Radars send out short pulses (bursts) of electromagnetic wave fields. The pulses bounce off particles in the atmosphere and return back to the radar dish. A computer processes the returned signals and, through algorithms, can make conclusions about what kinds of particles it “saw,” including the speed of motion (the Doppler effect) towards or away from the radar. Non-polarimetric Doppler radars (including the NOAA radars before polarimetric upgrade) transmit horizontally polarized electromagnetic waves, which only give a measure of the horizontal dimension of the cloud particles (snow, ice pellets, hail and rain). This one-dimensional picture makes it difficult to tell the difference between precipitation types.
With dual-pol technology, the picture becomes two-dimensional because the radar sends both horizontally and vertically polarized electromagnetic waves. As these perpendicular fields bounce off an object and are received back at the radar, a computer program separately processes information about the horizontal and vertical dimension of the particles. This cross-section now gives forecasters a measure of the size and shape of the object.
Research to Operations
NSSL scientists helped develop the Weather Surveillance Radar - 1988 Doppler (WSR-88D) radars, also known as NEXt-generation RADar (NEXRAD). Since the first Doppler weather radar became operational in Norman in 1974, NSSL has worked to extend its functionality and effectiveness, and proved to the NOAA National Weather Service (NWS) that Doppler weather radar was a crucial forecasting tool. The NWS now has a network of 158 NEXRADs.
NSSL continues to develop and improve algorithms that detect and notify forecasters of hail, severe thunderstorms, tornadic circulations, downbursts and gust fronts. Research to operations activities also include developing and improving signal processing techniques.
TECHNICAL EXPLANATION OF NEXRAD
A NEXRAD site consumes approximately 50.8 kilowatts of energy. That includes the air conditioners and/or heaters to control the temperature of the hardware. The transmitter itself takes 15 kilowatts--roughly the same amount as 13 clothes irons.
NEXRAD radar draws its power from the normal power grid. However, it has a very robust UPS system to keep the radar operating even in the event of a loss of power. Imagine if the radar stopped operating because of the storm it was monitoring knocking out its power!
THE TRANSMITTER (KLYSTRON)
Although a NEXRAD site consumes about 50.8 kilowatts of power, the klystron (pictured to the left) is a special (and expensive) unit responsible for converting standard commercial power to the 750,000 watts of coherent energy that is transmitted in each pulse. It consumes 7.5 watts, gets hit with 47,000 volts, and amplifies it into the 750,000 watts needed for each pulse. The klystron takes the place of the magnetron used in older radar systems.
The peak effective power of a NEXRAD site is, as mentioned above, 750 kilowatts. However, the transmitter is only active between 0.05% and 6.1% of the time. As mentioned in the brief introduction the radar is only transmitting for about 7 seconds out of each hour--or 0.19% of the time. As such, the average output power of a NEXRAD site is 0.0019 * 750,000 = 1458 watts. The specifications put the average output power is 1300 watts. In reality, the exact average output power depends on the mode in which the radar is operating.
The width of the beam is directly proportional to the distance from the dish based on the calculation Width (in feet) = Distance (in miles) X 100.82. Essentially, the width of the beam increases by 100.82 feet for each mile the distance to the radar increases. Thus at 5 miles, the beam width would be 5 x 100.82 = 504.1 feet. The table to the right shows the beam width at 50 mile intervals.
Of course, not all the energy is contained within the focused beam. Outside the beam, as defined by the "1/2 power points," there is still some amount of energy, but it quickly tapers off. Sidelobe energy is energy emitted directly from the dish in directions other than that of the intended beam. This results in false returns from nearby targets, though the returns are so weak that they generally do not affect the performance of the radar
NEXRAD uses pulse repetition frequencies of between 318 and 1304 pulses per second, which translates to pulse repetition times of between 3.144 milliseconds and .766 milliseconds (766 microseconds). Therefore, the range of NEXRAD is between 471.6km and 114.9km
What is Level II Radar Data?
There are two different sets of data that NEXRAD sites put out. One is named NIDS and one is named Level II data. There are many advantages to Level II produced radar images. Level II, for example, are shown with 128 colors instead of 15 providing more detail in intensity. This allows the ability to delineate certain phenomena (e.g. gust fronts, hook echoes) that are sometimes hard to see using NIDS data. The Level II data is available in 0.5 dBZ increments, while NIDS reflectivity data is only available in 5.0 dBZ increments. This lower precision in the NIDS data causes large errors in any estimates of precipitation and precipitation rate.
Base Reflectivity, Base Velocity & Spectrum Width
- Base Reflectivity is one of the basic quantities that Doppler radar measures. Color intensity corresponds to the amount of radiation that is scattered or reflected back to the radar by whatever targets are located in the radar beam at a given location. These targets can be hydrometeors (snow, rain drops, hail, cloud drops or ice particles) or other targets (dust, smoke, birds, airplanes, insects).
- Base Velocity is the average radial velocity of the targets in the radar beam at a given location. Radial velocity is the relationship between the target's motion and the direction of the radar beam. Positive values (warm colors) denote out-bound velocities that are moving away from the radar. Negative values (cool colors) are in-bound velocities that are moving towards the radar.
- Base Spectrum Width is a measure of velocity dispersion within the radar sample volume. The primary use of this product is to estimate turbulence associated with mesocyclones and boundaries.
|Steerability||360 deg||-1 to +45 deg|
|Normal Scan||360 deg||+0.5 to +19.5|
|Max rotation rate||30 deg/sec||30 deg/sec|
|15 deg/sec2||15 deg/sec2|
|Mechanical Limits||360 deg||-1 to +60 deg|
|Positioning Error (max)||+/-0.2||+/-0.2|
|Pedestal Type: Elevation over Azimuth|
The RPG (Radar Products Generator) takes base data (reflectivity, velocity, and spectrum width) from the RDA and generates user-requested meteorological and hydrological products.
|Reflectivity Base Product||Velocity Base Product|
Signal Detection Capabilities (at 0 dB SNR)
|Signal Description||Signal Parameters|
|Minimum required signal detection, short pulse||-7.5dBZe at 50 km|
|Typical Dectection (for Ze=200*R1.6)||-10 dBZe at 50 km (rainfall of 0.01mm/hr)|
|Minimum required signal detection, long pulse||-23.0dBZe at 25 km|
|Point target detection||RCS = 4 cm2 at 100 km|
International Civil Aviation Organization airport code
The ICAO airport code or location indicator is a four-character alphanumeric code designating each airport around the world. These codes are defined by the International Civil Aviation Organization, and published in ICAO Document 7910: Location Indicators are used by air traffic control and airline operations such as flight planning. They differ from IATA codes, which are generally used for airline timetables, reservations, and baggage tags. For example, the IATA code for London's Heathrow Airport is LHR and its ICAO code is EGLL. Most travelers usually see the IATA code on baggage tags and tickets and the ICAO code is used among other things by pilots, air traffic control and flight-tracking services such as FlightAware. In general IATA codes are usually derived from the name of the airport or the city it serves, while ICAO codes are distributed by region and country.
ICAO codes are also used to identify other aviation facilities such as weather stations, International Flight Service Stations or Area Control Centers, whether or not they are located at airports.
List of NEXRAD WSR-88D sites and their coordinates.