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• What Is NEXRAD Doppler Radar?
• How Are NEXRAD Images Displayed?
• How Does Conventional Radar Work?
• How Does Doppler Radar Work?
• How Does NEXRAD Doppler Radar Work?
• What NEXRAD Products Are Available Through AccuWeather?
• What Are The Different Modes Used By NEXRAD Doppler Radar?

What Is NEXRAD Doppler Radar?

NEXRAD (NEXt generation of weather RADar) Doppler radars have recently been installed throughout the country to replace the outdated network of conventional weather radars that were installed in the late 1950s. AccuWeather is offering access to the full network of nearly 150 NEXRAD Doppler radars, as each site is installed and test data made available from it by the government. NEXRAD's ability to detect wind patterns in storms and provide real time rainfall amounts are revolutionizing the way we keep up with and ahead of rapidly changing weather.

Each NEXRAD radar generates dozens of data types, including higher resolution reflectivity data, storm total rainfall amounts, wind speeds and direction, wind gusts and much more, including early tornado detection capability.

AccuWeather has the most NEXRAD products available (20 basic products plus value-added mosaics, StormTimer (tm) and numerous other value-added products) from the most NEXRAD sites, all in real time. AccuWeather's NEXRAD products will provide you with superior resolution, more detail, higher accuracy and a wide range of information than ever before available from existing radar technology.

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How Are NEXRAD Images Displayed?

AccuWeather makes NEXRAD data available in a scientific version which displays the full level and detail of NEXRAD data with a complete key, indicating the radar site and mode and the maximum intensity level detected. We have developed value-added NEXRAD displays in all resolutions by taking the unaltered NEXRAD data from the National Weather Service and adding visually pleasing base map features and keys. This makes this high-tech data easier for you to understand.

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How Does Conventional Radar Work?

Radar is an acronym that stands for RAdio Detection And Ranging. A weather radar consists of a parabolic dish (it looks like a satellite dish) mounted on a tower of up to five or so stories tall. The dish emits a pulsed beam of microwave radiation (analogous to a radar signal, only it is pulsed rather than continuous, and the signal has a shorter wavelength than radio signals). Most weather radars operate at wavelengths of either 5 cm or 10 cm.

The signal is emitted in periodic pulses rather than continuously. The radar goes through a sequence in which it emits a burst of microwave radiation, then listens for any returned signal, then emits another burst, then listens once again. The radar very rapidly switches from sending out the signal to listening for any returned signal to sending out the signal again in quick succession.

The burst of radiation travels out through the lower atmosphere and is scattered by particles in the atmosphere such as rain droplets and ice crystals. When the burst of microwave radiation encounters such particles, the signal's energy is scattered in all directions, and some of it is absorbed by the particle. A portion of the energy that is scattered is reflected back to the radar. This reflected signal is then received by the radar during its listening period, and is processed into the color digital display we are accustomed to seeing as radar data.

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How Does Doppler Radar Work?

Doppler radar uses the "Doppler effect" to determine the movement of the particles (rain drops, hail, ice crystals) that are reflecting the radar's signal back to the radar. Movement either toward or away from the radar along the radar beam, or radial, is all that can be detected, hence the term radial velocity.

This movement is detectable because the frequency of the reflected signal is shifted, due to the movement of the rain or ice particles as they are blown around by the wind. This frequency shift is termed the Doppler effect after the Austrian physicist Christian Doppler, who discovered it.

We experience this frequency shift in our lives every day. Picture yourself at a railroad crossing, waiting for the train to pass so you can drive on. The train in blowing its whistle as it approaches, and you can hear the pitch of the sound changing as the train nears. The pitch increases as the train approaches and then decreases as the train leads. You are standing still and the train, the source of the sound signal, is moving. The frequency of the train whistle's sound wave is being shifted due to the movement of the train, the source of that sound wave. The magnitude of the frequency shift is determined by the speed of the train.

As long as there is wind blowing the rain drops around, the frequency of the reflected signal returned to the radar will be shifted. Doppler radar detects that frequency shift and measures its magnitude; conventional radar does not.

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How Does NEXRAD Doppler Radar Work?

