The Rising Storm

n an otherwise pleasant autumn afternoon over the rolling plains of eastern Kansas, a lone cloud begins to build and darken. The wind quickly freshens, the temperature drops and, just minutes later, a howling thunderstorm is dumping torrential rains on the grasslands below.

The storm grew well behind a cold front, a hundred miles away from where weather watchers would expect it to develop. Needless to say, it hadn’t been forecasted with very much accuracy.

Such storms, known to atmospheric scientists as “elevated convection thunderstorms,” have been identified as a distinct weather event only in the last 25 years. Because forecasters know little about their triggers, they’re tough to predict, making them a problem of more than just academic interest.

In short, elevated convection storms can be killers. Capable of living three times longer than conventional thunderstorms, they often produce the types of massive rainfalls and rapid flooding that cost lives.

The storm on the plains didn’t develop without scientific notice. Earlier this year a team of MU atmospheric scientists and students traveled eastern Kansas and western Missouri to chase elevated convection storms, studying their birth and life cycle. The Kansas storm was one of a half-dozen solid research hits. The researchers positioned themselves under where the storm would grow and launched radiosonde-equipped weather balloons into the sky. As the radiosondes radioed back a pressure and temperature profile of the changing atmosphere, NOAA’s newest and most precise weather radars captured images.

Their data represent the first such measurements taken before, during and after elevated convection events. Combined with the radar data, the research team, led by Neil Fox and Patrick Market, both professors of atmospheric science at MU, are on their way to improving predictions of heavy rain, the leading cause of weather-related fatalities.

conventional thunderstorms, studied for a hundred years, are born when the sun heats the earth. This causes air near the surface to rise into the less dense upper atmosphere. Clouds form as warm air carrying moisture moves into the cooler air. As the warm air rises, it cools. Then, the moist water vapor condenses, releasing energy that keeps the air warmer than its surroundings, so that it continues to rise. If enough instability is present in the atmosphere, this process repeats itself long enough for cumulonimbus clouds to form, which support lightning and thunder.

An elevated convection thunderstorm is different. It occurs when moist convection originates at an altitude above the ground, usually in the lower troposphere — 8,000 to 10,000 ft. high. In this case, air is lifted by either a moving cold or warm front, or terrain in mountainous areas.

Predicting elevated convection thunderstorms is hard because the storms can occur in cold air more than 100 miles behind a front. The thunderstorms most people experience, and what forecasters tend to expect, generally occur inside the warm, moist-air masses along fronts.

Until recently these storms seemed rare anomalies. The phrase “elevated convection” only entered the research lexicon in 1991 when scientists began to wonder if unexpected thunderstorms were caused by a different set of atmospheric conditions. In the early research, hard data was sparse. The storms seemed to occur when the air near the ground was relatively cool and stable, but the air aloft was unstable. Horizontal instability, like a low-level jet stream, seemed to be a factor, too.

“We knew the process of storm formation was different with elevated convection storms,” Fox says. “We weren’t sure exactly how these storms got going and what promoted their formation into heavy rain producers. On a very basic level, we researchers weren’t even sure where they are got their moisture from.”

The need to better understand these storms was critical. A typical thunderstorm lasts from 30 minutes to an hour. An elevated storm may live up to three hours, potentially making it a heavy rainfall and flood producer. Researchers aren’t even sure when in an elevated storm’s lifetime it is most likely to produce heavy rainfall and which part of the storm is likely to be most active.

Making these storms even more dangerous is that they can occur any time. Classic thunderstorms tend to die at night due to the loss of heating sunlight. Elevated convection storms seem to be fueled by different influences, meaning they can produce heavy amounts of rain when most people are sleeping and can’t hear warnings.

More than 125 people die in flash floods each year, according to the National Weather Service. That compares with an average of 73 deaths for lightning, 68 for tornadoes and 16 for hurricanes. While the number of fatalities for tornadoes and hurricanes has decreased because of better forecasting, the number of fatalities due to flash flooding has remained constant. The MU research, funded by the National Science Foundation, aims to do better.

mizzou’s atmospheric sciences team was a natural fit for elevated convection research. Market and his students had previously chased “thundersnow,” a rare weather event that yields huge snowfalls in a short time. Chasing snow had accustomed Market and the student team to hard service — they often huddled for hours in unheated vans so their radiosondes would be at the right temperature for launching into snow-laden clouds. They took a similar hands-on approach to elevated convection.

“The data needed for a better forecast could be as simple as determining the warming or moistening of the lower troposphere, perhaps by a low level jet stream,” Fox says. “It could be that cooling or drying of the middle to upper troposphere causes the destabilization of the atmosphere. We do not know which of these factors is dominant with elevated convection. We have to go out and find it.”

Forecasters don’t know exactly what to look for to predict an elevated convection storm, which is why the pressure and temperature profiles sent back by the weather balloons are so important.

Like Mizzou’s Storm Chase Team, which researches tornado-bearing supercells in spring and summer, MU’s elevated convection team is a big group effort. Atmospheric science graduate students, hunkered down in the cement basement of MU’s Agriculture Building, use an array of sophisticated computer models and algorithms, radars, satellite imagery and weather station reports to make the best prediction possible of a potential elevated convection event. They direct the in-field team to the best location.

