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Abstract
The ice-nucleating properties of cloud seeding materials produced by burning acetone solutions of AgI-NaI, AgI-KI and AgI-NH4I have been tested in a wind-tunnel cloud-chamber test facility.
The AgI aerosol produced from 5% AgI-NH4I solutions yields nuclei active at temperatures as warm as −4C and shows an efficiency of 1012 nuclei per gm AgI at −5C. Aerosols from similar solutions of AgI-KI and AgI-NaI produce no ice crystals at temperatures warmer than −6.5 and −8C, respectively. Temperatures of −7.5 and −9C are required to obain efficiencies of 1012 nuclei per gm for these two latter solutions.
The greater ice nucleating ability of the NH4I solution at high temperatures in attributed tentatively to the production of relatively pure AgI crystals rather than to increased particle size but may also result from minor NH4I contamination.
Calculations of the droplet-aerosol particle collision rates show that contact nucleation could explain all ice formation observed at higher chamber temperatures, although other processes may occur simultaneously. On the other hand, the calculations also show that direct ice deposition or condensation-freezing is likely the dominant mechanism below −12C.
The implications of the laboratory results for field applications are discussed briefly. It is concluded that the AgI-NH4I system offers sufficient advantages to justify its adoption in field projects.
Abstract
The ice-nucleating properties of cloud seeding materials produced by burning acetone solutions of AgI-NaI, AgI-KI and AgI-NH4I have been tested in a wind-tunnel cloud-chamber test facility.
The AgI aerosol produced from 5% AgI-NH4I solutions yields nuclei active at temperatures as warm as −4C and shows an efficiency of 1012 nuclei per gm AgI at −5C. Aerosols from similar solutions of AgI-KI and AgI-NaI produce no ice crystals at temperatures warmer than −6.5 and −8C, respectively. Temperatures of −7.5 and −9C are required to obain efficiencies of 1012 nuclei per gm for these two latter solutions.
The greater ice nucleating ability of the NH4I solution at high temperatures in attributed tentatively to the production of relatively pure AgI crystals rather than to increased particle size but may also result from minor NH4I contamination.
Calculations of the droplet-aerosol particle collision rates show that contact nucleation could explain all ice formation observed at higher chamber temperatures, although other processes may occur simultaneously. On the other hand, the calculations also show that direct ice deposition or condensation-freezing is likely the dominant mechanism below −12C.
The implications of the laboratory results for field applications are discussed briefly. It is concluded that the AgI-NH4I system offers sufficient advantages to justify its adoption in field projects.
Abstract
No abstract available.
Abstract
No abstract available.
Abstract
Four seasons of hail data were gathered on a randomized cloud seeding project aimed at reducing hail damage and increasing rainfall in western North Dakota. Hail on seed days was generally less severe than on no-seed days. Statistical tests of data from passive hail indicators do not permit rejection of the null hypothesis at the 90% confidence level, but application of rank tests to crop-hail insurance loss data indicates that the seeding reduced crop damage from hail.Post-analyses of related data indicate that 1) the ratio of rainfall amount to hail energy was greater for seed days than no-seed days, and 2) radar characteristics of seeded storms differ from those of unseeded storms. In addition, case studies of 34 storms indicate that damaging hail was usually suppressed when their updraft areas were seeded continuously.
Abstract
Four seasons of hail data were gathered on a randomized cloud seeding project aimed at reducing hail damage and increasing rainfall in western North Dakota. Hail on seed days was generally less severe than on no-seed days. Statistical tests of data from passive hail indicators do not permit rejection of the null hypothesis at the 90% confidence level, but application of rank tests to crop-hail insurance loss data indicates that the seeding reduced crop damage from hail.Post-analyses of related data indicate that 1) the ratio of rainfall amount to hail energy was greater for seed days than no-seed days, and 2) radar characteristics of seeded storms differ from those of unseeded storms. In addition, case studies of 34 storms indicate that damaging hail was usually suppressed when their updraft areas were seeded continuously.
