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

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 quantify their patterns better, but traditional extreme value analyses based on radar records were found to be too inaccurate for quantifying the extreme intensities required 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 the coastal and orographic effects on extreme precipitation of durations between 10 min and 24 h. Combining 11 years of radar data with 10 min rain gauge data in the southeastern Mediterranean, we obtain estimates of the once in 100 years precipitation intensities with similar to 26 % standard error, which is lower than those obtained using traditional approaches on rain gauge data. We identify the following three distinct regimes which respond differently to coastal and orographic forcing: short durations (similar to 10 min), related to peak convective rain rates, hourly durations (similar to 1 h), related to the yield of individual convective cells, and long durations (similar to 6-24 h), 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.