Abstract

No abstract available.

Abstract

Rangno and Hobbs’s (RH) paper is a scattergun attack on the Israeli rain enhancement experiments that has no basis, as shown in this comment. This approach, unfortunately, has served to complicate rather than clear up questions relating to the Israeli experiment. The only area of agreement concerns some microphysical aspects of the Israeli clouds. But even there RH and the author disagree on their ramifications for cloud seeding effects. The existence of coalescence and ice multiplication in some of the Israeli clouds in no way precludes enhancement of precipitation, even from those clouds.

The claims of RH with respect to the Israeli experiments are many and varied. They claim and/or imply that (a) there is no physical basis for glaciogenic seeding in Israel; (b) the seeding was not conducted properly; (c) the experimental design was violated; (d) the evaluation was done selectively to obtain the highest effects; and (e) all the yet unexplained (by RH) seeding effects are due to type I errors (a lucky draw), twice in a row.

All of RH’s arguments are refuted, as described in detail in this comment. Therefore, the following can be concluded.

• The Israeli I and Israeli II cloud seeding experiments were executed and analyzed faithfully according to their experimental designs.

• The statistical analyses are valid and done according to the experimental design.

• The results show significant positive seeding effects in northern Israel.

• The magnitude of the seeding effect is plausible with respect to operational and physical considerations.

• The results of the Israeli I and Israeli II experiments confirm each other.

• Intermediate results of the Israeli III experiment in the south are in line with the previous results there.

• Target–control analyses of the operational seeding that followed the Israeli II experiment in the north show yet another replication of significant positive seeding effects there.

• There is mounting evidence that the desert dust is responsible for the difference in the seeding effects between north and south Israel.

The Israeli rain enhancement project remains an example of the strength and robustness of results that are obtained as an outcome of long-range planning in a scientific effort in the field of rain enhancement that has been continuous and consistent for three and a half decades thus far.

Abstract

A special method has been developed for the study of cells that are embedded in convective rain systems. This method consists of a package of computer programs that use pattern recognition techniques on three-dimensional digital radar data to identify the rain cells, track them with time, and calculate their properties. The product of the computations is a comprehensive database of physically meaningful properties of rain cells, which can be used to infer the internal structure and the dynamics of convective rain systems.

The cell-tracking method has been applied to the summer convective clouds of south Florida for the following purposes: (i) derivation of the relationship between the echo top height and the precipitation characteristics (e.g., area, water yield, rain intensity and duration of the rain cells); (ii) study of the microphysical behavior of cumulus clouds in relation to their cell properties; (iii) evaluation of the effect of seeding on cumulus clouds on the cell scale; and (iv) examination of cloud-to-ground lightning discharges in relation to convective cell intensity.

The cell-tracking method is also currently being used in rain enhancement projects in Texas in the United States, in Israel and in South Africa. The cell-tracking method, its products and their use in meteorological research are described in this paper.

Abstract

Enhancement of precipitation by cloud-seeding operations has been reported in many studies around the world in the last several decades. On the other hand, suppression of rain and snow by urban and industrial air pollution recently has been documented and quantified. Here it is shown that the two effects are the opposite sides of the same coin, demonstrating the sensitivity of clouds to anthropogenic aerosols of different kinds. This is done by analyzing the rainfall amounts in northern Israel during the last 53 years and explaining the changes there as the combined opposite effects of precipitation suppression by air pollution and enhancement by glaciogenic cloud seeding. Time series based on precipitation from rain gauges were analyzed for seeded and nonseeded days and periods in the experimental control and the target areas. The response variable is Ro, the orographic enhancement factor, which is the ratio of gauge-measured rainfall in inland hilly areas (500–1000 m) to the rainfall at the upwind coasts and plains. The results show that for the whole period of 1950–2002 the Ro of the hilly areas decreased by 15%. In the early nonseeded period (1950–60) Ro was found to be higher than the nonseeded days of the following period, which was the randomized experimental period (1961–74). This result apparently shows the effect of the increasing pollution. Factor Ro had an identical decreasing trend during the seeded days of the experimental period and through the subsequent fully operationally seeded period (1975–2002). However, the trend line of Ro was shifted upward by 12%–14% for the seeded rain time series in comparison with the unseeded time series. Thus, the opposite effects of air pollution and seeding appear to have nearly canceled each other in recent years, leading to the false impression that cloud seeding is no longer effective. However, the findings here suggest that if the operational seeding were to stop, Ro would decrease further by about 12%–14%. The sensitivity of Ro to both seeding and pollution effects was greatest in the areas with the greatest natural orographic enhancement factor and was practically nonexistent in areas in which Ro is near unity. This result suggests that the orographic clouds are the most sensitive to air pollution as well as to cloud-seeding effects on clouds and precipitation, in agreement with the large susceptibility of precipitation from such short-living shallow clouds to aerosols. Based on previous studies and on the results of this paper, it is suggested that the proposed mechanism is the most likely explanation to the observations, and no alternative explanations such as long-term trends in the cross-mountain moisture flux were found probable. It is certain that additional research is required.

Abstract

Statistical analyses suggest that cloud seeding has caused a net increase of rainfall only in northern Israel. These analyses also identify the reported desert dust as a detrimental factor for the seeding effectiveness. This paper deals with the question of what role the interaction of desert dust and the dynamic properties of the clouds plays in the determination of divergent seeding effects in Israel.

