Doppler weather radar data—like those shown in the animation above and often featured in weather broadcasts—can tell us much more than how much rain will fall on your weekend cookout.
Here at BirdCast, we turn weather radar data into information on the numbers and flight directions of birds aloft in order to expand the understanding of migratory bird movement. We have been hard at work serving you products, like the new migration forecast and live migration maps. After several years (and hundreds of posts) describing migration, species on the move, and unique migration events, we want you to have a better understanding of what happens at BirdCast. In this article, we’ll give you a brief overview and a behind-the-scenes look at the ways we apply radar data to study bird migration.
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MORE THAN WEATHER
Radar, an acronym for radio detection and ranging, was originally developed to detect enemy aircraft in World War II. As the decades have passed, it has become a valuable tool for studying all kinds of objects in the atmosphere. One of radar’s most common applications is the detection of meteorological events—especially thunderstorms, tornados, and hurricanes. But there are other applications. The same radars that meteorologists use to estimate rainfall rates are equally adept at detecting other objects in the atmosphere, like birds.
From the earliest years of radar development, engineers and operators noticed dots on their screens that confused them. These targets did not correspond to any weather condition or aircraft and were dubbed “angels” in honor of their transient nature. And confusion about these targets arose for good reason: radar captures information about anything in the atmosphere that can scatter its pulsed microwave energy back to it. Meteorological phenomena share the sky with objects both human and biological, from aircraft and Saharan dust to hordes of mayflies and forest fire smoke plumes. These pioneering engineers and operators were the first to realize that radars could detect objects well beyond the intended range of aircraft and meteorological phenomena.
Sorting out the identity of these “angels” was incredibly important, especially during wartime, where ignoring a detection could literally mean life or death. Edward Gray and David Lack, skilled field observers and premier ornithologists, in short order, hypothesized that the returns could be caused by birds. Eventually, the pair determined that birds were the culprit. So began the era of radar ornithology.
Fast forward five decades to the 1990s, when technological advances of Next Generation Radar (NEXRAD) became operational in the U.S., allowing the field of radar ornithology to truly take flight. By this time, many knew that a large component of targets on radar were birds aloft on their bi-annual migrations, but the computer power, data storage, and computer algorithms needed to parse out birds from meteorological phenomena in radar data did not exist. Data were difficult to access and manipulate, and computers took hours—days, on occasion—to retrieve and process information of ornithological importance. In recent years, faster, more efficient, and more effective computing power, coupled with an invaluable upgrade in NEXRAD technology, provided the tools to extract avian information. In 2013, NOAA deployed a new feature to U.S. radars called “ dual polarization ” (dual-pol, for short)—pulses of microwaves emitted at two angles instead of one. Radars outfitted with dual-pol allow for better discrimination of targets. For meteorologists, dual-pol data provide an ability to distinguish partially frozen sleet from droplets of rain; for ornithologists, they allow us to more easily separate birds from precipitation and other flying creatures.
…BUT, HOW DO WE KNOW THESE ARE BIRDS?
Equipped with computing power and advanced radar technology, we can begin to provide detailed data on avian activity in the atmosphere
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