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- Author or Editor: A. S. Dennis x
- Journal of Applied Meteorology and Climatology x
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Abstract
First radar echoes on a randomized cloud seeding project in North Dakota appeared closer to cloud base and at higher temperatures on seed days than on no-seed days. The average first echo temperature was near −11°C on no-seed days and −7°C on seed days.
Abstract
First radar echoes on a randomized cloud seeding project in North Dakota appeared closer to cloud base and at higher temperatures on seed days than on no-seed days. The average first echo temperature was near −11°C on no-seed days and −7°C on seed days.
Abstract
The number per unit area of radar echoes from isolated showers, as a function of echo radius r, is found by analysis of data from several localities to he proportional to exp (−βr), where β varies with time and place inversely as the intensity of convective activity. Calculated values of β range from 0.8 (n mi)−1 for intense convection to 1.7 (n mi)−1 for light shower activity occurring in trade-wind cumuli.
Abstract
The number per unit area of radar echoes from isolated showers, as a function of echo radius r, is found by analysis of data from several localities to he proportional to exp (−βr), where β varies with time and place inversely as the intensity of convective activity. Calculated values of β range from 0.8 (n mi)−1 for intense convection to 1.7 (n mi)−1 for light shower activity occurring in trade-wind cumuli.
Abstract
Conditions within 16 cumulus congestus clouds were measured on aircraft penetrations near the −5C level and precipitation developments during the subsequent 45 min were noted. Significant correlations exist between the in-cloud conditions and the time of onset and intensity of precipitation. The degree of spatial correlation among the fields of temperature, vertical motion and water content within a given cloud proved more important to subsequent developments than did the mean values of these variables.
Abstract
Conditions within 16 cumulus congestus clouds were measured on aircraft penetrations near the −5C level and precipitation developments during the subsequent 45 min were noted. Significant correlations exist between the in-cloud conditions and the time of onset and intensity of precipitation. The degree of spatial correlation among the fields of temperature, vertical motion and water content within a given cloud proved more important to subsequent developments than did the mean values of these variables.
Abstract
Salt seeding took place below one end of a line of stratocumulus clouds with 350 lb of NaCl released. Cloud base was 9000 ft and cloud tops were at 15 to 18,000 ft above sea level. Cloud top temperature was near −2C and updraft speeds below the hue were near 3 m sec−1. The resulting shower was monitored by radar with the total rainfall being estimated at 280 acre feet. No rain fell from the unseeded portion of the cloud line or from any other clouds within 50 mi.
Abstract
Salt seeding took place below one end of a line of stratocumulus clouds with 350 lb of NaCl released. Cloud base was 9000 ft and cloud tops were at 15 to 18,000 ft above sea level. Cloud top temperature was near −2C and updraft speeds below the hue were near 3 m sec−1. The resulting shower was monitored by radar with the total rainfall being estimated at 280 acre feet. No rain fell from the unseeded portion of the cloud line or from any other clouds within 50 mi.
Abstract
The height and temperature of first radar echoes observed on 49 test cases in a randomized cloud seeding project have been related to seeding treatment and to five cloud parameters, namely, cloud base height, cloud base temperature, maximum radar echo height, and the height and speed of maximum updraft as computed with a cloud model.
First echo height above cloud base in unseeded clouds averaged 11,000 ft and was strongly correlated with maximum updraft speed. First echo height in clouds seeded with silver iodide averaged 6900 ft. It was most closely correlated with cloud base temperature, suggesting that first echoes tend to appear at some fixed temperature. First echoes in salt seeded clouds appeared on an average 5300 ft above cloud base and showed no significant correlation with any of the cloud parameters mentioned.
It is concluded that both salt and silver iodide seeding can cause first echoes to appear closer to cloud base in unseeded clouds, with salt probably having a more pronounced effect in summertime convective clouds.
Abstract
The height and temperature of first radar echoes observed on 49 test cases in a randomized cloud seeding project have been related to seeding treatment and to five cloud parameters, namely, cloud base height, cloud base temperature, maximum radar echo height, and the height and speed of maximum updraft as computed with a cloud model.
