Reply on RC1

Francesco Marra

doi:10.5194/hess-2021-395-ac1

Abstract

The yearly exceedance probability of extreme precipitation of multiple durations is crucial for infrastructure design, risk management and policymaking. Local extremes emerge from the interaction of weather systems with local terrain features such as coastlines and orography, however multi-duration extremes do not follow exactly the patterns of cumulative precipitation and are still not well understood. High-resolution information from weather radars could help us better quantifying their patterns, but traditional extreme-value analyses based on radar records were found too inaccurate for quantifying the extreme intensities for impact studies. Here, we propose a novel methodology for extreme precipitation frequency analysis based on relatively short weather radar records, and we use it to investigate coastal and orographic effects on extreme precipitation of durations between 10 minutes and 24 hours. Combining 11 years of radar data with 10-minute rain gauge data in the southeastern Mediterranean, we obtain estimates of the 1 in 100 years intensities with ~22 % standard error, which is lower than those obtained using traditional approaches on rain gauge data. We identify three distinct regimes, which respond differently to coastal and orographic forcing: short durations (~10 minutes), related to peak convective rain rates; hourly durations (~1 hours), related to the yield of individual convective cells; and long durations (~6–24 hours), related to the accumulation of multiple convective cells and to stratiform processes. At short and hourly durations, extreme return levels peak at the coastline, while at longer durations they peak corresponding to the orographic barriers. The distributions tail heaviness is rather uniform above the sea and rapidly changes in presence of orography, with opposing directions at short (decreasing tail heaviness, with a peak at hourly durations) and long (increasing) durations. These distinct effects suggest that short-scale hazards such as urban pluvial floods could be more of concern for the coastal regions, while longer-scale hazards such as flash floods could be more relevant in mountainous areas.

Highlights

Knowledge of the yearly exceedance probability of extreme precipitation intensities at multiple spatiotemporal scales is crucial for infrastructure design, weather-risk management and policymaking (Chow et al, 1988; Kleindorfer and Kunreuther, 1999)
Combining 11 years of weather radar data with 10-minute rain gauge data in the southeastern Mediterranean, we 560 obtain estimates of 100-year return levels characterized by standard errors in the order of ~22%, which is lower than the standard errors obtained using traditional approaches on rain gauge data
At short and hourly durations, the scale parameter peaks at the coastline, while at longer durations it tends to peak a few kilometers inland corresponding to the orographic barriers

Summary

Introduction

Knowledge of the yearly exceedance probability of extreme precipitation intensities at multiple spatiotemporal scales is crucial for infrastructure design, weather-risk management and policymaking (Chow et al, 1988; Kleindorfer and Kunreuther, 1999). 30 the 100-year return levels are intensities characterized by 1% yearly exceedance probability, and exceeded on average once in 100 years (Katz et al, 2002). This task usually requires specific approaches to decrease the stochastic uncertainties characterizing the observation of extremes (e.g., Koutsoyiannis et al 1988; Buishand, 1991; Burlando and Rosso, 1996) and simplified conceptual models to account for the multiple spatiotemporal scales required by many impact studies (Sivapalan and Blöschl, 1998; Svensson and Jones, 2010)

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