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How NEXRAD sees the atmosphere


Look! Is it a bird? Is it a plane? Is it rain?
The clues about birds

HOW NEXRAD SEES THE ATMOSPHERE

     NEXRAD has greatly improved the detection of meteorologic events such as thunderstorms, tornadoes and hurricanes. When rain, snow, or hail intercepts a radar's EM pulses, they can scatter considerable amounts of energy. The amounts of returned energy, an indication of the density of targets, and the location of the targets reflecting the energy, are used to assemble a radar image.
     Below is a NEXRAD "base reflectivity" image from 20 Sept. 2001 at 2259 EDT (21 Sept., 2001 Greenwich Mean Time [GMT]). The image shows extensive precipitation in eastern PA, NW NJ, Long Island and southern NY. The location of the colored radar echoes indicates where the rain is in the atmosphere, and the various colors indicate rain density. 
     Target density, in this case, rain density, is measured in decibels (dBZ), a relative scale used to represent other forms of wave energy, like sound. The color scale and corresponding dBZ values are shown in the information bar on the right side of the image. 
     Regions of light and dark blue (5-10 dBZ) show areas of lighter precipitation, while areas of orange and red (45-50 dBZ) are strong, to occasionally severe thunderstorms.
     The image has a "blocky," irregular appearance, typical of precipitation. Also, notice the rapid changes in reflectance values over short distances. This is characteristic of rain events, especially when they contain embedded thunderstorms. 

LOOK! IS IT A BIRD? IS IT A PLANE?
IS IT RAIN?

     Each NEXRAD can operate in two modes, "Precipitation" or "Clear Air." The radar operates in "Clear Air" mode when precipitation is not detected in its scanning area. 
     The radar is very sensitive in this mode, and can detect very small targets, such as pollen, smoke and dust. Aerial biota like insects, birds and bats also reflect the radar's electromagnetic energy, though not usually the magnitude that rain can. Maximum values for birds and insects are usually 30-35 dBZ.
     The reflectivity image below from 11 May, 2001 at 0001 EDT (0401 GMT) shows the radar in "Clear Air" mode. It shows intense nocturnal bird migration well under way over western Long Island and over most of NJ. 
     The information plate on the right side of the image shows dBZ values and corresponding birds/km3 values. Dr. S. A. Gauthreaux of Clemson University, used counts of nocturnal migrants observed flying across the disk of the full moon to produce a conversion from dBZ to birds/km3. Each pixel on the image represents a volume of space equal to one cubic kilometer. 
     Interestingly, NEXRAD cannot distinguish between different types of precipitation, or even if targets are precipitation, birds, insects, or dust. Reflectivity and other NEXRAD images however, contain many clues about the nature of radar echoes. 

THE CLUES ABOUT BIRDS

     (1) The image above shows that the radar is in Clear Air mode, so the echoes are probably not rain. Note however, that birds and other biota can be detected when the radar is in either mode, and that rain is sometimes present even when the radar is still in Clear Air mode.
     (2) The image above shows a fine stippling pattern typical of birds and insects, in contrast to the "blocky" appearance of precipitation. 
     (3) The transitions between high reflectivity areas at the center, and low reflectivity areas of towards the outer boundary, are smooth and somewhat circular. This pattern is indicative of bird migration.
     (4) Maximum values observed during bird or insect migration are usually no higher than 30-35 dBZ. 
     (5) We also use speed and direction of target movement to discriminate between birds and other entities in the atmosphere. Remember that NEXRAD is a Doppler radar system. For every base reflectivity image NEXRAD produces, a corresponding "base velocity" image is generated that represents target speed and direction. 

     The image below is the base velocity image corresponding to the reflectivity image shown above. Green/blue hues in the scale represent targets moving toward the radar (negative values, measured in knots). Orange/red hues represent targets moving away from the radar (positive values). 
     Notice the line of grayish green and red hues representing velocities close to zero that divides the bright hues. Think of this line as an axis perpendicular to the direction of target movement. In this image, we can see that most targets are moving NE at 20-36 knots, light blue and dark greens toward the station, orange and red away from the station. On that evening, winds measured by weather balloon were blowing from the NW at 8 knots. 
     Birds typically fly at least 10-15 knots faster than wind speeds, and not necessarily downwind. On the other hand, insects, precipitation, and other aerial entities, are wind-blown. They generally move at the same speed and in the same direction as the wind. Targets moving at least 10 knots faster than the prevailing wind, or moving in crosswind or headwind conditions, are almost always birds.