NEXRAD radar - How it works

NEXRAD radar is live NEXRAD Level III data from the National Weather Service. Such data is provided by the NOAA WSR-88D radar dome in Lincoln, Illinois. There are 153 such stations located around the United States.

Central Illinois Weather Forecast Office

The Central Illinois office with WSR-88D dome. Photo credit: NOAA

NEXRAD (Next Generation Radar) obtains weather information (precipitation and wind) based upon returned energy. The radar emits a burst of energy. If the energy strikes an object (rain drop, bug, bird, etc), the energy is scattered in all directions. A small fraction of that scattered energy is directed back toward the radar.

This reflected signal is then received by the radar during its listening period. Computers analyze the strength of the returned pulse, time it took to travel to the object and back, and phase shift of the pulse. This process of emitting a signal, listening for any returned signal, then emitting the next signal, takes place very fast, up to around 1300 times each second.

The WSR-88D Doppler radar is operated in one of two modes -- clear air mode or precipitation mode. In clear air mode, images are updated every 10 minutes. In precipitation mode, images are updated every four to six minutes. Modern radars do not "sweep" the sky but update their images all at once. Television stations which display a sweeper line on their radar screens are simply using a gimmick.

Clear Air mode is used when there is no rain within the range of the radar. In this mode, the radar is in its most sensitive operation state. This mode has the slowest antenna rotation rate which permits the radar to sample a given volume of the atmosphere longer. This increased sampling increases the radar's sensitivity and ability to detect smaller objects in the atmosphere than in precipitation mode.

When precipitation is occurring, the radar does not need to be as sensitive as in clear air mode as rain provides plenty of returning signals. At the same time, meteorologists want to see higher in the atmosphere when precipitation is occurring to analyze the vertical structure of the storms. This is when the meteorologists switch the radar to precipitation mode.

NOAA Doppler Radar

Radar image and dBZ

Approximate rainfall rates

This site provides the short range base reflectivity image from the radar. This image shows precepitation out to 128 miles. How the radar builds an image of the sky is by taking snapshots in slices. These slices are called "tilts" as the radar changes its angle to the horizon. The base tilt is taken at the lowest elevation (.5 degrees)

"dBZ" stands for decibels of Z, Z being the reflectivity factor. Reflectivities in the range between 5 dBZ and 75 dBZ are detected when the radar is in precipitation mode. Reflectivities in the range between -28 dBZ and +28 dBZ are detected when the radar is in clear air mode.

Rainfall rate estimates are hourly rainfall rates only and are not the actual amounts of rain a location receives. The total amount of rain received varies with intensity changes in a storm as well as the storm's motion over the ground.

Also, thunderstorms can contain hail which is often a good reflector of energy. Typically, a hailstone is coated with a thin layer of water as it travels through the thunderstorm cloud. This thin layer of water on the hailstone will cause a storm's reflectivity to be greater, leading to a higher dBZ and an over estimate the amount of rain received.

Value of 20 dBZ is typically the point at which light rain begins. The values of 60 to 65 dBZ is about the level where ¾" hail can occur.

Clutter

Ground Clutter is the most common false echo and is usually seen in every radar image. When atmospheric conditions are such where there are low-level inversions (air temperature increasing with height instead of the typical decreasing with height) ground clutter can be very pronounced.

Ground clutter is the easiest false echo to recognize since it does not move in any organized fashion, it has no kind of structure to it that is similar to real precipitation, and it is usually close to the radar. The ground clutter in clear air mode is often more prevalent than precipitation mode. This is because the radar is in its most sensitive operation thereby "seeing" smaller objects such as dirt, dust and bugs.

NOAA Doppler Radar - Anomalous Images

Explanation of radar key

Note: Symbols are NOAA software-generated and subject to false positives. Some, all, or none of these images may appear on our radar.

Tornado Vortex Signature - Doppler radar signature in the radial velocity field indicating intense, concentrated rotation - more so than a mesocyclone. Like the mesocyclone, specific criteria involving strength, vertical depth, and time continuity must be met in order for a signature to become a TVS. Existence of a TVS strongly increases the probability of tornado occurrence, but does not guarantee it. A TVS is not a visually observable feature.

Mesocyclone - A storm-scale region of rotation, typically around 2-6 miles in diameter and often found in the right rear flank of a supercell (or often on the eastern, or front, flank of an HP storm). The circulation of a mesocyclone covers an area much larger than the tornado that may develop within it. Properly used, mesocyclone is a radar term; it is defined as a rotation signature appearing on Doppler radar that meets specific criteria for magnitude, vertical depth, and duration. Therefore, a mesocyclone should not be considered a visually-observable phenomenon (although visual evidence of rotation, such as curved inflow bands, may imply the presence of a mesocyclone).

3D Correlated Shear - A 3DCO means a moderate mesocyclone is possible, and some inner-cloud rotation has been detected on at least two radar slices vertically. A 3DCO needs to be watched for persistence and growth.

Tornado Warning - Issued by our local National Weather Service, a tornado warning means a tornado has been seen by a trained spotter or strong evidence exists on radar. If you are in a warning area, take immediate appropriate acton. In a home or building, move to a pre-designated shelter, such as a basement. If an underground shelter is not available, move to an interior room or hallway on the lowest floor and get under a sturdy piece of furniture. Keep windows closed. Houses do not explode due to air pressure differences. Stay away from windows during severe storms. Flying debris could shatter the glass and cause injury. Do not try to outrun a tornado in your car. Instead, leave it immediately and find shelter. If a sturdy building is not available, then get out of the vehicle and lay down in a low spot on the ground not subject to flooding, protecting the head and neck. Mobile homes, even if tied down, offer little protection from tornadoes and should be abandoned. Occasionally, tornadoes develop so rapidly that advance warning is not possible. Remain alert for signs of an approaching tornado. Flying debris from tornadoes causes most deaths and injuries.