Radar Images

Radar Images

Radar Image Interpretation – some points

  1. The intensity of echoes tends to decrease with increasing distance from the radar, because:The radar beam widens with distance, therefore the volume of the beam that is filled with rain is decreased – this reduces the echo intensity.The radar beam becomes attenuated, that is it loses power slightly when passing through very heavy rain, thus reducing the echo intensity further out from the radar. It is essential to keep these limitations in mind when interpreting the outer parts of the radar images.
  2. The radar may sometimes detect faint echoes from targets that are not rain, hail or snow, such as aircraft, swarms of insects, flocks of birds or even the ground or the sea surface (see text above).
  3. A weak echo may not mean that it is raining at the ground because under some certain conditions light rain can evaporate completely before reaching the ground.
  4. The intensity of very light rain or drizzle may be underestimated because of the absence of large droplets.

Radio Detection and Ranging

In RADAR (Radio Detection and Ranging), radio pulses are transmitted, in a highly focused beam, by a radar antenna and are reflected off objects and returned to a receiver at the antenna.

The radar’s computer determines the direction of the rain from the azimuth (orientation) of the antenna, and the distance to the rain from the time taken for the radar signal to return to the receiver.

The “reflecting power” of the rainfall (which depends on the size of the raindrops and their concentration) is also calculated, thus providing an estimate of rainfall intensity unit used is the dBz.

In short the display produced by the radar’s computer gives a horizontal map of where rain is falling and an estimate of how heavily it is falling.

This particular radar image shows the rate of rainfall in millimetres per hour of the ‘echoes’ produced.

On the right hand side, underneath the date and time – which is in UTC (known also as GMT) and which is one hour behind Malta Winter Time, and two hours behind Malta Summer Time, one finds a table showing the rate of rainfall. The bright red means that rain is falling at the rate of 100 mm per hour, and the darkest blue means that the rain is falling at the rate of 0.1mm per hour.

One has to bear in mind that clouds move and this means that in one hour 30 mm of precipitation is spread over a very large area. It is only if the cloud stops completely over one area that all the 30mm fall into that area.

Coloumn Max:  A display, in plan, of the maximum value of the reflectivity above each point at different altitudes of the area being observed, consequently, it is the maximum of PPIs (Plan Position Indicator) in the individual point.

Buildings and hills in the vicinity of the radar produce what is known as “ground clutter” on the radar image which is highlighted by a dotted circle. This results from the radio energy that is reflected back to the radar from these targets. To remove these unwanted reflections a filter is applied but it may also remove reflections from rain in the same area if these rain reflections are weaker than the ground clutter reflections.

Under highly stable atmospheric conditions, like calm and clear nights or when there is a warm southerly wind blowing, the radar beam can be refracted almost directly into the sea at some distance from the radar, resulting in an area of intense looking echoes. This “anomalous propagation” phenomenon detects “sea return”, a phenomenon similar to ground clutter except that the echoes come from the sea surface.

The radar does not scan directly overhead. Therefore, close to the radar site, data is not available due to the radar’s maximum tilt elevation of 60 degrees. This area is commonly referred to as the radar’s “Cone of Silence”.