an experiment to assess the hydrological value of a portable X-band radar

Transcription

1 ERAD 12 - THE SEVENTH EUROPEAN CONFERENCE ON RADAR IN METEOROLOGY AND HYDROLOGY RAINSCANNER@Barcelona: an experiment to assess the hydrological value of a portable X-band radar Marc Berenguer1, Shinju Park1, Daniel Sempere-Torres1, Jens Didszun2, Marcus Pool2, Monika Pfeifer2 1Centre of Applied Research in Hydrometeorology, Technical University of Catalonia. Barcelona, Spain. marc.berenguer@crahi.upc.edu 2Gematronik Weather Radar Systems, Selex Systems Integration GmbH. Neuss, Germany. Marc Berenguer 1. Introduction RAINSCANNER is a small, economic, X-band radar manufactured by SELEX Systems Integration that is very attractive for users such as municipalities and local water authorities, or as a complement to regional radar networks. This work presents a collaborative project focusing on the exploration of the use of RAINSCANNER for hydrometeorological purposes in a Mediterranean area. During the experiment, one RAINSCANNER unit was deployed in the Fabra Observatory (414 m amsl), covering the city of Barcelona and operated from October 11 to May 12. The main goal of the experiment has been to evaluate a mixed QPE production scheme combining X- and C-band radars and to assess the benefit of very-high space and time resolution (m and 1 minute compared to 1km and - minutes) for issuing rainfall warnings in an urban environment. The main challenges in this region are the orography (the hilly terrain results in ground clutter and beam blockage), and the high rainfall intensities of autumn convective situations (that may produce path and radome attenuation, specially at X-band). This paper presents the first results of the experiment and illustrates the performance of the developed QPE scheme over a rainfall event occurred during the experiment. Also, the last section demonstrates the use of RAINSCANNER for producing intense rainfall warnings in an urban context. 2. The experiment RAINSCANNER@BCN The project consisted in the operation of a X-band radar RAINSCANNER RS9 in the Fabra Observatory, covering the city of Barcelona (see Fig. 1). During the experiment, observations from the Catalan Weather Service s Creu del Vent Cband radar have been used in the processing of RAINSCANNER observations and to demonstrate the advantages of the high-resolution of the generated products (the main characteristics of these two radars are summarized in Table 1). Also, in the city of Barcelona (green area under the RAINSCANNER coverage in Fig. 1), the observations of the network of 24 rain gauges of CLABSA (the company in charge of the sewer system of Barcelona) have been used for the validation of the generated QPE products. 47 UTM 46 k m 46 km 4 Barcelona stream level gauge raingauge operational radar UTM 4 Fig. 1. Left: Location of the site of the RAINSCANNER unit during the experiment and the rest of the instrument in the domain: C-band radars (white circles), rain gauges (orange triangles) and stream level gauges (blue squares). The red shaded area shows the -km coverage of the Creu del Vent C-band radar of the Catalan Weather Service and the blue shade shows the -km coverage of the RAINSCANNER unit during the experiment. Right: Visibility of the RAINSCANNER unit from the Fabra Observatory over the city of Barcelona.

