Narrow atmospheric streams of water vapor that deliver heavy rains are more commonly associated with floods and debris flows in northern California than with flash floods in southern California
September 26, 2017 — Around the world, huge streams of water vapor known as atmospheric rivers can generate heavy rain or snow as they flow over land. Just a few atmospheric rivers, each typically more than 2,000 kilometers long and less than 1,000 kilometers wide, can deliver up to half of the annual precipitation on the U.S. West Coast, on average.
Atmospheric rivers are common over the northeastern Pacific Ocean and California, where their saturated air rises and cools over mountain ranges. This process results in precipitation that can alleviate droughts but can also cause floods, flash floods, and debris flows. In a new study, Young et al. investigate the relationship between atmospheric rivers and these potentially dangerous events.
The researchers collaborated with the Center for Western Weather and Water Extremes at the Scripps Institution of Oceanography to gather statewide reports of floods, flash floods, and debris flows from the National Centers for Environmental Information from 2004 to 2014. These reports were then cross-referenced with a catalog of atmospheric rivers that made landfall in California during those years.
The research team found a total of 1,415 reports of floods, flash floods, and debris flows that occurred on 364 unique days over the 10-year period. Some, but not all, of these events occurred in conjunction with an atmospheric river located along the California coast on the same day or the day before.
Most of the 1,415 event reports were flash floods that occurred during the warm season in southern California, but these events were usually not associated with atmospheric rivers. However, most of the flood and debris flow reports were linked to atmospheric rivers and occurred in northern California during the cold season.
The researchers also investigated the meteorological conditions associated with concurrent atmospheric rivers and event reports. They found that debris flows and floods occur when large-scale weather patterns cause atmospheric rivers to hit California’s coastal mountain ranges at a right angle. This maximizes uphill flow and the resulting precipitation.
Overall, these findings could help improve understanding of the risks of floods, flash floods, and debris flows. The scientists suggest that future research could examine the impact of precipitation at a finer scale, looking at hourly rates instead of 48-hour rates.