PNNL

Abstract

Global climate models project that extreme precipitation events will increase in intensity during the 21st century at large spatial scales, with several studies suggesting that the most extreme events will exhibit the highest rate of intensification. The identification of the causes of this phenomenon nevertheless remains tenuous, in part because some studies have used methods that may not be appropriate for diagnosing long-term changes in intensity. Using a large ensemble of North American regional climate simulations, we confirm statistical evidence that intensification occurs more rapidly with warming for more extreme events of hourly-to-daily durations at finer regional scales. The larger increases in more extreme events are found to be primarily due to atmospheric circulation changes. Thermodynamically induced changes are found to have relatively uniform effects across extreme events and regions. In contrast, dynamical changes that are reflected in vertical velocity weaken moderate precipitation events over western interior regions of North America, and enhance them elsewhere. The weakening effect decreases and even reverses for more extreme precipitation events, whereas there is further intensification over other parts of North America, creating an “intense gets intenser” pattern over most of the continent.