University of Auckland X-Band Rain Radar Page

PPI or plan position indicator is the scan which first comes to mind when thinking of radar and it is the most important for most radar applications. The dish rotates in azimuth (side to side) at a certain fixed elevation angle. Reflectivity data is found along with range and azimuth information. At sufficiently low elevation angles, this shows the horizontal structure of a rain event. Problems with this scanning procedure are ground clutter, beam blocking, incomplete beam filling, attenuation, beam overshooting and bright band contamination.

Ground clutter is the result of radar returns from non-meteorological targets such as buildings and hills. When being operated in PPI, the ground clutter can be reduced by increasing the elevation angle so that less of the beam contacts the ground. Increasing the elevation angle too far introduces problems with beam overshooting and possibly bright band contamination. Using a higher elevation angle means that a greater vertical range is being observed. As rain’s vertical structure is not uniform, this can lead to greater errors in surface rainfall estimation. For example, rain can evaporate leading to rain being observed at altitude and no rain being observed at ground level. Wind drift can also be significant, with rain observed above a specific location on the surface falling to the ground at a great distance from this position. A clutter map is made during dry periods so that the position of ground clutter is established. Areas in the image where clutter is above a certain reflectivity threshold are masked. Ground clutter can easily be distinguished by looking at the PPI animation. Targets which do not move between images are ground clutter targets.

Beam blocking is caused by non-meteorological targets that take up a significant proportion of the radar’s angle of vision. This causes all targets behind the blocking target to have diminished or no return. This is not seen at the current Mangakino site.

Incomplete beam filling becomes a problem at long ranges as the sample volume becomes quite large. The beam spreads out with range so at large distances the volume it takes up may not be completely filled with raindrops. If this is the case, the rainfall at this range will be underestimated.

Beam overshooting occurs when the beam passes over the top of the precipitation so that less intense (or no) rain echo is seen even though it could be raining substantially at the ground. This is a problem at long ranges or at high elevation angles. Due to the fact that the Earth is spheroidal, the height of the beam above the ground increases even if a 0° elevation is used. We have used two elevation angles at the Mangakino site so that precipitation closer to the radar is found from the high elevation scan and that which is far from the radar from the low elevation scan. This significantly reduces the ground clutter seen close in and reduces the beam overshooting problem at greater distances from the radar.

Bright band contamination can occur at long ranges during stratiform precipitation. The bright band is an enhanced layer of reflectivity around the 0°C isotherm. At long ranges or high elevation angles, the bright band can be intercepted on a PPI scan so that a ring of high reflectivity can be seen.