“These narrow rivers move a significant amount of water through the air, even though they take up a relatively small area,” said Carolyn Reynolds, Ph.D., head of the Probabilistic Prediction Research Office at NRL. “Large atmospheric rivers can transport as much moisture as the Amazon.”
On average, just a few atmospheric river events are responsible for 30 to 50 percent of the annual rainfall on the West Coast, according to the National Oceanographic and Atmospheric Administration.
The significant moisture, high surface winds, and associated severe weather these planetary features bring makes being able to predict them important to the Navy and Marine Corps. For example, the 2010 Maya Express contributed to Washington, D.C.’s “Snowmaggedon,” which dropped about 5 feet of snow in the area over approximately four days.
“What makes them a little bit confounding to model is how dynamic they are,” Reynolds said. “They can last up to several days, they can pulse, and small details on their edges can have a big impact on how they evolve.”
Reynolds said current weather models can be off by hundreds of kilometers even for short forecasts of just a few days.
“There is a lot about them we still need to understand, especially the ocean and air interactions,” she said. “We have a basic knowledge, but still have to get to the details.”
Reynolds and James Doyle, Ph.D., senior scientist in NRL’s marine meteorology division use sophisticated methods called adjoints to discern how the forecasts of these rivers are very sensitive to small changes in the structure of the rivers.
Reynolds said the adjoint tools are not unique, but are among the most complete and highest fidelity in the world. The team’s adjoints will be put to the test in 2020 when NRL will collaborate with other agencies to get live data on atmospheric rivers.