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Tarolli P.,University of Padua | Borga M.,University of Padua | Morin E.,Hebrew University of Jerusalem | Delrieu G.,Laboratoire dEtude des Transferts en Hydrologie et Environnement
Natural Hazards and Earth System Science | Year: 2012

This work analyses the prominent characteristics of flash flood regimes in two Mediterranean areas: the North-Western Mediterranean region, which includes Catalonia, France and Northern Italy, and the South-Eastern Mediterranean region, which includes Israel. The two regions are characterized by similarities in the hydro-meteorological monitoring infrastructure, which permits us to ensure homogeneity in the data collection procedures. The analysis is articulated into two parts. The first part is based on use of flood peak data, catchment area and occurrence date for 99 events (69 from the North-Western region and 30 from the South-Eastern region). Analysis is carried out in terms of relationship of flood peaks with catchment area and seasonality. Results show that the envelope curve for the South-Eastern region exhibits a more pronounced decreasing with catchment size with respect to the curve of the North-Western region. The differences between the two relationships reflect changes in the effects of storm coverage and hydrological characteristics between the two regions. Seasonality analysis shows that the events in the North-Western region tend to occur between August and November, whereas those in the South-Eastern area tend to occur in the period between October and May, reflecting the relevant patterns in the synoptic conditions leading to the intense precipitation events. In the second part, the focus is on the rainfall-runoff relationships for 13 selected major flash flood events (8 from the North-Western area and 5 from the South-Eastern area) for which rainfall and runoff properties are available. These flash floods are characterised in terms of climatic features of the impacted catchments, duration and amount of the generating rainfall, and runoff ratio. Results show that the rainfall duration is shorter and the rainfall depth lower in the South-Eastern region. The runoff ratios are rather low in both regions, whereas they are more variable in the South-Eastern area. No clear relationship between runoff ratio and rainfall depth is observed in the sample of floods, showing the major influence of rainfall intensity and the initial wetness condition in the runoff generation for these events. © Author(s) 2012.

Hazenberg P.,University of Arizona | Hazenberg P.,Wageningen University | Torfs P.J.J.F.,Wageningen University | Leijnse H.,Royal Netherlands Meteorological Institute | And 2 more authors.
Journal of Geophysical Research: Atmospheres | Year: 2013

This paper presents a novel approach to estimate the vertical profile of reflectivity (VPR) from volumetric weather radar data using both a traditional Eulerian as well as a newly proposed Lagrangian implementation. For this latter implementation, the recently developed Rotational Carpenter Square Cluster Algorithm (RoCaSCA) is used to delineate precipitation regions at different reflectivity levels. A piecewise linear VPR is estimated for either stratiform or neither stratiform/convective precipitation. As a second aspect of this paper, a novel approach is presented which is able to account for the impact of VPR uncertainty on the estimated radar rainfall variability. Results show that implementation of the VPR identification and correction procedure has a positive impact on quantitative precipitation estimates from radar. Unfortunately, visibility problems severely limit the impact of the Lagrangian implementation beyond distances of 100 km. However, by combining this procedure with the global Eulerian VPR estimation procedure for a given rainfall type (stratiform and neither stratiform/convective), the quality of the quantitative precipitation estimates increases up to a distance of 150 km. Analyses of the impact of VPR uncertainty shows that this aspect accounts for a large fraction of the differences between weather radar rainfall estimates and rain gauge measurements. Key Points A new Lagrangian based VPR identification methodRadar rainfall uncertainty estimation from spatial precipitation variabilityImproved weather radar based surface rainfall estimation ©2013. American Geophysical Union. All Rights Reserved.

