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Abstract
Ice particle concentrations have been measured from the NOAA/NCAR Explorer sailplane in unmixed and mixed updraft regions within northeast Colorado cumulus congestus clouds, and compared with the concentrations predicted from measured ice nucleus spectra. The clouds investigated were “cold, continental” cumulus with droplet populations of ∼1000 cm−3 and cloud base temperatures between +7 and −7°C. The concentrations of ice particles within unmixed updraft regions, exclusive of ice particles so large that they almost certainly entered the unmixed region by sedimentation or recycling, are consistent with those expected on the basis of the ice nucleus spectra, suggesting that primary ice nucleation is the dominant mechanism active within these regions. Millimetric size ice particles found within unmixed updraft cores presumably enter through sedimentation or recycling. The data do not indicate an important role for ice multiplication in these clouds.
Abstract
Ice particle concentrations have been measured from the NOAA/NCAR Explorer sailplane in unmixed and mixed updraft regions within northeast Colorado cumulus congestus clouds, and compared with the concentrations predicted from measured ice nucleus spectra. The clouds investigated were “cold, continental” cumulus with droplet populations of ∼1000 cm−3 and cloud base temperatures between +7 and −7°C. The concentrations of ice particles within unmixed updraft regions, exclusive of ice particles so large that they almost certainly entered the unmixed region by sedimentation or recycling, are consistent with those expected on the basis of the ice nucleus spectra, suggesting that primary ice nucleation is the dominant mechanism active within these regions. Millimetric size ice particles found within unmixed updraft cores presumably enter through sedimentation or recycling. The data do not indicate an important role for ice multiplication in these clouds.
Abstract
The characteristics of entrainment in and below 12 developing cumulus congestus clouds in the north-eastern Colorado area were investigated using measurements obtained with the NCAR/NOAA sailplane, supporting aircraft and rawinsondes. A region of moist adiabatic ascent was found in eight of the most vigorous clouds sampled. A gradual increase was noted in the equivalent potential temperature and the ratio of the liquid water content to the adiabatic value from the edge of the updraft region inward to the moist adiabatic core. Previous measurements and conceptual and theoretical models of entrainment are discussed in the context of the present set of measurements.
The moist adiabatic core was positioned off-center with respect to the boundaries of the updraft region. The measurements supported previous conceptual cloud models in which the updraft acts as an obstacle to the horizontal wind thereby causing the environmental air to flow around the upshear portion of the cell, protecting that region from entrainment. A turbulent wake would be expected to occur in the down-shear portion of the cell, producing increased turbulence and mixing in that region.
Abstract
The characteristics of entrainment in and below 12 developing cumulus congestus clouds in the north-eastern Colorado area were investigated using measurements obtained with the NCAR/NOAA sailplane, supporting aircraft and rawinsondes. A region of moist adiabatic ascent was found in eight of the most vigorous clouds sampled. A gradual increase was noted in the equivalent potential temperature and the ratio of the liquid water content to the adiabatic value from the edge of the updraft region inward to the moist adiabatic core. Previous measurements and conceptual and theoretical models of entrainment are discussed in the context of the present set of measurements.
The moist adiabatic core was positioned off-center with respect to the boundaries of the updraft region. The measurements supported previous conceptual cloud models in which the updraft acts as an obstacle to the horizontal wind thereby causing the environmental air to flow around the upshear portion of the cell, protecting that region from entrainment. A turbulent wake would be expected to occur in the down-shear portion of the cell, producing increased turbulence and mixing in that region.
Abstract
Temperature measurements from aircraft, taken with a reverse flow sensor and Rosemount probe, were compared in a penetration through a cloud to determine the effect of sensor wetting on the deduced entrainment. The temperature measured using the Rosemount probe was depressed relative to that measured with the reverse flow probe, leading to an underestimate of the equivalent potential temperature and an overestimate of the deduced entrainment. These results were used to identify possible errors in recent entrainment studies that used temperature as an indicator of entrainment.
Abstract
Temperature measurements from aircraft, taken with a reverse flow sensor and Rosemount probe, were compared in a penetration through a cloud to determine the effect of sensor wetting on the deduced entrainment. The temperature measured using the Rosemount probe was depressed relative to that measured with the reverse flow probe, leading to an underestimate of the equivalent potential temperature and an overestimate of the deduced entrainment. These results were used to identify possible errors in recent entrainment studies that used temperature as an indicator of entrainment.
Abstract
Some cloud microphysical measurements made from the sailplane The Explorer are presented. Drop sizes and concentrations and ice particle sizes, concentrations, shapes, and, in some cases, internal structures are determined from in situ photographs taken with a recently developed particle camera. The data from photographs taken in northeastern Colorado during the early stages of the development of spring and summer cumulus clouds and in their precipitation suggest that the first precipitation forms primarily by the ice process rather than by condensation–coalescence. Ice particles were frequently photographed in these clouds, but above the freezing altitude, water drops > 50 μm radius were very rare.
