A scientist has developed a new equation to better predict the behavior of atmospheric rivers

When torrential rains and strong winds hit densely populated coastal regions, entire cities can be wiped out, but governments and residents can take precautions.

Many of these coastal floods are caused by atmospheric rivers—regions of concentrated water vapor carried by strong winds, sometimes called “rivers in the sky.” Meteorologists are watching them, but being able to predict exactly how an atmospheric river might behave based on its underlying physics will offer more accurate predictions.

In paper published in Nature Communications, Senior author Da Young, associate professor of geophysical sciences at the University of Chicago, and first author Hing Ong, a postdoctoral fellow formerly in Young’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 improve the accuracy of atmospheric river forecasts, especially for extreme weather events and in the context of climate change. This improved process-level understanding also supports clearer communication of extreme weather forecasting results.

A global phenomenon

Atmospheric rivers are long, narrow regions of concentrated water vapor accompanied by strong winds that carry moisture from the tropics to the poles. They can transport 15 times more water than flows through the mouth of the Mississippi River, and they can bring heavy rain, snow and strong winds. Atmospheric rivers bring up to half of California’s annual precipitation.

While the West Coast of North America is particularly susceptible to extreme precipitation, carried by atmospheric rivers nicknamed the “Pineapple Express” as they originate around Hawaii, these rivers in the sky are found all over the world.

On average, at any point there are five in the northern midlatitudes and five in the southern midlatitudes, moving from west to east. Not all of them are powerful enough to cause devastating floods and landslides; weaker systems can be beneficial by replenishing reservoirs and alleviating droughts.

Atmospheric rivers are an important element 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 characterizing, monitoring, tracking and evaluating them to help communicate their level of hazard. But what was missing was a way to determine the evolution of the atmospheric river.

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

This indicator is sufficient for the development of tracking and monitoring algorithms, but to solve 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 certain rules or principles.

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

Deriving a basis for answering these questions required the team to develop a quantity that combines the amount of water vapor and energy strong winds into one variable: integral kinetic energy of steam (IVKE).

The new equation is as efficient and effective as IVT for tracking and monitoring atmospheric rivers. But it has “the added advantage of being an intuitive first-principles governing equation,” Young said, “that can tell us what makes the atmospheric river stronger, what dissipates it, and what causes it to spread eastward— in real time. “

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

Using this new framework, Young’s team found that the force of atmospheric rivers is mainly increased because potential energy is converted into kinetic energy. Rivers weaken due to condensation and turbulence and move eastward due to the horizontal movement of kinetic energy and moisture by air currents.

Weather and changing climate

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

This approach increases the confidence in forecasts, especially for extreme events, and helps in diagnosing model performance, which ultimately guides the improvement of forecasting models.

The role of climate change in the evolution of atmospheric rivers is also an interesting topic. “We know that as the climate changes, the amount of water vapor increases,” Yang said. “Assuming that the circulation doesn’t change, you can expect that the individual atmospheric river might get stronger.”

The study did not examine this connection, but it will be one of the team’s next steps. A new researcher in Yang’s lab, Aidy Zhang, will use the new framework to study how climate change affects atmospheric rivers by harnessing the kinetic energy of steam.

This research is a new field for Young, though not that far removed from his experience focusing on convective storms in the tropical atmosphere. Before joining the University of Chicago, Ian lived in California for 15 years, which sparked his interest in atmospheric rivers. And “now that I live in higher latitudes“I should pay more attention to these mid-latitude storms,” ​​he said.