Robert D. Elliott

On Convection Bands Within Pacific Coast Storms and Their Relation to Storm Structure

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 ba...

The Development of Quantitative Relationships between Orographic Precipitation and Air-Mass Parameters for Use in Forecasting and Cloud Seeding Evaluation

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.

The Water Balance of Orographic Clouds

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.

Randomized Cloud Seeding in the San Juan Mountains, Colorado

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 1960s 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 ...

A Diagnostic Technique for Targeting during Airborne Seeding Experiments in Wintertime Storms over the Sierra Nevada

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.

Heat, Water, and Vorticity Balance in Frontal Zones

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 ...

Santa Barbara Pyrotechnic Cloud Seeding Test Results 1967–70

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 ∼1...

Effects of Seeding on the Energy of Systems

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 precipitatio...