Weather disasters such as floods and tornadoes can take a huge toll on people and their possessions. The 2011 tornado outbreak in the southern United States, for example, killed 348 people and made the record books as the largest tornado outbreak in history.
Despite the importance of anticipating these events, however, today’s weather models can accurately forecast only two or three days ahead.
A group of University of Wisconsin-Madison climate researchers has now identified an intense weather pattern called a “superposed jet.” Accurately forecasting this pattern, they think, may make it possible to anticipate severe weather events as much as a week ahead of time.
“This gives emergency managers and the public an ample amount of time to prepare for an extreme event and better confidence in that event actually occurring,” said Andrew Winters, a doctoral student in the Department of Atmospheric and Oceanic Studies at UW-Madison. Winters is studying jet stream features as part of professor Jon Martin’s research team.
Winters and his colleagues began researching jet streams by studying the 2010 Nashville flood. They found heavy rainfall was caused by both an extremely humid air mass moving northward out of the Gulf of Mexico and a remarkably strong jet stream above Earth’s surface.
A jet stream is a thin ribbon of rapidly moving air that moves east around the globe. It is produced by a clash between cold Arctic air and hot tropical air at mid-latitudes, the areas of the globe between the tropics and polar regions. Jet streams are crucial to our weather patterns and play an important role in determining the occurrence of extreme weather.
Typically, the Northern Hemisphere is characterized by two jet streams: the polar jet, which largely affects our weather in the mid-latitude, and the subtropical jet, which sits further to the south over portions of the Gulf of Mexico and northern Mexico. Although a jet stream circles around the globe, the pattern is not a perfect ring. Instead, it has troughs and ridges similar to a wave.
Around each jet stream there is a motion of circulation, exchanging air masses between the ground and the atmosphere. Thus, the trough of the jet wave sucks the air from below upward, leaving a low-pressure atmospheric system near the ground that results in wet and cold weather. The peak of the wave creates a high-pressure atmosphere above the ground and is responsible for dry and warm weather.
Winters explains extreme weather happens when troughs or ridges of jet streams stay in an area for a long period of time. For example, if the trough part lingers in an area for days at a time, the resulting low-pressure system would keep dumping rain over the same area and could cause flooding. On the other hand, if a region is beneath the ridge for a long time, the high pressure system would remain stable and encourage drought.
In the Nashville case, the atmospheric pattern remained stationary for nearly two and a half days, which allowed heavy rain and flooding.
But things can get even more dramatic. Martin’s group has identified the impact of a superposed jet on the evolution of extreme events such as the Nashville flood and the 2011 tornadoes.
A superposed jet is formed by the positioning of a polar jet and a subtropical jet on top of one another. Since this combined jet has more intense circulation than a single jet, it could greatly increase the extremely humid air mass and have a greater influence on the formation of extreme precipitation over a region.
Martin’s group shows that, although these superposed jets are relatively rare phenomena, when they do play a role, their contribution to extreme events is clearly evident.
Specifically over North America, these superposed jets occur no more than a handful of times per month, but are most common over the southwestern United States and off the East Coast. Worldwide, this feature occurs most frequently over the western Pacific Ocean, just off the coast of Japan. In the Northern Hemisphere, late fall, winter and early spring months see the majority of superposed jets.
Superposed jets have never been identified and examined before. By using data in the upper atmosphere—such as wind speed—gathered from global forecast models, Martin’s group is able to identify the location of superposed jets using a set of self-developed criteria.
Forecasting will always be an uncertain business. For example, while one superposed jet may anticipate severe weather, others may not. Thus, Martin’s group, according to Winters, needs to understand the formation process of superposed jets in order to more confidently assess the effects of these powerful streams. “We have already known a lot about our atmosphere, but not enough,” Winters said.
