Central AtlanticStorm Investigators

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NEXRAD Radar

DEFINITION:

     During the 1990's, most National Weather Service stations across the U.S. replaced their WSR-57 or WSR-74 radars (the number corresponding to the year of manufacture) with NEXRAD (NEXt generation weather RADar) Doppler Radars. Unlike the old radars, the new NEXRAD radars can see the Speed and Direction of the water droplets in the air, not just the Intensity. In addition, the new NEXRAD radars have greater sensitivity for things that normally wouldn't show up on the old radars, like snow or non-precipitation air movement.

     There is no doubt that these new radars have saved lives, allowing the National Weather Service to sometimes detect tornado formation before the funnel reaches the ground. With algorithms (equation-based checklists) implanted into the radar's software, NEXRAD can detect TVS (Tornado-Vortex Signatures), Mesocyclones, and Hail. Mesocyclones are indicative of severe weather and point to areas where tornados may form; a TVS indicates where the radar thinks a tornado is forming.

LIMITATIONS:

     Although these new weather radars are leaps and bounds ahead of their predecessors, they do have their limitations. Despite greater sensitivity, some of the data is unreliable past 124 nm (Nautical Miles). Much of the limitations are from weather radars in general and are not the fault of the Doppler construction or computer algorithms.

     The chart below, taken from a NEXRAD manual, shows the ranges and limitations of the new radars.

The (0.5 Elev) Note at the top of the chart indicates that the data quoted is for a radar elevation of 0.5 degrees, the standard (and lowest) elevation which the radar is scanning.

Notice that the manual for the NEXRAD itself admits that Velocity Products (showing the speed and direction of molecules, very important to tornado detection) including TVS, Hail, and Mesocyclone detection are unusable outside 124nm.

Reflectivity (Intensity) echoes are possible to 248nm, but the software algorithms to make sense of the data can't work past 186nm. Beyond that only the general shape of storms can be made out.

The blue numbers on the right-hand side of the chart show the actual elevation in feet of the radar beam at 124, 186, and 248nm. Because the radar has to be elevated slightly up from ground level, to avoid ground clutter (trees, buildings, and mountains which will show up on the radar), the beam is very high in the atmosphere outside a few miles. This is caused by geometry and is made worse by the curvature of the Earth.

Although radars are supposed to show what's happening close to the ground, even at 124nm the beam is already almost 15,000 feet in the air. At 248nm, it is 50,500 feet in the air, high above almost all precipitation and even tall thunderstorms! This height change is the reason that precipitation may appear on a radar over an area in which the ground station is reporting no precipitation. The radar is either actually reporting the precipitation inside the cloud, or, if the cloud has a high base, precipitation can fall from the cloud through the radar beam but evaporate before it hits the ground. This precipitation is called 'virga.'

The blue numbers on the left-hand side of the chart show the width of the beam in miles. The beam starts out from the radar a few feet across, but spreads out as it gets farther away from the radar. By the time it is at 248nm, it is 5 miles wide.

This means that the intensity that it reports at this far out will be a data block which is 5 miles wide. Severe weather or intense rainfall could easily be hidden inside the data. This is what causes radar data to appear more "blocky" near the outside range of the radar. See Figure 1.a.

In addition, there are several other factors which could limit the usefulness of the data. Intensity echoes near the outside of the radar range are not only "blocky" because of beam width (see above) but also can be inaccurate due to attenuation, or interruption of the beam, by intense areas of precipitation which stop the radar beam from reaching its maximum range. This attenuation, along with "range folding" is serious business in the Velocity data, and in fact any data "behind" blocking agents or radar range is marked unknown, usually deleniated by a purple or grey color.

Although the beam is elevated 0.5 degrees for normal scanning, this still won't make it over high mountains, and radars in mountainous areas are severely limited when the beam hits the mountains. Examples of this can be seen in the Northwest states and, to a lesser extent, even in the Appalachians. I recall that Asheville NC was worried about not being covered by a radar until the decision was made to place one at GSP (Greenville - Spartanburg, SC). Western North Carolina was outside the range of the radar beams from Roanoke and Columbia; the radar beam from Bristol couldn't make it through the mountains. This left the area in a radar "shadow" until the GSP decision came through.

LAND COVERAGE:

As far as land coverage is concerned in the continental U.S., NEXRAD coverage is fairly good. When deploying the radars at various NWS offices, the government had to make sure that all areas were covered while trying to stay within their budget. Assuming a range limitation of 143mi. (this is the default range of the radar that you would see on a radar screen (or Intellicast NEXRAD image), as discussed above, the United States range markers for NEXRAD Radars look something like Figure 2.a.

Most of the United States is covered, with a concentration of radar sites in the eastern US, and a sparsity in the Rockies.

All of the Eastern US is covered, but some areas of the midwest and western Southeast are covered by only one or two radars, meaning that if one radar goes out (at any time 5-10 are out in the US) then severe weather detection could be a problem in these areas. Much of the West is still without doppler radar. Severe weather detection in these areas will have to be done with the old, out-of-date WSR's. Luckily much of this region is in the Rocky Mountain or desert areas where severe weather is somewhat rare. California is by far the best covered area in the West.

It is important to note that this map assumes full 143mi. coverage. Some reports indicate that some data becomes unreliable after as little as 62nm, and certainly other factors, as discussed above, can limit the usefulness of the data. And as discussed above, considerable "holes" exist in the NW US, the Rockies, and the Appalachians because of the mountains; these are not indicated on the map. In addition, NEXRAD radars, like most electronic equipment, have a habit of losing functionality or going out altogether on a fairly frequent basis.For Severe Weather Detection, Figure 2.a. is a best-case scenario.

Here in the Carolinas we are fairly well covered by NEXRAD weather radars, although the area near Hickory, NC, is near the edge of several radar ranges. See Figure 2.d. As noted above, the NEXRAD at GSP was not originally in the plans but a congressional hearing prompted its construction, making residents of Western North Carolina very happy. The Carolina Area Storm Investigators HeadQuarters is blessed to be very near the Raleigh radar site for optimum coverage.

FURTHER RESOURCES:

WW2010: Radar Meteorology is an excellent tutorial on NEXRAD and how to interpret its products.

The opinions and facts presented here are only those of the Jesse Ferrell. However, special thanks are in order to Dr. Edward Brotak of UNCA, whose textbook and teachings provided inspiration for this page.