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Abstract
Laboratory measurements of the rate of ice-crystal growth by sublimation are found to be in good agreement with Houghton's (1950) calculations.
With use of Houghton's data for the growth of ice crystals at relatively warm temperatures, and Vonnegut's (1949) data for the reaction rate of silver-iodide particles as sublimation nuclei, it is shown that it is extremely unlikely that summer convective clouds can be overseeded with ground generators.
Abstract
Laboratory measurements of the rate of ice-crystal growth by sublimation are found to be in good agreement with Houghton's (1950) calculations.
With use of Houghton's data for the growth of ice crystals at relatively warm temperatures, and Vonnegut's (1949) data for the reaction rate of silver-iodide particles as sublimation nuclei, it is shown that it is extremely unlikely that summer convective clouds can be overseeded with ground generators.
It is shown, by use of Langmuir's precipitation growth data and Ludlam's hailstone heat economy data, that the glaze-ice thunderstorm charge generation mechanism suggested by E. J. Workman and the author is consistent with the thunderstorm cell development pattern observed by Workman and the author. The environment of charge separation is found theoretically. It also is shown that the initiation of precipitation in Midwestern thunderstorms without the involvement of ice crystals is consistent with Langmuir's cloud-droplet growth theory.
It is shown, by use of Langmuir's precipitation growth data and Ludlam's hailstone heat economy data, that the glaze-ice thunderstorm charge generation mechanism suggested by E. J. Workman and the author is consistent with the thunderstorm cell development pattern observed by Workman and the author. The environment of charge separation is found theoretically. It also is shown that the initiation of precipitation in Midwestern thunderstorms without the involvement of ice crystals is consistent with Langmuir's cloud-droplet growth theory.
Abstract
The time of onset of the initial electrification in a thunderstorm cell has been correlated with the appearance of the initial radar (3 cm) precipitation-echo. The results show that precipitation is a necessary, but not sufficient, condition for the onset of thunderstorm electrification. The presence of radar-detectable precipitation does not lead to thunderstorm electrification, unless the precipitation echo evidences rapid vertical development. When this condition is fulfilled, the appearance of the initial electrification is almost coincident with the appearance of the initial radar precipitation-echo. On days when no precipitation echoes were present, no electric fields significantly different from the fair-weather positive fields were observed, although the clouds noted ranged from small fair-weather cumulus to clouds of considerable depth and active convection.
Abstract
The time of onset of the initial electrification in a thunderstorm cell has been correlated with the appearance of the initial radar (3 cm) precipitation-echo. The results show that precipitation is a necessary, but not sufficient, condition for the onset of thunderstorm electrification. The presence of radar-detectable precipitation does not lead to thunderstorm electrification, unless the precipitation echo evidences rapid vertical development. When this condition is fulfilled, the appearance of the initial electrification is almost coincident with the appearance of the initial radar precipitation-echo. On days when no precipitation echoes were present, no electric fields significantly different from the fair-weather positive fields were observed, although the clouds noted ranged from small fair-weather cumulus to clouds of considerable depth and active convection.
Abstract
The magnitude and location of the charge centers involved in lightning discharges can be determined from the value of the accompanying gradient change at seven points at the ground. Instrumentation, calibration, observation, and analysis techniques for thus locating thunderstorm charge-centers are described. The results of the analysis are presented.
The thunderstorm is found to be bipolar, with the negative center at a mean height of 25,000 ft msl or −16C, and with the positive center located about 2000 ft above it. The occasional occurrence of a positive center below the negative center is shown by the record of the leader processes of ground discharges and from other evidence.
Comparison is made with the work of Malan and Schonland in South Africa. While the results are in substantial agreement, important differences in the nature of streamer processes and the relative location of the negative centers tapped by successive elements of a single ground discharge are noted.
The cold environment found for many of the negative charge centers (as low as −33C) strongly suggests that these centers might not have been produced by a glaze-ice mechanism. Riming and, perhaps, sublimation are probably the important processes for precipitation growth at such low temperatures.
Abstract
The magnitude and location of the charge centers involved in lightning discharges can be determined from the value of the accompanying gradient change at seven points at the ground. Instrumentation, calibration, observation, and analysis techniques for thus locating thunderstorm charge-centers are described. The results of the analysis are presented.
The thunderstorm is found to be bipolar, with the negative center at a mean height of 25,000 ft msl or −16C, and with the positive center located about 2000 ft above it. The occasional occurrence of a positive center below the negative center is shown by the record of the leader processes of ground discharges and from other evidence.
Comparison is made with the work of Malan and Schonland in South Africa. While the results are in substantial agreement, important differences in the nature of streamer processes and the relative location of the negative centers tapped by successive elements of a single ground discharge are noted.
The cold environment found for many of the negative charge centers (as low as −33C) strongly suggests that these centers might not have been produced by a glaze-ice mechanism. Riming and, perhaps, sublimation are probably the important processes for precipitation growth at such low temperatures.
