Geometry

Due to the measurement geometry and the scanning angle, the height of the measurements taken from varies heavily.

The figure below illustrates how a single pulse can misinterpret four showers by scanning 1) evaporation below the precipitating cloud, 2) growth of the hydrometeors falling from the cloud, 3) partial beam filling from the upper parts of the cloud or 4) complete overshooting of the beam in respect to the precipitating cloud.

 

Evaporation

In the radar image below the radar on the left (Zaventhem, Belgium) detects precipitation over the Ardennes at high altitude while there is no precipitation near ground as can be seen from the radar image on the right (Wideumont). This effect is caused by evaporation, ie. the radar beam catches the evaporating layer and the precipitation never falls to the ground.

 

Overshooting

Overshooting creates ambiguity on the edges of the radar network. In the figure below you can see that while the radar does see up to the shaded area in Northern Finland, it is overshooting the precipitating cloud for the last 100km and does not "see" the rain at all. This case is typical for winter conditions and a common problem in the higher latitudes.

 

Attenuation

Attenuation of the radar beam was discussed in more detail in the previous modules. The example below shows attenuation in a radar composite from Southern Finland. The two radars on the southern coast line in Vantaa and Anjalankoski (ANJ) see the precipitating cloud very differently because the ANJ radar is attenuated by the heavy rain band over the Gulf.