Backscattered Energy

As pulse volumes within the radar beam encounter targets, energy will be scattered in all directions. A very small portion of the intercepted energy will be backscattered toward the radar. The degree or amount of backscatter is determined by target

size (radar cross section)
shape (round, oblate, flat, etc.)
state (liquid, frozen, mixed, dry, wet)
concentration (number of particles per unit volume)

Scattering Regimes

We are concerned with two types of scattering, Rayleigh and non-Rayleigh. Rayleigh scattering occurs with targets whose diameter (D) is small (D < /16) compared to the wavelength of the transmitted E-M energy (Doviak and Zrnic, 1984; Battan, 1973). The WSR-88D's wavelength is approximately 10.7 cm (4.21 in), so Rayleigh scattering occurs with targets whose diameters are less than or equal to about 7 mm or ~0.4 inch. Raindrops seldom exceed 7 mm so all liquid drops are Rayleigh scatters.

Non-Rayleigh scattering occurs when:

Targets have a diameter that is large (D > /16) compared to the wavelength of the radar's transmitted signal (i.e., Mie Scattering) or
Turbulence between air masses with significantly different indices of refraction occurs on scales comparable to (or smaller than) the radar wavelength (i.e., Bragg Scattering).

Targets that are Mie scatters will not produce the linear increase in reflectivity with size that Rayleigh scatters do. Nearly all hailstones are Mie, not Rayleigh, scatterers due to their larger diameters. However, since the vast majority of targets sampled by the WSR-88D are raindrop size or smaller, the Rayleigh assumption is used in all computations of radar reflectivity.

Bragg scattering, while possible with WSR-88D radars, is rare. When Bragg scattering is detected by the WSR-88D, it will generally result in reflectivity below 0 dBZ due to differences in refractive index gradients. Examples of Bragg Scattering and below 0 dBZ returns would be outflow boundaries, gust fronts, and synoptic scale cold fronts.

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