Doppler weather radar is one of the most useful tools available to track the weather in almost real time. Since the technology came into regular use in the 1950s, researchers have made incredible advances in the abilities of basic weather radar. Not only does weather radar track precipitation, but it can also detect bugs, bats, dust, road traffic, tornado debris, and even the setting sun.
Doppler radar works by sending out a beam of microwave radiation across the horizon at a slight tilt (between 0.5 and 19.5 degrees of tilt, depending on the weather that day) that bounces off of objects and returns to the radar site. The speed and intensity of the return pulse tells the radar how far away and how intense the precipitation is. The result is called a "radar return," and these returns are translated into a color-coded gradient that you see on a radar image when you check for storms in your area.
The National Weather Service recently upgraded all 160 radar sites across the United States to include "dual polarization" (dual-pol) as the next great advance in weather radar technology. Up until a couple of years ago, Doppler radar only sent out a horizontal pulse of radiation to detect precipitation. This allows the radar to see where the precipitation is, how intense it is, and how fast it's moving, but it doesn't allow the radar to see the shape and size of what the beam is bouncing off of. Dual polarization sends out an additional vertically-oriented pulse of radiation, which, in conjuction with the horizontal beam, allows the radar to determine the size and the shape of the object off of which it was reflected.
This allows the radar to distinguish between all sorts of objects, including differently sized raindrops, hail, snow, sleet, swarms of insects, birds, bats, dust storms, and even identifies flying debris to help meteorologists determine if there's a tornado on the ground.
One of the most common non-precipitation events detected by weather radar are swarms of bugs, bats, and birds that come out to feed (and be fed on) around and after sunset. These swarms often appear as a fuzzy ball that radiates out about 50-100 miles away from the radar site.
Here's an example of bugs showing on the Melbourne, Florida radar a little after midnight on Monday. The following is a "base reflectivity" image, which is the product used to show precipitation. On the left side of the image there's a batch of light rain north of Tampa, and across much of eastern Florida (centered around the radar in Melbourne), all of the grayish/bluish returns indicate bugs. Millions of bugs.
We can tell they're bugs (and probably some bats, too) thanks to dual-pol data. The following image shows the hydrometeor classification product for the same view as the radar image above. Hydrometeor classification shows the viewer what the radar's algorithms determine is responsible for bouncing the radar beam back to the radar itself, whether it's rain, heavy rain, hail, snow, ice, or biological.
In this case, you can see that the rain showers north of Tampa are identified as light rain ("RA") by the hydrometeor classification product. All of the returns near Melbourne on the eastern side of the state are, indeed, biological ("BI") returns.
Another great example of biological returns on weather radar occurred near San Antonio, Texas last night. The National Weather Service tweeted out this great graphic showing a colony of bats emerging from the Bracken Cave — containing the largest bat colony in the world — just after sunset.
The Mexican free-tailed bats have returned to South Central Texas. Doppler radar picked up the emergence. pic.twitter.com/439NBOpEMt
— NWS San Antonio (@NWSSanAntonio) March 24, 2014
The rising and setting sun can also cause a sharp "spike" to appear on radar imagery on a clear day. These spikes occur when the angle of the sun matches up with the angle of the radar beam, and the sun's rays shows up as a long, narrow return. A vivid example of sunset spikes can be seen in the radar image to the left, with radar sites from Texas to Montana simultaneously registering the radiation from the setting sun.
It means that if the radar happens to be pointed directly at the sun, it will receive some additional radiation in the [beam] that it did not send out. However, the radar doesn't know that this radiation wasn't radiation that it emitted. So it interprets it as just normal signal bouncing back. Since the emission from the sun is continuous, the return is interpreted as a continuous beam stretching out from the radar in the direction of the sun. And that is where those spikes come from.
Weather radar can also detect the leading edge of air masses like cold fronts, dust storms, and thunderstorm cold pools (also called "outflow boundaries"). I explained outflow boundaries in a post about shelf clouds a few weeks ago — essentially, when a thunderstorm "exhales" rain-cooled air, the colder, denser air builds up beneath the thunderstorm in what's called a cold pool. If the cold pool starts racing out ahead of the storm, it acts like a small-scale cold front called an outflow boundary.
Due to scattering of the radar beam, as well as insects and birds that get caught along the leading edge of the outflow boundary, they can show up on weather radar as a fine line moving away from a thunderstorm. The above image is an excellent example of a bunch of outflow boundaries on radar, seen across southern Alabama and Mississippi on September 16, 2013.
Doppler weather radar is also incredibly useful for confirming whether or not a rotating thunderstorm is producing a tornado, not only through the velocity (wind, basically) imagery, but also because the radar can detect debris swirling around in the tornado itself.
The EF-5 tornado that struck Moore, Oklahoma on May 20, 2013 provides a classic radar view of a major tornado producing incredible damage in a populated area. This first image a double-product view of the same location. The left image is base reflectivity, showing precipitation. The right image is base velocity, showing winds; the tight red-green couplet is the intense rotation of the tornado.
On the base reflectivity image, you can see a zoomed-in view of the classic "hook echo" that usually presents itself when a supercell thunderstorm produces a tornado. The pendant at the edge of the hook is the tornado itself — the ball of dark purple isn't precipitation, but rather tens of thousands of pieces of debris — wood, roofs, trees, cars, appliances, you name it — swirling around inside the tornado.
Above is the hydrometeor classification image I discussed above regarding bugs in Florida, but for the Moore tornado. The radar correctly identifies the majority of the thunderstorm as heavy rain and hail, but the ball right at the end of the hook is identified as debris in the tornado.
Weather radar is an incredible tool for everyone from the general public to forecasters issuing life-saving warnings to those in a storm's path. The next time you look at the radar, take a look around and see if you can spot something other than rain.