A Recent Example of Doppler Weather Radar's Limitations

After the recent tornado in Gainesville, Georgia, it was stated that the tornado was not detected by Doppler radar. Many questions arose during a spotter training session as to how and why Doppler radar could fail to detect a tornado. The fact is that Doppler radar has range limitations. One limitation results from the Earth's curvature and the other pertains to the widening of the beam with increasing distance from the radar tower. With the first limitation in mind, as the radar beam moves away from the radar antenna it does not follow the curvature of the Earth. As a result, the radar beam scans higher in the atmosphere as the distance from the radar increases. What does this mean to the radar operator? If there are two similar storms with one located 30 miles from the radar antenna and the other at 70 miles, the radar gets two very different pictures. If both storms have a tornado signature at an altitude of 5,000 feet, then the radar may have no problem detecting the tornado signature in the storm at 30 miles but will likely miss or "overshoot" the signature in the storm at 70 miles (figure 2). The second limitation results from the fact that as the radar beam moves away from the antenna the beam becomes progressively wider. This produces what is known as an aspect ratio problem. The result is that a tornadic circulation may not be detected because it is either too large or too small compared to the beam width at a given distance (figure 1). It is because of these limitations that you, the spotter, remain a VITAL part of the warning decision process. Those spotters who live beyond 50 miles away from the Doppler radar tower in Jasper County, South Carolina should keep this in mind for the purpose of recruiting new spotters. The greater the number of spotters we have beyond 50 miles, the more effective the National Weather Service will be in issuing timely warnings for severe weather.


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