Abstract
An electronic hailstone momentum sensor has been developed which records hailstone impacts on magnetic tape. The instrument and the programs for analyzing the recorded data are described. Sensors were operated during six hailstorms in 1969 and recorded 624 hailstone impacts. Derived frequency distributions of hailstone size show the median diameter for all stones recorded to be near 0.9 cm. Significant variations exist among storms. Values of the equivalent radar reflectivity factor Z e have been computed for the hailshafts sampled, for X-hand and S-band radar, and for both wet and dry hailstones. The values range up to 70 dBz (107.0 mm6 m−3) and show good agreement with radar-measured values. The results suggest that dual-wavelength radar systems are not much better than S-band sets alone for estimating hailstone size.
Abstract
An electronic hailstone momentum sensor has been developed which records hailstone impacts on magnetic tape. The instrument and the programs for analyzing the recorded data are described. Sensors were operated during six hailstorms in 1969 and recorded 624 hailstone impacts. Derived frequency distributions of hailstone size show the median diameter for all stones recorded to be near 0.9 cm. Significant variations exist among storms. Values of the equivalent radar reflectivity factor Z e have been computed for the hailshafts sampled, for X-hand and S-band radar, and for both wet and dry hailstones. The values range up to 70 dBz (107.0 mm6 m−3) and show good agreement with radar-measured values. The results suggest that dual-wavelength radar systems are not much better than S-band sets alone for estimating hailstone size.
Abstract
Magnetic tape records for radar observations of 80 moving one-hour test cases in a three-way randomized (no-seed, salt, silver iodide) cloud seeding experiment have been analyzed in terms of echoing areas and radar-estimated rainfall amounts. Individual test cases ranged from non-precipitating cumulus up to moderate thunderstorms with echoing areas exceeding 100 km2 and rainfall estimated at 3000 kT in 1 h.Out of numerous predictor variables, cloud depth is found to be the best single predictor for both echoing area and radar-estimated rainfall. The echoing area and radar-estimated rainfall are very closely correlated. A cube-root transformation of the radar-estimated rainfall improves the correlation between cloud depth and the radar-estimated rainfall for the no-seed (control) sample to 0.91. For clouds of a given depth, both the echoing area and radar-estimated rainfall are larger in seeded than in unseeded cases. The differences between no-seed and salt cases are of marginal statistical significance, but the differences in echoing area and rainfall between no-seed and silver iodide cases are significant at the 1% level. The indicated effects, expressed as a percentage of the echoing area or radar-estimated rainfall in the no-seed cases, decrease with cloud depth.A comparison of no-seed and AgI cases with the aid of a one-dimensional steady-state cloud model shows that AgI seeding may have led to increases in maximum cloud height averaging 600 m.It is concluded that seeding affected the precipitation in the Cloud Catcher test cases through both the microphysical processes and the cloud dynamics.
Abstract
Magnetic tape records for radar observations of 80 moving one-hour test cases in a three-way randomized (no-seed, salt, silver iodide) cloud seeding experiment have been analyzed in terms of echoing areas and radar-estimated rainfall amounts. Individual test cases ranged from non-precipitating cumulus up to moderate thunderstorms with echoing areas exceeding 100 km2 and rainfall estimated at 3000 kT in 1 h.Out of numerous predictor variables, cloud depth is found to be the best single predictor for both echoing area and radar-estimated rainfall. The echoing area and radar-estimated rainfall are very closely correlated. A cube-root transformation of the radar-estimated rainfall improves the correlation between cloud depth and the radar-estimated rainfall for the no-seed (control) sample to 0.91. For clouds of a given depth, both the echoing area and radar-estimated rainfall are larger in seeded than in unseeded cases. The differences between no-seed and salt cases are of marginal statistical significance, but the differences in echoing area and rainfall between no-seed and silver iodide cases are significant at the 1% level. The indicated effects, expressed as a percentage of the echoing area or radar-estimated rainfall in the no-seed cases, decrease with cloud depth.A comparison of no-seed and AgI cases with the aid of a one-dimensional steady-state cloud model shows that AgI seeding may have led to increases in maximum cloud height averaging 600 m.It is concluded that seeding affected the precipitation in the Cloud Catcher test cases through both the microphysical processes and the cloud dynamics.
