Interpreting Weather Radar Information |
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Weather radar is a remote sensing instrument using microwave
energy between X-band (3 cm wavelength) and S-band (10 cm
wavelength). A short pulse of high power microwave energy
is produced by a magnetron in the transmitter system and this
energy is focused by an antenna system into a narrow beam.
This pulse of energy travels through the atmosphere at the
speed of light (3 x 10 8 ms-1). When
a target such as a raindrop is encountered, some of the energy
is scattered of which a minute fraction is in the direction
back to the antenna system were a sensitive receiver system
is used to process and amplify this received power into useful
data. From the azimuth and elevation information on the pointing
direction of the antenna, the time between transmitting and
receiving and the power of the received signal, the target
location can be determined as well as its intensity or reflectivity.
dBZ is the unit used we use for reflectivity in meteorology.
dBZ is related to the number of drops per unit volume and
the sixth power of their diameter and it can be related to
rainfall rate through an empirical relationship called a Z-R
relationship.
In the table below a guideline on the interpretation of dBZ
factors are given.
|
dBZ
|
Rain
rate
(mmh -1) |
Comments
|
10 |
~0.2 |
Significant
but mostly non-precipitating clouds |
20 |
~1 |
Drizzle, very
light rain |
30 |
~3 |
Light rain
|
40 |
~10 |
Moderate rain,
showers |
50 |
~50 |
Heavy rain,
thundershowers, some hail possible |
60 |
~200 |
Extremely heavy
rain, severe thunderstorm, hail likely |
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Please take note that various atmospheric and environmental
conditions can negatively affect radar data and caution should
be exercised when interpreting the information. Some of these
effects include:
- returns from mountains and other non-meteorological targets,
- attenuation of the radar signal when viewing weather echoes
through areas of intense precipitation(with C-band radars),
- temperature inversions in the lower layers of the atmosphere
which bend the radar beam in such a way that ground clutter
is observed where normally not expected,
- the "bright band" which is a layer of enhanced reflectivity
caused by the melting of ice particles as they fall through
the OoC level in the atmosphere and which can
result in over-estimation of rainfall.
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