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Robert D. Elliott
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Robert D. Elliott

Abstract

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Robert D. Elliott

Abstract

This review provides a sketchy background of orographic weather modification activities prior to the 1960s, followed by a more critical review of major orographic projects carried out and reported in the scientific literature during the past 25 years. In the earlier of these major projects, evaluation of results had been based largely upon comparisons of seeded and nonseeded precipitation experimental units stratified by various sounding-derived parameters in an attempt to amplify the physical significance of the seeding effects within various sub-types of orographic clouds.

The later major projects are still underway with no final evaluations having been presented. However, a wealth of significant data analyses have been reported that provide important insights into the various natural and seeding precipitation mechanisms. Much of this is attributable to the new observational tools in use, which include airborne and ground microphysical sensors, doppler radar, and microwave radiometers.

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Robert D. Elliott

Abstract

Maps of hourly precipitation have been prepared for storms during the 1957–1960 Santa Barbara randomized seeding program. In non-seeded storms, they showed that approximately N–S oriented precipitation bands could be tracked eastward across the area. Similar maps for seeded cases showed that the bands were obscured by a strong stationary E–W oriented orographic band (the mountain range is oriented E–W).

Hourly station reports were arrayed in a table for each hour where row averages revealed the amplitude of the orographic effect and column averages that of the band effect. Row variance is related to the energy of the orographic precipitation-producing circulations, column variance to the band energy, and the residual variance, obtained by subtracting row and column variances from the total variance, to the energy of smaller-scale convective circulations. Attention was confined to the 7 hours of heaviest precipitation in each system.

In comparing seeded to non-seeded periods, the mean precipitation rate was more than double, and the total variance was almost five times as great. The proportion of the total variance in orographic form was more than double, the band variance was essentially the same, while the convective variance was less than a third of the non-seeded proportion.

It is concluded that the distribution of energy was shifted from smaller to larger scale circulation systems in going from non-seeded to seeded cases. The practical implications with respect to cloud seeding are discussed and illustrated by the results from two seasons of single generator tests made in the 1957–1959 period in the San Gabriel watershed near Los Angeles.

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Robert D. Elliott

Some general characteristics of synoptic developments in the 500 mb vorticity field are discussed with particular attention devoted to sudden injections of vorticity and to changes in circulation patterns downwind therefrom. Attention is then focused on vorticity injections generated at low latitudes, but not too low to interact in an important way with the middle latitude circulation. Composite maps based upon numerous cases are shown which reveal the development of large scale anomalous patterns in the westerlies during the 3 days subsequent to low latitude vorticity injection. The forecasting implications are discussed.

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Robert D. Elliott

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In connection with cloud seeding projects in Pacific Coast states, detailed analyses have been prepared of many storms. From this material, average values of significant parameters have been computed and are employed to construct a model of a typical cold-front-type occlusion moving into the California coast from the west, a type of storm responsible for a substantial fraction of California precipitation.

The model shows some deviations from the classical cold-front-type occlusion, such as the absence of a discernible elevated warm-front surface within 150 mi of the surface cold front. The system is discussed as a steady-state mechanism wherein the generation of liquid moisture in vertical currents is balanced by its removal through precipitation, primarily in the instability zone near the surface front, and by evaporation, primarily in the altostratus zone in advance of the instability zone.

The system being treated as an engine for the production of precipitation reaching the ground, its efficiency is discussed on the basis of simplified calculations of growth of precipitation in relation to ice-forming nuclei supply. It is shown that in this storm model efficiencies are normally well below 100 per cent, and that the introduction of artificial ice-forming nuclei can raise this efficiency markedly.

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Robert D. Elliott and Robert F. Strickler

An analysis of a series of cloud seeding projects conducted in three different Pacific-slope watershed areas, each during several seasons, is presented. Seven separate project-seasons are involved in each of which identical seeding procedures were used. Silver iodide smoke generators were operated at ground level for a total of well over 10,000 generator-hours during these operations.

