Radar Meteorology

the use of radar for meteorological observations and measurements based on the scattering of radio waves by, for example, hydrometeors, dielectric discontinuities accompanying atmospheric phenomena, and aerosol particles. Moreover, man-made reflectors in the form of metallized chaff with a length of approximately λ/2, where λ is the radar wavelength, are dispersed in the atmosphere to provide meteorological information. Additional data are obtained from the special radar reflectors and transponders that are sent aloft in sounding balloons.

The reflection of radio pulses in the troposphere from turbulent layers and layers exhibiting temperature inversions was first observed in 1936 by the Americans R. Colwell and A. Friend at medium and short waves. The first reports of the detection of precipitation by means of radar sets operating at centimeter wavelengths came from Great Britain in early 1941. In 1943, A. Bent and others organized the first operational observations of showers and thunderstorms. Also in 1943, V. V. Kostarev in the USSR started measurements of wind speed and direction in the high layers of the atmosphere by tracking the motion of sounding balloons carrying passive reflectors.

Clouds, precipitation, and regions with large gradients of temperature and water vapor can be detected with radar sets, as well as ionized tracks of lightning discharges and other atmospheric phenomena. Radar observation can provide information on the position, movement, structure, shape, dimensions, and physical properties of the objects detected. When radio waves are scattered from cloud and precipitation particles and the size r of these particles is small in comparison with the wavelength λ (Rayleigh scattering), the magnitude of the radar signal is ~r⁶/λ⁴. Such strong dependence of the value of the reflected signal on particle size implies that regions with larger particles stand out in radar observations of clouds and precipitation; radar pictures therefore do not always coincide with visual dimensions. The intensity of the scattered signals decreases sharply with increasing λ, but at millimeter and shorter wavelengths the signal is also strongly attenuated. The frequency range of meteorological radar sets is therefore restricted, and the sets usually operate at centimeter and millimeter wavelengths.

Empirical relations, which permit a determination of the variation in the intensity and amount of precipitation in the region scanned by the radar set, have been established between the average power of the reflected signals and the intensity of precipitation. Greater accuracy in the measurement of the intensity of precipitation and water content of clouds is achieved by measuring the attenuation of radio waves, an attenuation measured by dual-frequency radar sets. If λ is comparable to the particle size, the scattering is no longer governed by the Rayleigh law; when the frequency dependence of radio-wave attenuation is known, it therefore becomes possible to use the measurements of reflected signals at several wavelengths to estimate the size of precipitation particles. For nonspherical particles, the scattering probability depends on the particle’s shape and orientation. The shape of cloud and precipitation particles and, consequently, their state of aggregation can be judged by the degree of depolarization of the reflected signals.

The motion of scatterers leads to a shift in the frequency of the reflected signals as a consequence of the Doppler effect. By measuring the Doppler shift in the frequency and in other parameters of the spectrum of radar signals reflected from clouds, precipitation, large aerosol particles, and artificial scatterers, it is possible to investigate the structure of different motions in the atmosphere, such as wind, turbulence, and streamline updrafts. With the aid of sensitive radar sets, regions of highly varying refractive index can be detected. These regions are associated with the formation of stable layers in the ground and boundary layers of the atmosphere and with regions of intense clear-air turbulence at altitudes up to 10–15 km. The intensity of turbulence in a clear sky is estimated from the magnitude of reflected signals and from the width of the signals’ spectrum due to the Doppler shift.

Through the use of radar in meteorology, current data on the wind at different altitudes can be obtained under any weather conditions. Wind speed and direction are calculated from the measured coordinates of a pilot balloon. The wind analysis is often read simultaneously with measurements of the temperature, pressure, humidity, and other atmospheric parameters; radar sets have therefore been developed for a comprehensive scanning of the atmosphere. Such sets make it possible to determine the coordinates of a radiosonde from the signals of its transponder and to receive telemetric information on meteorological elements.