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Introduction to Radar Ornithology
   WSR-88D weather surveillance radar or NEXRAD (NEXt generation RADar) is a doppler radar system that has greatly improved the detection of meteorologic events such as thunderstorms, tornadoes and hurricanes. An extensive network of NEXRAD stations provides almost complete radar coverage of the continental United States, Alaska, and Hawaii. The range of each NEXRAD is 124 nautical miles.

NEXRAD stations in the continental U.S. with their coverage ranges

   The word radar is an acronym for "RAdio Detection And Ranging". Radars transmit microwave signals into the atmosphere and then listen for return signals. If the transmitted signal intercepts an object, most of the energy is scattered, but some will be reflected back to the radar receiver. The quantity that a radar measures is the returned energy, which is converted to a quantity called reflectivity. The amount of reflected energy can be used to estimate the number or density of targets in the atmosphere. Because microwaves travel through air at a known velocity, time between transmission and reception of the reflected energy can be used to estimate the distance of a target from the radar. Reflectivity is often represented by the symbol "z." NEXRAD displays measures of reflectivity in dBZ or decibels, that are represented as different colors on the radar image.

   Rain, snow, and hail can reflect considerable amounts of energy and thus are detectable by radar. Additionally, aerial biota such as insects, birds and bats, and particulates such as smoke, dust, and pollen also reflect radar signals. These reflected signals produce radar images.

NEXRAD reflectivity scan showing large precipitation event

dBZ Precip (mm/hr) ND 05 10 15 20 0 25 1 30 2 35 5 40 12 45 28 50 63 55 144 60 328 65 747 70 1701 75 3871 Base Ref 124nm (Elev 1)=0.5 deg 1 km GSP : Greer SC 34.88N 82.22W 06/16/99 22:34 Precip Mode VCP 21 Max: 66 dBz

dBZ Birds /km3 ND -28 -24 -20 -16 -12 -8 -4 0 4 58 8 65 12 81 16 123 20 227 24 489 28 1148 Base Ref 124nm (Elev 1)=0.5 deg 1 km GSP: Greer SC 34.88N 82.22W 10/18/99 00:43 UTC Clear Mode VCP 32 Max: 59 dBZ

    NEXRAD is a Doppler radar, and it can track the velocity and direction of targets moving relative to the radar station. This is called the radial velocity. The following radial velocity image shows movement of the bird and insect targets from the previous base reflectivity image. From the radar tutorial remember that the green hues in the scale represent targets moving toward the radar, and the red hues in the scale represent targets moving away from the radar. Notice the line of grayish hues representing velocities close to zero that divides the green and red hues. Think of this line as an axis perpendicular to the direction of target movement. In this case see that most targets are moving south and south-southeast at 30-50 knots (the brightest greens and reds).

kts. ND -64 -50 -36 -26 -20 -10 -1 0 10 20 26 36 50 64 RF Radial Vel 124nm (Elev 1)=0.5 deg 1 km GSP: Greer SC 34.88N 82.22W 10/18/99 00:43 UTC Clear Mode VCP 32 Max: -71 kts +58 kts

dBZ Birds /km3 ND -28 -24 -20 -16 -12 -8 -4 0 4 58 8 65 12 81 16 123 20 227 24 489 28 1148 Radial Vel 124nm (Elev 1)=0.5 deg 1 km Greer SC 34.88N 82.22W 09/12/99 23:42 09/13/99 00:42 Clear Mode VCP 32 Max: 47 dBz

a NEXRAD Volume Coverage Pattern (VCP 21, precip mode)

   Birds migrating at night are typically most dense around 1500 ft (~500 m) above the ground. Some may fly higher, up to 10 or 20000 ft.

Typical distribution of migrants with altitude

   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.

   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 image of birds overflying the Ft. Polk, Louisiana area.

dBZ Birds /km3 ND 5 59 10 71 15 109 20 227 25 602 30 1788 35 40 45 50 55 60 65 70 75 Base Ref 124nm (Elev 1)=0.5 deg 1 km POE: Ft. Polk LA 31.16N 92.98W 04/22/99 17:41 Precip VCP 32 Max: 43 dBz

   The reflectors nearest the station are "ground clutter" caused by RADAR returns from objects on the ground such as trees, buildings and even vehicles.

Recent Publications in Radar Ornithology

abstract Gauthreaux, Sidney A. Jr. 1992. The use of weather radar to monitor long term patterns of trans-Gulf migration in spring. In: J. M. Hagan and D. W. Johnston (eds.) Ecology and conservation of neotropical migrant landbirds. Pp. 96-100. Smithsonian Institution Press, Washington, DC. citation Gauthreaux, Sidney A. Jr. 1995. Radar Ornithology: Tracking Bird Migration by Radar. Wildbird 9(3): 38-39. abstract Gauthreaux, Sidney A. Jr. 1996. Neotropical migrants : Definition, status, trends, and their relationship to the Gulf of Mexico. pp 18-26. In: Proceedings: 15th Annual Gulf of Mexico Information Transfer Meeting, December 1995. U.S. Dept. Of the Interior, Minerals Management Service, Gulf of Mexico OCS Region, New Orleans, LA. OCS Study MMS 96-0056. citation Gauthreaux, Sidney A. Jr. and Kevin R. Russell. 1996. Monitoring purple martin roosts with weather radar. Purple Martin Update 7(1):2-6. citation Leshem, Yossi and Sidney A. Gauthreaux, Jr. 1996. Proposal to develop a global network to predict bird movements on a real-time and daily scale by using radars. Bird Strike Committee Europe BSCE-23/WP 50. London, May 13-17, 1996. citation Gauthreaux, Sidney A. Jr. and Jesus Haro. 1997. WSR-88D VAD wind profile data influence by bird migration over the southwest United States. NOAA Technical Memorandum NWS WR-244. U.S. Dept. Of Commerce, pp.1-10. abstract Russell, Kevin R., David S. Mizrahi and Sidney A. Gauthreaux Jr. 1998. Large scale mapping of Purple Martin migratory roosts. Journal of Field Ornithology 69:316-325. abstract Gauthreaux, Sidney A. Jr. and Carroll G. Belser. 1998. Displays of Bird Movements on the WSR-88D: Patterns and Quantification. Weather and Forecasting 13:453-464. abstract Gauthreaux, Sidney A. Jr., David S. Mizrahi, and Carroll G. Belser. 1998. Bird Migration and Bias of WSR-88D Wind Estimates. Weather and Forecasting 13:465-481. citation Gauthreaux, Sidney A. Jr. and Kevin R. Russell. 1998. Weather surveillance radar quantification of roosting purple martins in South Carolina. Wildlife Society Bull. 26 (1):5-16. citation Mizrahi, David S., Kevin R. Russell, and Sidney A. Gauthreaux Jr. 1998. Mapping Purple Martin Roosts Across the Eastern United States Using National Weather Service Radar, WSR-88D. Purple Martin Update 8(2):26-29. abstract Russell, Kevin R. and Sidney A. Gauthreaux Jr. 1999. Spatial and temporal dynamics of a purple martin pre-migratory roost. Wilson Bull. abstract Gauthreaux, Sidney A. Jr. and Carroll G. Belser. 1999. Bird Migration in the region of the Gulf of Mexico. The Ostrich. Proceedings of the International Ornithological Congress XXII, Durban, South Africa, 16-22 August 1998 citation Gauthreaux, Sidney A. Jr. and Carroll G. Belser. 1999. Reply to Black and Donaldson (1999), 'Comments on "Displays of Bird Movements on WSR-88D: Patterns and Quantification."' Weather and Forecasting 14:1041-1042.