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May-June 2012

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Weather Queries

What is the “three-body scatter spike” or “hail spike” seen on weather radar displays and sometimes mentioned by television meteorologists when severe thunderstorms are in the area?

Steve Olson

Salina, Kansas

The United States NWS operates a network of WSR-88D Doppler radars. On a standard WSR-88D radar display portraying intense thunderstorms, with the antenna usually pointing a few degrees above horizontal, a hail spike will sometimes appear. The hail spike, sometimes also called a flare echo, is invariably seen in conjunction with a very strong radar echo, that is, one with very high reflectivity, most often observed at altitudes of four to eight kilometers (13,000 to 26,000 feet) above ground. The spike itself has much lower reflectivity than the very strong echo, begins at or beyond the range of the strong echo, and lies along the same radials that pass through the high-reflectivity core. Reflectivity along the hail spike decreases with increasing range from the radar. The spike is most evident when there is no significant precipitation beyond the range of the strong echo. The hail spike is considered a sufficient, but not necessary, condition for the occurrence of large hail; it has some predictive value, because the appearance of the spike often occurs shortly before damaging hail reaches the ground.

Figure 1 shows a hail spike southwest of the Topeka, Kansas, WSR-88D radar. The radar location is at the black circle near the top of the image. The elevation angle of the antenna is 6.4 degrees. A very strong echo lies about 20 miles southwest of the radar near the center of the image. At this range, the high-reflectivity core is roughly four kilometers (13,000 feet) above ground. Notice several contiguous pixels colored purple in the figure. These represent an equivalent reflectivity factor greater than 70 dBZ (decibels) as shown by the color bar at the top. Directly beyond this high reflectivity core and along the same radials, there is a slowly narrowing spike of reduced reflectivity, mostly 5-25 dBZ, that extends out into the blackness, where there is no significant reflectivity. This is the hail spike. It is an artifact and does not represent precipitation falling at ranges beyond the potent storm at the center of the image. Why, then, do we see it?

Caption: Figure 1. A portion of a scan at 6.4° elevation angle, taken by the WSR-88D radar at Topeka, Kansas, at 9:09 a.m. CDT, June 2, 2008. A severe storm with large hail is at the center of the image. The core of maximum reflectivity is shown by the four contiguous purple pixels. Along the radials through this core, a hail spike extends toward the southwest. The width of the spike and the reflectivity values decrease with increasing range, which is typical.

Caption: Figure 1. A portion of a scan at 6.4° elevation angle, taken by the WSR-88D radar at Topeka, Kansas, at 9:09 a.m. CDT, June 2, 2008. A severe storm with large hail is at the center of the image. The core of maximum reflectivity is shown by the four contiguous purple pixels. Along the radials through this core, a hail spike extends toward the southwest. The width of the spike and the reflectivity values decrease with increasing range, which is typical.

Figure 2 helps explain the hail spike. The radar is at the lower left corner of the diagram. The radar emits pulses of electromagnetic energy at 10-centimeter wavelength along the line marked R. The precipitation echo, nearly 13 kilometers high, is outlined by the heavy, solid contour. Within that echo is a core C of very high reflectivity. Researchers have confirmed that such high reflectivity is caused by large hail–hail probably coated by liquid water or spongy hail, the outer shell of which is slushy ice, soaked with liquid. The presence of liquid increases the reflectivity markedly over what would be observed with dry hail of the same shape and size. The energy of the radar pulse scatters (bounces) off the hail, and some of the energy is sent back to the radar. The emitted and backscattered (reflected) energy both travel at the speed of light, and so the distance from the radar to the hail core C and back is easily determined from the total travel time. Half the total distance is the range of the hail core from the radar. In Figure 1, the range was approximately 20 miles, as noted above.

Caption: Figure 2. Diagram of the radar signal (pulse) path responsible for the three-body scatter spike (TBSS). See text for details. This figure last appeared in Lemon, L.R., 1998: The Radar “Three Body Scatter Spike”: An Operational Large-Hail Signature, Weather and Forecasting, 13, 327–340.

Caption: Figure 2. Diagram of the radar signal (pulse) path responsible for the three-body scatter spike (TBSS). See text for details. This figure last appeared in Lemon, L.R., 1998: The Radar “Three Body Scatter Spike”: An Operational Large-Hail Signature, Weather and Forecasting, 13, 327–340.

