Chapter 2 RADAR RAINFALL MEASUREMENTS
2.4. Quantitative Precipitation Estimation
2.4.1. Reflectivity Measurements
2.4.1.1 Beam Blocking
When a radar beam strikes a fixed object like a building or mountain, it is said that the beam has undergone blocking. Correcting for beam blocking is not part of the study but is still a major source of error within precipitation estimation and it will be worthwhile understanding these effects better. In Figure 2-3, two types of beam blockage are possible. The first is, 1) partial beam blocking and the second 2) total beam blocking (Bech et al., 2002). Figure 2-3 only illustrates the concept of beam blocking. Partial beam blocking can results in power losses and precipitation can be severely under-estimated. Total beam blocking, block one hundred percent of the power and no precipitation can be detected beyond that range. Each radar has its own
“finger print” when it comes to beam blocking that is dependent on the surrounding topography.
Figure 2-3: A graphic illustration of 1) partial and 2) total beam blocking. It is assumed that atmospheric refraction and the curvature of the earth has no effect on the radar beam.
Examples of beam blocking in a radar scan are shown in Figures 2-4 and 2-5. Figure 2.4
displays instantaneous composite reflectivities as measured by the East-London C-band radar in 2001. A section of shallow stratiform precipitation is present to the east of the radar over the Indian Ocean. Sectors of reduced to missing reflectivities are observed within this region of
1 2
stratiform precipitation. This is due to the terrain and structures surrounding the radar obscuring the lower elevation scans. This makes detecting low targets such as stratiform precipitation at long ranges very difficult.
Figure 2-4: Instantaneous reflectivities at the East-London radar showing an example of beam blocking to the east of the radar.
Figure 2-5 illustrates the result when beam blocking is present in a precipitation field.
The 24 hour precipitation field from the C-band radar at Irene was generated shortly after the S- band radar was constructed. The S-band radar is located around 100m to the south of the C-band radar and the resulting blockage caused by the S-band radar contributes to severely under- estimated precipitation totals, which are clearly visible on the radar display. A large spike of apparently increased precipitation almost due south of the radar is also observed which is caused by RLAN interference.
The Irene S-band itself experiences partial beam blockage to the west of the radar due to the construction of large warehouse structure visible on the horizon from the Irene radar site.
The structures are illustrated in Figure 2-6 with photographs taken from the top of the 15m tall radar tower. Figure 2-6 (a) illustrates and points out the largest building of the 3 structures with the arrow mark A. The structures located to the left and right from the structure in Figure 2-6 (a)
Figure 2-5: A 24 hour accumulation of the C-band radar after the installation of the new S- band radar at Irene. The S-band radar causes total beam blocking to the south of the radar resulting in severe under-estimation of precipitation, the large spike of apparently increased precipitation is caused by RLAN interference.
Figure 2-6: Images of the large warehouse structure as seen from the top of the S-band radar tower. The buildings are the cause of partial beam blocking as well as reflection of the electromagnetic wave.
(a)
(c) (b)
are represented by Figure 2-6 (b) and Figure 2-6 (c). Figure 2-6 (b) also has a tower visible mark with the arrow T.
Figure 2-7 illustrates the effects these structures can have on precipitation estimates. On the 15th of December 2010, a day of significant flooding with wide spread precipitation, the 24 hour accumulation illustrates definite power losses to the west of the radar with low precipitation measurements. The effect from the partial beam blockage is aggravated with an increase in range.
A problem that is unique to the Irene S-band radar, in a South African context at least, is that the metal construction of warehouse buildings to the west of the radar not only causes blockages of the radar beam but also acts as a reflector and reflects electromagnetic radiation.
Figure 2-8 displays 3 spiked like features to the west of the radar. During this time only storms to the north and east of the radar were present. The warehouse building reflected radiation to the east and the radar measured a power return with the receiver of the antenna pointing to the west, resulting in a false echo being detected. Figure 2-9 shows the effect it can have on the precipitation product.
Figure 2-7: A 24 hour accumulation of precipitation for the Irene S-band Radar. The partial beam blocking to the west of the radar causes under-estimation to be observed (red ellipse).
Figure 2-8: The radar at Irene detecting storms to the east and north. Reflectivity to the west is a result from radiation being reflected off the buildings (Figure 2-6) towards the storms to the east while the radar is facing west.
Figure 2-9: The effects the reflection (shown in Figure 2-8) can cause on the precipitation field.
Clearly the effect of beam blocking can be severe. Normally these blockages only affect the lowest level of the volume scan but it is also the most useful scan in terms of precipitation estimation at ground level (Collier, 1996). Thus most operational QPE’s include correction algorithms for example, Harold et al. (1974), Kitchen et al. (1994), Joss and Lee (1995), Fulton et al. (1998), Chumchean et al. (2006) and Zhang et al. (2011). Zrnic and Ryhzkov (1998) also point out the advantage that Dual-Polarised products (like differential phase) can have over single polarisation radars in terms of estimating precipitation under beam blocking conditions.