BISMARCK, N.D. (KFYR) - The advanced technology of weather radars is invaluable for meteorologists, however, there are some limitations to the current network of National Weather Service radars. Specifically, there’s some precipitation that these radars can’t detect because of obstructions or far distances from radar sites.

As mentioned earlier in our radar series, the radar beam increases in altitude, or height, as it travels farther away from the radar site. Radars start off at a 0.5° elevation angle and tilt upwards all the way to 19.5° 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. This can sometimes be a problem as radars cannot detect precipitation all the way down to ground level far away from the radar site.

There are two different volume coverage patterns (VCP) that weather radars use. Clear air mode is used when there’s no or very little precipitation in the area, allowing for slower scanning of the atmosphere, and, therefore, more sensitivity. There are also fewer elevation angles when using clear air mode as the radar only goes up 4.3°. On the other hand, precipitation mode is used for better precipitation detection, especially with towering thunderstorms in the area, as this VCP uses many more elevation angles all the way up to 19.5° to get a better 3-D picture of the surrounding precipitation. But notice that in both VCP charts below, the radar beam increases in height away from the radar site and, therefore, we have less data about precipitation the farther it is from the radar site.

Another limitation of radars is that they can’t detect precipitation directly above them, in the so-called “cone of silence,” because the radar beam goes out at an angle and radars can only go up to an elevation angle of 19.5°. So, when a storm is near or directly over the radar, the data becomes very limited, resulting in an ominous-looking hole that can serve as a blind spot for dangerous ongoing or developing weather. Meteorologists can work around this issue by switching to the data from other nearby radars in order to help fill in the missing data within the “cone of silence.”

155 National Weather Service radars across the country provide fairly good coverage for detecting precipitation, however, there are some notable gaps, as circled on the map below. Areas where radars can see precipitation at 4,000 feet or below, as shown in the yellow color, are locations that have good radar coverage. This includes much of central and eastern North Dakota thanks to the Minot, Bismarck, Grand Forks, and Aberdeen National Weather Service radars. However, the farther west you go in North Dakota, the higher the radar beam gets from the Bismarck and Minot radars, and the Glasgow and Billings, Montana radars are a bit too far away to help. Therefore, we’re left with a gap in the coverage with a sizeable area in parts of western North Dakota and eastern Montana where the radar cannot see precipitation below 6,000 feet. Furthermore, National Weather Service radars cannot sample the atmosphere below 10,000 feet in some parts of far western North Dakota and for a decent area of southeast Montana. This doesn’t mean that there’s no radar coverage in far western North Dakota or in parts of eastern Montana — it just means that there’s not as much radar data in these areas making it harder to detect precipitation, especially in the lower levels of the atmosphere.

Other notable, heavily populated cities across the country that have poor radar coverage from National Weather Service radars include Charlotte, NC; Columbus, OH; and Harrisburg, PA; along with other gaps in parts of tornado alley that can pose a problem to rural communities.

And sometimes it’s not the far distance between radar sites that results in poor radar coverage in a certain area. Mountains can also block the radar beam, which is a big problem in the western U.S. with the Rockies, Cascades and the Sierra Nevada mountain ranges resulting in large gaps in where we can detect precipitation with radars.

Radar gaps and far distances from radar sites can be especially problematic for severe weather, as being able to detect low-level rotation within thunderstorms is vital for determining if a tornado will form. But also in the winter, when stratiform clouds are usually much lower to the ground, the radar can sometimes miss this precipitation by the radar beam overshooting it.

The issue of poor radar coverage in far western North Dakota was brought to light in 2018 when an EF-2 tornado struck Watford City, killing one. In this case, the Minot and Bismarck radars were sampling the tornadic storm at least 10,000 feet above the ground, which is a challenge since scanning storms with radar this high up doesn’t provide meteorologists with as good of a view of low-level rotation within the storms, which could indicate tornado development. And Watford City isn’t alone when it comes to locations that can experience tornadoes and are in similar radar coverage gaps.