First echo height above cloud base in unseeded clouds averaged 11,000 ft and was strongly correlated with maximum updraft speed. First echo height in clouds seeded with silver iodide averaged 6900 ft. It was most closely correlated with cloud base temperature, suggesting that first echoes tend to appear at some fixed temperature. First echoes in salt seeded clouds appeared on an average 5300 ft above cloud base and showed no significant correlation with any of the cloud parameters mentioned.
It is concluded that both salt and silver iodide seeding can cause first echoes to appear closer to cloud base in unseeded clouds, with salt probably having a more pronounced effect in summertime convective clouds.
Abstract
Silver iodide seeding of convective clouds in a number of experiments in South Dakota has yielded mixed results, including both increases and decreases in rainfall and, apparently, decreases in hail. The results vary with the synoptic situation and with location with respect to the silver iodide release point; there is tentative evidence that they can extend upwind. Two dynamic effects of seeding are postulated, namely, increased growth of seeded clouds and the suppression of neighboring clouds at distances of some tens of miles.
Abstract
Silver iodide seeding of convective clouds in a number of experiments in South Dakota has yielded mixed results, including both increases and decreases in rainfall and, apparently, decreases in hail. The results vary with the synoptic situation and with location with respect to the silver iodide release point; there is tentative evidence that they can extend upwind. Two dynamic effects of seeding are postulated, namely, increased growth of seeded clouds and the suppression of neighboring clouds at distances of some tens of miles.
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Abstract
Ten seasons of commercial cloud seeding in Santa Clara County, California, are evaluated using target and control stations selected in 1955. Comparison of the linear regressions of target rainfall upon control rainfall, using seasonal totals, for the seeded winters and for ten immediately preceding ones gives evidence for a net increase in target precipitation. Examination of the rainfall distribution within the target area, which was not a part of the original evaluation plan, shows that seeding effects are concentrated downwind of the highest part of the Santa Cruz Mountains, which form the southwestern boundary of the target area, and suggests that they are limited to periods of convective instability.
Abstract
Ten seasons of commercial cloud seeding in Santa Clara County, California, are evaluated using target and control stations selected in 1955. Comparison of the linear regressions of target rainfall upon control rainfall, using seasonal totals, for the seeded winters and for ten immediately preceding ones gives evidence for a net increase in target precipitation. Examination of the rainfall distribution within the target area, which was not a part of the original evaluation plan, shows that seeding effects are concentrated downwind of the highest part of the Santa Cruz Mountains, which form the southwestern boundary of the target area, and suggests that they are limited to periods of convective instability.
Abstract
A 3-year randomized crossover seeding experiment has been conducted in South Dakota to test effects of artificial nucleation upon supercooled convective clouds of spring and early summer. The associated rainfall observations have been analyzed by several statistical techniques. The principal conclusions are: 1) on days with isolated showers, rainfall has been heavier in the seeded target area than in the unseeded target area; 2) on days with widespread convective activity and southwesterly winds aloft, rainfall has been lighter in the seeded target area than in the unseeded target area, except for the regions 10–20 miles east of the Black Hills; and 3) on days with widespread convective activity and northwesterly winds aloft, rainfall has been lighter in the seeded than in the unseeded target area.
Abstract
A 3-year randomized crossover seeding experiment has been conducted in South Dakota to test effects of artificial nucleation upon supercooled convective clouds of spring and early summer. The associated rainfall observations have been analyzed by several statistical techniques. The principal conclusions are: 1) on days with isolated showers, rainfall has been heavier in the seeded target area than in the unseeded target area; 2) on days with widespread convective activity and southwesterly winds aloft, rainfall has been lighter in the seeded target area than in the unseeded target area, except for the regions 10–20 miles east of the Black Hills; and 3) on days with widespread convective activity and northwesterly winds aloft, rainfall has been lighter in the seeded than in the unseeded target area.
