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  • Author or Editor: Robert D. Elliott x
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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

Abstract

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Robert D. Elliott
and
Einar L. Hovind

Abstract

The data source for this study is a collection of five years of serial upper-air soundings taken during storms at five stations in the Southern California coastal and offshore region, along with supporting aerial, radar, and surface precipitation observations.

Detailed analyses of frontal systems revealed mesoscale motions and processes which are an important and integral part of the frontal structure. In particular, the flow pattern within the prefrontal precipitation region is found to be characterized by waves aloft and a matching cell structure below, with wavelength of 200 to 300 km and with crusts oriented parallel to the front. Within these cells are found small convection bands with which are associated sharp peaks in the precipitation distribution. The overall pattern slopes aloft over the front, and this slope, along with horizontal and vertical mixing, is an essential element in the dynamic balance within the frontal zone.

The intensity of the mesoscale vertical motion responsible for clouds and precipitation in the frontal zone appears to have some association with the degree of convective activity. In the stronger fronts, vertical velocity peaks of 20 cm sec−1 or more are found to be the rule.

The thermal balance is dominated largely by vertical differences in horizontal advection which are balanced by convective heat exchange. A strong, low-level current of warm air from the south overridden by cold air from the west determines to varying degrees the convective instability and results in a considerable amount of available potential energy being converted directly into convection-scale kinetic energy, thus by-passing its conversion to the cyclone-scale kinetic energy.

In fronts possessing greater stability, vertical velocities are less and the eastward movement of the front is greater, suggesting that in these cases the broad-scale deformation of the cyclone-scale thermal pattern, which controls the deepening and the occlusion processes, is accelerated.

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Robert D. Elliott
and
Einar L. Hovind

Abstract

Pacific storms entering Southern California have been intensely sampled and subjected to detailed investigation through a storm study program in the Santa Barbara area during the 1960–63 (inclusive) winter storm seasons.

One result which has emerged from the analyses of precipitation and upper-air data was the discovery that organized convection bands were a common feature within the main precipitation region. These bands were detected from storm precipitation distributions, which, through quasi-objective methods, have been separated into the following three components: storm mean motion precipitation, orographic precipitation, and convection band precipitation.

The typical convection bands appear to be 20 to 40 miles wide, centered some 30 to 60 miles apart, oriented along the upper shear vector (between winds in the convective cloud layers and the adjacent layer above), and moving along a direction of the lower shear vector. There is evidence that the increased convective activity within the bands is associated primarily with the destabilization of the air mass through differential thermal advection.

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Robert D. Elliott
and
Einar L. Hovind

Abstract

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
Russell W. Shaffer

Abstract

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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Robert D. Elliott
,
Pierre St. Amand
, and
John R. Thompson

Abstract

Tests of the effectiveness of ground-released pyrotechnics in enhancing precipitation in storms in Santa Barbara County were conducted during the three winter seasons of 1967–68, 1968–69 and 1969–70. The mode of operation and the type of pyrotechnic device remained fixed through the three years in order to develop a large sample of data. The observation unit employed was a convective band embedded within a general storm system. A series of pyrotechnic candles of the LW-83 formulation were ignited just prior to and during the passage of convective bands over the seeding site, located on a 3500-ft mountain ridge in the Santa Ynez mountains. The bands were detected upwind of the test area and tracked into the test area by use of telemetered raingages and weather radar. Out of a total of 85 bands, 43 were seeded and 42 not-seeded. The selection of bands to seed was made on a random basis following declaration of the approach of a seedable band.

Over 60 recording raingages extending over an area of ∼1500 mi2 provided the basic evaluation data. Soundings taken with a GMD-1 system just prior to band passage into the test area provided useful air mass documentation. The cases were stratified by stability and 500-mb temperature categories.

The statistical analysis shows that there was a statistically significant difference between the distributions of seeded and not-seeded band precipitation totals for stations distributed over a several hundred square mile area downwind of the point source of nuclei. Indications were that precipitation was increased by 50% or more. The effect was greatest in the case of the warmer and more unstable categories.

