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)

We are concerned with two types of scattering, Rayleigh and non-Rayleigh. Rayleigh scattering occurs with targets whose diameter (D) is much smaller (D < /16) than 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.

Nearly all hailstones are non-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.

Next Page: Practice Exercise #1
or proceed on to: Section 2: Radar Equation and Reflectivity