The Anisotropic Multifractal Model and Wind Turbine Wakes

International audienceA typical routine in wind field resource assessment, at the most basic level, consists of first to third order statistics of times series data. The quality of the time series data can range between 0.05 to 600 seconds. More often than not the frequency of data will be the latter of the two since it is the cumulative power over long periods of time that define the financial return from turbines and thus high-resolution data is deemed unnecessary. It is now evident that such coarse time series data are no longer sufficient for a representa- tive assessment of the wind and that estimations based on such data are associated with inaccurate power curve pre- diction and turbine damage. In particular it has been sug- gested that such problems are due to a lack of understand- ing of the somewhat intermittent nature of the wind velocity fields and the small-scale fluctuations thus associated. In order to address this there has been a significant increase in research involving coupled mesoscale-microscale mod- els and stochastic downscaling methods. Our contribution is a demonstration that a good knowledge of small-scale variability is essential for a better understanding of the at- mospheric boundary layer. We discuss the applicability of the stochastic anisotropic multifractal model to the complex conditions of wind farm potential and operational sites

Analyses multifractales et spatio-temporelles des précipitations du modèle Méso-NH et des données radar

International audienceDans le cadre des multifractals universels, il est possible de caractériser la variabilité spatio-temporelle de la pluie sur une grande gamme d'échelle à l'aide de trois paramètres invariants d'échelles. Dans cette étude, nous avons estimé ces paramètres multifractals sur des simulations numériques effectuées avec le modèle méso-échelle Méso-NH (développé par Météo - France et le Laboratoire d'Aérologie), et des images radar composites, couvrant le même événement pluvieux, à savoir un orage particulièrement violent, dit de type Cévenol, ayant eu lieu sur la partie sud de la France du 5 au 9 septembre 2005. La comparaison des résultats montre que les deux types de données présentent des domaines d'invariance d'échelle relativement similaires, et dont les propriétés sont en accord avec les modèles de précipitation spatio-temporels unifiés et scalant les plus simples. Néanmoins l'évaluation de leurs exposants conduit à des valeurs parfois fortement différentes

Performance of Green roof in stormwater management regarding high-resolution precipitation fields

International audienceAt the basin scale, green roofs can represent an efficient tool to manage stormwater in urbanized areas. Widely implemented and/or completed with additional sustainable urban drainage systems, they show a positive impact on urban runoff: decrease and slowdown of the peak discharge and decrease of runoff volume. To assess green roof performances at this scale, a specific module dedicated to simulate their hydrological behaviour has been developed in the Multi-Hydro rainfall-runoff model. Its distributed structure gives the opportunity to test the susceptibility of green roof response regarding spatial distributions of precipitation. Based on radar rainfall fields, an ensemble of 50 realistic downscaled rainfall fields with a resolution of 10 m in space has been generated by using multifractals downscaling technique. Simulations have been conducted on a small urban catchment close to Paris (France) where most of the buildings roofs are assumed to easily accept the implementation of green roof. Although green roof confirm their ability to reduce urban runoff, these results illustrate that peak discharge reduction seems to be clearly dependant of spatial distribution of precipitation. Implementation of green roof can also produce concomitance situations and higher peak discharges than those produced by impervious roofs

Green roof and storm water management policies: monitoring experiments on the ENPC Blue Green Wave

International audienceCurrently widespread in new urban projects, green roofs have shown a positive impact on urban runoff at the building/parcel scale. Nevertheless, there is no specific policy promoting their implementation neither in Europe nor in France. Moreover they are not taken into account (and usually considered as an impervious area) in the sizing of a retention basin for instance. An interesting example is located in the heart of the Paris-East Cluster for Science and Technology (Champs-sur-Marne, France). Since 2013 a large (1 ha) wavy-form vegetated roof (called bleu green wave) is implemented. Green roof area and impervious areas are connected to a large retention basin, which has been oversized. The blue green wave represents a pioneering site where an initially amenity (decorative) design project has been transformed into a research oriented one. Several measurement campaigns have been conducted to investigate and better understand the hydrological behaviour of such a structure. Rainfall, humidity, wind velocity, water content and temperature have been particularly studied. The data collected are used for several purposes: (i) characterize the spatio-temporal variability of the green roof response, (ii) calibrate and validate a specific model simulating its hydrological behavior. Based on monitoring and modeling results, green roof performances will be quantified. It will be possible to estimate how they can reduce stormwater runoff and how these performances can vary in space and in time depending on green roof configuration, rainfall event characteristics and antecedent conditions. These quantified impacts will be related to regulation rules established by stormwater managers in order to connect the parcel to the sewer network. In the particular case of the building of a retention basin, the integration of green roof in the sizing of the basin will be studied

