Mapping the past: identifying analog precipitation years with reference rainfall metrics

Stormwater control measures (SCMs) are important for urban climate adaptation and mitigation strategies. Current design approaches use historical rainfall estimates to determine the design case; however these historical rainfall records do not adequately capture the observed nonstationarity resulting from climate change. While there are climate change projections, these have not easily or consistently been brought into practice. There is an opportunity to simulate climate change impacts analytically through analogous observations of other climate regions or past observations, as well as provide a method to express hydrologic observations and model predictions. This study aims to identify past years of rainfall in Philadelphia, Pennsylvania, USA that are analogous to projected precipitation under various climate change scenarios (three socioeconomic pathways projecting three future 25-year periods). Analogous rainfall years were identified using a novel metric that has previously been used to identify reference years of rainfall The analogous years and future projections both indicate the same precipitation risks in the form of larger, extreme events (approximately 10 mm of additional rainfall for the region’s 2-year, 24-hour event on average and preferential selection of the wettest years on record with climate change intensification). However, future temperature projections remain unprecedented in comparison to observations from the analogous precipitation years, where climate change projections consistently indicate a + 2–6 °C anomaly in monthly maximum air temperature from the baseline of 1950–2000 but the analogous years show little to no difference. In all, this work provides the two-fold outcome of a practical means for communicating the risks of climate change with recallable observations, and the potential to indicate observations that could be used in hydrological simulations to model the risks of climate change to water resources systems.