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Arthur L. Rangno

Abstract

A six-season, randomized-by-season cloud seeding experiment consisting of three seeded seasons and three non-seeded seasons was conducted by Colorado State University (CSU) during the middle and late 1960's in the Wolf Creek Pass region of the San Juan Mountains of southwest Colorado. The results of the seeding have been reported in a series of papers as having produced statistically significant increases in precipitation at Wolf Creek Summit when the 500 mb temperature was ≥−23°C. Furthermore, it has been reported that increases in precipitation produced statistically significant increases in the runoffs from three target watersheds when compared to the runoffs from three control watersheds.

In this paper the results of the Wolf Creek Pass Experiment (WCPE) are reexamined. It is shown that the three non-seeded seasons occurred during meteorological conditions which brought “warm aloft” (500 mb temperatures ≥ −23°C) storm days with unusually light precipitation over a wide region of Colorado, northern New Mexico, southern Utah and northern Arizona. This bias produced high values of seed/no-seed precipitation ratios at Wolf Creek Summit which led to the misperception of large increases in precipitation due to cloud seeding.

It is also shown that nearly all central and southwest Colorado watersheds with similar exposures to the target watersheds for the WCPE had high runoffs during the three seeded seasons compared to the three control watersheds chosen. Hence, the increases in runoff reported from the three target watersheds were part of a large-scale pattern due to natural causes rather than to cloud seeding.

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Arthur L. Rangno

Abstract

Images of frozen drops with pieces missing were collected on two days of airborne sampling in shallow supercooled stratiform frontal clouds in the coastal waters of Washington State. In those limited regions where ice appeared to be newly formed, ice fragments with rounded portions accounted for about 5% of the total ice particle concentrations. These results are in rough agreement with the body of literature on laboratory experiments concerning the freezing of drops in free fall that have suggested a modest, though not insignificant, role for the fragmentation of freezing drops on total ice particle concentrations when larger supercooled drops are present.

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Arthur L. Rangno and Peter V. Hobbs

Abstract

Evidence is presented that the passage of an aircraft through supercooled clouds can produce high concentrations of ice particles (> 1000 L−1 at −8°C in one case). These Aircraft Produced Ice Particles (APIPs) are characterized, initially, both by their high concentration and very uniform size distribution. The ice particles are contained in a cylindrical-like volume of air that is initially oriented along the flight track of the aircraft; the diameter of the cylinder is ∼300 m after 5 min of elapsed time. Possible mechanisms for APIPs are discussed.

In view of these findings, care must be taken in the interpretation of data on clouds that have been penetrated by aircraft. It seems likely that some previous observations of abnormally high concentrations of ice particles in clouds were due to APIPs. Also, since APIPs mimic some of the effects produced by deliberate artificial seeding, it could complicate the evaluation of cloud seeding experiments.

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Arthur L. Rangno and Peter V. Hobbs

Abstract

Some significant differences exist between the precipitation amounts for Climax 2NW (a station situated near the center of the target area in the Climax I and Climax II cloud seeding experiments) that have been used in previous analyses of the Climax experiments and NOAA published values for this station. Some of the interpolated 500 mb temperature assignments used in previous analyses also differ in category (i.e., ≥ − 20°C or < − 20°C) from those derived from NOAA sounding data.

Using the NOAA data for precipitation at Climax 2NW, and the same control stations and series of statistical tests (which, in order of increased sophistication, are the seed/no-seed single, double and covariate ratios) used by the Climax Experimenters, the following results are obtained for interpolated 500 mb temperatures ≥ − 20°C at Climax 2NW. For the exploratory Climax I experiments: seed/no-seed single, double and covariate ratios of 1.65, 1.14 and 1.25, respectively (compared to seed/no-seed single and double ratios of 2.17 and 1.32 for Climax 2NW and a seed/no-seed covariate ratio of 1.34 for the “target group mean” precipitation reported by the Climax Experimenters). For the confirmatory Climax II experiment: seed/no-seed single, double and covariate ratios of 0.90, 1.04 and 0.90, respectively (compared to seed/no-seed single and double ratios of 1.24 and 1.17 for Climax 2NW and a seed/no-seed covariate ratio of 1.27 for the “target group mean” precipitation reported by the Climax Experimenters).

