Weather Radars - Data Types - Reflectivity (in Decibel or DBZ)

Reflectivity (in Decibel or DBZ)

Return echoes from targets ("reflectivity") are analyzed for their intensities to establish the precipitation rate in the scanned volume. The wavelengths used (1–10 cm) ensure that this return is proportional to the rate because they are within the validity of Rayleigh scattering which states that the targets must be much smaller than the wavelength of the scanning wave (by a factor of 10).

Reflectivity perceived by the radar (Z_e) varies by the sixth power of the rain droplets' diameter (D), the square of the dielectric constant (K) of the targets and the drop size distribution (e.g. N of Marshall-Palmer) of the drops. This gives a truncated Gamma function, of the form:

|K|2 N₀e D D6dD

Precipitation rate (R), on the other hand, is equal to the number of particles, their volume and their fall speed (v) as:

R = N₀e D (D3/6) v(D)dD

So Z_e and R have similar functions that can be resolved giving a relation between the two of the form:

Z = aRb

Where a and b depend on the type of precipitation (snow, rain, convective or stratiform), which has different, K, N₀ and v.

• As the antenna scans the atmosphere, on every angle of azimuth it obtains a certain strength of return from each type of target encountered. Reflectivity is then averaged for that target to have a better data set.
• Since variation in diameter and dielectric constant of the targets can lead to large variability in power return to the radar, reflectivity is expressed in dBZ (10 times the logarithm of the ratio of the echo to a standard 1 mm diameter drop filling the same scanned volume).