Storm-top divergence also may be important. Again, use neighboring radars for distant storms to view the lower levels. A fundamental concept of radar is that the radar beam becomes wider with range. Some information, especially for velocity estimates, may be returned from side lobes outside the defined radar beam. In addition, dBZ values may be averaged to slightly less than reality.
Aspect ratio deals with the beam width versus range problem.
If the phenomena is very small compared to the sample volume, then the phenomena could essentially be invisible to or more likely not well sampled by the radar. To illustrate aspect ratio, consider Fig. Retrieved velocities from the most distant circulation would be averaged to near zero, although the radar likely would resolve some sense of circulation depending on exactly how the beam intercepted the rotation. This example is exaggerated. Even at distant ranges, circulations usually are large enough and the beam small enough to allow for at least some degree of resolution.