The Israeli Rainfall Enhancement Experiment A Physical Overview

Abstract

     This paper will attempt to give a summaryof the findings obtained during the period in which the two Israeli (I and II) cloud seeding experiments were conducted.  It includes a synthesis of all the information obtained through extensive measurements of cloud and cloud system properties ranging from mesoscale characteristics of the cloud systems to the microphysical structure of the cloud elements that form these systems.
     It is suggested that the positive effects on rainfall, under seeding, obtained in the course of these two experiments can be attributed to the following:
     1.  The organized winter cloud systems responsible for most of the raim in Israel almost always form in the cold sector of the prevailimg low pressure systems. They consist of cumulus cloud elements which are typically continental in nature with a fairly high degree of colloidal stability.
     2. The modal values of the cloud top temperature distributions on rain days are in the range of -15 to -22 C. Cloud temperatures are fairly uniformly distributed around 5 to 8 C.
     3.  The nature of the cloud droplet spectra in these clouds is such that ice crystal formation and its subsequent growth by riming seems to be the major rainforming process rather than that of the collision-coalescence mechanism.
     4.  On the average, the chain of events from initial ice crystal formation through graupel formation to the arrival of raindrops on the ground, can be described on a quantitative basis to suggest that seeding for "static" effects can increase the precipitation efficiency of the clouds in the range of top temperatures of -10 to -20 C and particularly in the range of -15 to -20 C.
     Statistical analyses of the rainfall data taking into account physical measurements lend the Israeli experiments the strongest support, namely that of physical plausibility. They suggest that:
     1.  The positive effects of seeding on all days can be stratified according to the daily mode of the cloud top temperature distributions to indicate a gradual and systematic increase of detectable seeding effects (and their statistical significance) in clouds converging from both ends of the cloud top temperature spectrum towards the most amenable range of clouds withbetween -15 to -21 C. Thus, confirming both our field studies and microphysical predictions.
     2. The area of maximums eeding effect, at a distance of 35-50 km downwind from the line of seeding, which has consistently been found to exist, can be attributed to the known patterns of turbulent diffusion of the seeding material released at cloud base altitudes.
     While far from being complete the above studies and analyses provide a fair basis for understanding and accepting the statistical results and thus also indicate which criteria should be used to transfer this knowledge.