Interpreting Radar Images - Frequently Asked Questions
2. I measured 20 mm of rain between 7 pm and 8 pm, but the radar imagery only showed a rainfall rate of 2 mm per hour. Why the difference?
3. The radar shows that it is raining , but it is dry where I am.
4. My area of interest is covered by two (or more) radars. However, the echoes from the different radars show quite different shapes and rainfall intensities.
5. The radar is showing some echoes that don't look like rain.
6. Areas of widespread rain appear and disappear on the 512km image. Why is that?
1. It is raining where I am but no rain is showing on the radar.
 The effects of the curvature of the earth on weather radar. |
1.1 The intensity of echoes tends to decrease with increasing distance from the radar. This is because:
i). The radar beam broadens with distance, thus decreasing the proportion of the beam which is filled with rain, which reduces the echo intensity;
ii). The radar beam becomes
further from the ground with distance
(partly because of the Earth's curvature, and partly because the beam
is angled upwards by a fraction of a degree), thereby missing the lower
parts of the rain. A horizontal radar beam detects raindrops at a
height of 1 kilometre above the Earth's surface from rain that is 100
kilometres away from the radar. It detects raindrops at a height of 3
kilometres from rain that is 200 kilometres away, and at a height of
around 7 kilometres at a distance of 500 kilometres from the radar. In winter especially,
the rain clouds can be below the radar beam at a distance of more than
200 kilometres from the radar, and hence the radar beam will overshoot the rain.
As a result, the radar image will not show any rain even though at the
ground level it may be raining at the time.
Due to the reasons described above, interpreting the radar images at
extended ranges can be difficult and you should take great care in
using these images. With practise, however, you will find them useful.
iii). The beam can lose power slightly when passing through very heavy rain, thus reducing the echo intensity further out from the radar.
Thus precipitation that is occurring some distance away from the
radar might not show at all, or may show with a reduced intensity. The
presence of significant echoes at large range probably indicates the
presence of large amounts of rain at high levels above the ground (e.g.
a thunderstorm). At these distances the radar echoes are likely to be
reflections caused by ice rather than rain drops, where the
relationship between reflectivity and rainfall rate is different.
1.2 The presence of
mountains within the range of the radar can block
part or whole of the radar beam, thus significantly reducing the echo
intensity from rain on the other side of the mountains.
1.3 Because of the variations in rain closer to the radar site and in the refractive index of the air, attempts to correct for these limitations have not been very successful. Consequently the estimation of rainfall rates using the radar imagery should be used only as a very rough guide.
2. I measured 20 mm of rain between 7 pm and 8 pm, but the radar imagery only showed a rainfall rate of 2 mm per hour. Why the difference?
The radar reflectivity is strongly dependent on the diameter of raindrops in the cloud not the amount of rain drops and therefore rainfall rates. Tropical maritime rainfall consists of very many moderate sized raindrops so that the reflectivity is much less than for similar rainfall rates in continental area rain clouds. The later rain clouds typically consist of very large raindrops but much less in number.
3. The radar shows that it is raining , but it is dry where I am.
The radar may sometimes detect faint echoes from non-precipitation
targets such as aircraft, areas of smoke/ash from large fires, swarms
of insects, flocks of birds or even the surface (when unusual
atmospheric conditions bend the radar beam back down to the surface!).
Permanent echoes are created when the radar beam reflects off ground
features and buildings, normally within about twenty kilometres of the
radar site, but mountain ranges further away can sometimes generate
permanent echoes.
Remember that due to the Earth's curvature, the radar beam becomes
higher above the ground the further it travels from the radar. Thus a
weak echo may not mean that it is raining at the ground because under
some circumstances light rain aloft can evaporate completely before
reaching the surface.
The intensity of drizzle may be underestimated because of the lack of large droplets.
4. My area of interest is covered by two (or more) radars. However, the echoes from the different radars show quite different shapes and rainfall intensities.
The main reasons for differences include the different distances and angles from the radar transmitters, the presence of topography and differences in the frequency and angle of the radar beam. Also the collection period of the two radars may well have been different.
5. The radar is showing some echoes that don't look like rain.
Sometimes the refractive index of the air is such that the radar
beam
becomes "bent" and reflects the ground or ocean surface some distance
away from the radar. This is known as Anomalous Propagation and occurs
usually when there are strong temperature inversions present.
Electronic processing of the returned signal usually detects the
steadier reflections from ground clutter, but reflections from the
waves on water are more likely to appear like true rain echoes.
Near sunrise and sunset the radar antenna momentarily scans the sun. On
occasions this can be seen as a pencil line radiating out from the
centre of the image in the direction of the sun.
6. Areas of widespread rain appear and disappear on the 512km image. Why is that?
The 512km range images are unique as they are a mosaic of all available radar images and sometimes several radars may contribute to the overall view. Some radars in the Bureau network are windfinding radars, which means that there are certain times of the day in which they do not provide input to the 512km radar views (click here for more information about the routine windfind time periods). As a result, when widespread rain is evident in the region it can sometimes disappear from the radar image between one frame and the next. This happens because the nearby radar has switched to windfinding mode. Similarly rain can sometimes appear after the windfinding radar switches back to its normal mode. The Bureau's windfinding radar sites are shown on the radar home page map as blue squares.