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E. J. Smith

Abstract

Analysis of three cloud seeding experiments over areas of Australia suggests that results varied with cloud-top temperature; when cumulus and similar clouds had top temperatures ≲−10C rainfall was increased by seeding, but when the cloud tops were warmer rainfall was decreased.

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E. J. Smith and K. J. Heffernan

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E. J. Smith and K. J. Heffernan

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E. J. Smith, E. E. Adderley, and F. D. Bethwaite

Abstract

A cloud-seeding experiment was conducted in South Australia from 1957 to 1959 inclusive in which clouds over one of two areas were seeded with silver-iodide smoke released from an aircraft, with random choice of area. No precipitation increases due to seeding were detected.

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E. J. Smith, E. E. Adderley, and D. T. Walsh

Abstract

A cloud-seeding experiment was conducted in the Snowy Mountains of Australia from 1955–1959 inclusive. The objective was to determine if silver-iodide smoke released from an aircraft into clouds could increase the precipitation over a selected area. The method involved a comparison of the precipitation in a target area and that in a control area during randomized periods of seeding and no seeding. Over the five years, the ratio of the precipitation in the target to that in the control area was higher in seeded than in unseeded periods. Three statistical tests are presented which show that the seeded periods are different from the unseeded periods at significance levels of 0.03, 0.09 and 0.03 (one sided). This supports a positive seeding effect. Other analyses both detract from and support this contention. The net result is that the experiment in inconclusive. Further, improved experiments are proposed.

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E. J. Smith, E. E. Adderley, and F. D. Bethwaite

Abstract

From 1958 to 1963 an experiment was conducted in the New England region of Australia, in which clouds were seeded with silver iodide smoke released from an aircraft. Clouds over two areas were seeded, with random choice of area. Rainfall measurements in the two areas suggest that seeding increased the rainfall during the first year, but no net changes in rainfall could be detected in subsequent years. The seeding appears to have increased the variability of rainfall.

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R. E. Smith and R. J. Hung

Abstract

A three-dimensional, nine-element, high-frequency cw Doppler sounder array has been used to detect ionospheric disturbances during periods of severe weather, particularly during periods with severe thunderstorms and tornadoes. One typical disturbance recorded during a period of severe thunderstorm activity and one during a period of tornado activity have been chosen for analysis in this note. The observations indicate that wave-like disturbances possibly generated by the severe weather have wave periods in the range 2–8 min which place them in the infrasonic wave category.

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R. J. Hung and R. E. Smith

Abstract

Gravity waves with wave periods of 13 to 15 min and horizontal phase velocities of 90 to 220 m s−1 were present in ground-based observations of the upper atmosphere during time periods when tornadoes were occurring and gravity waves with wave periods of 20 to 25 min and horizontal phase velocities of 100 to 200 m s−1 were detected when a hurricane was present. Combinations of available neutral atmosphere data and model parameter values were used with a group ray tracing technique in an attempt to locate the sources of these waves. Computed sources of the waves with periods of 13 to 15 min were located within 50 km of the locations where tornadoes touched down from 2 to 4 h later. In the case of the waves with periods of 20 to 25 min it was found that the computed location of the source was roughly where the hurricane would be located 3 h after the waves were excited. The applicability of the present study to a tornado and hurricane warning system is noted.

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Vincent E. Larson and Adam J. Smith

Abstract

In various practical problems, such as assessing the threat of aircraft icing or calculating radiative transfer, it is important to know whether mixed-phase clouds contain significant liquid water content. Some mixed-phase clouds remain predominantly liquid for an extended time, whereas others glaciate quickly.

The glaciation rate of mixed-phase layer clouds is thought to depend on various factors, including number concentration of snow crystals, terminal velocity of snow crystals, and crystal habit type. This paper attempts to quantify some of these factors by deriving scaling laws (i.e., power laws) for the mixing ratio and sedimentation flux of snow at cloud base. The scaling laws are derived from the governing equation for snow mixing ratio. They neglect aggregation of snow crystals and accretion of supercooled liquid by snow crystals. The scaling laws permit arbitrary exponents and prefactors of the mass–diameter and fall speed–diameter power laws, allowing flexibility in crystal habit properties.

The scaling laws are tested using idealized large-eddy simulation (LES) of three thin, midlevel layer clouds. The scaling laws agree adequately with the LES over one order of magnitude for snow flux and over two orders of magnitude for snow mixing ratio. They indicate, for instance, that in the present LES, cloud-base snow flux and snow mixing ratio increase faster than linearly with increasing cloud thickness and supersaturation with respect to ice.

By varying the exponents and prefactors of the scaling laws, one may explore the sensitivity of glaciation rate to habit type. The relationship is complex, but, for the cloud cases examined, dendrites tend to glaciate cloud more rapidly than plates.

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Stephen E. Kenney and Phillip J. Smith

Abstract

A diagnostic analysis of energy conversion processes is undertaken to determine 1) the impact of latent heating and 2) the importance of scale in the release of available potential energy (APE). Data from moist and dry forecasts of an intense extratropical cyclone by the Drexel Limited Area Mesoscale Prediction System model are used to study the role of moist processes. Scale dependence is investigated by partitioning the quantity into an area average and eddy component and then separating the eddy component into long-wave and short-wave regimes.

The results show that the APE release is substantially larger in the moist case than in the dry case and that this difference is most pronounced in the major precipitation regions and above 500 mb. Also, the long-wave regime dominates over the short-wave regime, with the long-wave component comprising 96% of the eddy APE release.

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