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Stratocumulus
This is the most common cloud type in Britain and is composed
entirely of liquid water droplets. Because the droplets are very
small (around 10-20 microns in diameter), they tend to give a fairly
low radar signal.
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Stratocumulus with drizzle
If stratocumulus is more than a few hundred metres thick, then larger
`drizzle' drops (around 100-200 microns in diameter) can grow. These
give a larger return to the radar, but usually evapourate before they
reach the surface.
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Altocumulus
This mid-level cloud occurs below freezing and is typically composed
of both ice crystals and supercooled liquid water droplets. The radar
signal is usually dominated by the larger ice crystals, which tend to
be several hundred microns across.
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Cirrus
Cirrus clouds are composed purely of ice crystals and in radar images
are characterised by their classic `fallstreak' structure.
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Rain
Frontal or `stratiform' rain looks like this. Ice crystals nucleate
high in the atmosphere and grow as they fall. At the melting level
(2.4 km in this example) the radar reflectivity at increases sharply
because the dielectric constant of liquid water is higher than that
of ice. The effects of attenuation by rain are also visible as
vertical swaths of anomalously cloud-free air above the regions of
highest rainfall rate, but this is only really a problem for
high-frequency radars such as the 94 GHz Galileo.
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Showers
When the rainfall is generated by convection the rainfall rate is
much more variable. The melting level is visible at 2 km in this
example.
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Insects
On hot summer days insects are often visible in the lowest 1 or 2 km.
They can often be seen carried up from the surface in convective
plumes, and in this example have formed a layer at around 800 metres
(lidar observations confirmed that no cloud was present on this
occasion). In the tropics insects are much more of a problem for
cloud radar.
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Glitched data
You may have spotted occasions when the radar data looked like this -
it's not a real cloud at 9 km, but occasionally the radar detector
gets out of synch with the transmitter, and the transmit pulse
appears at 9 km (as well as cirrus echos `folding' and appearing at
low levels). We are hoping to move to a new data acquisition system
in the next few months which should cure this problem.
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