Azimuth Distortion and Resolution

To understand the concept of azimuth distortion (also referred to as “beam width distortion”), a knowledge of the definition of the radar beam is imperative. The beam is defined such that, in most cases, any meteorological target (precipitation) with which it comes in contact will backscatter a detectable amount of energy to the radar. What will the radar display as it scans through a target in azimuth?

The finite beam width of the radar results in echoes being extended in the azimuth direction – called azimuth distortion. 

 

Figure 1. Azimuth distortion in the horizontal plane


It is apparent that azimuth distortion will result in an enlargement of a target by approximately one-half beam width on either side, so total enlargement is equal to one beam width. Azimuth distortion is the most significant contributor to overestimation of precipitation coverage by radar. Azimuth distortion increases as a function of range (see figure 1).

Echo enlargement is not restricted to the PPI display, however. As you scan upward in RHI mode in an attempt to determine the base and top of a target, the echo will be stretched by one-half beam width above and below. This is why a half-beam width correction should be applied when determining tops and bases of echoes. The range of the storm must be known before this can be done properly.

In summary, when viewing echoes on the PPI display, both range and azimuth distortions are inherent, as shown in Figures 1 and 3. Azimuth distortion is the most significant, particularly at long range. On the RHI display distortion due to beam width causes erroneous heights for echo base and top (see Figure 2). The echo is spread by about half a beam width on either side of the target. The echo is also distorted vertically important when estimating Thunderstorm heights. 
 Figure 2. Display effects of range and azimuth distortions (shaded regions show fictitious echo)

 

Every time a target is detected by the radar beam, regardless of its real position, radar always assumes it at the center of the beam.

Azimuth resolution = one beamwidth

Since azimuth distortion stretches targets by one-half beam width on either side, two echoes at the same range from the radar must be at least one beam width apart in order to be detected separately. This minimum separation in azimuth necessary for targets to be resolved individually defines the azimuth resolution (also called “beam width resolution”).

Since the diameter of the beam increase with range, the ability of the radar to discern separate targets in azimuth decreases with range. This produces an apparent decrease in echo coverage as a line or area of cells moves towards a radar site, and an apparent increase in coverage as a line or area of cells moves away from the radar. A linearly oriented group of cells may appear to be a solid line of echoes at 200 km from the radar and a broken line of echoes at 50 km from the radar, even if the line’s characteristics had remained unchanged during this time (Figure 3). A false areal coverage trend may be reported in cases when distance between the cells and the radar changes. 


Figure 3. Azimuth distortion on the radar display

For a beam width of 1.6 degrees azimuth distortion is approximately 1.5 km at 50 km range, half on either side of the target. That is, at a range of 50 km, two cells must be more than 1.5 km apart to appear as two echoes on the scope. Similarly, at 300 km range, azimuth distortion is about 8 km. That is, two cells must be more than 8 km apart to appear as two echoes on the scope.

Azimuth distortion results in targets separated by less than a beamwidth appearing as one echo. So two separate echoes would be seen as one elongated echo at further distance.

As it moves near radar, even if nothing changes, they would look more separated. They then gradually merges again as they move pass the radar.