Nowcasting research in the PLURISK project
Extreme weather conditions (extreme local rain storms) might induce severe floods and related socio-economical impacts on the urban environment (Belgian cities). While floods along rivers have been studied already extensively, quantification, forecasting, control and management of inundations along sewer systems and urban rivers have to face particular difficulties. They need fine-scale (local, short duration) rainfall estimation and nowcasting (=short-term forecasting in real time). They also require involvement of local authorities, which typically have low capacity in setting up risk quantification, forecasting, control and management systems.
The PLURISK research project aims to support these local authorities and develops methodologies and software for:
- Nowcasting of fine-scale extreme rainfall, integrating national (C-band) and local (X-band) radar technology, numerical weather prediction and lightning detection. A nation-wide system is developed for the real-time forecasting (for the next hours) of local rainfall. The system aims to support any Belgian city or urban community with nowcast results. The forecast uncertainty will be estimated using the stochastic ensemble prediction system STEPS (see below).
- Two-dimensional fine-scale modelling, mapping and nowcasting of inundations in urban areas. It will allow urban inundations to be nowcasted and visualised at street and house level, together with the uncertainty (due to uncertainty in the rainfall nowcast and the uncertainty in the local inundation modelling).
- Socio-economic risk quantification of urban inundations, incl. material and immaterial (social, ecological) damage assessment, quantification of risk perception (awareness), coping capacity and recovery capacity, impacts on built heritage, and uncertainty estimation on the risk quantifications.
- Risk communication and flood risk warning based on nowcasting results. The project mainly focuses on the generation and dissemination of flood risk warnings, but also extreme rainfall warnings are addressed.
- Risk reduction by combining prevention/management. New sustainable management strategies are developed by better interfacing spatial planning, eco-management and urban water management (e.g. green - blue water; role of landscape architecture; restoration of biodiversity in urban areas incl. ecotechnologies on buildings and very dense urbanized areas) and considering the services of biodiversity for human population (ecosystem services). Future climate conditions are taken into account through the knowledge obtained from the previous BelSPO project CCI-HYDR.
The Short Term Ensemble Prediction System STEPS is an operational nowcasting system developed by a joint collaboration between the Australian Bureau of Meteorology and the UK MetOffice (see Seed, 2003; Bowler et al., 2006). STEPS is based on the extrapolation of radar images using the motion field estimated on a short sequence of images. To reproduce the dynamic scaling of rainfall, i.e. the observation that the lifetime of rainfall features is dependent upon their spatial scale, the radar fields are decomposed into a multiplicative cascade. The different levels of the cascade stochastically evolve in time to account for the uncertainty in the temporal evolution of rainfall. The uncertainty in the rainfall estimation by the radar and the unknown future evolution of the velocity field is also considered within a stochastic framework. Ensemble nowcasts of rainfall accumulations up to 2-3 hours in the future are produced together with a number of probabilistic products for the exceedance of rainfall thresholds. STEPS also allows blending the nowcasts with downscaled NWP, which can be useful for forecasts beyond 3 hours in the future. The ensemble nowcasts will be used as inputs into urban hydrological models to assess their sensitivity to the rainfall input.
Seed. 2003. A dynamic and spatial scaling approach to advection forecasting. Journal of Applied Meteorology 42(3):381-388.
Bowler NE, Pierce CE, Seed AW. 2006. STEPS: A probabilistic precipitation forecasting scheme which merges an extrapolation nowcast with downscaled NWP. Quarterly Journal of the Royal Meteorological Society 132:2127-2155.
Contact person for this topic: Maarten Reyniers