Snowpack and other official precipitation records are examined and target-to-control-area comparisons made in order to bring out the effects of the seeding and apply statistical tests thereto.

It is concluded that this evaluation procedure, as applied to the available data, is capable of discerning the effectiveness of cloud seeding in increasing precipitation over a period of several seasons, but is not capable of bringing into focus the many details of interest to the cloud seeder.

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Robert D. Elliott and Einar L. Hovind

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A significant question bearing on the prediction of orographic precipitation and the seeding of orographic clouds is what fraction of the water condensed over an orographic barrier falls on the barrier as precipitation. This has been treated in a rather inadequate manner to date, largely because of lack of basic data.

Through the use of abundant storm-sounding data taken upwind of two Southern California orographic barriers and data from the corresponding mountain recording raingage networks, comparisons of computed condensation and observed precipitation have been made for a number of winter storms over a four-year period. The results indicate that approximately one quarter of the orographically produced condensate fell as precipitation on the watersheds.

A breakdown into air mass stability on the basis of the inflow rawinsonde data showed that, for similar orographic flow conditions, more precipitation was produced by unstable air masses than by stable air masses.

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Robert D. Elliott and Theodore B. Smith

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The basic problems involved in the investigation of the effects of “blocking action” upon the circulation pattern of the northern-hemisphere westerlies are defined and the mode of attack outlined. By application of turbulence principles seasonal variations in lateral mixing and northward heat transport across the westerlies are related to variations in the extent of blocking action. Evidence is presented indicating that blocking action, lateral mixing, and northward heat transport are all greater in the winters of years when strong north–south thermal gradients exist and vice versa. The year-to-year variations in these quantities exhibit a long-period and relatively large amplitude climatic swing with a period of roughly 12–14 years. Considerable year-to-year variation in the radiative properties of the earth's surface and atmosphere, or in incoming radiation, is thus implied.

An analysis of heat convergence through the lateral turbulence process shows in general the heart of the westerlies losing heat, particularly from the east coasts of the continents to the Aleutian and Icelandic Lows. This mechanism is apparently the means by which the large amount of heat picked up by the air from the oceans off the east coasts is dispersed.

A distinction is made between the turbulent eddy sizes identified with unstable, migratory systems and those of blocking action, the latter representing dynamically quasistable systems capable of producing effects upon the general circulation of the westerlies at great distances both upstream and downstream and over a period of many days.

Effects resulting from the intrusion of stable, persistent high-pressure cells into the normal westerly flow of the general circulation of the atmosphere in the characteristic manner of blocking action are investigated by means of empirical, semi-statistical procedures. These processes are found to cause the development of long stable wave patterns in the atmosphere downstream from the point of inception. These stationary patterns may occasionally exist for periods of as long as a month. The wave lengths are found to be longer than those generally considered representative of stationary wave patterns. Presumably this difference is due to the distorting effect of the larger amplitude of the long waves associated with the blocking action process.

A theory of the formation of these blocking high cells is suggested based on the accumulation of heat in low latitudes and the necessity for the readjustment of the general circulation to redistribute this heat. One means of dissipation of the blocking cells is shown to be the formation of a wave pattern in the atmosphere which is out of phase with the stable wave pattern formed by the blocking high.

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Robert D. Elliott and Russell W. Shaffer

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The physical basis for a relationship between orographic precipitation and air-mass characteristics, wind flow pattern and gross terrain features is outlined. Consideration is also given to the manner in which the precipitation falls from cloud and is caught in a rain gage. A model is developed which is employed in conjunction with numerous storm sounding data to establish semi-empirical relationships between precipitation at four mountain stations in Southern California and upwind air-mass characteristics.

The sounding sites were several hours upwind of the mountain stations. This arrangement makes it possible to employ the relationships established for short term quantitative precipitation forecast purposes as well as for cloud seeding evaluation. It is believed that the general method employed is applicable in other climatic zones.

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