[Broken External Image]:
http://aaron.ou.edu/dow/images/dimmit_1s.jpg
These utility trucks carrying a radar dish are familiar sights on the Great Plains, and have been spotted nationwide participating in other meteorological studies. There are two basic DOWs, built sometime around 1993. They are the brainchild of OU (now NCAR) scientist Josh Wurman. The units are now based in Boulder, Colorado and are owned by a partnership of universities.
The radars are used to gather extremely high resolution (both temporal and spatial) data of storm structure. From this data it is possible to make theories that give us a better understanding of how tornadoes and other severe weather forms.
A third DOW (Rapid-DOW) has recently been built using a phased-array radar. It allows a near-continuous look at a storm, and can be identified by its large, flat panel rather than the familiar radar dish.
A new fleet of mobile radars (actually, two) is now in existence, thanks to a partnership between NSSL, OU, TTU, and TAMU. These are called SMART Radars, and may be mistaken for DOWs. They're based in Norman, Oklahoma, where the DOWs used to reside.
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More information on DOWs
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More information on SMART Radars
(Thanks to Kevin Scharfenberg for some corrections)