Because the radar wavelength and the hail size are roughly comparable, a special kind of scattering of the radar pulse occurs–Mie scattering, named after Gustav Mie, one of the scientists who first examined this phenomenon. The hail core scatters some energy in all directions, not only back toward the radar. Some of this energy is directed at the ground. The energy of greatest importance for generating a hail spike is included in the cone-like shape extending down from the hail core. The height of the hail core above ground is h. The distance from the hail core to the edge of the oval area at the ground is r. The wet ground redirects some of the energy received from the hail core back toward the sky. A small fraction of this energy reaches the hail core whence it came, and a fraction of that bounces back to the radar.

The technical name for a hail spike is three-body scatter spike (TBSS), because the energy pulse originating at the radar is scattered three times before it returns to the radar: by the hail core, by the ground, and by the hail core a second time. Each time the pulse is scattered, the energy density in the pulse is greatly diminished. By the time it gets back to the radar after three scatterings, it is many orders of magnitude weaker than when it was emitted. Were it not for the very high reflectivity of the water-soaked hail, there would not be enough power left to measure by the time the emitted pulse returned to the radar.

Consider the pulse energy in a TBSS that takes the shortest path out from the radar and back. It travels a distance R to the hail core, h to the ground, h back to the hail core, and R back to the radar. When this part of the pulse is mapped on a radar display, it will appear at range R + h, as shown in Figure 2. If R + h lies beyond the range of the precipitation echo, there will be space between it and the beginning of the hail spike. That is the case in Figure 2, but not in Figure 1.

Consider the pulse energy that takes a longer path out from the radar and back. The diagram assumes that energy backscattered for angles smaller than θr will be too miniscule for the radar to measure. Thus the longest path taken by pulse energy that is included in a hail spike is R + r + r +. So the far end of the hail spike is at range R + r.

In the previous issue (March/April 2012), this column focused on a major upgrade of the WSR-88D radar, now in progress: the incorporation of dual-polarization capability. Dual-polarization sensing will detect three-body scattering much more readily than the original 88D radar. Values of differential reflectivity are quite high (greater than 2.5 dB) for large, rain-soaked hail; values of the correlation coefficient are unusually low (near 0.8) for giant hail. (See the previous issue for explanation of these terms.) Three-body scattering depicted on dual-polarization radar products will normally overwhelm any signal from precipitation beyond the hail-producing storm and in the same direction from the radar, thereby isolating the spike on the radar display.

The most tell-tale sign of a TBSS with dual-polarization radar is a very low (< 0.60) correlation coefficient.

I thank Paul T. Schlatter, NOAA Coordination Officer, National Weather Service, for help in answering this question.

I observe precipitation daily for the Community Collaborative Rain, Hail, and Snow (CoCoRaHS) network. Does mist count as a trace of precipitation, even though it was barely enough to moisten the windshield on the car and left nothing in the gauge? This was definitely not fog.

Peter Boddie

Littleton, Colorado

If you can see mist collecting on the windshield, it's falling, and any droplets falling to the ground count as precipitation, in your case, a trace. The Glossary of Meteorology, published by the American Meteorological Society, defines precipitation as “all liquid or solid phase aqueous particles that originate in the atmosphere and fall to the earth's surface.” This excludes dew and frost, which form on the ground. The Glossary considers mist and drizzle to be equivalent, namely, the droplet diameter for both ranges from 0.2 to 0.5 millimeters. This size range is somewhat arbitrary, in that droplets smaller than 0.2 millimeters are observed to fall. Cloud or fog droplets lie in the size range from a few micrometers to many tens of micrometers (1,000 micrometers = 1 millimeter). These droplets remain suspended in the air; the larger ones do so because of an updraft. At night it is especially easy to tell whether the droplets are falling. Just shine a flashlight straight up and observe the droplets. If they are suspended or moving about randomly, it's fog and doesn't count as precipitation. If they are slowly falling in unison, it's definitely precipitation (drizzle or mist).

Did you know that it's now easier than ever to submit questions to this column? Just use our new email address: weatherqueries@gmail.com. If you attach photos, please be sure that they have a resolution of at least 300 dpi. Photos taken with 5-megapixel or higher digital cameras will normally satisfy this requirement. Also be sure to include your name and the city or town where you live (or the nearest town if you live in a rural area). Weatherwise cannot answer questions in print without this information.

Weatherwise Contributing Editor THOMAS SCHLATTER is a retired meteorologist and volunteer at NOAA's Earth System Research Laboratory in Boulder, Colorado. Submit queries to the author at weatherqueries@gmail.com, or by mail in care of Weatherwise, Taylor & Francis, 325 Chestnut St., Suite 800, Philadelphia, PA 19106.

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