Impact of multiple radar reflectivity data assimilation on the numerical simulation of a flash flood event during the HyMeX campaign

Ida Maiello,Sabrina Gentile,Pier Paolo Alberoni,Nicoletta Roberto,Rossella Ferretti,Luca Baldini,Frank Silvio Marzano,Errico Picciotti

doi:10.5194/hess-21-5459-2017

Abstract

Abstract. An analysis to evaluate the impact of multiple radar reflectivity data with a three-dimensional variational (3-D-Var) assimilation system on a heavy precipitation event is presented. The main goal is to build a regionally tuned numerical prediction model and a decision-support system for environmental civil protection services and demonstrate it in the central Italian regions, distinguishing which type of observations, conventional and not (or a combination of them), is more effective in improving the accuracy of the forecasted rainfall. In that respect, during the first special observation period (SOP1) of HyMeX (Hydrological cycle in the Mediterranean Experiment) campaign several intensive observing periods (IOPs) were launched and nine of which occurred in Italy. Among them, IOP4 is chosen for this study because of its low predictability regarding the exact location and amount of precipitation. This event hit central Italy on 14 September 2012 producing heavy precipitation and causing several cases of damage to buildings, infrastructure, and roads. Reflectivity data taken from three C-band Doppler radars running operationally during the event are assimilated using the 3-D-Var technique to improve high-resolution initial conditions. In order to evaluate the impact of the assimilation procedure at different horizontal resolutions and to assess the impact of assimilating reflectivity data from multiple radars, several experiments using the Weather Research and Forecasting (WRF) model are performed. Finally, traditional verification scores such as accuracy, equitable threat score, false alarm ratio, and frequency bias – interpreted by analysing their uncertainty through bootstrap confidence intervals (CIs) – are used to objectively compare the experiments, using rain gauge data as a benchmark.

Highlights

In the last few years, a large number of floods caused by different meteorological events have occurred in Italy
The Point-Stat Tool of MET (Model Evaluation Tools) application (DTC, 2013), developed at the DTC (Developmental Testbed Centre, NCAR), has been used to objectively evaluate the 12 h accumulated precipitation produced by Weather Research and Forecasting (WRF) on both domains
The observations used for the statistical evaluation were obtained from the DEWETRA platform of the Department of Civil Protection and the comparison has been performed over the central Italy target area using about 3000 rain gauges with a good coverage throughout the Italian territory

Summary

Introduction

In the last few years, a large number of floods caused by different meteorological events have occurred in Italy. I. Maiello et al.: Impact of multiple radar reflectivity data assimilation the peculiar distribution of land and water, and due to the Mediterranean Sea temperature, which is warmer than that of the European northern seas (Baltic Sea and North Sea). Maiello et al.: Impact of multiple radar reflectivity data assimilation the peculiar distribution of land and water, and due to the Mediterranean Sea temperature, which is warmer than that of the European northern seas (Baltic Sea and North Sea) These factors can produce severe meteorological events: for example, if precipitation persists over urbanized watersheds with steep slopes, devastating floods can occur in a relatively short time

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Results

Conclusion