Radar Glossary
- Azimuth:
- A direction in terms of the 360° compass; north at 0°, east at 90°,
south at 180°, and west at 270°.
- Clear-air echos:
- Radar returns from cloud- and precipitation-free (optical clear) air.
Clear-air echos are caused by Bragg scatter and insects. Bragg scatter comes
from small-scale fluctuations of the refractive index (i.e. turbulent
fluctuations of humidity).
- Depolarisation ratio (LDR):
- Ratio between the reflectivity received at vertical
polarisation, but transmitted with horizontal polarisation, and the reflectivity
at horizontal polarisation. Depolarisation of the horizontal polarisied pulse is
caused by asymmetric particles which are not aligned horizontal. Depolarisation
of rain is normally very small (app. -40 dB). It is high (-20 to -10 dB) for
melting snow and water coated hail or graupel.
- Differential reflectivity (ZDR):
- Ratio between the reflectivity of a horizontal polarised
pulse and the reflectivity of a vertical polarised pulse. ZDR depends on the
asymmetry of the shape, the orientation and the falling behaviour of the
particles. ZDR is positive for oblate raindrops, zero or slightly negative for
hail and graupel. Note that ZDR is strongly biased by differential attenuation
during the passage of the radar pulse through heavy rainfall.
- Doppler velocity (v):
- Reflectivity-weighted average velocity of targets in the pulse volume.
Determined by phase measurements from a large number of successive pulses. Also
called radial velocity. Gives only the radial component of the velocity vector.
It is generally assumed that raindrops and other particles are advected with the
wind and have no own motion except their falling velocity. Motion towards the
radar are negative (blue colours), motions away from the radar are positive (red
colours).
- Elevation:
- The vertical pointing angle of the antenna; 0° is horizontal, 90°
is vertical.
- PPI (Plan Position Indicator):
- Display on which radar signals are shown in plan position with range and
azimuth angle displayed in polar coordinates, forming a
map-like display.
- Range bin:
- Discrete element along a single radial of radar data at which the received
signals are sampled. Range bins from POLDIRAD data are spaced at multipes from
150 m intervals. See also sample volume.
- Reflectivity factor (Z):
- Integral over the backscatter cross-section of the particles in a pulse
volume. For particles small compared to the wavelength the scatter cross-section
is D6, where D is the diameter of the particle. Radars are
calibrated in the way to give directly (assuming the dielectric constant of
water) the reflectivity factor from the received backscattered energy. Units for
the reflectivity factor are mm6 m-3 or the logarithmic
value of this in dBZ.
- RHI (Range Height Indicator):
- Display on which radar signals are shown with height as the vertical axis
and range as the horizontal axis, forming a vertical cross section of a cloud or
precipitation system.
- Sample volume or pulse volume:
- The volume in which the radar data for one range bin are
measured. Defined by the width of the radar beam (app. 1°) and half the
length of the transmitted pulse. 1 µs pulses are 150 m deep, 2 µs
pulses are 300 m deep.
- Spectral width (w):
- A measure of the dispersion of velocities within the pulse volume. Standard
deviation of the velocity spectrum. Spectral width depends among others from the
turbulence within the pulse volume.
This glossary is based on the glossary of Ronald Rinehart's book "Radar for Meteorologists".
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