Abstract
Rainfall data collected at 67 gages in a 2750 mi2 target area during a four-year randomized cloud seeding experiment in North Dakota have been stratified in a variety of ways and subjected to several kinds of statistical tests. Some stratifications related to cloud model predictions were possible for only the last two years when a rawinsonde station was operated as part of the project. Monte Carlo experiments simulating 500 reruns of the four-year experiment have been used to establish significance levels for the tests within each data stratification.
The analysis provides significant evidence that seeding convective clouds on a determinate set of days leads to 1) an increase in the frequency of rainfall events at the individual target gages, 2) an increase in the average rainfall recorded per rainfall event, and 3) an increase in total rainfall on the target. The set of days to which this evidence applies is those days with dynamic seedability; that is, days for which a cloud model predicted an increase in cloud top height under the influence of silver iodide seeding. Rainfall observations on days when the cloud model predicted no increase in cloud height show no significant differences between seed and no-seed days.
The possibility of bias has been checked by comparing the frequencies of wet and dry days and the averages of several meteorological variables for seed and no-seed days within each stratification, by cross-checking the stratifications, and by comparing rainfall on seed and no-seed days over an area of roughly 50,000 square miles surrounding the target area. There is no obvious bias to account for the significant differences between seed and no-seed days with dynamic seedability.
It is tentatively concluded that dynamic effects, including rainfall increases, were produced by light to moderate silver iodide seeding from below cloud base. The potential rainfall increase resulting from seeding below selected clouds on days with dynamic seedability is estimated at one inch per growing season.
Abstract
Rainfall data collected at 67 gages in a 2750 mi2 target area during a four-year randomized cloud seeding experiment in North Dakota have been stratified in a variety of ways and subjected to several kinds of statistical tests. Some stratifications related to cloud model predictions were possible for only the last two years when a rawinsonde station was operated as part of the project. Monte Carlo experiments simulating 500 reruns of the four-year experiment have been used to establish significance levels for the tests within each data stratification.
The analysis provides significant evidence that seeding convective clouds on a determinate set of days leads to 1) an increase in the frequency of rainfall events at the individual target gages, 2) an increase in the average rainfall recorded per rainfall event, and 3) an increase in total rainfall on the target. The set of days to which this evidence applies is those days with dynamic seedability; that is, days for which a cloud model predicted an increase in cloud top height under the influence of silver iodide seeding. Rainfall observations on days when the cloud model predicted no increase in cloud height show no significant differences between seed and no-seed days.
The possibility of bias has been checked by comparing the frequencies of wet and dry days and the averages of several meteorological variables for seed and no-seed days within each stratification, by cross-checking the stratifications, and by comparing rainfall on seed and no-seed days over an area of roughly 50,000 square miles surrounding the target area. There is no obvious bias to account for the significant differences between seed and no-seed days with dynamic seedability.
It is tentatively concluded that dynamic effects, including rainfall increases, were produced by light to moderate silver iodide seeding from below cloud base. The potential rainfall increase resulting from seeding below selected clouds on days with dynamic seedability is estimated at one inch per growing season.
Abstract
A methodology is described for testing the simulation of tropical convective clouds by models through comparison with observations of clouds and precipitation from earth-orbiting satellites. Clouds are divided into categories that represent convective cores: moderately thick anvil clouds and thin high clouds. Fractional abundances of these clouds are computed as a function of rain rate. A three-dimensional model is forced with steady forcing characteristics of tropical Pacific convective regions, and the model clouds are compared with satellite observations for the same regions. The model produces a good simulation of the relationship between the precipitation rate and optically thick cold clouds that represent convective cores. The observations show large abundances of anvil cloud with a strong dependence on rain rate, but the model produces too little anvil cloud by a factor of about 4 and with a very weak dependence on the rain rate. The observations also show probability density functions for outgoing longwave radiation (OLR) and albedo with maxima that correspond to extended upper-level cold clouds, whereas the model does not. The sensitivity of the anvil cloud simulation to model parameters is explored using a two-dimensional model. Both cloud physical parameters and mean wind shear effects are investigated. The simulation of anvil cloud can be improved while maintaining a good simulation of optically thick cloud by adjusting the cloud physics parameters in the model to produce more ice cloud and less liquid water cloud.
