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Abraham Gagin

Abstract

In static-mode seeding two assumptions are usually made: a deficiency in concentrations of natural ice crystals is the reason for delay, or even failure, of precipitation formation in certain cloud conditions; and, moderate increases in ice crystal concentrations, obtained by glaciogenic seeding of such clouds, will result in rainfall enhancement either by making the already existing process of rain formation more effective or by inducing precipitation formation in clouds that otherwise would not have precipitated naturally.

The basic assumption behind seeding for dynamic effects is that increased cloud buoyancy, achieved through conversion of supercooled water to ice by seeding, will cause an increase in cloud depth, which in turn will result in stronger rainfall intensities, areas and durations.

These basic assumptions are examined in terms of physical and statistical analyses of data from Israeli II (a static-mode seeding project) and FACE-2 (a dynamic-mode seeding project).

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Daniel Rosenfeld and Abraham Gagin

Abstract

Several important factors that govern the total rainfall from continental convective clouds were investigated by tracking thousands of convective cells in Israel and South Africa. The rainfall volume yield (R vol) of the individual cells that build convective rain systems has been shown to depend mainly on the cloud-top height. There is, however, considerable variability in this relationship. The following factors that influence the R vol were parameterized and quantitatively analyzed: 1) Cloud base temperature—it is shown that when other factors are fixed, a 50% increase in the absolute humidity of the cloud base will nearly double the R vol. 2) Atmospheric instability—cells in a more unstable atmosphere will rain much less (up to a factor of 5) than cells which are forced to grow to a similar maximum height in a more stable atmosphere. We suggest that more stable cells rain more because they grow more slowly, so that there is enough time for the cloud water to be converted into precipitation particles. 3) The extent of isolation of the cell—it is shown that isolated cells precipitate only about one-third of the R vol of highly clustered cells, having the other factors be identical.

It is also shown that a strong low level forcing increases the duration of R vol of clouds reaching the same vertical extent.

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K. Ruben Gabriel and Abraham Gagin

Abstract

This paper reports separate analyses of daytime and nighttime precipitation based on data from recording raingages of the second Israeli randomized experiment. These analyses seemed important because there are a number of hypotheses on the differential effects of AgI seeding during the day and at night, and because about half of the seeding of this experiment was done at night. Our findings are unfortunately equivocal because of the large variability of the date which had to be broken down by categories of modal cloud-top temperatures and 12-b (day/night) periods. The increase of precipitation efficiency that had been noticed (on 24-h data) to occur in the −12° to −21°C window appears to have been larger at night, but the difference is not significant. The increase in the number of rain events for warmer clouds may have been only a daytime effect, but again, the present data are not conclusive.

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Abraham Gagin and K. Ruben Gabriel

Abstract

Earlier Published analyses of the second Israeli randomized experiment (1969–75) were restricted to 24-h data; this paper provides more details which are based on continuous time data from recording raingages. The present analyses confirm that when cloud tops were warmer than −21°C, seeding increased the efficiency of precipitation. In the −21° to −11°C window, both amount and duration of rainfall increased by some 50%, but no extra rain events appeared. Extra rain events were apparently initiated by seeding when cloud-top temperatures were warmer (−11°C and above); however, this did not significantly increase the amount of rainfall. No effect of seeding was found when cloud tops were colder than −21°C. It appears that seeding makes the existing process of rain formation more effective and also inducts precipitation formation in some clouds that would not have precipitated naturally.

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Abraham Gagin, Daniel Rosenfeld, William L. Woodley, and Raul E. Lopez

Abstract

Volume scan radar studies incorporating the use of an elaborate method of defining and tracking convective rain cells through their lifetime have been used to

a) Explore and verify, in quantitative terms, the basic tenet of the technique of cloud seeding aimed at producing dynamic effects. This technique relates increases in the depth of convective cells, assumed to occur due to this type of seeding, to corresponding increases in the treated cells' rainfall intensity, area and duration of precipitation and, consequently, to the total yield of rainfall volume.

b) Employ the data gathered on the gross properties of rainfall of convective cells, namely their heights, intensities, precipitation areas and their durations and total rain volume, to estimate the effect of seeding, if any, on their properties.

These studies suggest that seeding convective cells for dynamic effects affected the preceding properties of these cells in a manner that resulted in increases in their total rainfall and that the positive changes in these properties could be predicted from the changes in maximum cell height following seeding.

The effect of seeding appears to be strongest for cells treated early in their life cycle with a substantial amount of AgI (i.e., more than 600 g). Seeding effects of 22% increases in cell heights and over 100% increases in cell rain volume are indicated under such seeding conditions. The significance levels of these results are found to be 2.1% and 0.6%, respectively.

The positive effects produced by seeding on the AgI treated cells may have resulted in a compensating negative effect on the smaller untreated clouds forming in the vicinity of these treated cells.

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