Also important is the correlation of the events in the atmosphere to what weather radars are seeing. Radar is the tool of choice for short-term predictions of storms and issuing warnings.

The Mizzou team will compare their balloon data with advanced radar returns from sites in Topeka, Kansas City and Springfield to see if the advanced radar can detect something of use in better forecasts.

Faculty members in MU’s Atmospheric Sciences Program take pride in the amount of research experience afforded both graduate and undergraduate students. Blending of graduates and undergraduates is not an accident: The idea is to build teamwork and communication skills into the learning experience.

“We bake student involvement into all stages of the research,” Market says. “In most other programs, students rarely get an opportunity for hands-on experience. We’ve found that putting students out in the atmosphere and observing and collecting data is a tremendous way to help the students match what they learn in the classroom with what is happening in the environment. Students in many atmospheric science programs can get a degree without looking out the window. Not at MU.”

Using weather balloons to get a snapshot of elevated convection events is great for basic science, but isn’t a practical forecasting tool. To help forecasters actually get a jump on dangerous storms, the MU team is partnering with the National Weather Service to scan elevated convection events with their recently upgraded weather radar. Under the arrangement, the researchers get to compare their atmospheric soundings with radar data, while forecasters get a precise analysis of what is really happening in the atmosphere.

The partnership is valuable for both parties. Current-generation Doppler radar has proven to be an invaluable tool for forecasters, allowing them to see inside storms and track precipitation and rotation in the atmosphere. But in some ways it has proven too good, often showing extraneous and confusing things. Birds, for example.

To provide faster and more precise imagery of storms, NOAA has upgraded its national radar network with dual-polarization, or dual-pol, technology. It’s the most significant enhancement made to the nation’s radar network since Doppler radar was installed in the early 1990s.

By providing better information about the type of precipitation in the atmosphere — its intensity, size and location — dual-pol radars can increase the accuracy of forecasts and allow for more accurate and timely warnings.

The dual-pol upgrade was made possible by new software and a hardware attachments to the radar dish, additions that allow it to send and receive both horizontal and vertical pulses of energy that provide a detailed two-dimensional picture. Conventional Doppler radars, on the other hand, send out just a horizontal pulse of energy, giving forecasters only a one-dimensional picture of what is in the air.

With conventional Doppler, in other words, it’s either precipitation or not precipitation: The system can’t tell the difference between rain, snow, or hail. Dual-pol radar helps forecasters clearly identify rain, hail, snow, ice pellets and other objects, thus improving forecasts for all types of weather.

“Dual-pol gives us what we’ve needed,” Fox says. “Now we can distinguish the light from the heavy rain in a very accurate way. We can build a more complete picture of the storm’s structure and how it is moving and developing.”

This type of weather radar will eventually become an important tool to predict elevated convection, the researchers say. There’s a good chance it will also be able to very accurately determine the location and intensity of rain shafts, allowing forecasters to precisely predict where heavy rain — and potential flooding — will occur.

“Dual-pol gives us what we’ve needed,” Fox says. “Now we can distinguish the light from the heavy rain in a very accurate way. We can build a more complete picture of the storm’s structure and how it is moving and developing.”

This will eventually allow scientists and forecasters to pinpoint where heavy rain is likely to cause floods, he says. “It will give meaningful information to people to keep them out of places where water can rise so rapidly there is no practical way of escape. This targeted information will also allow emergency services to be better prepared, too.”

Getting severe weather reports right is important in less obvious ways as well, Fox says. “Imprecise forecasts lead to public apathy. Right now, all we can do is to issue watches and warnings over a large geographic area — one that may or may not be impacted by the heavy rainfall. Too many inaccurate warnings and people ignore all warnings.”

Fox’s post-doctoral work was in radar interpretation and flash flood warnings. He studied under Chris Collier, a prominent atmospheric scientist at the University of Salford, in Manchester, U.K., who helped build the United Kingdom’s weather radar network. Radar interpretation in the U.K. is oriented toward flash-flood forecasting.

By combining this season’s radar data with that obtained by the radiosondes, Fox and Market’s team hope to create a three-dimensional model of an elevated convection event from its inception to dissipation. With the model they can better analyze influences like the low level jet stream, which seems to create an inflow of moist air into the storm.

Fox is an expert in using radar to follow precipitation carried by winds, an area of enquiry that wasn’t practical until the latest generation of weather radar. These currents may hold the key to seeing the early stages of elevated thunderstorm formation.

“Probably more important is that we can use the radar to track the development of the precipitation,” Fox says. “The new capabilities allow us to determine where drops are large or growing, where there is ice and where the ice is melting. All these things help us figure out where the rain is being formed, where it is heaviest, and where energy is being used or released during the precipitation process. This helps us understand the convection process, as well as downbursts.”

eastern kansas and western Missouri in late summer and early fall are the bulls-eye for elevated convection storms. More than a dozen storms can occur in this area, giving the researchers opportunities to be in the right place to gather data. So far, MU researchers have gathered data from six storms. That doesn’t sound like much, but it’s actually a dramatic increase over what was previously available. “We’ll be working backward through the storm’s life to find what triggered its birth and life and when it’s likely to produce a downburst,” Fox says. “We now have some insight into the features that forecasters should look for.”

Reader Comments

Anthony Lupo wrote on June 21, 2015

Great story Randy and Sean about our 7th ranked meteorology department.

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