2 ERAD 12 - THE SEVENTH EUROPEAN CONFERENCE ON RADAR IN METEOROLOGY AND HYDROLOGY 3. The QPE scheme Table 1. Main characteristics of the RAINSCANNER unit in Fabra Observatory and the Creu del Vent C-band radar used in this study. RAINSCANNER CDV C-band radar Frequency (MHz) 94 4 Diameter of the reflector (m).9. Beam width ( ) Range resolution (m) Maximun range (km) 16 Time resolution (min) 1 6 #azimuthal measurements #elevations 1 16 The basis of the developed QPE scheme for the processing of RAINSCANNER radar is that of the EHIMI system (see Corral et al. 9), which is operationally generating the QPE products of the radar network of the Catalan Weather Service (composed of 4 C-band radars). C-band raw radar data are processed to produce Cartesian rainfall maps according to a chain of algorithms including: (i) reduction of the effects of beam blockage by the orography using the approach of Delrieu et al. (199), (ii) clutter elimination by combining the techniques of Berenguer et al. (6) and Sánchez-Diezma et al. (1), (iii) identification of the type of precipitation and extrapolation of elevated reflectivity measurements to the surface according to a double Vertical Profile of Reflectivity (VPR) as described by Franco et al. (8), and (iv) conversion of reflectivity into rain rate using an adaptive Z-R relationship depending on the type of rainfall. The scheme developed to generate improved QPE products from RAINSCANNER observations take advantage of (1) their very high-resolution (of the order of around m and every minute), and (2) the volumetric observations of the regional C-band radar network. Fig. 2 shows the main steps of the processing that is applied for the generation of QPE products from RAINSCANNER observations: 1) Monitoring of signal stability: uses the concepts of the method of Sempere-Torres et al. (3), based on the intensity of ground echoes. 2) Beam blockage mitigation: by compensating reflectivity measurements with the power loss simulated over a Digital Elevation Model with the algorithm of Delrieu et al. (199); Fig. 3 shows the map of the simulated losses due to beam blockage and expected ground echoes). 3) Ground clutter identification and substitution: Based on a simplified version of the algorithms described above for regional radars. 4) Extrapolation of elevated reflectivity measurements to the ground: the VPR estimated from the observations of the regional C-band radar network is used to extrapolate RAINSCANNER observations to the surface with the algorithm of Franco et al. (6), using different VPRs for the different types of precipitation. The classification of precipitation types is obtained from the volumetric observations of regional radars (Franco et al. 8). ) Z-R transformation: done with an adaptive Z-R relationship depending on the type of precipitation. 6) Other: In the future, monitoring of signal attenuation will be applied by comparing RAINSCANNER observations with those of the regional C-band radars. QPE production scheme high-resol. raw data m x 1 o 1 minute 1 elevation Corrections: +Beam blockage. +Signal stability* +Clutter removal & substitution +Attenuation +VPR extrapolation* +Double Z-R* high-resol. QPE high-resol. rainfall warnings Rainscanner (*) including C-band information m x m 1 minute C-band radar 3D raw data 1km x 1 o 6 minutes 16 elevations Corrections: +Beam blockage +Signal stability +Clutter removal & substitution Corral et al. (9) Processed 3D data Signal stability VPR Precipitation classification Franco et al. (6) Fig. 2. Algorithms implemented in the QPE scheme used to generated improved high-resolution rainfall maps and warnings.

3 ERAD 12 - THE SEVENTH EUROPEAN CONFERENCE ON RADAR IN METEOROLOGY AND HYDROLOGY energy loss [%] Fig. 3. Left: Simulated clutter map due to the interception of the radar beam with the terrain. The orange triangles show the location of the rain gauges used for the validation of the QPE products. Right: expected energy loss due to the orography. 4. RAINSCANNER vs. Creu del Vent C-band radar As mentioned above, the QPE scheme developed uses volumetric information of a regional C-band radar in the processing of the high resolution RAINSCANNER observations. Figure 4 shows the lowest elevation PPIs for the Creu del Vent C-band radar and for the RAINSCANNER unit over a common domain. Theses PPIs show a widespread precipitation system with embedded convective cells and a developing squall line. The vertical development of the system as captured from Creu del Vent radar is presented in Fig.. The bright band peak is visible at a height around 3 km and the development of convection extends up to around km. 6 A - - B Reflectivity (dbz) Fig. 4. PPI observations for 27 October 11 at 12:48 UTC over the RAINSCANNER domain corresponding to the CDV C- band radar of the Catalan Weather Service for an elevation of. (left), and with the RAINSCANNER unit located in the Fabra Observatory (right). The grey shading on the right panel shows the area where RAINSCANNER suffers from total beam blockage. The violet arrows show the directions most affected by path attenuation. The comparison of the measurements of the two radars (Figs 4 and ) shows the following elements: 1) Despite of the clear differences in the sensitivity of the two radars, there is a clear correspondence between the echoes in the PPIs from the two radars. 2) Significant differences in the precipitating areas can be attributed to path attenuation for the two radars (the two arrows in the panels of Fig. 4 show the areas most affected by attenuation).

4 ERAD 12 - THE SEVENTH EUROPEAN CONFERENCE ON RADAR IN METEOROLOGY AND HYDROLOGY height 3) The high resolution of RAINSCANNER observations permits a more defined depiction of small convective cells and of the reflectivity gradients. 4) The location of the RAINSCANNER unit, on the slopes of the main hill over Barcelona, permitted an excellent coverage of the city of Barcelona (within km from the radar) and the cities along the coast, which were the main targets of the experiment. Toward the NW, a sector of about 1 of the RAINSCANNER domain is totally blocked (as can also be seen in the beam blockage simulations of Fig. 3) by the summit distance Reflectivity (dbz) height normalized VPR [db] Fig.. Left: Vertical cross section along the axis A-B (Fig. 4 left) observed on 27 October 11 at 12:48 UTC. Right: Vertical Profiles of Reflectivity over the RAINSCANNER domain; the thick red line corresponds to the mean apparent VPR.. Results The rainfall event occurred on -6 October 11 has been used to illustrate the performance of the processing chain applied to RAINSCANNER unit over the city of Barcelona. This situation produced a rather homogeneous rainfall accumulation field (Fig. 6); rain gauge observations range between 22 and 41 mm in 24 hours. Although radar amounts were slightly overestimated, the time series of radar rainfall estimates match remarkably well those for the observations at collocated rain gauges (see Fig. 7). 7 y CL7 CL1 CL8 CL12 CL4 CL13 CL14 accumulation [mm] R [mm] x 1 3 G [mm] Fig. 6. Left: Estimated rainfall accumulation map around the RAINSCANNER location. The full circles show the location of 17 rain gauges of the CLABSA netwrk and the accumulated rainfall records. Right: Scatter plot between radar rainfall estimates and rain gauge accumulations at gauge locations.

