WMOD00399

• Title: WMO Training Workshop on Weather Modification for Meteorologists - Lecture Notes
• Date: 12/1/1979
• Weather Modification - Doc Type: Report

- 4::1 -

radars additional calculations have to be made using the continuity property of flows
in the atmosphere. Results obtained using these systems are very interesting and will
doubtless influence the progress of ideas on the structure of movements in clouds.

3.7 There are, nevertheless, substantial limits to this method which reduce
the likeli'hood of its being used widely in cloud modification work. Examples are the
complex instrumentation used, the relatively small zone in which such measurements
can be carried out and the low measurement accuracy. The latter is due to the need to
use a working regime in which the antennae make a non-synchronous survey of the
studied space, this in order to increase the method's efficiency. As a result, the
dato used in the wind vector calculation for a given point in the spatial network are
on radial wind components measured at times more than one minute apart. When large
wind gradients are concerned, this can lead to substantial errors in the determination
of the resulting wind vector.

3.8 Of equal importance 1S the complexity of the actual observation
procedure and the time-consuming data processing requirements. For this reason one
of the main directions of the work in this field is to increase the efficiency of
observation and information processing. Only sufficiently efficient observations will
enable the transfer to be made from analysis and interpretation of individual measure-
ments to generalizations and experimental conclusions based on fully detailed factual
data, in other words to a study of thl~ actual dynamic properties of clouds.

3.9 For the above reasons, many researchers feel that despite the obvious
advantages of multi-point Doppler systems, it is best to continue work using a single
Doppler radar which enables many phenomena to be detected in connexion with cumulo-
nimbus clouds, e.g. meso-scale circulations, inflow areas and large up-drafts.
Donaldson (1970) proposed a set of cri.teria whose use enables eddies to be detected
reliably from a single radar's data. The main indicators are: t~e presence of a
significant tangential shift in the radial speed (up to 2.10-2s- ) and sufficient
steadiness in the value and sign of this shift in time. Two additional indicators
are an unvarying quality in the field of the shift area when the observation angle
is changed and a greater vertical extent of the shift area by comparison with its
diameter in the horizontal section. These or similar criteria are often used in the
analysis of images obtained by Doppler radars of the fields of radial speeds in large
cumulonimbus clouds. Much work has been carried out recently on this, especially
with a view to improving the instrumentation and methods for processing and presenting
data. The distributions obtained by this instrumentation for the radial components
of the divergence of the wind field in clouds and the vertical flow speeds calculated
according to these data seem to correspond ~'ell to the real flows in these clouds.
There is no doubt that the inclusion of Doppler radar observations in the PEP
programme would considerably broaden and strengthen its purpose and should enable
the possibilities of increasing the precipitation from specific clouds to be analysed
from a sound physical basis.

3.10 Considering the fact that Doppler radars have not yet become standard
observational techniques as have ordinary radar, without which it would be difficult
to imagine carrying out a study of cloud physics and weather modification, one
should bear in mind the possibility of extending the functions of non-coherent radars