TOWARDS TIME AND SPACE EVOLVING EXTREME WIND FIELDS

摘要

Time and space evolving extreme wind fields are needed to produce accurate and realistic extreme hydraulic loads. To fulfill this need, a semi-parametric method based on the theory of max-stable processes has been proposed that can be used to determine the time and space evolving wind fields associated with a given return value of wind speed at a specified reference location. The method uses wind velocity time series over a grid of locations and their location-specific extreme value distributions in order to 'lift' observed extreme events (storms) into yet unobserved and even more extreme events. Because the lifted fields are needed to force hydrodynamic models, they need to be continuous in time and space and to extend over a few days before the peak of the storm in order to allow the spin up of the hydrodynamic models. Thus, even if the original values of the wind fields are not above the threshold used to determine the location-specific extreme value distributions at a given location and instant, they still need to be lifted. This is achieved by augmenting the location-specific extreme value distributions by the empirical distributions of the observations and lifting also the values below the thresholds. Mainly as a consequence of this procedure, the resulting lifted wind fields have low temporal and spatial wind speed gradients away from the peak of the storm. These gradients have raised concerns about the validity of the fields and it has been suggested that the lifting by means of a simple scaling factor may be preferable. To address these concerns, a thorough validation of the lifted fields has been carried out assessing the ability of the lifted fields to reproduce extreme hydraulic conditions. The main recommendation of this study is that a lifting method defined as the combination of the method based on the theory of max-stable processes and a fixed factor method be considered further for the determination of temporally and spatially varying hydraulic loads.