Borga M.,University of Padua | Anagnostou E.N.,University of Connecticut | Anagnostou E.N.,Hellenic Center for Marine Research | Bloschl G.,Vienna University of Technology | Creutin J.-D.,Laboratoire dEtude des Transferts en Hydrologie et Environnement
Environmental Science and Policy | Year: 2011

The management of flash flood hazards and risks is a critical component of public safety and quality of life. Flash-floods develop at space and time scales that conventional observation systems are not able to monitor for rainfall and river discharge. Consequently, the atmospheric and hydrological generating mechanisms of flash-floods are poorly understood, leading to highly uncertain forecasts of these events. The objective of the HYDRATE project has been to improve the scientific basis of flash flood forecasting by advancing and harmonising a European-wide innovative flash flood observation strategy and developing a coherent set of technologies and tools for effective early warning systems. To this end, the project included actions on the organization of the existing flash flood data patrimony across Europe. The final aim of HYDRATE was to enhance the capability of flash flood forecasting in ungauged basins by exploiting the extended availability of flash flood data and the improved process understanding. This paper provides a review of the work conducted in HYDRATE with a special emphasis on how this body of research can contribute to guide the policy-life cycle concerning flash flood risk management. © 2011 Elsevier Ltd.

Zanon F.,University of Padua | Borga M.,University of Padua | Zoccatelli D.,University of Padua | Marchi L.,CNR Research Institute for Geo-hydrological Protection | And 3 more authors.
Journal of Hydrology | Year: 2010

A Mesoscale Convective System in North-Western Slovenia produced up to 350-400mm in 12h on 18 September, 2007. The region impacted by the storm shows significant differences in climatic and geologic properties at short distances. Owing to such variability, extreme flooding concentrated over the Selška Sora watershed at Železniki (103.3km2), outside the area which received the highest precipitation. Hydrometeorological analyses of the storm are based on accurate analysis of C-band weather-radar observations and data from a rain gauge network. Detailed surveys of high-water marks and channel/floodplain geometry, carried out two months after the flood, are used for hydrologic analyses of the Selška Sora flood. These include estimation of peak discharge at 21 sites. Unit peak discharges range from 5 to 7m3s-1km-2 in basins characterised by size up to approximately 25km2. Higher unit peak discharges (>10m3s-1km-2), estimated in a few smaller basins, are influenced by intense sediment transport. Observed rainfall, estimated peak discharges, and observer notes on timing of peak discharge are used along with a distributed hydrologic model to reconstruct hydrographs at multiple locations. Examination of the rainfall distribution and flood response shows that the extent and the position of the karst terrain provided a major control on flood response in the region impacted by the storm. Use of the distributed hydrological model together with the post-flood survey observations is shown to provide an accurate description of the flood. Water balance and response time characteristics are examined for selected catchments, showing that event runoff coefficient ranged between 17% and 24% for different catchments. The quality of the peak discharge simulation at the 21 surveyed sites is substantially degraded when using spatially-uniform rainfall over the area covering all the surveyed sub-catchments, mainly due to rainfall volume errors introduced by using the spatially uniform value. On the other hand, the influence of rainfall spatial averaging at the scale of the sub-catchments generally has a very limited effect on runoff modelling, showing that rainfall spatial organisation was not able to overcome the catchment dampening effect for this flood. © 2010 Elsevier B.V.

Sicart J.E.,Laboratoire dEtude des Transferts en Hydrologie et Environnement | Hock R.,University of Alaska Fairbanks | Hock R.,Uppsala University | Ribstein P.,University Pierre and Marie Curie | Chazarin J.P.,IRD Montpellier
Journal of Glaciology | Year: 2010

In mountain environments, longwave radiation provides large amounts of melt energy for high-albedo snow surfaces and can dominate in the energy balance of snow or glacier surfaces under cloudy skies. This study examines the atmospheric controls of sky longwave radiation at Glaciar Zongo, Bolivia (16°15' S, 5060 ma.s.l.) over an entire year to derive a parameterization suitable for melt studies. Tropical glaciers are characterized by a pronounced seasonality of longwave radiation, due to cloud emissions during the wet season that strongly enhance the small emissivity of the thin and dry clear-sky atmosphere at very high altitudes. Clear-sky radiation is well simulated as a function of air temperature and humidity, but changes in humidity atmospheric profiles from daytime to night-time entail different optimized coefficients for hourly and daily data. Cloud emission, which enhances clear-sky emissivity by up to 55%, with an average of 20%, is estimated using daily atmospheric transmissivity for solar radiation. Partial correlations show that in high mountains cloud emissions control the variations of longwave radiation, far more than clear-sky emissivity and temperature of the emitting atmosphere. An independent test on Glaciar Antizana in the humid tropics of Ecuador (0°28' S, 4860 ma.s.l.) indicates that the parameterization is robust for the Central Andes.

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