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A. Gagin

Abstract

Detailed measurements of the ice particle properties of winter continental cumulus clouds, performed by instrumented aircraft, are described and analyzed. The data presented, based upon measurements in 69 such clouds in the cloud summit temperature range of −5 to −25°C, suggests that the temperature dependence of ice crystal concentrations, at 300 m below cloud tops, corresponds to the mean ice nucleus temperature spectrum within a range of variation of about one order of magnitude. This rather limited discrepancy is proposed to be attributed partly to very limited processes of secondary production of ice crystals and partly to our inability to account, instrumentally, for the activation of nuclei through such mechanisms as contact nucleation.

These findings also suggest that cold continental cumuli do not provide conditions for noticeable effects due to ice crystal multiplication mechanisms. In particular, it is shown that the characteristics of the cloud droplet spectra, in such clouds, are the key factor in determining the ice budget of the clouds as they affect the timing and the nature of the riming processes that were proposed to lead to ice crystal multiplication. These findings are in good agreement with recent laboratory studies suggesting a strong link between temperature and droplet characteristics during the riming process, and ice crystal multiplication.

Consequently and in view of the shown dependence of graupel concentrations on temperature in cold continental cumuli, some conclusions are drawn regarding the formation of either natural or artificially induced precipitation in such colloidally stable clouds. These conclusions seem to be in agreement with observations elsewhere of precipitation likelihood as a function of cloud top temperature.

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A. Gagin
and
J. Neumann

Abstract

The Second Israeli Randomized Cloud Seeding Experiment was conducted during the six rainfall seasons November–April 1969–75. Its primary purpose was to examine the possibilities of enhancing rainfall in the catchment area of Lake Kinneret which serves as the principal reservoir of the Israel National Water Carrier. The present report constitutes the final statistical evaluation of this experiment. It has been preceded by a series of preliminary reports, some of which have dealt with specific subject areas.

The study shows positive overall results, of 13 and 18% increases of precipitation under seeding in the total North and Catchment areas, respectively. The corresponding significance levels are 2.8 and 1.7%. These findings may be attributed to the following combination of reasons:

1) The clouds are cold (winter) continental cumuliform clouds, associated mostly with cold fronts and post-frontal bands, and have microstructures which seem to suggest that the rain-forming processes most frequently operate through the growth of ice crystals, followed by the more rapid growth of riming by graupel particles rather than through the all-water processes of collision-coalescence.

2) The water budget of the prevailing cumuliform clouds (average base temperature of 8°C) is such that the addition of moderate concentrations of ice crystals probably increases the precipitation efficiency of these clouds.

3) The distributions of cloud-top heights and temperatures indicate a very high predominance of cumuliform clouds, with tops in the range of −15 to −20°C. Such clouds have been found to be most amenable to seeding for rainfall enhancement.

The above findings are supported by both experimental and theoretical evidence.

The detailed statistical tables indicate that the overall increases in rainfall can also be tested with regards to their physical plausibility. Thus, it will be shown that (i) the attempt to target the seeding effects, on the basis of diffusion studies of the seeding agents, to a predesignated area, seems to have been successful; (ii) increases in daily rainfall under seeding are very large when the mode of the cloud top temperature distribution is in the range −15 to −21°C—under such conditions, increases of 46% at an 0.5% significance level (a 90% confidence interval for the increase of 13–113%) were detected; and (iii) stratification of the data according to daily precipitation in the control area shows that the days with large precipitation contributed much less to the end result. The effect of seeding is most pronounced and consistent on most of the days (82%) when the daily rainfall is ≤15 mm. The mean rainfall was 8.1 mm.

It is suggested that the repeatability of significant and positive effects on rainfall under seeding, as demonstrated by the various statistical analyses, constitutes a measure of confirmation of the similar results obtained in the Israeli Experiment 1. The physical plausibility of these results adds further support to the statistically strong indications of a positive effect on rainfall.

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B. Terliuc
and
A. Gagin

Abstract

Measurements of cloud condensation nuclei (CCN) conducted both from the ground and from an instrumented aircraft during two winter seasons, utilizing a thermal diffusion chamber, suggest that.

  1. CCN spectra prove to always have, in both rainy and dry weather, continental-like characteristics. Their concentrations were shown to depend on wind direction and found to be surprisingly high, even in air masses with a partial maritime trajectory. The existence of local sources appears to have a limited and a generally minor effect on the characteristics of CCN spectra.

  2. On the average, CCN concentrations tend to decrease by a factor of 2 during the first three days of a rainy spell. Consequently, cloud droplet concentrations, computed for cumulus clouds assumed to have base updraft velocities of 250 cm sec−1, seem to show a similar trend but are always within the range of 180–950 cm−2.

  3. Since these variations do not seem to correlate with rainfall, they are not dominant in their effect on the relative efficiency of the rain-forming processes. However, the ever-present high concentrations of CCN, and thus, of cloud droplets, must have a significant effect on the nature of these processes in view of the resulting high colloidal stability of local clouds.

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N. Chodes
,
J. Warner
, and
A. Gagin

Abstract

An experiment is described, in which the rate of growth of cloud droplets, in a thermal diffusion chamber, is measured accurately to yield the value of the condensation coefficient. The data obtained from measurements of 40 individual droplets grown at four different supersaturations (0.40,0.49,0.62 and 0.72%) suggests that the values of the condensation coefficient are 0.037, 0.033, 0.031 and 0.033, respectively; the average value is 0.033 with a standard deviation of 0.005. Probable sources of error are discussed and rejected.

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A. Gagin
,
D. Rosenfeld
, and
R. E. López

Abstract

Radar volume scan studies employed two radars simultaneously to measure heights and other echo characteristics of convective cells. Maps produced from the date were used to identify and track cells, whose properties were then related to one another by regression analysis, with the following conclusions: 1) Cell rainfall and area are strongly related. 2) Rainfall intensity, maximum precipitation area, duration and rate of precipitation, and total rain volume are strongly dependent on cell-echo height. Equations defining these relationships are shown to take the form of power laws. 3) Times taken for cells to attain maximum lifetime properties are shown to be interrelated and to be proportional to other basic properties of the cells.

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William L. Woodley
,
John A. Flueck
,
Ronald Biondini
,
Robert I. Sax
,
Joanne Simpson
, and
Abe Gagin

The Florida Area Cumulus Experiment (FACE) is a long-term program to determine the potential of dynamic seeding for increasing convective rainfall over a fixed target area. The first phase of FACE (FACE-1) provided strong indications for increased, seeding induced rainfall. The second phase, FACE-2 (beginning in June 1978 and ending in August 1980), was conducted in an attempt to confirm these indications of a positive seeding effect. The criteria for confirmation in FACE-2 were published in a NOAA Technical Report prior to program commencement. A clarification and sharpening of these confirmatory criteria are discussed in this paper. In addition, a minority position of what is to constitute confirmation in FACE-2 involving the use of linear predictor models also is discussed. This paper was written and accepted for publication before the treatment decisions of FACE-2 were known.

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