BISMARCK, ND (KFYR) – Last week, we took you inside the Bismarck weather radar to show how it works to send out radio frequency energy to detect precipitation. This week, we’ll be talking about how the radar samples the atmosphere from a variety of different angles to get a complete 3-D picture of the current conditions around the radar site.
The problem that weather radars sometimes encounter is that the radar cannot scan precipitation all the way down to ground level. It starts off at a 0.5° elevation angle and tilts upwards all the way to 19° above horizontal. When you factor in the curvature of the Earth, the radar beam is actually sampling the atmosphere higher and higher the farther away you go from the radar site. The issue of weather radars missing precipitation below the radar beam usually occurs in the winter when clouds are much lower that produce snowfall compared to towering cumulonimbus thunderstorm clouds in the summer.
We can calculate the beam height of the weather radar to determine where in the atmosphere it’s sampling precipitation using some trigonometry. The general rule is that at a 0.5° elevation angle, the radar beam rises about one mile up for every 120 miles that it travels out (or for metric units, one kilometer up for every 120 km away from the radar site). So the farther away you go from the radar site, the higher in the atmosphere the radar is sampling and not getting as comprehensive of a picture of the precipitation in that area.
With this limitation in mind, the surrounding 120 miles from a radar site is generally where we have the best coverage of precipitation, as this is where the radar can sample the lowest 5,280 feet (or one mile) of the atmosphere.
However, radars can detect precipitation further than 120 miles from the radar site, it’s just not as useful due to how high in the atmosphere the radar beam is at that point.
The height of the radar itself also plays a role in determining where in the atmosphere it is scanning. The Bismarck weather radar is located at the National Weather Service office near the airport, which is a lower-lying part of the city. Therefore, the radar has to be positioned 65 feet above the ground to ensure that any surrounding topography does not interfere with the beam at its lowest elevation angle, 0.5°.
Another thing we need to take into account when scanning the atmosphere with weather radars is the different types of precipitation that meteorologists are trying to detect. For instance, the intensity of the return signal to the radar from raindrops is approximately five times greater than the returned radio frequency energy from snowflakes that have comparable sizes. Therefore, the radar operates in different “modes” for various types of precipitation and that can be controlled by meteorologists at the National Weather Service.
Chauncy Schultz, science and operations officer at the Bismarck National Weather Service, said: “When we’re dealing with finer precipitation, lighter precipitation, say snowflakes, for example, they’re pretty small. It’s harder to detect that if the radar is spinning too fast, so the way we handle this is through what we call volume coverage patterns.
Snowfall, for example, forms a lot closer to the ground than, say, thunderstorm clouds do. But we also know it’s more sensitive, it’s harder to detect. So we operate the radar in a little slower fashion and it spins around more slowly, giving it more time to pick up on the small snowflakes, small raindrops when we’re dealing with lighter precipitation. And then it might go up to 1.5° and 3° slices above the elevation. There’s no need to go really high in the sky when we’re dealing with [light] rain and snow because all of that precipitation forms relatively near the ground.
When we’re dealing with thunderstorms, on the other hand, we know that the updrafts in thunderstorms are rising really to 50,000 feet in the worst-case scenarios. And we know that the thunderstorms are developing, evolving very quickly. So in that case, it’s more important for us to get a fast, full volume scan, is what we would call it, and the radar makes all of its revolutions and goes up in elevation scans.
So, during thunderstorms, we’re going to employ what we call a volume coverage pattern that is faster and it goes up from 0.5° and it adds more elevation slices too. So we’re actually building a bigger 3-D picture of the radar during thunderstorms. And again, we can control that as meteorologists from the Weather Service, what volume coverage pattern that the radar is actually operating in. And we do that based on the type of weather phenomena that we’re expecting and that we’re observing.
If we’re operating in that mode that is for snowfall or light rain, that takes about six minutes for a complete radar picture to be determined. If we’re operating in thunderstorm mode, or spinning the radar a lot faster, maybe getting a little less fine details higher up in the sky, but we’re going all the way high up in the sky and really getting those fine details of thunderstorms, it can be completed in about four minutes.
And we actually have new technology with the radar where we can actually halfway through its elevation slices when it’s going up and around, drop the radar back down and scan that lowest elevation slice that’s right near the earth to get a sense of what might be going on with rotation in the clouds, potential tornadic activity right near the ground.”
Next week on Morse Code of Weather, we’ll go into more details on how motors and equipment within the radar allow it to operate 24/7/365.
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