Abstract
A methodology is described for testing the simulation of tropical convective clouds by models through comparison with observations of clouds and precipitation from earth-orbiting satellites. Clouds are divided into categories that represent convective cores: moderately thick anvil clouds and thin high clouds. Fractional abundances of these clouds are computed as a function of rain rate. A three-dimensional model is forced with steady forcing characteristics of tropical Pacific convective regions, and the model clouds are compared with satellite observations for the same regions. The model produces a good simulation of the relationship between the precipitation rate and optically thick cold clouds that represent convective cores. The observations show large abundances of anvil cloud with a strong dependence on rain rate, but the model produces too little anvil cloud by a factor of about 4 and with a very weak dependence on the rain rate. The observations also show probability density functions for outgoing longwave radiation (OLR) and albedo with maxima that correspond to extended upper-level cold clouds, whereas the model does not. The sensitivity of the anvil cloud simulation to model parameters is explored using a two-dimensional model. Both cloud physical parameters and mean wind shear effects are investigated. The simulation of anvil cloud can be improved while maintaining a good simulation of optically thick cloud by adjusting the cloud physics parameters in the model to produce more ice cloud and less liquid water cloud.
Abstract
Boundary layer wind data observed by a Doppler lidar and sonic anemometers during the mornings of three intensive observational periods (IOP2, IOP3, and IOP7) of the Joint Urban 2003 (JU2003) field experiment are analyzed to extract the mean and turbulent characteristics of airflow over Oklahoma City, Oklahoma. A strong nocturnal low-level jet (LLJ) dominated the flow in the boundary layer over the measurement domain from midnight to the morning hours. Lidar scans through the LLJ taken after sunrise indicate that the LLJ elevation shows a gradual increase of 25–100 m over the urban area relative to that over the upstream suburban area. The mean wind speed beneath the jet over the urban area is about 10%–15% slower than that over the suburban area. Sonic anemometer observations combined with Doppler lidar observations in the urban and suburban areas are also analyzed to investigate the boundary layer turbulence production in the LLJ-dominated atmospheric boundary layer. The turbulence kinetic energy was higher over the urban domain mainly because of the shear production of building surfaces and building wakes. Direct transport of turbulent momentum flux from the LLJ to the urban street level was very small because of the relatively high elevation of the jet. However, since the LLJ dominated the mean wind in the boundary layer, the turbulence kinetic energy in the urban domain is correlated directly with the LLJ maximum speed and inversely with its height. The results indicate that the jet Richardson number is a reasonably good indicator for turbulent kinetic energy over the urban domain in the LLJ-dominated atmospheric boundary layer.
Abstract
Boundary layer wind data observed by a Doppler lidar and sonic anemometers during the mornings of three intensive observational periods (IOP2, IOP3, and IOP7) of the Joint Urban 2003 (JU2003) field experiment are analyzed to extract the mean and turbulent characteristics of airflow over Oklahoma City, Oklahoma. A strong nocturnal low-level jet (LLJ) dominated the flow in the boundary layer over the measurement domain from midnight to the morning hours. Lidar scans through the LLJ taken after sunrise indicate that the LLJ elevation shows a gradual increase of 25–100 m over the urban area relative to that over the upstream suburban area. The mean wind speed beneath the jet over the urban area is about 10%–15% slower than that over the suburban area. Sonic anemometer observations combined with Doppler lidar observations in the urban and suburban areas are also analyzed to investigate the boundary layer turbulence production in the LLJ-dominated atmospheric boundary layer. The turbulence kinetic energy was higher over the urban domain mainly because of the shear production of building surfaces and building wakes. Direct transport of turbulent momentum flux from the LLJ to the urban street level was very small because of the relatively high elevation of the jet. However, since the LLJ dominated the mean wind in the boundary layer, the turbulence kinetic energy in the urban domain is correlated directly with the LLJ maximum speed and inversely with its height. The results indicate that the jet Richardson number is a reasonably good indicator for turbulent kinetic energy over the urban domain in the LLJ-dominated atmospheric boundary layer.
