December 8, 2016

Using weather radar imagery


Using weather radar imagery


The advent of color computer video and high-speed internet connections has made radar imagery not only available to pilots but has created an attractive way to supplement the standard weather briefing.

There are a myriad of products available, but most of them do not paint a full picture of the precip situation. As a result, it is imperative that we understand what each product has to offer.

First of all, realize that there are inherent limitations of radar imagery as it exists today. Radar energy is affected in many ways as it attempts to reach precipitation, and during its return to the radar site.

In other words, the accuracy of the radar image is dependent on the accuracy of the radar energy gathered to depict precipitation returns on that radar image. So what are the limitations of radar to accurately depict precip?

Simply put, if we look at how radar energy travels to and from a potential target (e.g. precip), we see that reflection, refraction, scattering and diffraction of that energy are all potential problems.

Reflection is probably the most easily understood as it is commonly experienced when looking in the mirror. Unless you are a vampire, you should see a reflection of yourself! However, when radar energy hits a reflective surface at an angle of incidence other than straight ahead, energy can be reflected at an angle such that it will not return to the radar site.

Refraction, on the other hand, is not a problem of how the radar energy hits a potential target, but what that energy travels through on its way to and from the target. For example, put a pencil in a glass of water and it will appear to kink to a different angle within in the water.

In this case, light moves at a different speed through the more dense water, so the visible energy we see is refracted. Waves of radar energy will behave similarly as they travel through varying densities of the atmosphere.

Scattering of radar energy is again a function of the atmosphere in which the energy travels. Not all the energy directed at a target will reach it, and not all the energy directly reflected back will reach the radar site.

A practical example of scattering is seen in the blue sky. Earth’s atmosphere scatters blue and violet wavelengths more readily than the rest of the visible spectrum and so the sky appears blue. Pollution further scatters different light wavelengths sometimes giving the sky an orange or red cast as well.

Diffraction of radar energy is seen as an expansion of the radar energy as it tries to go around solid objects. The “pilots halo” (or “glory”) is one example of diffraction that is visible to the pilot. In this example, light is expanded so that the individual color wavelengths are visible in a rainbow-style circle on the side of the airplane opposite the sun. With radar energy, an object partially blocking the path of the beam will cause diffraction of the beam at the edges of the object.

So the four problems mentioned above impact radar energy so that the picture “painted” is potentially inaccurate, or at least incomplete. The most common effect is due to scattering and absorption of radar energy, and we refer to this phenomenon as attenuation.

Basically, the more precip the radar energy has to pass through, the more is reflected back, resulting in less energy available to see precip that is farther away. Whole bands of precip can be obscured from radar “sight” as a result.

Refraction is also problematic but is not directly observable on a radar image. If the atmosphere’s non-standard characteristics cause the radar beam to refract, the tops of precip can either be missed entirely, or painted as being lower than they really are.

High precip tops are most susceptible to these errors as they are farther away from the radar source. In extreme cases of what is called super-refraction, radar beams can come into contact with the ground, creating a clutter of radar returns. Temperature inversions close to the ground can create this effect.

Also, rapidly cooled air behind t-storms can cause false returns that we refer to as anomalous propagation, making it appear as if there is more rain behind an approaching storm than there actually is.

Keeping in mind these various problems with radar identification of precip, lets turn our attention to the imagery products available to pilots.

Four good radar imagery sources exist for pilot use: base reflectivity, composite reflectivity, composite radar, and composite radar summary. All of these may be available with or without time looping and enhanced color imaging, and reflectivity maps having several modes (clear air, precip, PPI, RHI, and echo tops) that may also be useful.

It also suffices to say that the conventional composite radar maps are of little use in light of color enhanced radar summary maps so we won’t talk about them here.

Picking the right reflectivity map can be tricky. I’ve seen many pilots in the weather room intently studying a clear-air mode base reflectivity map and wondered what exactly they thought they were looking at. Bearing in mind that the whole purpose of looking at a base reflectivity map is to find precip reaching the ground, its best to start with the precip mode map.

They are labeled, so look for the “precip mode” label to know you have the right one. An additional concern with reflectivity maps, other than using the correct one, are that attenuation of the radar energy is a possibility at the outer rim of the radar’s range, so the absence of precip returns near the rim of the map (e.g. the edge of the radar coverage) does not necessarily mean there is no precip at those locations!

Other problems with reflectivity maps include false returns due to insects and birds, topography, reflected energy and airborne objects.

A swarm of insects can appear as a blob of returns beyond the central ground clutter of a base reflectivity map. Birds and bats taking off from roost can create co-centric rings of radar echoes. Airborne objects are usually fleeting in nature but some occasionally show up on elevated reflectivity maps as significant radar events.

For example, the Columbia Space Shuttle break-up on reentry created a radar echo similar in appearance to a squall line. Topography and buildings close to the radar site have the effect of blocking narrow bands of radar energy that can give the false impression of gaps in the precip where none actually exist.

So base reflectivity maps are generally ok for finding precip reaching the ground, but be careful to use the correct map. For example, if it’s precip tops you are interested in, then use the echo tops map.

In fact, the best overall solution to all the issues surrounding the use of reflectivity maps is to use composite radar summaries. Especially in the case of color enhanced summaries, all the information you could ask for is right at your fingertips.

On these maps, multiple radar site data is consolidated into one chart. As an additional feature, echo tops and direction and speed of cell movement will be indicated. Coding is also added to indicate hail potential (“HAIL”), cell rotation (“MESO”), Tornados (“TVS”), and Hook echoes (“HOOK”). If a thunderstorm or tornado watch is in effect, it will be shown as well by the outlining of the effected area.

One great advantage of the radar summary is that severity detection by echo form is usually easier than when using reflectivity maps. Bow echoes of fast moving large convective systems will be readily apparent and warn of locally strong winds and heavy precip.

Hook echoes, one type that is also easy to see using the base reflectivity map, will be readily apparent on the radar summary and warns of potential tornadic activity. Squall lines will appear as skinny long lines with strong leading edge radar echoes and usually 100 plus miles of trailing precip.

Since strong precip leads to attenuation problems, the radar summary may provide better overall precip depiction since storms can be depicted from data from multiple radar sources, effectively viewing the precip from more than one angle.

Radar imagery is indeed useful for pilots making that go-no go weather decision. But understanding exactly what the picture is telling you is critical to making the correct decision.

Ask yourself questions like, “Is that just a strong narrow band of showers, or is there precip that the radar isn’t showing? Is that a gap in the precip I can fly through, or is it really a blocked radar beam? Is there really precip showing on the map or is this simply a flock of birds taking flight from roost in the morning? Etc. etc.

Realize that just because the radar imagery shows something is there, doesn’t always make it so, and that sometimes things that really are there don’t show up at all!

This month’s Pilot Primer is written by Donald Anders Talleur, an Assistant Chief Flight Instructor at the University of Illinois, Institute of Aviation. He holds a joint appointment with the Professional Pilot Division and Human Factors Division. He has been flying since 1984 and in addition to flight instructing since 1990, has worked on numerous research contracts for the FAA, Air Force, Navy, NASA, and Army. He has authored or co-authored over 160 aviation related papers and articles and has an M.S. degree in Engineering Psychology, specializing in Aviation Human Factors.