When the overall precipitation is considered, including the between-band (not-seeded) component, the net increase is about 32%. Precipitation between bands was not significantly changed by seeding.

A computerized seeding-area-of-effect model was employed to predict an envelope of areas of seeding effect for the various categories of seeded bands. The bulk of the stations for which seeded precipitation distributions were significantly different from the not-seeded distributions fell within these areas.

The test results show the value of seeding winter convective orographic systems with this pyrotechnic device. The test results also demonstrate the value of employing the convection band as a natural unit of seeding and of observation. The sensitivity of the statistical evaluation was greatly enhanced through the use of this approach.

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Robert M. Rauber
,
Robert D. Elliott
,
J. Owen Rhea
,
Arlen W. Huggins
, and
David W. Reynolds

Abstract

A diagnostic technique for targeting during airborne seeding experiments has been developed for the Sierra Cooperative Pilot Project (SCPP). This technique was used operationally during SCPP for real-time guidance to aircraft, providing 1) the location and orientation of the seeding line required to target ice particles created by seeding to a specified ground location and 2) an estimate of the areal coverage of the seeding effect on the ground. Procedures to use this technique as a real-time guidance tool during seeding operations in Sierra wintertime storms are discussed.

Three evaluation studies of the targeting method are presented. These include 1) comparisons of diagnosed wind fields with those measured by aircraft; 2) comparisons of ice particle growth rates and habits within seeded cloud regions with those used in the targeting computations; and 3) comparison of radar echo evolution within seeded cloud regions with calculated particle trajectories.

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Robert D. Elliott
,
Russell W. Shaffer
,
Arnold Court
, and
Jack F. Hannaford

Abstract

An introduction section discusses aspects of the basic design of the CRBPP that are criticized by Rangno and Hobbs (RH). Individual sections reply to the following five aspects of our analysis discussed by RH: diffusion of seeding agent, anomalously high nucleus counts, statistical analysis based on 6 h blocks, duration alteration, and the question of multiplicity in the analysis. The concluding section states that the analyses support the concept that cloud-top nucleation dominates the water balance under stable orographic conditions.

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Robert D. Elliott
,
Russell W. Shaffer
,
Arnold Court
, and
Jack F. Hannaford

Abstract

A five-year randomized cloud seeding program conducted in the San Juan Mountains of southwestern Colorado by the Bureau of Reclamation was completed in April 1975. The test design included randomization of the seeding by 24 h experimental days, and other features such as operation only during suitable cloud and wind conditions, and suspension to avoid adverse effects on the public and environment.

Previous experimentation by Grant near Climax, Colorado, during the 1960's had identified conditions under which clouds seeded with silver iodide would produce more snow than similar, untreated clouds. The purpose of the Colorado River Basin Pilot Project was to determine whether the experimental procedure applied at Climax would be effective in an operational mode. The objectives were twofold: 1) to test the physical concepts of weather modification potential, and 2) to test the practical weather modification potential for an operational technology in the continental weather systems that bring snow to the San Juans.

A formal statistical analysis based on precipitation data for 71 experimental treated days and 76 experimental control days found no significant difference between precipitation, gage by gage, on seeded and unseeded days, even after deletion of 22 control days suspected of contamination by previous seeding.

An a posteriori analysis, based on 6 h time blocks (in place of the 24 h day called for in the experimental design), indicates that positive seeding effects may have been achieved during periods of warm cloud-top temperatures, as expected from the Climax experiment. These positive effects may have been overbalanced in the experiment by decreases in snowfall due to seeding unfavorable cloud types.

The results of the a posteriori analysis suggest that an operational seeding program, flawlessly carried out with perfect forecasts and no periods of suspension, could increase 15 October–15 May precipitation by 10–12% in various portions of the drainage basin. The resulting average precipitation increase during the snow accumulation season, put into a hydrologic streamflow model developed as part of the evaluation, yields a potential increase in annual runoff of the San Juan River of 197 000 000 m3. In the Rio Grande Basin, on the downwind side of the crest, the potential increase in annual runoff is 186 000 000 m3.

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