Abstract
Global ozone observations from the Microwave Limb Sounder (MLS) aboard the Upper Atmosphere Research Satellite (UARS) are presented, in both vertically resolved and column abundance formats. The authors review the zonal-mean ozone variations measured over the two and a half years since launch in September 1991. Well-known features such as the annual and semiannual variations are ubiquitous. In the equatorial regions, longerterm changes are believed to be related to the quasi-biennial oscillation (QBO), with a strong semiannual signal above 20 hPa. Ozone values near 50 hPa exhibit an equatorial low from October 1991 to June 1992, after which the low ozone pattern splits into two subtropical lows (possibly in connection with residual circulation changes tied to the QBO) and returns to an equatorial low in September 1993. The ozone hole development at high southern latitudes is apparent in MLS column data integrated down to 100 hPa, with a pattern generally consistent with Nimbus-7 Total Ozone Mapping Spectrometer (TOMS) measurements of total column; the MLS data reinforce current knowledge of this lower-stratospheric phenomenon by providing a height-dependent view of the variations. The region from 30°S to 30°N (an area equal to half the global area) shows very little change in the ozone column from year to year and within each year.
The most striking ozone changes have occurred at northern midlatitudes, with the October 1992 to July 1993 column values significantly lower than during the prior year. The zonal-mean changes manifest themselves as a slower rate of increase during the 1992/93 winter, and there is some evidence for a lower fall minimum. A recovery occurs during late summer of 1993; early 1994 values are significantly larger than during the two previous winters. These results are in general agreement with variations measured by the Nimbus-7 TOMS and Meteor-3 TOMS instruments at midlatitudes. However, the southern midlatitudes exhibit less of a column ozone decrease (relative to the north) in the MLS data (down to 100 hPa) than in the TOMS column results. The timing and latitudinal extent of the northern midlatitude decreases appear to rule out observed CIO enhancements in the Arctic vortex, with related chemical processing and ozone dilution effects, as a unique cause. Local depletion from CIO-related chemical mechanisms alone is also not sufficient, based on MLS CIO data. The puzzling asymmetric nature of the changes probably requires a dynamical component as an explanation. A combination of effects (including chemical destruction via heterogeneous processes and QBO phasing) apparently needs to be invoked. This dataset will place constraints on future modeling studies, which are required to better understand the source of the observed changes.
Finally, residual ozone values extracted from TOMS-minus-MLS column data are briefly presented as a preliminary view into the potential usefulness of such studies, with information on tropospheric ozone as an ultimate goal.
Abstract
Global ozone observations from the Microwave Limb Sounder (MLS) aboard the Upper Atmosphere Research Satellite (UARS) are presented, in both vertically resolved and column abundance formats. The authors review the zonal-mean ozone variations measured over the two and a half years since launch in September 1991. Well-known features such as the annual and semiannual variations are ubiquitous. In the equatorial regions, longerterm changes are believed to be related to the quasi-biennial oscillation (QBO), with a strong semiannual signal above 20 hPa. Ozone values near 50 hPa exhibit an equatorial low from October 1991 to June 1992, after which the low ozone pattern splits into two subtropical lows (possibly in connection with residual circulation changes tied to the QBO) and returns to an equatorial low in September 1993. The ozone hole development at high southern latitudes is apparent in MLS column data integrated down to 100 hPa, with a pattern generally consistent with Nimbus-7 Total Ozone Mapping Spectrometer (TOMS) measurements of total column; the MLS data reinforce current knowledge of this lower-stratospheric phenomenon by providing a height-dependent view of the variations. The region from 30°S to 30°N (an area equal to half the global area) shows very little change in the ozone column from year to year and within each year.
