The Flare Echo: Reflectivity and Velocity Signature

Abstract

The characteristics and causes of a radar artifact called a flare echo are described. The spike or flare-shaped echo typically has reflectivities <20 dBZ, and approaching Doppler velocities. It extends radially 10?20 km downrange of some intense radar storm echoes. Zrni? recently proposed a three-body scattering scenario to explain its occurrence, which consists of scattering by the hydrometeors to the ground, backscattering by the ground to the hydrometeors and scattering by the hydrometeors to the radar. In addition he developed relationships that predict the behavior of the flare reflectivities and velocities. The data presented here support Zrni?'s three-body scattering explanation and relationship, indicating that the flare echo power is dependent on the inverse cube of the distance from the large hydrometeors to the ground. The flue Doppler velocities depend on the radial velocity and fall speed of the hydrometeors responsible for producing the flare. However, it was found that Zrni?'s theory did not fully address anomalies observed for scattering paths directly below the large hydrometeors and the contribution of their radial velocities to the flare velocities. In this paper flare echo data from Colorado and Alabama are compared. The Colorado flares are typically more intense, extensive, and longer lasting and are highly likely to be associated with large (≥ 2 cm) hail and can thus be used as a warning signature. However, this use is not transferrable to Alabama storms where surface hail rarely occurs with flare echoes. In fact, there is evidence that large raindrops may sometimes cause the flare in Alabama. The flare echo may cause difficulties for unaware researchers using multiple Doppler techniques to synthesize wind fields. It is also a potential problem for forecasters interpreting the data and computer algorithms searching for velocity features such as downbursts and gust fronts. The flare velocities may prove useful for nowcasting microbursts.