Quantities measured by the radars
10 May 2011



Quantities measured by the radars


Radar reflectivity factor, Z

A quantity related to the scattered power detected by the receiver. The name is sometimes shortened to 'radar reflectivity' but this is actually a different quantity. It is the basic measure of the calibrated received signal recorded by a radar.

Signal to noise ratio in reflectivity

The signal to noise ratio in Z. It is particularly used with the 1.275 GHz radar where it is useful for showing refractivity features such as convection cells.


The radial velocity of scatterers along the line of sight from the radar to the scatterer.

Spectral width

The width of the spread of velocity. It gives an indication of the degree of turbulence within the measured scattering volume.

Differential reflectivity, Zdr

Differential reflectivity is the difference in the energy returned from horizontally polarised pulses compared to that returned from vertically polarised pulses. It is expressed as a logarithm. Small rain drops are spherical, so scatter equally at both polarisations, meaning Zdr is close to 0 dB. Large raindrops and ice particles are oblate, looking bigger in the horizontal direction than in the vertical, so produce a Zdr greater than zero. Zdr is used with radar reflectivity to estimate rain rate more accurately.

Linear depolarisation ratio, LDR

The linear depolarisation ratio represents the fraction of energy reflected back in the opposite polarisation to that transmitted by a radar. It is usually too small to be measured, but a detectable signal indicates the presence of tumbling non-spherical particles. It is a particularly good indicator of the melting layer.

Differential phase shift, phidp

This is the difference in phase between a horizontally-polarised beam and a vertically-polarised beam. In heavy rain, the large rain drops cause a horizontally-polarised beam to travel slightly slower than a vertically-polarised beam and results in a phase shift between them. In a radar plot it looks like a 'search-light' effect behind regions of heavy rain. In contrast, hail stones are nearly spherical so do not cause a differential phase shift. This helps to distinguish hail storms from heavy rain showers.

  1.275 GHz 3 GHz 35 GHz 94 GHz
Scanning Y N N N
Continuous operation N N Y Y
Radar reflectivity factor Y Y Y Y
Signal to noise in reflectivity Y N Y N
Velocity Y Y Y Y
Spectral width (of velocity) Y Y Y Y
Differential reflectivity Y Y N N
Linear depolarisation ratio Y Y Y N
Differential phase shift N Y N N
Contact: Hooper, David (STFC,RAL,RALSP)