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ThunderstormsMore than a thousand thunderstorms rage across Earth's surface at
any moment. These are convective systems—they
form when relatively warm, moist air near the ground rises through
cooler air aloft, like water boiling in a teakettle. When the right conditions are in place, a fair-weather cumulus cloud can blossom into a dangerous cumulonimbus (thunderstorm cloud) in less than an hour. Most thunderstorms are not severe. They produce a little wind, spit out some lightning, and deposit rains that are often beneficial. Some parts of the world get most of their precipitation from thunderstorms. However, if the air is strongly unstable and the wind varies greatly with height, then a thunderstorm can become severe. The strongest thunderstorms produce winds above hurricane force, hailstones larger than golf balls, and even tornadoes. For over 30 years, NCAR scientists have studied thunderstorms across the planet, from the tropics to the U.S. Great Plains. Thunderstorm typesSome thunderstorms form as isolated cells as small as 5–10 miles (8–16 kilometers) wide. Others are accompanied by fellow storms along a narrow squall line or are scattered across a broad area.
Most individual storms last only an hour or so. The more organized and intense systems can survive much longer. The largest thunderstorm clusters, which can span over 100 miles (160 km), are called mesoscale convective systems. During the summertime, many of these systems develop in the afternoon across the U.S. High Plains. Often they march east into the Midwest during the night and continue into the next morning. NCAR scientists have identified preferred corridors of movement that can persist for weeks at a time. A large field study called BAMEX, led by NCAR in 2003, examined mesoscale convective systems across the Midwest in detail. The most intense thunderstorms are supercells. The air flow in and out of these storms is unusually strong and organized, helping keep the storm potent for up to six hours or more. Supercells produce the largest hail and the most violent tornadoes. NCAR researchers have carried out pioneering work on the structure and evolution of supercells. They've learned how low- and mid-level winds interact to sustain supercells and how a supercell concentrates large-scale rotation to help spawn a tornado. In a year-2000 project called STEPS, NCAR scientists and collaborators studied low-precipitation supercells, which drop little rain but can produce large hail and high winds. Forecasting and observationWeather forecasters can alert people to the risk that severe storms may form in a general area. They still can't predict more than an hour or so in advance exactly where these cells will develop. However, new forms of radar can spot a developing storm quickly. NCAR's Auto-nowcaster software takes advantage of that ability to project the movement and evolution of thunderstorms up to an hour out. The Auto-nowcaster has tracked weather at events where pinpoint weather is crucial, such as the 2000 Sydney Olympics. NCAR is also part of a team developing the Weather Research and Forecasting computer model (WRF). With a resolution as fine as 2.5 miles (4 km), WRF (pronounced "worf") is one of the first large-scale models that can realistically simulate thunderstorm complexes. Since October 2004, WRF has been used by the National Weather Service in crafting its public forecasts. NCAR and its collaborators have a long history of field research on thunderstorms. The center has sent aircraft, radiosondes (weather balloons), and ground-based instruments in and near thunderstorms for decades. In the last 10 to 15 years, mobile weather observing labs and portable Doppler radars, such as Doppler on Wheels, have become a key part of storm research.
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