Answers to Frequently Asked Questions about 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(NEXt generation of weather RADar) Doppler radars have recently been installed throughout the country to replace the outdated network of conventional weather radars that were installed in the late 1950s.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 137 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.

Figure 1 NEXRAD Doppler Radar Compared To Conventional Radar
Attribute	NEXRAD Doppler Radar	Conventional Radar
Number of products	20 basic products	1 product: reflectivity at one elevation angle
Update frequency	5, 6, or 10 minutes	2 minutes
Data Resolution	.6 x .6 miles to 143 miles	2.5 x 2.5 miles to 286 miles .6 x .6 miles to 66 miles
Range	286 miles for Composite Reflectivity; 143 all others	286 miles
Levels of data	16 levels for reflectivity products	6 levels
Number of sites	137 sites in continental U.S.	128 sites
Land coverage	Approximately 95%	Approximately 75%
Population coverage	Approximately 97%	Approximately 90%

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. In the extremely sensitive clear air mode, Base Reflectivity data is gathered at levels between -28 dBZ and +28 dBZ (negative values arise from the logarithmic equations used to convert the power of the signal returned to the radar by scatterers in the atmosphere to reflectivity values). At these levels, the NEXRAD Doppler radar is capable of detecting dry frontal boundaries, blowing dust, very light snow and drizzle with precise detail. When in precipitation mode, the radar's sensitivity is switched to detect thunderstorms outflow boundaries but also the resolution and detail to detect severe thunderstorms. The minimum detectable signal available with conventional radar is reflectivities of 13 dBZ.

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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.

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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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What Is Reflectivity Data And How Is It Gathered?

Reflectivity data shows the radar's signal reflected back to the radar by liquid and frozen droplets in the atmosphere. Base Reflectivity data is gathered at all elevation angles surveyed in each volume scan, but only four of these elevation angles, tilts 1 through 4, are available to users outside the National Weather Service. Figure 2 shows the elevation angles generally used by most NEXRAD sites. Elevation angles at some locations with unique geography may differ somewhat. For example, the Los Angeles, California NEXRAD is stationed at the top of a mountain. Tilt 1 for the Los Angeles radar will be between 0.00° and 0.40°.

Figure 2 Elevation Angles, Tilts 1 Through 4
Tilt Number	Clear Air Mode	Precipitation Mode
Tilt 1	0.50°	0.50°
Tilt 2	1.50°	1.45°
Tilt 3	2.50°	2.40°
Tilt 4	3.50°	3.35°

The Base Reflectivity, Tilt 1 product, reflectivity data gathered at the 0.50° elevation angle, is the product most like that available from conventional radar. The radar makes one 360° sweep at 0.50° above the local horizon, and the reflectivity data gathered from this sweep is what is displayed on the Base Reflectivity, Tilt 1 product. The elevation angles corresponding to Tilts 2 through 4 are listed in the table above. These are the same elevation angles as those used for the base velocity, Tilts 1 through 4 products.

Base Reflectivity, Tilt 1 shows the location and intensity of precipitation that is reaching the ground. That is the intensity of precipitation, be it rain or snowfall, that is being experienced at ground level. The higher elevation angles display the location and intensity of precipitation aloft which will, in most instances, eventually reach the ground (in situations where the lower levels of the atmosphere, near the ground, are eventually very dry, some of the precipitation aloft may evaporate before reaching the ground; this is known as virga).

Base Reflectivity, Tilt 1 is most useful for identifying where rain and/or snow are falling and how heavy the rain/or snowfall is. The radar does not delineate between rain and snow. The type of precipitation that is falling can be verified by noting the reports of surface observing stations within the coverage area of the radar.

Individual thunderstorm cells producing locally heavy rainfall and possibly hail can easily be identifies as small areas of very high dBZ level echoes and can be tracked by looping consecutive Base Reflectivity, Tilt 1 images in a time series. The radar does not delineate between thunderstorms and other non-thundering precipitation areas. Thunderstorms on NEXRAD Doppler radar can be verified as such, using lightning data. Areas of precipitation as indicated on radar that correspond with areas of lightning flashes as indicated in the lightning data for the same time period are indeed thunderstorms.

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. The radar is more sensitive when in clear air mode and is able to detect dry frontal boundaries, drizzle and snow (which typically shows up at reflectivities of 5 dBZ or lower) with greater precision and detail than when the radar is in precipitation mode.

The NEXRAD Doppler radar's sensitivity also allows users to view cold fronts, sea breeze fronts and thunderstorm outflow boundaries that have no precipitation associated with them and track their movement. Boundaries such as these appear as thin continuous lines on the Base Reflectivity product at reflectivities of 10 dBZ or lower. Other non-weather related phenomena are also detected periodically in NEXRAD reflectivity data due to the high sensitivity and high resolution of the radar, such as smoke plumes from grass and forest fires and movements of large flocks of migrating birds.

dBZ levels on the scientific version correspond to levels of precipitation intensity in terms of inches of rainfall per hour. The higher the dBZ level, the more intense the precipitation; they are directly proportional. "dBZ" stands for decibels of Z, Z being the reflectivity factor. The higher the reflectivity factor, the heavier the rainfall. This scale is logarithmic, so there is no factor one can use to multiply the dBZ level and arrive at the corresponding rainfall rate. Figure 3 shows approximate equivalents of dBZ and rainfall rates in inches per hour.

Figure 3 dBZ Levels With Equivalent Rainfall Amounts
dBZ level	Rainfall in inches/hour Precipitation Intensity
10 dBZ < .004	Very light rain or light snow
20 dBZ ~ .01	Light rain or moderate to heavy snow
30 dBZ ~ .08	Moderate rain or sleet showers
40 dBZ ~ .40	Moderate to heavy rain or sleet showers
50 dBZ ~ 1.75	Heavy thunderstorms
60 dBZ ~ 8.00	Intense to severe thunderstorms with hail

The 60 dBZ level would correspond approximately to 8 inches of rain per hour if all the precipitation were to fall as liquid rain. Sometimes there is hail falling in a storm with a reflectivity of 60 dBZ, so the precipitation that is falling is not all liquid. Above 60 dBZ, it is very difficult to arrive at a corresponding estimate for the rate of rainfall as there is likely hail mixed with the raindrops.

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