OBSERVATION OF CLOUDS AND RAIN USING RADARS

The exceptional role of atmospheric moisture both in the processes that form the meteorological situation and in the methodology for establishing weather forecasts deservedly places it at the center of attention of modern meteorology. However, the study even of such basic phenomena as the condensation of water in the atmosphere, cloud formation, and precipitation encounters serious obstacles because of the absence of convenient and effective methods for observing and measuring the water content in the atmosphere at various altitudes and in various phase states. Moreover, there are practically no convenient methods for determining such an extremely important characteristic—important, for example, for aviation—as the spatial extent of cloud formations. Therefore, every success in improving or creating new means for observing atmospheric moisture is highly significant. Especially important in this respect is the creation of methods that do not require raising apparatus into the air and that make it possible to conduct regular measurements from the earth’s surface. New possibilities of this kind are being created, in particular, as a result of the development of radar technology.

Earlier (see, for example, 1) data were published indicating noticeable absorption of microwaves by atmospheric moisture, as well as in fog, clouds, and rain; this can be used to study these objects by methods of absorption spectroscopy. In the two brief communications reviewed here\(^{2,3}\), certain very scanty data are presented from, apparently, fairly extensive but, as the authors indicate, classified investigations carried out in England in recent years. Unlike the preceding works, here the usual radar methods are already being directly applied for observing clouds and rain. In general terms, the radar technique is a direct modification of optical sounding of the atmosphere by a searchlight beam, proposed as early as 1905 by V. V. Kuznetsov and successfully developed in recent years by N. A. Kalitin and others (see, for example, 5). It consists in sending a ray of light (or microwave radiation) in some direction and, by measuring the intensity of the scattered radiation, judging the scattering capacity of the medium traversed by the ray. An essential difference is that, in the case of microwaves, the use of short-duration pulses and the comparatively great transparency of the scattering medium make it possible to obtain information easily about the scattering capacity in different directions and at different distances from the transmitter. (In the case of optical sounding this is possible only in clear weather.) But the small magnitude of the scattering capacity of clouds for microwaves is also the main difficulty facing radar methods. Since the measurements are made in the centimeter range (in communication 2, \(\lambda = 3\) cm and the pulse duration is \(1\) microsec), scattering by water droplets must obey Rayleigh’s law and, consequently, the scattering coefficient must be proportional to \(R^6/\lambda^4\), where \(R\) is the radius of the droplets. It is not surprising that, in the first instance, only “heavy rain” can be observed, i.e., rain with large droplets giving a large amount of precipitation (tropical showers, thunderstorms). At the same time, the successful observation of “heavy rain” makes it evident that further improvement of radar technology and a transition to shorter waves will make it possible to extend observations also to ordinary clouds and rain.

The photographs shown, borrowed from \(3\), illustrate the present possibilities of the method. In Fig. 1 one can clearly see a layer of clouds belonging, according to the author, to the type that occurs with a weakly expressed and slowly moving or quasi-

stationary front. In Fig. 2 the distant echo was obtained from a thunderstorm, while the nearer one was from rapidly developing cumulonimbus rain clouds (CuNb). Fig. 3 corresponds to the case of two strong thunderstorms—one directly in the area of the radar station, the other at a distance of 25–36 miles from it. A small isolated echo at an altitude of 17,000 feet was caused by an aircraft carrying out parallel meteorological measurements.

Fig. 1. Ground layer of clouds.

Fig. 2. Distant thunderstorm, extending upward to 12 km.

Fig. 3. Near and distant thunderstorms.

The figures show that modern apparatus already makes it possible, while located on the earth’s surface, to obtain information on the location and movement of heavy clouds and rains in an area extending up to 70 km in radius and up to 12 km in height. It is quite obvious that further development of the method must turn it into a powerful tool for investigating the formation of precipitation as well as the dynamics of atmospheric motions.

G. Rozenberg

REFERENCES CITED

1. V. L. Ginzburg, UFN 34, 469 (1948).
2. H. Whalley and G. J. Scoles, Nature 163, 372 (1949).
3. R. F. Jones, Nature 163, 728 (1949).
4. V. V. Kuznetsov, Izvestiya Akademii nauk, series V, 28, 4–5 (1905).
5. I. A. Khvostikov, Izvestiya AN SSSR, Physical Series 10, 403 (1946).