This question is investigated through analyses of the cloud seeding effectiveness in northern Israel (Israel-2 experiment and the operational seeding) stratified into days when the southern margins of the rain cloud system (SMR) locations were in the north or in the south and into “dust” and “no-dust” days.

The results indicate that the SMR plays an important role on dust days, where a seeding effect of 11% is indicated an days with the SMR in the south, and an effect of −11% is indicated on days with the SMR in the north. On no-dust days positive effects were indicated regardless of the location of the SMR. These results are consistent with the following observations.

1. The strongest interaction of desert dust with rain clouds in the north occurs on dust days when the, SMR is in the north.

2. When the SMR moves to the south, much of the desert dust is washed down by the intervening rain between the north and the SMR.

Abstract

Alpert et al. in a recent paper challenged the quantification of the suppression of orographic precipitation that was shown in two recent papers by Givati and Rosenfeld to occur in Israel. Their main claim was that the results were determined by the selection of the rain gauges. In this comment, it is demonstrated that when an objective selection of the rain gauges is applied to all of the rain gauges that were used by Alpert et al. and Givati and Rosenfeld, the outcome replicates the results of Givati and Rosenfeld and provides additional insights. At the final account, this comment further enhances the confidence that orographic precipitation has been suppressed over Israel. The direct evidence to the cause is still lacking.

Abstract

Experimental and operational cloud seeding in Israel has been conducted since 1961 by aircraft seeding of silver iodide (AgI) at cloud-base level along a line upwind of target areas. The first experiment, Israeli-1 (1961–67), had a crossover design. Its overall seeding effect was an enhancement of 15% of the rainfall in the target areas, significant at 0.9%. The second experiment, Israeli-2 (1969–75), had a generally similar crossover statistical design to that of Israeli-1 but with some modifications that allowed a separate evaluation of the north target area alone. The seeding effect obtained for the north alone was 13%, significant at 2.8%. Based on this, clouds in northern Israel have been seeded operationally since 1975, while randomized experimental seeding is continuing in the south as Israeli-3.

Recent analyses of both targets of the Israeli-2 experiment indicated that rainfall was not enhanced in the south target area. Preliminary and intermediate analyses of Israeli-3 do not indicate rain enhancement in the south either.

Reanalyses of the experiments, stratified by observations of dust haze, show an increase of 26% in Israeli-2 north on the 202 “no-dust” days and no effect on the remaining 182 “dust” days. According to Israeli-2 south and Israeli-3, the indicated seeding effects in the south were also more positive by 16% on the “no-dust” days than on the “dust” days.

The statistical evidence suggests that dust, blown from the Sahara-Arabian deserts bordering the target area to the south (or something else that is associated with the dust), plays an important role in the natural precipitation processes such that seeding is beneficial only when this dust is absent.

Abstract

Urban air pollution and industrial air pollution have been shown qualitatively to suppress rain and snow. Here, precipitation losses over topographical barriers downwind of major coastal urban areas in California and in the land of Israel that amount to 15%–25% of the annual precipitation are quantified. The suppression occurs mainly in the relatively shallow orographic clouds within the cold air mass of cyclones. The suppression that occurs over the upslope side is coupled with similar percentage enhancement on the much drier downslope side of the hills. The evidence includes significant decreasing trends of the ratio of hill to coast precipitation during the twentieth century in polluted areas in line with the increasing emissions during the same period, whereas no trends are observed in similar nearby pristine areas. The evidence suggests that air-pollution aerosols that are incorporated in orographic clouds slow down cloud-drop coalescence and riming on ice precipitation and hence delay the conversion of cloud water into precipitation. This effect explains the pattern of greatest loss of precipitation at the midlevel of the upwind slopes, smaller losses at the crest, and enhancement at the downslope side of the hills.

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.

Abstract

The accuracy of the estimation of Z–R relationships is evaluated for the Window Probability Matching Method (WPMM) and regression methods. The evaluation is based on experiments of random subsampling of disdrometer-obtained 1-min reflectivity Z and rain-rate R pairs. The simulation of the disparity between the radar and the rain gauge measurement volumes was done by 3-min time averaging of the reflectivity data. Geometrical mismatch and synchronization inaccuracies between the radar and rain gauges are simulated by desynchronization of dt minutes, that is, shifting the R and Z time series with respect to each other by dt minutes. The WPMM and bias-corrected regression methods have similar skill in estimating rainfall accumulation even when geometrical and synchronization errors are introduced. However, the WPMM has significant advantage in estimating the rain intensities when geometrical and synchronization errors are introduced to the radar–gauge-measured Z–R pairs for simulating real-world radar and rain gauge comparisons.

Regression-based Z–R relationships tend to overestimate the low rain intensities and underestimate the high rain intensities with the crossover at the estimated median rain volume intensity. This trend becomes more severe with the increased desynchronization. This reduction of the dynamic range of R does not occur when using WPMM.

Although rain gauge bias correction may render the overall rain accumulation insensitive to the power of the Z–R law, its appropriate selection has a major effect on the partition of rainfall amounts between weak and strong intensities or the partition between convective and stratiform rainfall.