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<br />- 42 - <br /> <br />characteristics motions. This research gave r1se to the important pra~tical <br />conclusion, which is now the basis for further work in this direction, :that the <br />spectrum of signal fluctuations in cloud and precipitation radar echoes. from a given <br />pulse volume repeats the distribution of the particles' radial speeds ~eighted by their <br />radar reflectivity in the pulse volume. In particular, the mean Doppl~r spectrum <br />displacement of the signal from the emitted pulse volume is proportional to the mean <br />radial speed of the scatterers in this volume which give the greatest qontribution to <br />the size of the reflected signal. Figuratively speaking, a Doppler radar can be <br />compared with an anemometer which can be placed at the selected point of a cloud <br />corresponding to the sounding direction and distance settings at which the signal 1S <br />propagated. The use of a multi-channel system for processing the signals and the <br />installation of an equal number of such instruments along the radar beam, and scanning <br />antennae along the azimuthal and vertical angles enable the volume of the cloud or <br />cloud system to be studied. <br /> <br />". <br /> <br />3.4 The simplest and most obvious application, which was therefore found <br />very early in this research, is the method of vertical sounding of clo~ds and <br />precipitation by Doppler radars. The radar accurately measures the ve~tical speed of <br />particle movement, but the calculation of the vertical air flow is obtained by <br />excluding from the measured value the speed of gravitational settling of these particles. <br />The latter can be estimated from the particle size determined by the r~dar reflect- <br />ivity if the phase state of the reflecting particles (raindrops, crystals) is known. <br />However, the estimates show that the vertical flow measurement accuracx does not <br />exceed 1-2 mis, and in complicated cases when the phase composition of :the scatterers <br />is not known, it can be much worse. As a result, this method of verti~al sounding <br />can only be used for studying large cumulonimbus clouds where the speeds of vertical <br />flow~ are expected to be great in comparison to the error. This method only provides <br />information in the cross-sections of clouds and precipitation which are' directly <br />above the radar installation site. However the likelihood that a cumuionimbus cloud <br />or, more importantly, a particular part of it which interests the researcher will <br />move over the radar installation site is very small, which is a disadvantage of the <br />method. This method could be substantially improved by installing the Doppler radar <br />on an aircraft, although this type of work is still in the initial stages of develop- <br />ment. <br /> <br />3.5 The most useful research on the use of Doppler measurements in weather <br />modification, is that directed towards the development of methods for obtaining <br />information on vertical motions or the horizontal wind vector in clouds and preci- <br />pitation using surface-based radar operating with angular scanning. Be~ause of this, <br />such systems have a greater possibility of carrying out the necessary measurements <br />in the selected meteorological target or in its zone. <br /> <br />3.6 Foremost in such attempts is the work actively carried out in the <br />U.S.A., France and other countries on the creation of systems consistirig of two or <br />three Doppler radars for obtaining data on wind vector distribution. The use of <br />several radars is necessary because the Doppler radar can only measure the radial <br />component of wind. When using three radars to measure three radial components in one <br />point of the cloud the speed vectors can be determined directly, where~s with two <br />