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Abstract
A description of the known physical properties of a thunderstorm reveals that active charge separation occurs during that stage of the storm's life-cycle in which the growth of graupel by the accretion of supercooled droplets is the dominant process. Laboratory experiments under simulated thunderstorm conditions show that a graupel pellet, growing by the accretion of supercooled droplets, acquires negative charge as a result of collisions with ice crystals. Other experiments show that when two ice formations are placed in rubbing contact, the ice which is warmer, or which contains trace amounts of contaminants, acquires negative charge. Further experiments suggest that the charge separation results from potential differences which arise during the resolidification of a liquid layer formed at the ice-ice contact.
Calculations indicate that the graupel pellets in a thunderstorm, as a result of the acquisition of the latent heat of supercooled droplets, will achieve temperatures several degrees warmer than coexisting ice crystals. Thus the graupel pellets will acquire negative charge as a result of rubbing contacts with ice crystals. The graupel pellets have much higher fall velocities than ice crystals, thus accounting for the polarity of the main thunderstorm dipole. Measurements suggest that the amount of charge separated per graupelcrystal collision is adequate to account for the magnitude of the charges of the main dipole.
Abstract
A description of the known physical properties of a thunderstorm reveals that active charge separation occurs during that stage of the storm's life-cycle in which the growth of graupel by the accretion of supercooled droplets is the dominant process. Laboratory experiments under simulated thunderstorm conditions show that a graupel pellet, growing by the accretion of supercooled droplets, acquires negative charge as a result of collisions with ice crystals. Other experiments show that when two ice formations are placed in rubbing contact, the ice which is warmer, or which contains trace amounts of contaminants, acquires negative charge. Further experiments suggest that the charge separation results from potential differences which arise during the resolidification of a liquid layer formed at the ice-ice contact.
Calculations indicate that the graupel pellets in a thunderstorm, as a result of the acquisition of the latent heat of supercooled droplets, will achieve temperatures several degrees warmer than coexisting ice crystals. Thus the graupel pellets will acquire negative charge as a result of rubbing contacts with ice crystals. The graupel pellets have much higher fall velocities than ice crystals, thus accounting for the polarity of the main thunderstorm dipole. Measurements suggest that the amount of charge separated per graupelcrystal collision is adequate to account for the magnitude of the charges of the main dipole.
Tests involving exposure of AgI smoke to bright sunlight show a decrease in concentration of effective nuclei (at −20°C) of approximately two orders of magnitude per hour. The concentration of effective nuclei is increased greatly (as much as two orders of magnitude) by the addition of a little ammonia vapor to the AgI smoke. Smoke samples which have been deactivated completely by exposure to ultraviolet light can be caused to form large numbers of ice crystals by the addition of ammonia vapor. If ammonia is added before exposure to light, the rate of decay is the same or greater, and the effectiveness cannot be restored by further addition of ammonia. The effect of ammonia is believed to be due to the adsorption of ammonia on the silver iodide surfaces or to the formation of an ammine of silver iodide.
Tests involving exposure of AgI smoke to bright sunlight show a decrease in concentration of effective nuclei (at −20°C) of approximately two orders of magnitude per hour. The concentration of effective nuclei is increased greatly (as much as two orders of magnitude) by the addition of a little ammonia vapor to the AgI smoke. Smoke samples which have been deactivated completely by exposure to ultraviolet light can be caused to form large numbers of ice crystals by the addition of ammonia vapor. If ammonia is added before exposure to light, the rate of decay is the same or greater, and the effectiveness cannot be restored by further addition of ammonia. The effect of ammonia is believed to be due to the adsorption of ammonia on the silver iodide surfaces or to the formation of an ammine of silver iodide.
Abstract
Extreme precipitation events, and the quantitative precipitation forecasts (QPFs) associated with them, are examined. The study uses data from the Hydrometeorology Testbed (HMT), which conducted its first field study in California during the 2005/06 cool season. National Weather Service River Forecast Center (NWS RFC) gridded QPFs for 24-h periods at 24-h (day 1), 48-h (day 2), and 72-h (day 3) forecast lead times plus 24-h quantitative precipitation estimates (QPEs) from sites in California (CA) and Oregon–Washington (OR–WA) are used. During the 172-day period studied, some sites received more than 254 cm (100 in.) of precipitation. The winter season produced many extreme precipitation events, including 90 instances when a site received more than 7.6 cm (3.0 in.) of precipitation in 24 h (i.e., an “event”) and 17 events that exceeded 12.7 cm (24 h)−1 [5.0 in. (24 h)−1]. For the 90 extreme events {>7.6 cm (24 h)−1 [3.0 in. (24 h)−1]}, almost 90% of all the 270 QPFs (days 1–3) were biased low, increasingly so with greater lead time. Of the 17 observed events exceeding 12.7 cm (24 h)−1 [5.0 in. (24 h)−1], only 1 of those events was predicted to be that extreme. Almost all of the extreme events correlated with the presence of atmospheric river conditions. Total seasonal QPF biases for all events {i.e., ≥0.025 cm (24 h)−1 [0.01 in. (24 h)−1]} were sensitive to local geography and were generally biased low in the California–Nevada River Forecast Center (CNRFC) region and high in the Northwest River Forecast Center (NWRFC) domain. The low bias in CA QPFs improved with shorter forecast lead time and worsened for extreme events. Differences were also noted between the CNRFC and NWRFC in terms of QPF and the frequency of extreme events. A key finding from this study is that there were more precipitation events >7.6 cm (24 h)−1 [3.0 in. (24 h)−1] in CA than in OR–WA. Examination of 422 Cooperative Observer Program (COOP) sites in the NWRFC domain and 400 in the CNRFC domain found that the thresholds for the top 1% and top 0.1% of precipitation events were 7.6 cm (24 h)−1 [3.0 in. (24 h)−1] and 14.2 cm (24 h)−1 [5.6 in. (24 h)−1] or greater for the CNRFC and only 5.1 cm (24 h)−1 [2.0 in. (24 h)−1] and 9.4 cm (24 h)−1 [3.7 in. (24 h)−1] for the NWRFC, respectively. Similar analyses for all NWS RFCs showed that the threshold for the top 1% of events varies from ∼3.8 cm (24 h)−1 [1.5 in. (24 h)−1] in the Colorado Basin River Forecast Center (CBRFC) to ∼5.1 cm (24 h)−1 [3.0 in. (24 h)−1] in the northern tier of RFCs and ∼7.6 cm (24 h)−1 [3.0 in. (24 h)−1] in both the southern tier and the CNRFC. It is recommended that NWS QPF performance in the future be assessed for extreme events using these thresholds.
