What scientists are just beginning to figure out, though, is why. This summer, a major government-sponsored field project called PECAN (Plains Elevated Convection At Night) has gathered data that researchers say will lead to breakthroughs in understanding nighttime storms.
"We already know we have the data we need. We're going to be able to analyze the data and map windfields and temperature and moisture, which will allow us to understand it to give forecasters a much better idea," said Conrad Ziegler, a research meteorologist at the National Severe Storms Laboratory (NSSL) and the study's principal scientist.
Forecasters can more accurately predict what storms will do in the daytime, largely because the air near the earth's surface in the daytime is fairly representative of what's above it, allowing surface-based measurements such as radar to predict when a storm's going to stir up.
"When the sun goes down it gets interesting," Ziegler said.
During the day, the sun heats the ground and the warm air rises and mixes with the air above it. But when the sun goes down, all bets are off. As the surface air cools, it doesn't mix much, if at all, with the air above it, rendering surface measurements unreliable at predicting what's going on in the atmosphere.
For decades, mobile radar, ground-based weather stations, and weather balloons (deployed at a rate of once an hour at the most) have been used to observe storm systems. But in order to fully understand how the environment might affect the storm after sundown, more frequent readings are needed at higher points in the lower atmosphere.
At the moment, forecasters can see systems developing, but "the challenge is knowing when they're going to become severe with hail or strong winds or a tornado," said NSSL scientist Dave Turner, who was in charge of deploying the equipment used in PECAN. "We don't understand nighttime storms well enough, so computers don't model it. I don't think we have the right instrumentation."
For the PECAN study, scientists and graduate students spent six weeks this summer using state-of-the-art equipment in order to capture as much data from as many storms as possible, focusing on large systems of nighttime storms called Mesoscale Convective Systems that often produce severe weather. Turner set up 10 Infrared and microwave devices that provided temperature, humidity, and wind profiles about every five minutes, including four that could be moved around.
"We certainly saw many examples of a rapidly evolving atmosphere," he said, noting that they went on over 30 storm-tracking missions.
In many cases, the storms followed the pattern the scientists and students predicted, but in some cases what happened "wasn't anywhere close" to what they had expected, Turner said.
Read more at Discovery News
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