Beam Filling and Reflectivity Differences

All things being equal, the signal return from a sample volume at a distant range will return less power than it would if it was located closer to the radar. The power density is lower at long ranges due to the increasing size of the pulse volume. The simplified radar equation, Equation (4), accounts for this with the R2 term in the numerator, which acts to normalize the reflectivity (Z). This prevents weak

Equation (4)

storms in close from appearing stronger than storms at great distances. However, this normalization of reflectivity values is only valid when the beam is completely filled with the corresponding drop-size distribution.

In Figure 4, two radars are sampling the core of a thunderstorm at different ranges. The drop-size distribution is such that the actual reflectivity is 60 dBZ. Notice that along the dotted line passing through both beams and the high reflectivity core, only "Beam B" is completely filled with 60 dBZ. Due to the longer range, "Beam A" is larger and, therefore, contains the 60 dBZ core plus weaker echoes surrounding it. The result is that there will be an averaging down of the 60 dBZ echo returns such that radar "A" will display a reflectivity value less than 60 dBZ at the range indicated by the dotted line. Lack of complete beam filling is oftentimes the reason why two radars will display different reflectivity values at the same altitude.

Figure 4: Example of beam filling differences. Radar beam "A" is wider along the dotted line than beam "B". The result is incomplete beam filling for beam "A" and reflectivity less than 60 dBZ being displayed. Beam"B" is completely filled and 60 dBZ will be displayed at the range indicated by the dotted line.

Next Page: Practice Exercise #2
or continue on to: Section 3: PRF, Rmax and Vmax