— We conclude that the Climax II experiment failed to confirm that precipitation can be increased by artificial seeding in the Colorado Rockies.

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Peter V. Hobbs and Arthur L. Rangno

Abstract

In a previous analysis by Hastay and Gladwell (1969) of the Skagit Cloud Seeding Project, the actual runoffs of the Skagit River during the two seeded years were compared to the runoffs predicted by a principal component (or covariate) analysis technique. It was concluded that seeding from ground generators with silver iodide increased the annual runoff of the Skagit River by at least 15% in the second year of the Skagit Project (the 1964 water year) and that this result was significant at the 0.005 (or higher) level. In this paper it is shown that this conclusion cannot be substantiated due to the inclusion in their analysis of a control river which behaved anomalously during the 1964 water year and on which the statistical significance of Hastay and Gladwell's result rests. Comparisons of the runoff of the Skagit river during the 1964 water year with the runoffs of two similarly situated rivers, with which the Skagit is well correlated historically, show no significant effects due to seeding.

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Peter V. Hobbs and Arthur L. Rangno

Abstract

Measurements and observations have been made on the development of ice in 90 cumulus (cumulus and cumulonimbus) and 72 stratiform (altocumulus, altostratus, nimbostratus, stratocumulus, and stratus) clouds. Ice particle concentrations significantly in excess of those to be expected from ice nucleus measurements (i.e., ice enhancement) were measured in 42 of the cumuliform and 36 of the stratiform clouds. For the complete data set, and for cloud top temperatures (TT) between −6° and −32°C, the maximum concentrations of ice particles (I max in L −1) in the clouds were essentially independent of TT(r=0.32). However, I max was strongly dependent on the broadness of the cloud droplet size distribution near cloud top. If the breadth of the droplet size distribution is measured by DT, such that the cumulative concentration of droplets with diameters ≥DT exceeds a prescribed value, then for −32≤TT≤−6°C:where n=8.4 and DO=18.5 μm for the cumuliform clouds and n=6.6 and DO=19.4 μm for the stratiform clouds.

When DT>D 0 and TT≤−6°C, initial concentrations of ice were intercepted near the tops of clouds in the form of clusters ∼5–25 m wide. These clusters form strands of ice which, with increasing distance from cloud top, widen and merge and may eventually appear as precipitation trails below cloud base.

In light of these findings, it is postulated that ice enhancement is initiated during the mixing of cloudy and ambient air near the tops of clouds and that it is postulated with the partial evaporation and freezing of a small fraction (∼0.1%) of the droplets approximately >20 μm in diameter. Contact nucleation might be responsible for the freezing of these droplets. Under suitable conditions, this primary mechanism for ice enhancement may be augmented by other ice-enhancement mechanisms (e.g., ice splinter production during riming, and crystal fragmentation).

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Peter V. Hobbs and Arthur L. Rangno

Abstract

Extremely high ice particle concentrations developed rapidly in the ascending tops of maritime cumulus congestus clouds after drizzle drops had already formed below this level by the collision–coalescence mechanism. In one building cloud with a top temperature no colder than −8°C, the ice particle concentrations increased from 0 to >350 L−1 within 9 min. In another cloud with a top temperature no colder than −13°C, the ice particle concentrations increased from ≤1 to ∼1100 L−1 within 12 min. Subsequently, the ice particle concentrations in these clouds decreased, even though the cloud top temperature of one of the clouds continued to decrease to −23.5°C.

The mechanism responsible for these prodigious increases in ice particle concentrations is not clear. The concentrations built too fast to be explained by the riming-splintering mechanism as it is presently formulated. It is suggested that high ice particle concentrations might form in localized pockets of high supersaturation with respect to water.

