WMOD00399

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

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2.

MEASUREMENT OF PRECIPITATION

2.1 Up to the present time the amount of precipitation falling over a
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particular area has been determined by measuring it at a series of points using pluvio-
graphs with further interpolation of the data over the whole area. The ~nterpolation
is necessary because of rain's variability caused by meteorological and ~rographical
factors. This method's accuracy depends on the accuracy of the point m~asurement at
ean pluviograph station, the number of pluviographs over the area and the method of
interpolation, as well as on the length of time during which the precipitation amount
is accumulated.

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2.2 The advantages of the radar method of areal precipitation measurement
are largely the same as those of most remote sounding methods, namely that the
measurement system does not influence the parameter bei~ measured and ~t can cover
areas which are difficult of access. It is also important that measurements can be
made over a large area from the one point where ~he radar is installed ahd where the
results are also collected. There are, of course, many factors which reduce the
accuracy of radar measurements and can limit the method's applicability pr else
complicate its use. In order to decide on the suitability of the radar method in
rain-stimulation work we shall consider its advantages and disadvantages'as applied to
PEP tasks. However, before proceeding with a discussion of areal rainfall measurement,
we shall first briefly discuss the characteristics of rain, or more genetally
precipitation, which have a direct connexion with the measurement problefu. Precipita-
tion can be divided into two main classes, prolonged extensive rainfall ~nd showers.
The first class is associated, for example, with slowly rising air masse$ on a front
or in the field of large-scale horizontal convergence, and the second, with unstable
air masses in which localized convergence occurs causing high vertical velocity. The
fact that showers result from the localized conveJgence explains why they consist,
at any given moment, of one or more individual precipitation zones within a cloud.
These zones have a lifetime of up to half an hour and horizontal dimensions of up to
5 km. In the longest showers (storms) or squalls, new cells devel~p as the old ones
decay. This leads to a considerable variability in the amount of areal precipitation
during the shower, in particular in directions which are perpendicular to that of its
movement. Steady precipitation has much less spatial and temporal variability but
may nevertheless vary in intensity by factors of 2 to 3 over distances of less than
20 km. The air currents have a considerable influence on the distribution of
precipitation, with the result that in hilly or mountainous terrain the precipitation
field may preserve its non-homogeneous structure for several hours. Thi~ leads to
the occurrence of orographic characteristics of areal cloud amount distribution.
Knowledge of the distribution of precipitation over the project target a~ea and of its
auto-correlation characteristics over the whole area (including possible control areas)
is extremely important when planning precipitation enhancement experiments. The
precipitation's degree of homogeneity determines to a significant extentithe possibility
of constructing a powerful randomized scheme for statistical assessment qf the
experimental results. The shape and disposition of target and control ateas can only
be selected when the natural precipitation distribution is known. Knowledge of how
the radar echo from precipitation is generated and develops in various c9nditions and
the amount of influence of meso-scale phenomena (orography, sea-breeze, lakes, etc.)
on its development, determines the optimum site selection and the prefer~ble method of
cloud seeding.