Evapotranspiration evaluation using three different protocols on a large green roof in the greater Paris area

Nature-based solutions have appeared as relevant solutions to mitigate urban heat islands. To improve our knowledge of the assessment of this ecosystem service and the related physical processes (evapotranspiration), monitoring campaigns are required. This was the objective of several experiments carried out on the Blue Green Wave, a large green roof located in Champs-sur-Marne (France). Three different protocols were implemented and tested to assess the evapotranspiration flux at different scales: the first one was based on the surface energy balance (large scale); the second one was carried out using an evapotranspiration chamber (small scale); and the third one was based on the water balance evaluated during dry periods (point scale). In addition to these evapotranspiration estimates, several hydrometeorological variables (especially temperature) were measured. Related data and Python programs providing preliminary elements of the analysis and graphical representation have been made available. They illustrate the space–time variability in the studied processes regarding their observation scale. The dataset is available at https://doi.org/10.5281/zenodo.8064053 (Versini et al., 2023).</p

Science communication for resilient cities: monitoring digital communication to be weather-ready

International audienceThe quality of science and technology communication has become more challenging due to the fact that access to information has hugely increased in terms of variety and quantity. This is a consequence of different factors, among others the development of public relations by research institutes and the pervasive role of digital media (Bucchi 2013; Trench 2008). A key question is how can we objectively assess science and technology communication? Relatively few studies have been dedicated to the definition of pertinent indicators and (Neresini and Bucchi 2011). This research aims to understand how communication strategies, addressed to the general public, can optimise the impact of research findings in hydrology for resilient cities and how this can be assessed. Indeed urban resilience to extreme weather events relies both on engineering solutions and increased awareness of urban communities as it was highlighted by the FP7 SMARTesT project and the experiences carried out in the framework of TOMACS (Tokyo Metropolitan Area Convective Studies for Resilient Cities) and CASA (Engineering Research Center for Collaborative Adaptative Sensing of the Atmosphere, supported by the U.S. National Science Foundation). The research will greatly benefit from the development of automated analysis of unstructured Big Data that allows the exploration of huge amounts of digital communication data: blogs, social networks postings, press articles... Furthermore, these techniques facilitate the comparison of socioeconomic trends with physical-environmental trends. We will also investigate case studies corresponding to several research projects under the umbrella of the Chair " Hydrology for resilient cities " : for example the Interreg NWE IVB RAINGAIN project, the KIC Climate Blue Green Dream project and worldwide collaborations such as TOMACS. All these projects involve awareness raising and capacity building activities aimed to stimulate cooperation between scientists, professionals (e.g. water managers, urban planners) and beneficiaries (e.g. concerned citizens, policy makers)

Quantifying the impact of small scale unmeasured rainfall variability on urban runoff through multifractal downscaling: A case study

International audienceThis paper aims at quantifying the uncertainty on urban runoff associated with the unmeasured small scale rainfall variability, i.e. at a resolution finer than 1. km. ×. 1. km. ×. 5. min which is usually available with C-band radar networks. A case study is done on the 900. ha urban catchment of Cranbrook (London). A frontal and a convective rainfall event are analysed. An ensemble prediction approach is implemented, that is to say an ensemble of realistic downscaled rainfall fields is generated with the help of universal multifractals, and the corresponding ensemble of hydrographs is simulated. It appears that the uncertainty on the simulated peak flow is significant, reaching for some conduits 25% and 40% respectively for the frontal and the convective events. The flow corresponding the 90% quantile, the one simulated with radar distributed rainfall, and the spatial resolution are power law related. © 2012 Elsevier B.V

