performed with the Canadian regional climate model

The implementation of a physically based parameterization scheme for computation of wind gusts in a numerical regional climate model (RCM) is described in this paper. The method is based on an innovative approach proposed by Brasseur [2001] that assumes that gusts occurring at the surface result from the deflection of air parcels flowing higher in the boundary layer. Our parameterization scheme is developed so as to use quantities available at each model time step: consequently, the gusts are also computed for each of these time steps. To illustrate the performances of this novel method, gusts simulated for two severe midlatitude windstorms with the Canadian RCM at various resolutions are compared with observed gust speeds. The study is carried out concurrently for the complex terrain of Switzerland and for the smoother topography of Belgium. A preliminary analysis indicates that this parameterization performs equally well over flat and over mountainous regions; it also responds properly in the strengthening as well as the weakening phases of wind storms. The storm-dependent results rely on the model configuration associated with the downscaling procedure, as well as on the accuracy of the simulated flow fields. The model response is dependent on the resolved topography distribution and height and on the types of lower boundary conditions that affect the stability of the boundary layer. The simulated gusts are generally more realistic at higher resolution over the complex topography of Switzerland but are less sensitive to resolution over the flat terrain as in Belgium. On the basis of these two storms, this study also shows that simple scaling coefficients relating gust speeds and resolution are not an appropriate method for addressing such issues