Flexible Experimental Units and Cloud Stimulations in the Italian Rain Enhancement Project

During the Italian Rain Enhancement Project a high density network composed by 80 automated rain gages were installed and operated in the experimental area by UCEA (Ministry for Agricultural Resources) in the period 1992-1994. Sites were 10 km apart; precipitations detected and continuously recorded at time intervals not shorter than 10 minutes. Such possibility stimulated very much to put into practice an envisaged experiment for a different approach in collecting pluviometric data for the final statistical evaluation of the Project using flexible experimental units (Flex units), the idea being that during the long time interval of the traditional 24 hours experimental units, meso-scale atmospheric disturbances might produce additional rain amounts not at all connected to cloud stimulation.

This alternate unit was defined as the rain collected in the target and control areas, during the effective time duration of seeding plus a time interval ?t considered necessary for the stimulated cloud droplets to grow to precipitable size. The range of variability of ?t was arbitrarily assumed from 1 to 3 hours. Two of the main objectives of the experiment are:

-to collect an alternate data set useful to the final statistical evaluation of the project; -to investigate in details the chain of physical events taking place in the experimental area, in terms of cloudiness and precipitation, since the beginning of seeding operation.

In order to reach those objectives, the features of Flex units distribution were matched into the larger scenario of Cloud Top (every 5 minutes) and Rain Totals images computed by weather radar in the same time interval.

Achievements from the Italian experiment have proposed explanations about some of the many aspects of rain stimulation; the optimization of the system, of course, still represents Tecnagro's future task.

Flex units with ?t in the range from 1 to 3 hours collected within the high density pluviometric network, showed time continuity in the pattern of stimulated rain and acceptable coherence with radar data.

But no significance came out from the statistic between target and control areas, very likely because the data set collected in the target area appears deprived by extended area effects spread off-shore on the open sea, while control area's pluviometry is suspected to have been contaminated depending on wind direction and speed.

Radar analyses showed objective visual evidences of cause-effect relationship between cloud seeding, cloud top development and stimulated rains, at time even with ?t extended up to 4 and 5 hours, therefore furnishing better insight into the thermodynamic taking place also outside the experimental area at distances much farther than expected with respect to seeding paths. Indirectly this may suggest how slow is the process by which stimulated cloud droplets grow to precipitable size. Anyway, horizontal wind speed, up-currents of orographic origin and the atmospheric instability are factors of paramount importance that may influence upon the distances where stimulated rain patterns could be located on the ground by radar and rain gages.

At the same time, the meso-analysis adopted in our experiment has shown how inadequate the traditional 24 hours unit might be when adopted in the statistic, as in most previous cloud stimulation projects.