Beam Blocking Spreading and Overshooting What happens to

Beam Blocking Spreading and Overshooting • What happens to the beam as it moves away from the radar.

Beam Blocking • This is the blocking of portions of the scan by nearby objects • All or some of the beam may be blocked, leading to no/lower returns • May change from day to day owing to changes in atmospheric refractivity • Blocked zones are soon identified through repeated examination of the output

Beam Spreading • Because it has a finite angular width the dimensions of the beam increase with range.

Beam Spreading • Beam spreading is noticeable on the standard displays – pixel size increases with range

Partial Beam Filling • As a result of beam spreading, often, at large range, the pulse volume is not completely filled with precipitation. • This is called partial beam filling • Thus, small-scale, intense cells at large range will appear weaker and their power averaged over a larger area on the display

Overshooting • The beam will also overshoot some targets, especially at large range • This limits the useful range at a particular tilt angle/low level CAPPI • 2. 0 km CAPPI becomes PPI surface at range greater than 135 km! Thus cell moving beyond 135 km may have appearance of weakening when in fact it is not • Impacts RDSS, VIL

Summary • Understanding what happens to the beam as it moves away from the radar and hits “cumulo- granite” are all important to properly interpreting radar.