5 ERAD 12 - THE SEVENTH EUROPEAN CONFERENCE ON RADAR IN METEOROLOGY AND HYDROLOGY CL1 CL7 CL14 CL8 R [mm/h] 1 R [mm/h] 1 : 6: 12: 18: : 6: 12: 18: Fig. 7. Time series of rain gauge measurements every minutes (orange line) and collocated radar rainfall estimates (black line) for the event of -6 November 11. The location of these 4 rain gauges is shown in Fig Rainfall warnings in an urban context The QPE products presented above have been implemented within the framework of a warning system to support the monitoring of urban floods (see e.g. Sempere-Torres et al. 1999). The system issues different warnings according to the exceedence of rainfall thresholds, set either based on operational criteria or on the probability of exceedence. Currently, we use 2 intense rainfall warnings (based on empirical rainfall thresholds) and 4 urban flood warnings based on the exceedence of rainfall for return periods of,, and years (an example is presented in Fig. 8). The display module with the rest of rainfall events of the experiment is available online at Fig. 8. Screen captures of the google-based display over the city of Barcelona for the event of 27 October 11. Left: reflectivity field. Right: rainfall warnings. The blue and green areas correspond to thresholds of 1 and 3 mm/h. Acknowledgments We thank the Catalan Weather Service for providing the data from the C-band radar at Creu del Vent and from their rain gauge networks. Also, thanks are due to the Catalan Water Agency and CLABSA (Clavagueram de Barcelona, SA) for their rain gauge data. We are grateful to the Royal Academy of Arts and Sciences of Barcelona and CLABSA for their support to the project. This work is part of the project FFGRad (CGL ) funded by the Spanish Ministry of Economy and Competitiveness (MINECO). The first author is supported by a Ramon y Cajal grant (RYC--621) of MINECO. References Berenguer, M., Sempere-Torres, D., Corral, C., and Sanchez-Diezma, R., 6: A fuzzy logic technique for identifying nonprecipitating echoes in radar scans. J. Atmos. Ocean. Tech., 23,

6 ERAD 12 - THE SEVENTH EUROPEAN CONFERENCE ON RADAR IN METEOROLOGY AND HYDROLOGY Corral, C., Velasco, D., Forcadell, D., and Sempere-Torres, D., 9: Advances in radar-based flood warning systems. The EHIMI system and the experience in the Besòs flash-flood pilot basin. Flood Risk Management: Research and Practice, Taylor and Francis, P. Samuels, Ed., Delrieu, G., Creutin, J. D., and Andrieu, H., 199: Simulation of Radar Mountain Returns Using a Digitized Terrain Model. J. Atmos. Ocean. Tech., 12, Franco, M., Sánchez-Diezma, R., and Sempere-Torres, D., 6: Improvements in weather radar rain rate estimates using a method for identifying the vertical profile of reflectivity from volume radar scans. Meteorol. Z., 1, Franco, M., Sánchez-Diezma, R., and Sempere-Torres, D., 8: Improving radar precipitation estimates by applying a VPR correction method based on separating precipitation types. th European Conf. on Radar in Meteorology and Hydrology, Helsinki, Finland. Sanchez-Diezma, R., Sempere-Torres, D., Creutin, J.-D., Zawadzki, I., and Delrieu, G., 1: An improved methodology for ground clutter substitution based on a pre-classification of precipitaion types. 3th Int. Conf. on Radar Meteorology, Munich, Germany, Amer. Meteor. Soc., Sempere-Torres, D., C. Corral, J. Raso, and P. Malgrat, 1999: Use of weather radar for combined sewer overflows monitoring and control. J. Environ. Eng., 12, Sempere-Torres, D., R. Sánchez-Diezma, M. Berenguer, R. Pascual, and I. Zawadzki, 3: Improving radar rainfall measurement stability using mountain returns in real time. 31st Conf. on Radar Meteorology, Seattle, WA, Amer. Meteor. Soc.,