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When torrential rains and powerful winds hit densely populated coastal regions, whole cities can be destroyed—but governments and residents can take precautions with sufficient warning.

Many of these coastal deluges are caused by atmospheric rivers—regions of concentrated water vapor carried along on strong winds, sometimes called "rivers in the sky." Meteorologists monitor them, but the ability to predict exactly how an atmospheric river might behave based on its underlying physics would offer more precise forecasts.

In a paper in Nature Communications, senior author Da Yang, assistant professor of geophysical sciences at the University of Chicago, and first author Hing Ong, a postdoctoral researcher formerly in Yang's group and now at Argonne National Laboratory, describe a new equation they developed to better understand the processes that drive atmospheric rivers.

They hope the new framework will enhance the accuracy of atmospheric river predictions, especially for extreme weather events and in the context of a changing climate. This improved, process-level understanding also supports clearer communication of extreme weather forecast results.

A global phenomenon

Atmospheric rivers are long, narrow regions of concentrated water vapor accompanied by strong winds that carry moisture from the tropics toward the poles. They can transport as much as 15 times the amount of water that flows through the mouth of the Mississippi River, and they can bring heavy rain, snow, and strong winds. Up to half of California's annual precipitation is brought by atmospheric rivers.

While the west coast of North America is particularly susceptible to extreme precipitation carried by atmospheric rivers—nicknamed a "Pineapple Express" when it originates around Hawaii—these rivers in the sky occur worldwide.

On average, there are five in the northern midlatitudes and five in the southern midlatitudes at any given point, moving west to east. They aren't all powerful enough to cause damaging floods and landslides; weaker systems can be beneficial, replenishing reservoirs and relieving droughts.

Atmospheric rivers are an essential element of the global climate, and understanding them will help improve the ability to forecast weather, manage water resources, and predict flood risk. Much of the existing research on atmospheric rivers involves characterization: monitoring, tracking, and rating them to help convey their hazard level. But what has been lacking is a way to determine an atmospheric river's evolution.

Atmospheric rivers are monitored using a metric called integrated vapor transport (IVT), which describes the amount and velocity of water vapor moving through the atmosphere.

This metric is enough to develop tracking and monitoring algorithms, but to address fundamental questions about the evolution of atmospheric rivers, scientists need a governing equation. This is a mathematical expression that describes how a system changes based on specific rules or principles.

A governing equation would let scientists ask big-picture questions, Yang said, such as: "What provides energy to form and sustain atmospheric rivers? And why do they move eastward?"

Deriving the framework to answer these questions required the team to develop a quantity that combines the water vapor amount and the energy of strong winds into one variable: integrated vapor kinetic energy (IVKE).

The new equation is as effective and efficient as IVT at tracking and monitoring atmospheric rivers. But it has "the added benefit of being an intuitive first principle-based governing equation," said Yang, "that can tell us what makes an atmospheric river stronger, what dissipates it, and what makes it propagate eastward—in real-time."

The breakthrough adds physical process–level understanding to the statistical analysis of atmospheric rivers. The working title of the paper that describes this versatile framework was "One Stone, Two Birds."

Using this new framework, Yang's team found that atmospheric rivers mainly increase in strength because potential energy converts into kinetic energy. The rivers weaken due to condensation and turbulence and travel eastward due to the horizontal movement of kinetic energy and moisture by air currents.

Weather and a changing climate

The National Oceanic and Atmospheric Administration (NOAA), the primary center responsible for weather forecasting, researches, monitors, and publicizes information on atmospheric rivers. Yang suggested that his team's new framework complements NOAA's IVT-based analyses, offering real-time diagnostics that provide a stronger physical basis for forecast results.

This approach boosts confidence in predictions, especially for extreme events, and aids in diagnosing model performance—ultimately guiding improvements in forecasting models.

The role of climate change in the evolution of atmospheric rivers is also a topic of interest. "We know that with climate change, the amount of water vapor is increasing," said Yang. "Under the assumption that the circulation doesn't change much, you may expect that the individual atmospheric river may get stronger."

The study did not investigate that relationship, but it will be one of the team's next steps. A new postdoctoral researcher in Yang's lab, Aidi Zhang, will use the new framework to study how climate change impacts atmospheric rivers using vapor kinetic energy.

This research is a new area for Yang, although not so distant from his expertise, focusing on convective storms in the tropical atmosphere. Before joining UChicago, Yang lived in California for 15 years, which piqued his interest in atmospheric rivers. And "now that I live in higher latitudes," he said, "I should pay more attention to these midlatitude storms."