Comparing California Atmospheric River Flood Events in a Current and Future Climate

The Oroville dam crisis was one of the most catastrophic dam emergencies in recent years. In 2017, there was an anomalous number of atmospheric rivers bringing large amounts of rain to California. This caused the levels of Lake Oroville to rise and both the main and emergency spillways to be damaged. As a result, the majority of the Feather River Basin had to be evacuated which included more than 180,000 people. In order to better prevent events like the Oroville dam crisis, it is necessary to analyze how the flood events associated with atmospheric rivers will change in a warmer future climate. Atmospheric rivers are long, concentrated swaths of water vapor in the atmosphere that are associated with many flood events that occur in California. A 4-km high-resolution convection-permitting regional climate model is used to look at future changes in flood characteristics in California atmospheric river flood events. There were 49 floods from 2000–2013 that are investigated in a future climate, based on comparing well with the Stage IV observations. A control simulation is compared to a pseudo-global warming (PGW) projection to see overall changes in the flood characteristics in a current and future climate. The results show that there are significant future increases in the rain rates during these specific flood events. Large increases in runoff and decreases in snow water equivalent were also observed in the PGW runs. There is also an elevation dependence in these quantities, with the largest differences between the control and PGW runs occurring at mid-elevations and the smaller changes occurring at the highest and lowest elevations. Integrated water vapor transport, a quantity used to define atmospheric rivers, is calculated and shows a large increase within future atmospheric river flood events. All these findings suggest that the flood events associated with atmospheric rivers in a future climate will become more intense, with a possibility of having a greater negative impact. The water management in California could potentially use the results of this work to make policy changes that better reflect the water balance that is expected in the future. If water managers take these changes into account when making decisions, it could potentially prevent another disaster like the Oroville dam crisis and save money, resources, and lives.