RADAR LIMITATIONS -- (beam height & resolution, effective resolution decreases with distance, radars do not see tornadoes)

The radar antenna is angled upward slightly (at 0.5 degrees) on its lowest level scan. This reduces ground clutter around the radar.

Ground clutter are objects like trees and buildings close to the radar which will produce a "signal" that looks much like precipitation.

Therefore, as the radar beam travels away from the radar, it is positioned higher up in the atmosphere.

In addition, the curvature of the earth also contributes to this effect.

Storms close to the radar can usually be thoroughly seen by the radar from bottom to top. However, the radar beam may only be able to intersect the mid- to upper levels of a thunderstorm that is farther away from the radar.

In the example to the left, the radar will be able to scan the lowest portion of storm "A" while in storm "B" the radar cannot see low enough to sample the lowest portion of that storm.

Many important features exist in, or near, the lower levels of thunderstorms. It is possible that the radar will not able to observe these low level features near, or within, a thunderstorm as the radar beam may pass above them.

A spotter can be used to compensate for this lack of low level data.

In addition to the beam height phenomenon explained above, we have to consider beam width resolution.

The large antenna dish size of NWS Doppler radars allows for a beam width of slightly less than one degree. This is quite important for providing high detail when examining a thunderstorm or other types of weather phenomena. It is the beam width and wavelength (not the radar's "power" rating) that are two of the most important radar characteristics used to assess the radar's ability to accurately detect or sample a thunderstorm.

Despite the narrow beam width of NWS Doppler radars, the beam width will become quite wide as the radar beam travels away from the radar (the radar's beam is conical in shape). For example, 60 miles away from the radar, the width of the radar beam will be approximately 1 mile wide. That means all elements (and motions/velocities of those elements) in the thunderstorm that are being sampled (or measured) are "averaged" together within that one mile wide area. This has large implications in the amount of detail we can see in thunderstorms that are a great distance from the radar. This reduction in beam width resolution will cause details to be "lost" at greater distances from the radar.

It is important to note that in almost all instances, Doppler radars do not see tornadoes. Only in rare cases does the radar actually detect a feature called a tornado vortex signature (TVS).

This occurs when the tornado is large enough and relatively close to the radar. When this occurs, the radar can, at times, see a small-scale intense rotation signature.

Most times, the radar actually detects the larger scale circulation in the thunderstorm within which the tornado forms. This larger scale circulation is called a mesocyclone. The radar operator needs to interpret the three-dimensional Doppler velocity display to correctly identify a mesocyclone. He or she will then factor in many other pieces of data (radar derived and environmental) to determine whether or not the mesocyclone will likely produce a tornado.

Complicating this decision-making process is the fact that the displayed velocity data will be dependent on how the radar beam intersects and samples the mesocyclone. How the radar beam samples the mesocyclone is dependent on its distance from the radar and the size of the mesocyclone. In the above example we see the radar beam which gets wider as it moves away from the radar. "A," "B," and "C" represent three identical circulation centers (mesocyclones) at different distances from the radar. Keep in mind that the radar will average increasingly larger sampling areas as the beam travels further from the radar. Therefore, the mesocyclone closest to the radar will be sampled best and will show the highest detail.

Depending on its size, mesocyclone "A" will be sampled multiple times as the radar beam rotates across the circulation. Conversely, in this example it is possible that mesocyclone "C" would only be sampled once by the centerline of the radar beam. The rotation in mesocyclone "C" would not be depicted as "clearly" as that in mesocyclone "A."

This is a result of the wind motion (within the circulation) that is toward the radar being averaged together with the wind motion moving away from the radar. The net result could be that the radar shows a velocity near zero in the mesocyclone (or other variations which are not indicative of the true nature of the mesocyclone -- depending on where and how often the radar beam actually intersects the mesocyclone). Spotters can be used to assess whether or not a thunderstorm is exhibiting any visual evidence of circulation that the radar may not be able to discern because of its distance.

Despite the superior resolution capabilities of NWS radars, limitations do exist. As the distance from the radar increases, a given storm will be seen with less detail. Or in other words, the effective resolution decreases. This is why it becomes increasingly important for the NWS to have spotters at greater distances from the radar. It should be noted that the above example of resolution degradation is more the exception than the rule. In this instance, meteorologists could use another radar situated closer to the storm.