So, what has been done to help fill in some of these gaps in the National Weather Service’s radar network?

There are 45 terminal doppler weather radars (TDWR) located at some of the major airports across the country and operated by the Federal Aviation Administration (FAA) as a way for air traffic controllers to better monitor precipitation close to airports for takeoff and landing conditions. These radars are also equipped with doppler capabilities allowing them to detect the speed and direction of precipitation within storms. They can sometimes be helpful for meteorologists to provide additional data that the National Weather Service network of radars might miss, or help provide information about precipitation or rotation closer to the ground than the NWS radars are able to do. These TWDRs operate at lower power than the National Weather Service network of radars and therefore cannot detect precipitation as far from the radar site, however, they can still be very valuable for precipitation detection within a less than 100 miles radius from the airports they are located at.

Additionally, satellite data can step in to help fill in weather radar gaps. Meteorologists are constantly relying on satellite data for sensing cloud cover, how tall clouds are and therefore the intensity of storms, lightning detection, how much water vapor is in the atmosphere, and many other capabilities. Satellite data is high-resolution across the country thanks to the GOES series of weather satellites.

Reports from spotters and the public in these radar gaps are also vital in order to provide meteorologists with the “ground truth” of what’s actually happening in that area.

To combat the limited radar coverage in far western North Dakota, the National Weather Service has recently begun to operate the Minot radar at a lower elevation angle than usual (lower than the normal 0.5° “base reflectivity” angle that most radars use) to better detect precipitation in the western part of the state.

Chauncy Schultz, science and operations officer at the Bismarck National Weather Service, said: “The Minot radar is one of the only radars in the country that actually scans at an elevation below 0.5°. So, thanks to research that was done within the last couple of years, we lowered the angle of the Minot radar to 0.3° to try and gather more information at a lower altitude over western North Dakota to help with the detection of weather that’s important to the western part of the state. It might not seem like that much of a difference, but it is when you start talking about elevation angles farther and farther from the radar as the earth curves.”

Additionally, the Atmospheric Resource Board within North Dakota Department of Water Resources operates two doppler radars in western North Dakota (one in Stanley and one in Bowman) to help provide additional information about precipitation in this part of the state. These radars are actually old National Weather Service radars that were decommissioned when the National Weather Service network of radars was upgraded in the late 1990s, and they were redeployed with the Atmospheric Resource Board. They are also equipped with doppler capabilities allowing them to detect the speed and direction of precipitation within storms in the area.

Darin Langerud, director of the atmospheric resource division at the ND Department of Water Resources, said: “The purpose of the radars originally was to and still is to support the cloud seeding program in western North Dakota that is conducted every summer during June, July and August. So, that was the original intent of getting the radars deployed to help in that effort.”

“As far as the radar gaps in western North Dakota go, we’ve been operating the Bowman radar year-round since 2011 to help fill a gap in coverage in the National Weather Service Radar Network. That’s been a nice bonus for the citizens who live in those areas,” Langerud added.

“In the wintertime, especially when precipitation, the clouds that produce it, are lower. They’re not as high as thunderstorms in the summer. We need radar that can fill in a gap and can do a better job of detecting the precipitation that’s lower to the ground,” Langerud said.

“I think it is unique and I’ve had some conversations with some colleagues of mine in some of the other western states. And this actually was a discussion, this was a genesis of some work that was going on and is going on currently in Colorado to help fill some gaps in radar coverage there, too, in the mountainous western part of the state where they had significant gaps in coverage. So, I think things like this can be very helpful to filling in these coverage gaps in the National Weather Service system,” Langerud added.

You can access the real-time data (updated every five minutes) from the Bowman and Stanley weather radars that are operated and maintained by the Atmospheric Resource Board, as well as the Williams County radar that is operated by the county, on the ND Department of Water Resources website.

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