Since the first units were placed along the Gulf Coast in the 1950s, ornithologists and birders have become increasingly aware of the power of using radar as a tool for understanding bird migration. In addition to detecting and depicting meteorological phenomena, this radar network can be used to watch and to track the movements of birds. In this feature we will provide some basics for how to interpret radar data, in particular how to understand the movements of birds on Weather Surveillance Radar – 1988D (WSR-88D). A second installment will discuss challenges in identifying biological targets and some locally interesting patterns visible on radar.
Radar (an acronym for radio detection and ranging) was originally developed and employed leading up to World War II, in particular to detect aircraft. However, from almost the earliest days of radar surveillance, movements of birds were known to appear in radar imagery; the term “angels” was originally applied to the patterns of birds on radar, before solid ground-truthing confirmed these patterns as birds. Over the last 70-80 years, technological advances in computing, electronics, and physics have produced a wonderful array of developments that make radars a powerful tool for studying aeroecology, in particular detecting density, location, direction, and speed of biological targets, such as birds, insects, and bats.
Simple basics of radar
A WSR-88D unit (hereafter radar) emits a pulse of electromagnetic radiation; the antenna emitting and receiving this pulse is stationed at one of several angles of elevation above the ground, so the energy gradually travels higher and higher above the surface of the earth under typical conditions as it moves away from the radar. This is important, so remember it for the next paragraph. As the radar scans, this beam of pulsed energy moves away from the radar until something interferes with it – a target. This target may be a bird, a rain drop, insects, or smoke particles – regardless, some of the energy of the initial pulse bounces back to the radar, representing the relative magnitude of the target/s scattering the return energy. When the radar receives this return energy, the radar has a location of the target relative to the radar as well as a degree of reflectivity in terms of how much scattered energy returns to the radar (base reflectivity image). Additionally, radar provides information on the direction and speed of target movements relative to the station itself (base velocity image). With these data, we can say something about the magnitude, position, extent, and speed of the targets detected on radar.
Reflectivity – magnitude, position, and extent of bird movements
Birds lift off for nocturnal migration usually about 30-45 minutes after sunset. They typically ascend to flight altitudes 1000-3000m above the ground (smaller bodied species are typically in the lower portion of this region below 1000m, larger bodied birds higher up to 3000m or above). During this ascent and during migratory flight, birds interact with pulses of energy emitted by the radar across the continental US. The pulses of energy scattering off of birds closer to the radar are lower in altitude, meaning that reflected energy will pass through and out of the layer of bird migration. The configuration of the radar combined with the biology of birds moving in the lower portions of the atmosphere result in displays of birds on radar that typically appear as a circular, halo, or donut-like pattern around the station in reflectivity imagery.
Velocity – direction and speed of bird movements
Interpreting velocity imagery from radar is a critical component of radar ornithology. Using the velocity image, one can compare speeds of targets to speeds of the prevailing winds. Whenever targets are moving across the wind or against the wind, or moving more than 10-15 knots faster than the wind, those targets are almost exclusively birds. Examining the winds aloft, particularly those at 925mb (those we discuss in our forecasts), you can determine the wind speed, compare it to the speed at which the objects are moving across the radar on the velocity imagery, and study which, if any, targets are traveling slower than birds should be traveling. Any targets drifting with the wind or not moving substantially faster than winds aloft we often label aerial plankton (including small insects, dust, pollen, and smoke) after Clemson University Radar Ornithology Laboratory’s use of the term in the original BIRDCAST project 1999-2001.
Because radar is a Doppler system, we talk about inbound and outbound targets relative to the radar station when we discuss radar detections of birds. Inbound targets will usually be represented by cool colors on radar legends, whereas outbound targets will be represented by warm colors. The most intense colors will represent targets moving fastest relative to the station, inbound or outbound, and visualizing an imaginary axis dividing the inbound and outbound targets is perhaps the best way to determine the general direction of movement from a velocity image. See this excellent description from David Mizrahi and NJ Audubon here.
Birds are not the only targets that radar detects. Precipitation appears as blocky, unevenly distributed patterns, very different from birds. But other biological targets like bats and insects appear in the same stippling pattern as birds, which makes distinguishing birds from bats, insects, and other aerial plankton challenging. Our next radar piece will discuss some characteristics that can be used to help separate birds.
Radar provides us an opportunity to quantify bird migration. By correlating radar reflectivity data and direct visual studies of nocturnal migrants passing across the disk of the full moon, Dr. Sid Gauthreaux and Carroll Belser developed a calibration curve for interpreting radar reflectivity (measured in dBZ) in terms of birds km-3. See more here.
The basic metrics are here, and this terminology is used in the BirdCast migration reports and BirdCast forecasts:
- Minimal migration: <5 dBZ — fewer than 59 birds per cubic kilometer
- Light migration: 5-10 dBZ — approximating 59-71 birds per cubic kilometer
- Moderate migration: 10-20 dBZ — approximating 71-227 birds per cubic kilometer
- Heavy migration: 20-30 dBZ — approximating 227-1788 birds per cubic kilometer
- Extreme: >30 dBZ — more than 1788 birds per cubic kilometer (actually occurs at some times in very rare circumstances)
Watch animation of Nexrad Radar:
To help visualize and better understand how you can use radar to examine migration, we posted an animation and discussion of a flight on 8 – 9 May 2009 here. Watching this animation will allow you to see differences between precipitation and migrating birds.
An excellent primer for radar ornithology is the Clemson University Radar Ornithology Laboratory website. All of the concepts described above can be found in much greater detail on that site.