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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SSP-3: On-site physical investigation of cloud seedability and occurrence of
favourable seeding situations, more detailed climatic/synoptic studies.

SSP-I. Eighteen Members offered sites, six of those were selected tor SSP-2 on
the basis of criteria involving

1. The physical characteristics of the site (size and degree of homogeneity).
2. The characteristics of the precipitation (annual amount; regime, including importance
of the ice phase; proportion liquid to solid at the ground; and length of. rainy season).
3. Availability of meteorological data (density of rain gauge network; length of record;
other data like radar, rawinsonde, etc.).
4. Facilities (airport, instrument flight support, lack of flight restrictio~s; communica-
tions; working conditions).

'":.

Precipitation amounts of 51-70 cm per annum, with most of it in liq~id form, a
length of the rainy season of 3-4 months, a gauge density of > 6 per 1000 km2~ and a pre-
cipitation record of > 40 years were given the highest scores.

The optimum amount of annual precipitation was chosen on the basis of two con-
siderations: a given increase of rain available during the growing season le~ds to the
highest increase in crop production if the rain is marginal. On the other hand less rain
normally implies higher rainfall variability. Thus, the duration of a randomized experiment
would be longer not only from the point of view of number of rainfalls but also because of
its higher variability. The chosen optimal rainfall amount is a compromise. i

SSP-2 addressed the statistical evaluation problem through numerical experiments
by the Australian Commonwealth Scientific and Industrial Research organizatio~ in which the
detection of an assumed 10 or 20% increase in rainfall above the natural variability of the
rainfall (as established by historic records) was assessed. In particular: with what con-
fidence level can the effect of the seeding be detected (above the natural variability) in
a five year experiment? The conservation of resources and the urgency of the problem made
it necessary to optimize the experiment in every way. The statistical experi~ent provided
a key contribution for an objective site comparison.

A typical statistical data set is shown in Figure 1 with information about the
Spanish site. For the PEP seeding phase a more elaborate statistical experiment will be
undertaken once the results of SSP-3 are available.

Site visits and climatological studies reduced the eight sites of SSP-l to two,
namely Australia and Spain. Answers to questionnaires sent out by the WMO S~cretariat
revealed a lack of potential support for SSP-3 in Australia. Thus, the body with the over-
all responsibility for PEP, the PEP Board, had to decide in 1977 that Spain be the only
site for the field part of the site-selection study. This situation, caused by the gener-
ally depressed world economy, is deplorable from the scientific point of view;because the
chances to come up with a good final PEP site are reduced. All our hopes arejnow with the
Spanish site.

The EC-Panel on Weather Modification stated about the Spanish site that:

(a) The results of the statistical simulation experiment on the surface rainfall data met
the requirements for SSP-3;

(b) The available data and basic meteorological facilities as well as logistic possibili-
ties met the requirements;

(c) The terrain in the experimental area was uniform and suitable.

The meteorological and synoptic characteristics of the Spanish site (together with the
Australian one) will be discussed in detail in the PEP Report No. 10. May it suffice to say
that the site is centered around Valladolid (Figure 2) and has an annual rainfall of 30-60 cm.