DBZ (meteorology)

Values above 20 dBZ usually indicate falling precipitation, but can also mean sleet, hail, or snow.

Principle

The radar reflectivity factor (Z) of precipitation is dependent on the number (N₀) and size (D) of reflectors (hydrometeors), which includes rain, snow, graupel, and hail. Very sensitive radars can also measure the reflectivity of cloud drops and ice. For an exponential distribution of reflectors, Z is expressed by:

${\displaystyle Z={\int }_0^{Dmax}{N}_0{e}^{-\mathrm{\Lambda }D}{D}^6\mathrm{d}D}$

As rain droplets have a diameter on the order of 1 millimetre, Z is in mm⁶m⁻³ (μm³). By dividing Z with the equivalent return of a 1 mm drop in a volume of a meter cube (Z₀) and using the logarithm of the result (because the values vary greatly from drizzle to hail), one obtains the logarithmic reflectivity L_Z, in dBZ:

${\displaystyle L_Z=10{\log }_{10}\frac{Z}{Z_0}}$

dBZ values can be converted to rainfall rates (R) in millimetres per hour using the Marshall-Palmer formula:

${\displaystyle \mathrm{R}={\left(\frac{{10}^{(dBZ/10)}}{200}\right)}^{\frac{5}{8}}}$