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Arthur L. Rangno and Peter V. Robbs

Abstract

Two statistical experiments, carried out in Israel, appeared for a time to have provided a unique demonstration of the ability of cloud seeding to increase rainfall. In this paper the authors examine the possibility that both experiments were compromised by type I statistical errors (i.e., “lucky draws” or false positives). It is concluded that in the first Israeli experiment a type I statistical error produced the appearance of statistically significant effects of artificial seeding on rainfall 1) in the buffer zone and the center target area, 2) in the coastal region of Israel, a few kilometers downwind of the seeding, and 3) in portions of Lebanon, Syria, and Jordan.

Analysis of the second Israeli experiment using the original crossover design produced a null result. However, when the two target areas were evaluated separately, naturally heavier rainfall over a wide region on days when the north target area was seeded produced the appearance of increases in rainfall due to seeding in the north target area, and when the south target area was seeded, the appearance of decreases in rainfall due to seeding was produced.

Target-control (as contrasted with crossover) evaluations of the second Israeli experiment for the north target area alone foundered when control stations were selected from a relatively small region of anomalously low seed/no-seed ratios that was situated within a much larger region of high seed/no-seed ratios, which included Lebanon, Jordan, and most of Israel. Thus, the north target area seed/no-seed ratios are not an isolated, seeding-induced anomaly. On the contrary, it is the low seed/no-seed ratios of the northern coastal control stations, selected after the experiment began, that are anomalous in a regional context and are virtually the only stations that yield an apparently statistically significant effect due to seeding in the north target area.

It is concluded that neither of the Israeli experiments demonstrated statistically significant effects on rainfall due to seeding.

Considerations of the rainfall climatology of Israel, recent reports concerning the microstructure of clouds in Israel, aid the relatively small amount of seeding carried out in the first Israeli experiment support the view that seeding was unlikely to have had significant effects on rainfall. Contrary to previous reports, clouds in Israel contain large cloud droplets, precipitation-sized drops, and considerable concentrations of natural ice particles at quite high temperatures, all of which should obviate attempts to increase rainfall by artificial seeding in wintertime air masses.

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Mark T. Stoelinga, Peter V. Hobbs, Clifford F. Mass, John D. Locatelli, Brian A. Colle, Robert A. Houze Jr., Arthur L. Rangno, Nicholas A. Bond, Bradley F. Smull, Roy M. Rasmussen, Gregory Thompson, and Bradley R. Colman

Despite continual increases in numerical model resolution and significant improvements in the forecasting of many meteorological parameters, progress in quantitative precipitation forecasting (QPF) has been slow. This is attributable in part to deficiencies in the bulk microphysical parameterization (BMP) schemes used in mesoscale models to simulate cloud and precipitation processes. These deficiencies have become more apparent as model resolution has increased. To address these problems requires comprehensive data that can be used to isolate errors in QPF due to BMP schemes from those due to other sources. These same data can then be used to evaluate and improve the microphysical processes and hydrometeor fields simulated by BMP schemes. In response to the need for such data, a group of researchers is collaborating on a study titled the Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE). IMPROVE has included two field campaigns carried out in the Pacific Northwest: an offshore frontal precipitation study off the Washington coast in January–February 2001, and an orographic precipitation study in the Oregon Cascade Mountains in November–December 2001. Twenty-eight intensive observation periods yielded a uniquely comprehensive dataset that includes in situ airborne observations of cloud and precipitation microphysical parameters; remotely sensed reflectivity, dual-Doppler, and polarimetric quantities; upper-air wind, temperature, and humidity data; and a wide variety of surface-based meteorological, precipitation, and microphysical data. These data are being used to test mesoscale model simulations of the observed storm systems and, in particular, to evaluate and improve the BMP schemes used in such models. These studies should lead to improved QPF in operational forecast models.

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