Range-normalized values of reflectivity, Z, can range over many orders of magnitude. To compress this large range of values for operational use, Z is displayed in decibels of Z, that is, dBZ. Converting Z to dBZ is simply done by using

Equation (5)

For example, if Z = 4000 mm⁶m^-3, then dBZ = 10(log₁₀ 4000) ~ 10 x 3.6 = 36 dBZ.

Due to the WSR-88D's sensitivity, reflectivities as low as -32 dBZ can be detected in clear air mode near the RDA. How can there be such a thing as a negative dBZ? If 0 < Z < 1, log₁₀Z < 0 and thus dBZ < 0. Very low reflectivity values indicate the presence of extremely small sized particles (e.g., ash, dust, etc.). The majority of reflectivity below 0 dBZ is the result of refractive index gradients, known as Bragg Scattering. Examples of Bragg Scattering and < 0 dBZ returns would be outflow boundaries, gust fronts, and synoptic scale cold fronts.

The WSR-88D can also detect reflectivity values as high as 95 dBZ. As an example, a one cubic meter volume containing just one 38.3 mm (~1.50 inch) diameter water-coated hailstone would yield a reflectivity value of approximately 95 dBZ. However, severe-sized hail frequently occurs with reflectivities less than 70 dBZ. This is a good indication that such large targets do not meet the Rayleigh approximation.

Table 4: dBZ and Z values

Next Page: Beam Filling and Reflectivity Differences