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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.
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.
Abstract
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.
Abstract
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.
Abstract
No Abstract Available.
Abstract
No Abstract Available.
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.
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.
Results of 1,460 replies to a questionnaire on the usefulness of various forecast aids are summarized. Figures are presented showing the forecast aids, charts and diagrams in the order of their indicated usefulness to forecasters. The analysis of the replies shows that differences between services are minor and mainly attributable to differences in requirements and data availability. Differences in experience and training are minor but indicate some small variations. For several forecast aids discrepancies are noted between a particular aid's usefulness and its use.
Results of 1,460 replies to a questionnaire on the usefulness of various forecast aids are summarized. Figures are presented showing the forecast aids, charts and diagrams in the order of their indicated usefulness to forecasters. The analysis of the replies shows that differences between services are minor and mainly attributable to differences in requirements and data availability. Differences in experience and training are minor but indicate some small variations. For several forecast aids discrepancies are noted between a particular aid's usefulness and its use.
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.
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.
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.
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.
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.
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.
