meteorological radar

(meteorology) A remote sensing device that transmits and receives microwave radiation for the purpose of detecting and measuring weather phenomena; includes Doppler radar, which is used to determine air motions (to detect tornadoes), and multiparameter radar, which provides information on the phase (ice or liquid), shapes, and sizes of hydrometeors.

A remote-sensing device that transmits and receives microwave radiation for the purpose of detecting and measuring weather phenomena. Radar is an acronym for radio detection and ranging. Today, many types of sophisticated radars are used in meteorology, ranging from Doppler radars, which are used to determine air motions (for example, to detect tornadoes), to multiparameter radars, which provide information on the phase (ice or liquid), shape, and size of hydrometeors. Airborne Doppler radars play a vital role in meteorological research. Radars are also used to detect hail, estimate rainfall rates, probe the clear-air atmosphere to monitor wind patterns, and study the electrification processes in thunderstorms that generate lightning discharges.

Commonly used, pulsed Doppler radar operates in the microwave region, with standard wavelengths of 10, 5, and 3 cm, referred to as S-, C-, and X-band radars, respectively. The electromagnetic radiation is focused into a narrow beam by illuminating a parabolic dish reflector with microwave energy provided by the radar transmitter. S-band radars require the use of large antennas (see illustration) to generate a narrow beam of microwave energy; transmit high power (peak power of 1 megawatt); and suffer relatively little attenuation as the radar beam passes through regions of heavy rain and hail. X-band radars use much smaller antennas to achieve similar narrow beams, and are highly portable. However, X-band radars suffer from attenuation when used to probe precipitation, which significantly limits their range. Attenuation results when the radar energy is either absorbed by the raindrops or reemitted from the raindrops in directions other than toward the radar. See also Antenna (electromagnetism); Doppler radar; Microwave.

Parabolic dish antenna of the CSU-CHILL 11-cm multiparameter Doppler radar operating at Colorado State University. The antenna is housed within a large, inflatable radome. (P. Kennedy, Colorado State University)

A pulsed Doppler radar typically emits 1000 electromagnetic pulses per second. These individual pulses are typically 1 microsecond (10⁻⁶ s) in duration. The Doppler radar provides information on the target's velocity, either toward or away from the radar when viewed along the radar beam. The Doppler shift, which is measured as a small difference between the frequency of the transmitted pulse and the frequency of the energy backscattered to the radar, provides a measure of the scatterer's radial motion. Scatterers in the case of meteorological radar include raindrops, ice particles (snowflakes), hailstones, and even insects, providing clear air returns. A Doppler radar also detects the amplitude of the backscattered signal, which can be used as a measure of storm intensity and as a means of estimating rainfall rates. See also Precipitation (meteorology); Scattering of electromagnetic radiation; Storm; Storm detection.

Dual-wavelength radar transmits electromagnetic energy at two wavelengths, and it also receives energy at both wavelengths. Typically, S- and X-band wavelengths are used. Dual-wavelength techniques were originally proposed to detect large hail. At S-band, hail is usually a Rayleigh target, whereas at X-band, hail is considered a Mie scatterer. Since radar energy is scattered in various directions by a Mie target, the power returned at the X-band wavelength is reduced relative to that at S-band. The presence of large hail is interpreted on the basis of the ratio of backscattered power at X-band to that at S-band. Dual-wavelength techniques are also used to estimate rainfall rates by comparing the backscattered power at a nonattenuating wavelength (S-band) to that at attenuating wavelengths (X-band). See also Hail.

Dual-polarization radar is able to transmit and receive both horizontally and vertically polarized radiation (the polarization of a radar beam is defined by the orientation of the electric field vector that comprises an electromagnetic wave). These radars are now used in meteorological research and have largely superseded dual-wavelength radars. A suite of multiparameter variables is being used to infer information on particle phase (ice or water), size, orientation, and shape.

Radar operating at a wavelength of 2 cm is in low Earth orbit (350 km or 210 m above the surface) and is used for mapping tropical precipitation. Understanding the amount and distribution of tropical rainfall is crucial for better understanding the Earth's climate. This space-borne radar and associated satellite was jointly developed by the United States (NASA) and Japan (National Space Development Agency); it is known as the Tropical Rainfall Measuring Mission (TRMM) satellite. Space-borne radar presents many challenging problems, including cost, size constraints, reliability issues, and temporal sampling. It is obviously impossible to continuously sample every precipitating cloud in the tropics from radar orbiting the Earth. But the TRMM satellite will help scientists develop a statistical distribution of rain rates within a certain area, and calculate the probability of a specific rain rate occurring. Based on this information, it will be possible to generate monthly mean rain amounts within areas of 10⁵ km². Such information will be vital for the verification of climate models. See also Satellite meteorology.

Radars used to probe the clear air, or regions devoid of clouds, are known as profilers. A profiler is essentially a Doppler radar that operates at much longer wavelengths compared to weather radar. Wavelengths of 6 m, 70 cm, and 33 cm are commonly used. In the case of a profiler, the reflected power is not only from hydrometeors but also from gradients in the index of refraction of air, which are caused by turbulent motions in the atmosphere. These turbulent motions in turn cause small fluctuations in air temperature and moisture content, which also change the index of refraction. A profiler can determine the airflow in the cloud-free atmosphere, roughly up to 10 km above the Earth's surface. Optical radars, called lidars, use lasers as the radiation source. At these short wavelengths (0.1–10 μm), the laser beam is scattered by small aerosol particles and air molecules, allowing air motions to be determined, especially in thin, high tropospheric clouds and in the Earth's boundary layer (approximately the lowest 1 km or 0.6 mi of the Earth's atmosphere). See also Aerosol; Hydrometeorology; Laser; Lidar.

In the late 1990s, the weather radars used by the National Weather Service to provide warnings of impending severe weather were updated from antiquated WSR-57 and WSR-74 noncoherent radars to NEXRADs (Next Generation Weather Radars). NEXRAD (WSR-88D) radars are state-of-the-art Doppler radars operating at a wavelength of 10 cm. Using NEXRAD's Doppler capability, weather forecasters are able to warn the public sooner of approaching tornadoes and other severe weather. In severe storms, a mesocyclone first develops within the storm. The mesocyclone may be 10 km (6 mi) or more wide, and represents a deep rotating column of air within the storm. Severe and long-lasting tornadoes are often associated with mesocyclones. The mesocyclone is readily detected by a Doppler radar such as NEXRAD. The entire continental United States is covered by the NEXRAD network, consisting of more than 100 radars. NEXRADs along the Gulf Coast, in Florida, and along the eastern seaboard provide warning information on land-falling hurricanes. About 60 of the nation's busiest airports are also equipped with Doppler radars. These radars (operating at a wavelength of 5 cm, known as Terminal Doppler Weather Radars) provide weather-related warnings to air-traffic controllers and pilots. One particularly dangerous weather condition is wind shear, which often occurs as a microburst or intense downdraft. Microbursts can severely affect the flight of landing and departing aircraft, and have been identified as a factor in many aircraft accidents. See also Radar; Radar meteorology; Tornado; Weather forecasting and prediction.