Impact de la variabilité non-mesurée des précipitations sur les débits en hydrologie urbaine : un cas d'étude dans le cadre multifractal

National audienceCet article utilise les propriétés multifractales d'un évènement pluvieux dans la région de Londres le 9 février 2009, pour mieux comprendre et quantifier 'incertitude associée à la variabilité spatio-temporelle des précipitations non mesurées par les radars météorologiques en bande C, dont la résolution estimée est de 1 km*1 km*5min, et comment elle se transfère aux prévisions des débits en réseaux d'assainissement. Le cas d'étude hydrologique est celui du bassin versant urbain de 900 hectares de Cranbrook (Londres). Les propriétés multifractales sont en accord avec le modèle spatio-temporel le plus simple, reposant sur un exposant d'anisotropie entre l'espace et le temps. Ceci permet de désagréger le champ de précipitation à l'aide de cascades multifractales spatio-temporelles. Un ensemble de champs de précipitations désagrégés réalistes est alors généré à l'aide des multifractals universels, puis l'ensemble des hydrogrammes correspondants est obtenu par un modèle urbain pluie-débit semi-distribué. Il apparait que les queues de probabilité issues de l'analyse de 100 échantillons de précipitation présentent un comportement en loi de puissance, qui est retrouvé sur les débits de pointe mais avec des exposants différent

Characterization of the Evapotranspiration flux on a Blue Green Solution (Blue Green Wave)

International audienceThe rapid growth of urban areas, jointly with the effects of climate change, is the major challenge to face the transition towards sustainable cities. Climate change leads to substantial modifications of the water cycle in cities, increasing the frequency of intense precipitation, drought and heat wave events. The replacement of natural surfaces by dark and impervious ones is the main cause of Urban Heat Islands (UHI) phenomenon. UHIs are microclimates characterized by significant temperature differences between inner cities and the surrounding rural areas. Part of a solution to tackle this issue is the re-naturalization of cities through the installation of Blue Green Solutions (BGS), such as green roofs, favoring the evapotranspiration (ET) process and thus reducing the air temperature. To benefit BGS implementation, it is crucial to understand the thermo-hydric processes that govern them. For this purpose, the ET process of a 1 ha green roof implemented in front of the Ecole de Ponts ParisTech (France) called Blue Green Wave (BGW) was studied to determine its possible cooling effect to mitigate UHIs. Therefore, three methods were tested and compared to estimate ET: (i) the water balance during dry periods through the difference on the soil moisture content measured via a wireless sensors network, (ii) the absolute humidity measured by a dynamic transpiration chamber, and (iii) a scintillometer to assess the sensible heat flux, which allows to deduce the latent heat flux by computing the energy balance. The wireless sensors demonstrated to assess correctly ET trends over long time periods, while the dynamic chamber allows to identify more precisely the ET behavior during shorter periods of measurement due to a better resolution. Indeed, ET computed via the water budget appeared significantly high compared to the values estimated by the dynamic chamber, and without showing an obvious daily pattern. In addition, ET trends estimated by both scintillometer and transpiration chamber methods were very close, but the corresponding values suffered from a significant difference. The divergence in ET flux computed by the three methods can be caused by: (1) errors in the sensible heat flux estimated by the scintillometer, leading overestimations of the latent heat flux; (2) noisy data of soil water content, induced by the rainfall events and the local soil characteristics where the sensors are implemented, and (3) modifications of the atmospheric conditions within the transpiration chamber. More generally, ET appeared higher in spring season and during the first days of summer, when high temperatures were reached and soil water content was enough to support ET without inducing a deficit for plants. Conversely, despite significant temperatures at the end of summer, ET rate was lower due to the lack of water content in the soil. This suggests that during summer, when the UHI intensity is stronger and the cooling effects of the green roofs are needed, the ET potential could not be sufficient. To go further in the space-time characterization of ET flux, additional experiments and multi-fractal analysis will be carried out soon