09 Aug 2021

09 Aug 2021

Review status: this preprint is currently under review for the journal HESS.

Coastal and orographic effects on extreme precipitation revealed by weather radar observations

Francesco Marra1, Moshe Armon2, and Efrat Morin2 Francesco Marra et al.
  • 1National Research Council of Italy, Institute of Atmospheric Sciences and Climate (CNR-ISAC), Bologna, 40129, Italy
  • 2The Fredy & Nadine Herrmann Institute of Earth Sciences, the Hebrew University of Jerusalem, Jerusalem, Israel

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.

Francesco Marra et al.

Status: open (until 24 Nov 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on hess-2021-395', Anonymous Referee #1, 13 Sep 2021 reply

Francesco Marra et al.

Model code and software

A Unified Framework for Extreme Sub-daily Precipitation Frequency Analyses based on Ordinary Events Francesco Marra, Marco Borga, Efrat Morin

Francesco Marra et al.


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Short summary
We present a new method for quantifying the probability of occurrence of extreme rainfall using radar data and use it to examine coastal and orographic effects on extremes. We identify three regimes, directly related to precipitation physical processes, which respond differently to these forcing. The presented methods and results are of interest for researchers and practitioners using radar for the analysis of extremes, risk managers water resources managers, climate change impact studies.