Radar-based rainfall nowcasting at European scale: long-term evaluation and performance assessment

This work studies the performance of CRAHI's algorithm for rainfall nowcasting at European scale using the mosaics produced within the EUMETNET project OPERA (with a resolution of 4 km and 15 minutes) in the framework of the European Civil Protection research project HAREN (www.haren-project.eu). Systematic evaluation has been carried out since June 2012, focusing on the space-time variability of the nowcasting skills, and its dependence on the scale of the forecasted precipitation systems and on rainfall intensities. Also, the probabilistic nowcasting technique SBMcast (Berenguer et al. 2011) has been adapted to the use of OPERA mosaics at European scale to assess the uncertainty in the produced nowcasts. The performance of this probabilistic technique has been evaluated over a number of cases. Finally, the work analyzes the usefulness of these nowcasts for hazard assessment at European scale, based on exceeding the regional rainfall thresholds used by the EUMETNET project METEOALARM.Peer ReviewedPostprint (published version

Adding value to the measurements of an X-band radar on Catalonian coast

Rainscanner@Barcelona is an experiment assessing the hydrological value of a small X-band radar in urban areas.Postprint (published version

Design and evaluation of a community and impact-based site-specific early warning system (SS-EWS): the SS-EWS framework

The recent extreme rainfall events in Spain such as the storm Gloria have highlighted the gaps in emergency communication, particularly the disconnect between the available impact-based early warning systems (IBEWSs) and the steps communities take during emergencies. This paper presents a community-centred framework named ‘site-specific early warning system’ (SS-EWS) to co-design and co-evaluate with communities an IBEWS for vulnerable locations within high-risk areas. The components of the framework guide communities in identifying and evaluating local impacts; establishing impact and advisory tables; deriving impact-based rainfall thresholds and warning levels; and configuring the SS-EWS with radar-based nowcasting and numerical weather prediction (NWP) models. A first implementation and evaluation of the SS-EWS have been done for a public school, two ford crossings and the city of Terrassa, Spain. The SS-EWS shows promising results in triggering location-based or site-specific warnings compatible with the reported impacts and proposing actions to reduce the local risk. Furthermore, the combination of NWP and radar-based nowcasting improved the capacity of the SS-EWS to monitor the evolution of the precipitation and capture highly intense rainfall. The SS-EWS can be a straightforward and cost-efficient complement for regional EWS to increase the preparedness of communities.The study has been carried out mainly in the H2020 project ANYWHERE (H2020-DRS-1-2015-700099). The authors extend their gratitude to the municipality of Terrassa and Vicenta Villar, the Civil Protection of Catalonia, the Consorcio de Compensación de Seguros (CCS) and Josep Maria Gibert for kindly providing data.Peer ReviewedPostprint (published version

Long-term analysis of gauge-adjusted radar rainfall accumulations at European scale

Monitoring continental precipitation over Europe with high resolution (2 km, 15 min) has been possible since the operational production of the OPERA composites from the European weather radar networks. The OPERA data are the essential input to a hazard assessment tool for identifying localized rainfall-induced flash floods at European scale, and their quality determines the performance of the tool. This paper analyses the OPERA data quality during the warm seasons of 2015–2017 by comparing the estimated rainfall accumulations with the SYNOP rain gauge records over Europe. To compensate the OPERA underestimation, a simple spatially-variable bias adjustment method has been applied. The long-term comparison between the OPERA and gauge point daily rainfall accumulations at the gauge locations shows the benefit of the bias adjustment. Additionally, the daily monitoring shows gradual improvement of the OPERA data year by year. The impact of the quality of the OPERA data for effective flash flood identification is demonstrated for the case of the flash floods that occurred from 29 May to 3 June 2016 in central Europe.Peer ReviewedPostprint (author's final draft

ReAFFIRM: Real-time Assessment of Flash Flood Impacts: a Regional high-resolution Method

