Ceaseless booms of thunder, racing winds, golf ball–sized hail, and, occasionally, the formation of destructive tornadoes are some of the effects of supercell storms. Thousands of these storms occur over the midwestern United States and other parts of the world each year. The damage they cause on or close to Earth’s surface is easily noticed, but these spring and summertime storms create true chaos kilometers above us.
Winds from these storms can lead to a turbulent plume of cirrus clouds that erupt well above the storm’s anvil-shaped cloud. These plumes usually form before the most severe weather on the ground, so understanding their formation could improve severe weather prediction and ultimately increase the precious time that people have to get to safety. Yet scientists have struggled to figure out exactly why they appear.
Recently, a team of atmospheric scientists found an explanation, potentially paving the way for better predictions of the most severe weather. In an article published in Science, researchers proposed that for speeding stratospheric winds, “the storm acts like a physical obstacle, similar to a river rock,” said Morgan O’Neill, an assistant professor at Stanford University and lead author of the study. When river water flows fast enough over a jagged rock, the water flow that follows is much more turbulent; when fast winds flow over these overshooting storm tops, they accelerate and crash down. The air from two distinct atmospheric layers, the troposphere and the stratosphere, is violently mixed, manifesting as a cirrus plume above the storm.
This process, known as a hydraulic jump, typically occurs because of a solid obstacle, like a mountain or river rock, making its formation in the atmosphere “a very special kind of hydraulic jump,” said O’Neill, because “the storm is fluid. It’s made of the same stuff” as the air it obstructs.
“We know that mixing occurs because things are a mess up there,” said Gretchen Mullendore, director of the Mesoscale and Microscale Meteorology Laboratory at the National Center for Atmospheric Research, who was not involved in the study. This article demonstrates the obstacle effect of overshooting tops, “which to my knowledge, has not been clearly identified previously. That’s a big deal,” said Mullendore.
Simulating a Storm
To better understand the underlying physics of the cirrus plume formation above supercell storms and to test their hydraulic jump idea, the researchers simulated an overshooting storm top using a sophisticated model that’s widely established in the atmospheric dynamics community called the Bryan Cloud Model 1. “The model is second to none,” said Kristopher Bedka, a meteorologist at NASA Langley Research Center who was not involved in the study. It simulated the storm at 50-meter resolution, allowing the researchers “to get at the detailed processes that hadn’t been shown in previous studies,” said Bedka.
The scientists simulated two storms: one that produced a cirrus plume and one that didn’t. The latter was used for them to get a reference point. The characteristics of this reference storm matched observations of an actual supercell storm that occurred over Oklahoma in 2011. The agreement indicates that the simulations have at least some basis in reality, rather than strictly being the output of models.
Through the simulations, the researchers also found that how these plumes are formed likely leads to much more water vapor entering the dry stratosphere than previously thought. A wetter stratosphere could also potentially damage the ozone layer, a layer that protects life from harmful radiation, propagating other changes to the atmosphere as a whole. Though this aspect of the findings “certainly has implications for stratospheric composition, there are many other factors that go into deciding whether this is going to increase estimates of ozone destruction,” said Jessica Smith, an atmospheric chemist at Harvard University who was not involved in the study.
Huge Step Forward, Followed by Smaller Steps
The study is a huge step forward, but it is still rough around the edges. For one, the simulation “really wasn’t quite as realistic as what we’d see in satellite images,” said Bedka.
Studies that simplify some of reality are useful when you’re testing whether a physical mechanism is even sensible. But the study’s granular details, like the exact estimate of water vapor entering the stratosphere, should be taken with a grain of salt.
Still, this work carves a clear path for further exploring the dynamics of supercell storms, whereas previous hypotheses had amounted to only disparate threads. “This study brings those threads together,” said Smith, “and weaves them into a more coherent, self-consistent framework.”
—Jordan Wilkerson (@JordanPWilks), Science Writer