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Migrating Birds
At its lowest scanning level a NEXRAD antenna is set at a 0.5 degree elevation angle. Therefore, as a radar beam travels farther from the antenna, it scans higher altitudes.
| a NEXRAD Volume Coverage Pattern (VCP 21, precip mode) |
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Birds migrating at night are typically most dense around 1500 ft (~500 m) above the ground. Some may fly higher, frequently to 15,000 ft. (especially in trans-gulf migration) and occasionally to 20000 ft.
| Typical distribution of migrants with altitude |
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When a radar beam scans this distribution of targets, reflectivity value rapidly increases with distance from the station. As the radar scans above the densest layers of the migration pulse, the reflectivity value gradually declines. The following figure represents a hypothetical snapshot of nocturnal migration. In this snapshot the most dense bird migration occurs where the black dots are most numerous. When the radar beam scans this area, the reflected energy from birds is 25-30 dBZ represented by the darkest green dots. Again, note the relationship between reflectivity and altitude. From ground level up to the most dense bird layer, the reflectivity value increases rapidly; from the most dense bird layer to much higher altitudes, the reflectivity value decreases gradually.
| Birds overflying a NEXRAD station and detected as different reflectivities. |
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Note that no reflectors were detected by the radar near to or far from the NEXRAD. This results in a "doughnut-shaped" pattern on the radar scan.
| Precipitation mode base reflectivity image of birds overflying POE Ft. Polk, Louisiana. | ||||||||||||||||||||||||||||
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The reflectors nearest the station are "ground clutter" caused by radar returns from objects on the ground such as trees, buildings and even vehicles.