The most striking ozone changes have occurred at northern midlatitudes, with the October 1992 to July 1993 column values significantly lower than during the prior year. The zonal-mean changes manifest themselves as a slower rate of increase during the 1992/93 winter, and there is some evidence for a lower fall minimum. A recovery occurs during late summer of 1993; early 1994 values are significantly larger than during the two previous winters. These results are in general agreement with variations measured by the Nimbus-7 TOMS and Meteor-3 TOMS instruments at midlatitudes. However, the southern midlatitudes exhibit less of a column ozone decrease (relative to the north) in the MLS data (down to 100 hPa) than in the TOMS column results. The timing and latitudinal extent of the northern midlatitude decreases appear to rule out observed CIO enhancements in the Arctic vortex, with related chemical processing and ozone dilution effects, as a unique cause. Local depletion from CIO-related chemical mechanisms alone is also not sufficient, based on MLS CIO data. The puzzling asymmetric nature of the changes probably requires a dynamical component as an explanation. A combination of effects (including chemical destruction via heterogeneous processes and QBO phasing) apparently needs to be invoked. This dataset will place constraints on future modeling studies, which are required to better understand the source of the observed changes.
Finally, residual ozone values extracted from TOMS-minus-MLS column data are briefly presented as a preliminary view into the potential usefulness of such studies, with information on tropospheric ozone as an ultimate goal.
Abstract
Time series of net and seasonal mass balances for three glaciers in western North America, one in the Pacific Northwest and two in Alaska, show various relationships to Pacific hemisphere climate indexes. During the winter season the two coastal, maritime-regime glaciers, over 2000 km apart, are affected almost identically, albeit inversely, by atmospheric and oceanic conditions in both the tropical and North Pacific. The two Alaska glaciers, only 350 km apart, have almost no coherence. Lag correlations show that in winter the maritime glaciers are influenced by concurrent conditions in the North Pacific, but by conditions in the tropical Pacific in August–September of the prior northern summer. The winter balance variations contain interannual El Niño–Southern Oscillation variability superimposed on North Pacific interdecadal variability; the interdecadal 1976–77 climate regime shift is clearly evident. The summer balances and the continental-regime glacier have a general lack of correlations, with no clear, strong, consistent patterns, probably a result of being influenced more by local processes or by circulation patterns outside the Pacific Ocean basin. The results show the Pacific Northwest is strongly influenced by conditions in the tropical Pacific, but that this teleconnection has broken down in recent years, starting in 1989. During the seven years since then (1989–95), all three glaciers have shown, for the first time, coherent signals, which were net mass loss at the highest rate in the entire record. The authors’ results agree with those of other recent studies that suggest these recent years are unusual and may be a signature of climate warming.
Abstract
Time series of net and seasonal mass balances for three glaciers in western North America, one in the Pacific Northwest and two in Alaska, show various relationships to Pacific hemisphere climate indexes. During the winter season the two coastal, maritime-regime glaciers, over 2000 km apart, are affected almost identically, albeit inversely, by atmospheric and oceanic conditions in both the tropical and North Pacific. The two Alaska glaciers, only 350 km apart, have almost no coherence. Lag correlations show that in winter the maritime glaciers are influenced by concurrent conditions in the North Pacific, but by conditions in the tropical Pacific in August–September of the prior northern summer. The winter balance variations contain interannual El Niño–Southern Oscillation variability superimposed on North Pacific interdecadal variability; the interdecadal 1976–77 climate regime shift is clearly evident. The summer balances and the continental-regime glacier have a general lack of correlations, with no clear, strong, consistent patterns, probably a result of being influenced more by local processes or by circulation patterns outside the Pacific Ocean basin. The results show the Pacific Northwest is strongly influenced by conditions in the tropical Pacific, but that this teleconnection has broken down in recent years, starting in 1989. During the seven years since then (1989–95), all three glaciers have shown, for the first time, coherent signals, which were net mass loss at the highest rate in the entire record. The authors’ results agree with those of other recent studies that suggest these recent years are unusual and may be a signature of climate warming.