Abstract
Extreme precipitation events, and the quantitative precipitation forecasts (QPFs) associated with them, are examined. The study uses data from the Hydrometeorology Testbed (HMT), which conducted its first field study in California during the 2005/06 cool season. National Weather Service River Forecast Center (NWS RFC) gridded QPFs for 24-h periods at 24-h (day 1), 48-h (day 2), and 72-h (day 3) forecast lead times plus 24-h quantitative precipitation estimates (QPEs) from sites in California (CA) and Oregon–Washington (OR–WA) are used. During the 172-day period studied, some sites received more than 254 cm (100 in.) of precipitation. The winter season produced many extreme precipitation events, including 90 instances when a site received more than 7.6 cm (3.0 in.) of precipitation in 24 h (i.e., an “event”) and 17 events that exceeded 12.7 cm (24 h)−1 [5.0 in. (24 h)−1]. For the 90 extreme events {>7.6 cm (24 h)−1 [3.0 in. (24 h)−1]}, almost 90% of all the 270 QPFs (days 1–3) were biased low, increasingly so with greater lead time. Of the 17 observed events exceeding 12.7 cm (24 h)−1 [5.0 in. (24 h)−1], only 1 of those events was predicted to be that extreme. Almost all of the extreme events correlated with the presence of atmospheric river conditions. Total seasonal QPF biases for all events {i.e., ≥0.025 cm (24 h)−1 [0.01 in. (24 h)−1]} were sensitive to local geography and were generally biased low in the California–Nevada River Forecast Center (CNRFC) region and high in the Northwest River Forecast Center (NWRFC) domain. The low bias in CA QPFs improved with shorter forecast lead time and worsened for extreme events. Differences were also noted between the CNRFC and NWRFC in terms of QPF and the frequency of extreme events. A key finding from this study is that there were more precipitation events >7.6 cm (24 h)−1 [3.0 in. (24 h)−1] in CA than in OR–WA. Examination of 422 Cooperative Observer Program (COOP) sites in the NWRFC domain and 400 in the CNRFC domain found that the thresholds for the top 1% and top 0.1% of precipitation events were 7.6 cm (24 h)−1 [3.0 in. (24 h)−1] and 14.2 cm (24 h)−1 [5.6 in. (24 h)−1] or greater for the CNRFC and only 5.1 cm (24 h)−1 [2.0 in. (24 h)−1] and 9.4 cm (24 h)−1 [3.7 in. (24 h)−1] for the NWRFC, respectively. Similar analyses for all NWS RFCs showed that the threshold for the top 1% of events varies from ∼3.8 cm (24 h)−1 [1.5 in. (24 h)−1] in the Colorado Basin River Forecast Center (CBRFC) to ∼5.1 cm (24 h)−1 [3.0 in. (24 h)−1] in the northern tier of RFCs and ∼7.6 cm (24 h)−1 [3.0 in. (24 h)−1] in both the southern tier and the CNRFC. It is recommended that NWS QPF performance in the future be assessed for extreme events using these thresholds.
Abstract
Television photos of smoke plumes an analyzed to estimate meridional wind shear on the space shuttle Challenger associated with the accident of Mission 51-L. Gust velocities were obtained by detailed examination of the debris trails. The shuttle exhaust trail was used to establish altitudes of significant features in the photographs. Wind data obtained from the photographs compare favorably with data obtained from a rawinsonde released 9 min after the launch of the shuttle.
Abstract
Television photos of smoke plumes an analyzed to estimate meridional wind shear on the space shuttle Challenger associated with the accident of Mission 51-L. Gust velocities were obtained by detailed examination of the debris trails. The shuttle exhaust trail was used to establish altitudes of significant features in the photographs. Wind data obtained from the photographs compare favorably with data obtained from a rawinsonde released 9 min after the launch of the shuttle.
