NWS Radar: Short-Comings of the Radar

Radar Spacing

While radar coverage is very good in central Alabama, there is still an inheirent problem with the radar when it comes to being able to analyze thunderstorms: the beams do not follow the curvature of the surface of Earth.

Take for example, a storm between the Birmingham (BMX) and Maxwell AFB (MXX) radars (see image to the right). If the storm is a good distance away from BMX, due to the fact that the radar beam is straight and doesn't follow the Earth's surface, the lowest elevation scan from BMX may only be able to "see" into the middle portion of the storm. If that same storm is closer to MXX, this will allow us to "look" into the lower portion of that same storm, possibly allowing us to detect a rotational cuplet closer to the Earth's surface. From this, we could infer that given the right circumstances, the formation of a tornado is possible.

Beam Spreading
Images courtesy of NWS SRH Jetstream

While often depicted as a cone with distinct edges, the radar beam is better visualized much like that of ordinary household flashlights. In a darkened room take a flashlight and, while standing 10 feet away or more, shine it on a wall. You will notice the bright area around the center of the beam but will also notice you can see the brightness fade farther away from the beam's center point. You will also notice the width of the beam spreads or decreases as you move toward or away from the wall.

The beam of energy transmitted from the doppler radar is no different. A conical shaped beam is formed as the energy moves away from the radar. And it is near the center line of the beam where most of the energy is located with the energy decreasing away from the centerline.

By convention we define the width of the beam as the distance between the two half power points - the point where there is a 50% reduction in the radar's transmitted energy. For the 88D radar, the angle between the two half power points is one degree. Outside of the half power points, the energy rapidly decreases.

It is easy then to see that the actual physical width of the radar beam depends upon the distance to the radar. The width of the beam expands at a rate of almost 1000 feet for every 10 miles of travel. At 30 miles from the radar, the beam is approximately 3,000 feet wide. At 60 miles, the beam is about 6,000 feet wide. At 120 miles the beam is nearly 12,000 feet or over two miles wide.

While often depicted as a cone with distinct edges, the radar beam is better visualized much like that of ordinary household flashlights. In a darkened room take a flashlight and, while standing 10 feet away or more, shine it on a wall. You will notice the bright area around the center of the beam but will also notice you can see the brightness fade farther away from the beam's center point. You will also notice the width of the beam spreads or decreases as you move toward or away from the wall.

The beam of energy transmitted from the doppler radar is no different. A conical shaped beam is formed as the energy moves away from the radar. And it is near the center line of the beam where most of the energy is located with the energy decreasing away from the centerline.

Refraction
Images courtesy of NWS SRH Jetstream

In addition to beam spreading, the beam also does not travel in a straight line. The beam is bent due to differences in atmospheric density. These density differences, caused by variations in temperature, moisture, and pressure, occur in both the vertical and horizontal directions and affect the speed and direction of the radar beam.

The more dense the atmosphere the slower the beam travels. Conversely, the less dense the atmosphere the faster the beam travels. These changes in density can occur over very small distances so it is common for the beam, as it spreads, to be in areas of different densities at the same time. The beam will bend in the direction of the slower portion of the wave.

The atmospheric density naturally decreases with increasing elevation and is primarily due to the decrease in air molecules, and consequently air pressure, as elevation increases. This means the top portion of a beam in the atmosphere can move faster than the bottom portion. Under normal atmospheric conditions, when there is a gradual decrease of pressure, temperature, and humidity with height, a radar beam's curvature is slightly less than the earth's curvature.

However, atmospheric conditions are never normal. If the decrease in density with height is more than normal (the actual density is less than normal) then the beam bends less than normal and climbs excessively skyward. This phenomenon is known as subrefraction . Subrefraction causes the radar to overshoot objects that would normally be detected. Distant thunderstorms might not be detected with subrefraction as well as under reporting the intensity as the beam hits only the top portion of the thunderstorm cloud.

Conversely, if the decrease in density with height is less than normal (the actual density is greater than normal) then the beam bends more than normal and is curved more toward the earth's surface. This phenomenon is called superrefraction . Superrefraction causes the radar beam to be closer to the earth's surface than what would occur in a normal atmosphere. This can lead to overestimating the strength of a thunderstorm as the beam would be detecting more of the core of the storm versus the weaker upper levels.

If the atmospheric condition that causes superrefraction bends the beam equal to or more than the earth's curvature then a condition called ducting, or trapping, occurs. Ducting often leads to false echoes also known as anomalous propagation or simply AP.