Flash floods evolve rapidly in time, which poses particular challenges to emergency managers. One way to support decision-making is to complement models that estimate the flash flood hazard (e.g. discharge or return period) with tools that directly translate the hazard into the expected socio-economic impacts. This paper presents a method named ReAFFIRM that uses gridded rainfall estimates to assess in real time the flash flood hazard and translate it into the corresponding impacts. In contrast to other studies that mainly focus on in- dividual river catchments, the approach allows for monitoring entire regions at high resolution. The method consists of the following three components: (i) an already existing hazard module that processes the rainfall into values of exceeded return period in the drainage network, (ii) a flood map module that employs the flood maps created within the EU Floods Directive to convert the return periods into the expected flooded areas and flood depths, and (iii) an impact assessment module that combines the flood depths with several layers of socio- economic exposure and vulnerability. Impacts are estimated in three quantitative categories: population in the flooded area, economic losses, and affected critical infrastructures. The performance of ReAFFIRM is shown by applying it in the region of Catalonia (NE Spain) for three significant flash flood events. The results show that the method is capable of identifying areas where the flash floods caused the highest impacts, while some locations affected by less significant impacts were missed. In the locations where the flood extent corresponded to flood observations, the assessments of the population in the flooded area and affected critical infrastructures seemed to perform reasonably well, whereas the economic losses were systematically overestimated. The effects of different sources of uncertainty have been discussed: from the estimation of the hazard to its translation into impacts, which highly depends on the quality of the employed datasets, and in particular on the quality of the rainfall inputs and the comprehensiveness of the flood maps.Peer ReviewedPostprint (published version

An inverse method to retrieve 3D radar reflectivity composites

Dense radar networks offer the possibility of getting better Quantitative Precipitation Estimates (QPE) than those obtained with individual radars, as they allow increasing the coverage and improving quality of rainfall estimates in overlapping areas. Well-known sources of error such as attenuation by intense rainfall or errors associated with range can be mitigated through radar composites. Many compositing techniques are devoted to operational uses and do not exploit all the information that the network is providing. In this work an inverse method to obtain high-resolution radar reflectivity composites is presented. The method uses a model of radar sampling of the atmosphere that accounts for path attenuation and radar measurement geometry. Two significantly different rainfall situations are used to show detailed results of the proposed inverse method in comparison to other existing methodologies. A quantitative evaluation is carried out in a 12 h-event using two independent sources of information: a radar not involved in the composition process and a raingauge network. The proposed inverse method shows better performance in retrieving high reflectivity values and reproducing variability at convective scales than existing methods.Peer ReviewedPostprint (author's final draft

The use of NWP forecasts to improve an ensemble nowcasting technique

Quantitative Precipitation Nowcasting (QPN) is one of the main applications of radar observations. On one hand, one of the most used nowcasting algorithms is Lagrangian extrapolation. It shows skill in specifying the timing and location of precipitation over short time periods, but shows low skill when using past precipitation trends to predict changes in precipitation intensity. On the other hand, Numerical Weather Prediction (NWP) models have roof skill at predicting the precise timing and location of precipitation, although they provide useful information about the intensity trends. It is therefore to investigate whether this additional information provided by NWP could be used to improve QPN. SBMcast (Berenguer et al., 2011) is an ensemble nowcasting algorithm based on Lagrangian extrapolation of recent radar observations. It generates a set of future rainfall scenarios (ensemble members) compatible with observations and preserving the spatial and temporal structure of the rainfall field according to the String of Beads model (Pegram and Clothier, 2001). This study shows the first results obtained with a methodology to constrain the spread of SBMcast ensembles with the additional information provided by NWP.Peer ReviewedPostprint (author’s final draft

A variational approach to retrieve 3D radar reflectivity composites

This study proposes an alternative methodology to obtain high-resolution radar reflectivity composites based on a variational approach considering different error sources in an explicit manner. The methodology retrieves the 3-dimensional precipitation field most compatible with the observations from the different radars of the network. With this aim, the methodology uses a model that simulates the radar sampling of the atmosphere. The model settings are different for each radar and include features such as the radar location, hardware parameters (beam width, pulse length…) and the scan strategy. The methodology follows the concept of an inverse method based on the minimization of a cost function that penalizes discrepancies between the simulated and actual observations for each radar. The simulation model is able to reproduce the effect of beam broadening with the distance and attenuation by intense precipitation. The methodology has been applied on two radars close to Barcelona (Spain).Peer ReviewedPostprint (published version

A methodology for attenuation correction based on the minimization of a cost function

Postprint (published version