NEXRAD Doppler radar processes the radar's reflected signal to determine the location and intensity of precipitation (reflectivity products), the wind speeds toward and away from the radar site (radial velocity products) and many other products. This information goes directly to AccuWeather's computer systems, which create easy to read color displays of the NEXRAD information for each individual radar, and combine information from all radars to create mosaic displays.

The Base Reflectivity, Tilt 1 product is the product most like that available from conventional radar; however, there are some significant differences. For one, NEXRAD Doppler radar data is gathered at a high resolution out to a farther distance from the radar site than conventional radar data, .6 by .6 of a mile resolution out to 143 miles for NEXRAD Base Reflectivity data, as opposed to similar resolution out to only 66 miles for conventional radar. And NEXRAD data depicts 16 data levels, as opposed to only 6 data levels that were available with conventional radar. Following is a table (Figure 1) outlining the significant differences between NEXRAD Doppler radar and conventional radar.

The NEXRAD Doppler radar is also much more sensitive than conventional radar, allowing users to see meteorological phenomena never before visible in radar data, such as blowing dust and dry frontal boundaries that have no associated precipitation.

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What NEXRAD Products Are Available Through AccuWeather?

The reflected signal of the NEXRAD Doppler radar is processed by computer programs to yield a wealth of information that has never before been available. For example, the strength of the reflected signal is an indication of the intensity of precipitation. NEXRAD Doppler radar uses this information to interpret how much rain has fallen and where. Rainfall totals for the past 1 hour, the past 3 hours and the entire duration of a particular storm are available. These precipitation accumulation products are potentially valuable to emergency managers, hydrologists and others concerned with flooding and flash flooding. Real-time reports of rainfall have never before been available with such detail and coverage.

NEXRAD Doppler radar also uses the strength of the reflected signal to interpret the amount of liquid water contained within the clouds (the Vertically Integrated Liquid Water product) which can be very useful in determining the relative strength and hail-producing potential of storms.

The radial velocity data is one of the most valuable products available from each NEXRAD Doppler radar. It is the radial velocity product that allows meteorologists to detect circulations inside thunderstorms that often precede the development of a tornado. The exact location of cold fronts and other meteorological boundaries, such as sea breeze fronts, can also be determined with the use of this data. Directions for interpreting radial velocity data are included in this manual in the chapter on Base Velocity.

Vertical wind profiles are also available from NEXRAD. These profiles allow meteorologists, aviators and others to monitor changes in wind speed and direction with height directly above the radar site. Such information can alert meteorologists about atmospheric changes that could result in severe weather. This information is also extremely valuable to aviators in determining the magnitude of vertical wind sheer.

If you are interested in NEXRAD Doppler Radar, you can have access to all of the sites via AccuData, the largest weather database. Visit our site for all of the details.

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What Are The Different Modes Used By NEXRAD Doppler Radar?

While conventional radar displays reflectivity data gathered at only one elevation angle from one 360° sweep, NEXRAD Doppler radar, also known as WSR-88D (Weather Surveillance Radar - 1988 Doppler), is operated in one of three possible volume scan strategies.

Clear Air Mode

A NEXRAD Doppler radar will typically operate in the clear air mode when little or no precipitation is being detected within the effective range of the radar for reflectivity data (460 km or 286 mi.). In clear air mode, the radar completes either seven or eight 360° sweeps at five unique elevation angles between .5° and 4.5° above the local horizon, comprising a "volume scan." NEXRAD data is updated every 10 minutes for all products except the three layers of Layer Composite Reflectivity, Vertically Integrated Liquid and Echo Tops products, which are not available when the radar is in clear air mode.

Precipitation Mode

When the radar detects a significant amount of precipitation, it is then operated in the precipitation mode. In this mode, the volume scan of the radar is comprised of 11 full 360° sweeps at nine unique elevation angles (between .5° and 19.5° above the local horizon). NEXRAD data is updated every 6 minutes when the radar is in precipitation mode and all products are available.

Severe Weather Mode

During severe weather or suspected severe weather situations, the radar is operated in severe weather mode. In this mode, the NEXRAD Doppler radar's volume scan is comprised of 16 full 360° sweeps at 14 unique elevation angles between .5° and 19.5°. Each volume scan is completed in five minutes. Severe weather mode is a sub-mode of precipitation operating mode and is not regularly delineated from precipitation mode.

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