When torrential rains and strong winds hit densely populated coastal regions, entire cities can be destroyed, but governments and residents can take precautions well in advance.

Many of these coastal floods are caused by atmospheric rivers – regions of concentrated water vapor driven by 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 provide more accurate forecasts.

In an article published on November 4 in Natural communicationsmain author Da Yangassistant 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 determine atmospheric rivers.

They hope the new framework will improve the accuracy of atmospheric forecasts on rivers, particularly for extreme weather events and in the context of climate change. This improved process-level understanding also allows for 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 transport moisture from the tropics to the poles. They can carry up to 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. Nearly half of California’s annual precipitation comes from atmospheric rivers.

While the west coast of North America is particularly susceptible to extreme precipitation carried by atmospheric rivers – nicknamed the “Pineapple Express” when they originate around Hawaii – these rivers in the sky are present around the world . On average, there are five in the northern mid-latitudes and five in the southern mid-latitudes at any given time, moving from west to east. Not all of them are powerful enough to cause devastating floods and landslides; weaker systems can be beneficial, replenishing reservoirs and alleviating droughts.

Atmospheric rivers are a critical part of the global climate, and understanding them will help improve the ability to predict weather, manage water resources, and predict flood risks. Much of the existing research on atmospheric rivers involves their characterization: their monitoring, monitoring and assessment to help indicate their level of danger. But what was missing was a way to determine the atmospheric evolution of a river.

“One stone, two birds”

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

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

A governing equation would allow scientists to ask general questions, Yang said, such as: “What provides the energy to form and maintain atmospheric rivers?” And why are they moving east?

To define the framework for answering these questions, the team had to develop a quantity that combines the amount of water vapor and the energy of strong winds into a single variable: the integrated vapor kinetic energy (IVKE). .

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

This breakthrough adds understanding at the level of physical processes to the statistical analysis of atmospheric rivers. The working title of the article describing this versatile framework was “One Stone, Two Birds.”

Using this new framework, Yang’s team discovered that the strength of atmospheric rivers increases primarily because potential energy converts to kinetic energy. Rivers weaken due to condensation and turbulence and move eastward due to the horizontal movement of kinetic energy and moisture caused by air currents.

Weather and climate change

The National Oceanic and Atmospheric Administration (NOAA), the main 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 builds confidence in forecasts, particularly for extreme events, and facilitates the diagnosis of model performance, thereby guiding forecast model improvements.

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 increases,” Yang said. “Assuming the circulation doesn’t change much, each atmospheric river can be expected to strengthen.”

The study did not investigate this relationship, but it will be one of the team’s next steps. A new postdoctoral researcher in Yang’s lab, Aidi Zhangwill use the new framework to study the impact of climate change on atmospheric rivers using the kinetic energy of steam.

This research is a new area for Yang, but not so far from his expertise, which focuses on convective storms in the tropical atmosphere. Before joining UChicago, Yang lived in California for 15 years, which sparked his interest in atmospheric rivers. And “now that I live at higher latitudes,” he said, “I should pay more attention to these mid-latitude storms.”

“Steam kinetic energy for the detection and understanding of atmospheric rivers.” » Ong, H. and Yang, D., Nat. Comm., November 4, 2024.

Funding: Packard Fellowship, National Science Foundation.

—Adapted from an article first published by the Division of Physical Sciences.