In a nutshell, Doppler radar is a specialized tracking system that uses the Doppler effect (Doppler shift) to determine the location and velocity of storms and precipitation.
Now, let’s crack open that shell and get to the nuts and bolts of how it came to be and how it works.
Doppler vs. conventional radar
Doppler radar has been one of the most important technological advances in hazardous weather prediction over the past 30-plus years. Forecasters using Doppler radar can issue more timely and accurate hazardous weather information than ever before.
U.S. Weather Bureau’s first experimental Doppler weather radar unit, obtained from the U.S. Navy in the 1950s. (Photo: NOAA)
Conventional radar provides information about the location and intensity of precipitation associated with a storm, while Doppler radar adds the capability to discern air motions within a storm. This helps meteorologists detect near-ground wind shears, which are dangerous to aircraft. Doppler radar technology also enables meteorologists to forecast the location and severity of weather with greater accuracy, which has resulted in improved public safety.
How Doppler radar works
The radar dome, which many people say looks like a big soccer ball or volleyball, protects a 28-foot-wide antenna. The antenna is made up of a transmitter and a receiver. The transmitter samples the environment by sending out a pulse of energy that reflects off objects (like raindrops) and scatters the energy pulse in all directions. Part of the energy reflects back to the antenna and is measured by the receiver.
The returned energy is then processed into three types of base data: reflectivity, velocity and spectrum width. Reflectivity is calculated from the fraction of signal that is reflected back to the radar. Velocity is calculated by measuring the Doppler shift from one pulse to the next to determine the speed with which the object is moving toward or away from the radar. Spectrum width is calculated based on how much variation there is in velocity readings in a given area.
Additionally, a host of derived products are computed from the base data. These include precipitation estimates, as well as severe weather parameters like tornadoes, hail and heavy rain. During a potentially hazardous weather event, many of these products will be examined by a forecaster to make decisions on issuing warnings.
The radar does not sample the entire atmosphere at once. Rather, the antenna makes a 360-degree rotation pointed at a particular elevation angle, then changes elevation and completes another rotation. The number and selection of elevation angles, along with the speed of the antenna’s rotation, varies based on the weather conditions.
During calm conditions, the antenna rotates slowly, completing a scan of five slices in 10 minutes. During active weather conditions, the antenna rotates faster, completing a scan of up to 14 elevation slices in 4.5 minutes. This rapid scan is crucial during developing severe weather as conditions can change by the minute.
History of Doppler radar
Doppler radar and severe storms research were joined in the early 1960s when the National Severe Storms Project began in Kansas City, and continue to this day at the National Severe Storms Laboratory (NSSL) in Norman, Oklahoma. The Union City, Oklahoma, tornado in May 1973 marked the beginning of NSSL intercept teams using research Doppler radars to collect measurements from severe thunderstorms.
WSR-88D (Doppler radar) imagery of Moore, Oklahoma, tornadic supercell, May 3, 1999. The National Weather Service rated the storm an EF5 based on storm damage. (Image: NOAA)
In the 1980s, the push to get Doppler radars into warning operations became well-organized as the NEXRAD (NEXt generation weather RADar) program formed. The first operational Doppler radar, the WSR-88D (Weather Surveillance Radar-88Doppler), was installed near Norman in 1988.
In the early to mid-1990s, a national network of over 100 WSR-88D Doppler radar sites was built as part of the modernization of the National Weather Service. Today, about 160 Doppler radars dot the U.S. landscape. In recent years, these radars have been retrofitted with dual-polarization technology, firing beams aligned both horizontally and vertically. Computer programs measure the differences between the vertical and horizontal wave returns to better detect different kinds of precipitation — small raindrops, large drops, hail and snow — and other echo returns like chaff, wildfire smoke, bats, birds, insects and airborne tornado debris.
Click here for more FreightWaves articles by Nick Austin.
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