Abstract
Some cloud microphysical measurements made from the sailplane The Explorer are presented. Drop sizes and concentrations and ice particle sizes, concentrations, shapes, and, in some cases, internal structures are determined from in situ photographs taken with a recently developed particle camera. The data from photographs taken in northeastern Colorado during the early stages of the development of spring and summer cumulus clouds and in their precipitation suggest that the first precipitation forms primarily by the ice process rather than by condensation–coalescence. Ice particles were frequently photographed in these clouds, but above the freezing altitude, water drops > 50 μm radius were very rare.
Abstract
The structure of ice precipitation in northeastern Colorado, collected within summer cumulus clouds and at the ground beneath thunderstorms, shows that the dominant precipitation formation mechanism is the riming of small ice particles, not liquid coalescence. While direct evidence of a stage involving the vapor growth of ice crystals is rare, such a stage is probably present in most cases.
Abstract
The structure of ice precipitation in northeastern Colorado, collected within summer cumulus clouds and at the ground beneath thunderstorms, shows that the dominant precipitation formation mechanism is the riming of small ice particles, not liquid coalescence. While direct evidence of a stage involving the vapor growth of ice crystals is rare, such a stage is probably present in most cases.
Abstract
The hailstone size (diameter) distributions measured by hailpads during the 1972-74 randomized seeding experiment of the National Hail Research Experiment are analyzed statistically for evidence of seeding effects and differences from year to year. Two approaches are taken, one comparing the entire empirical size distributions on seed days and on control days and the other comparing the mean diameters. The latter is based on the consistency with the exponential distribution (truncated at a prescribed minimum diameter), since the exponential distribution can be characterized completely by the difference between the mean diameter and the minimum diameter. Both approaches yield statistically significant results (10% level) only for 1974, when the hailstones were larger on seed days than on control days on the average. This may have resulted from the addition of seeding by rockets in 1974 or from differences in the hailpads used in that year. However, the physical hypothesis for the experiment predicted smaller stones on seed days; that tendency did appear in 1973 (though not significantly) and the difference was negligible in 1972.
Abstract
The hailstone size (diameter) distributions measured by hailpads during the 1972-74 randomized seeding experiment of the National Hail Research Experiment are analyzed statistically for evidence of seeding effects and differences from year to year. Two approaches are taken, one comparing the entire empirical size distributions on seed days and on control days and the other comparing the mean diameters. The latter is based on the consistency with the exponential distribution (truncated at a prescribed minimum diameter), since the exponential distribution can be characterized completely by the difference between the mean diameter and the minimum diameter. Both approaches yield statistically significant results (10% level) only for 1974, when the hailstones were larger on seed days than on control days on the average. This may have resulted from the addition of seeding by rockets in 1974 or from differences in the hailpads used in that year. However, the physical hypothesis for the experiment predicted smaller stones on seed days; that tendency did appear in 1973 (though not significantly) and the difference was negligible in 1972.
Abstract
Concurrent measurements from the CSU-CHILL multiparameter Doppler radar, the Office National d’Etudes et de Recherches Aérospatiales VHF lightning interferometer, and the National Lightning Detection Network, obtained during phase A of the Stratosphere–Troposphere Experiments: Radiation, Aerosols, Ozone (STERAO-A) field project, provided a unique dataset with which to study the relationships between convective storm microphysics and associated lightning. Two storms have been examined in detail in this study: 10 and 12 July 1996. Both storms were long lived, existing in some form for over 4 h apiece, and produced very low cloud-to-ground (CG) lightning flash rates, in particular negative CG flash rates (generally <1 min−1 and often no CG flashes for periods ranging from 10 to almost 30 min), during all or a portion of their lifetimes while simultaneously producing relatively high intracloud (IC) flash rates (>30 min−1 at peak). For both storms, radar reflectivity intensity and the production of hail were anticorrelated with the production of significant negative cloud-to-ground lightning. These observations are shown to be consistent with an elevated charge hypothesis and suggest a possible way of correlating updraft speed, hail, and storm severity to CG and IC flash rates.
Abstract
Concurrent measurements from the CSU-CHILL multiparameter Doppler radar, the Office National d’Etudes et de Recherches Aérospatiales VHF lightning interferometer, and the National Lightning Detection Network, obtained during phase A of the Stratosphere–Troposphere Experiments: Radiation, Aerosols, Ozone (STERAO-A) field project, provided a unique dataset with which to study the relationships between convective storm microphysics and associated lightning. Two storms have been examined in detail in this study: 10 and 12 July 1996. Both storms were long lived, existing in some form for over 4 h apiece, and produced very low cloud-to-ground (CG) lightning flash rates, in particular negative CG flash rates (generally <1 min−1 and often no CG flashes for periods ranging from 10 to almost 30 min), during all or a portion of their lifetimes while simultaneously producing relatively high intracloud (IC) flash rates (>30 min−1 at peak). For both storms, radar reflectivity intensity and the production of hail were anticorrelated with the production of significant negative cloud-to-ground lightning. These observations are shown to be consistent with an elevated charge hypothesis and suggest a possible way of correlating updraft speed, hail, and storm severity to CG and IC flash rates.
Abstract
An extensive statistical analysis is made of the precipitation data collected during the randomized seeding experiment conducted by the National Hail Research Experiment during 1972-74, aimed at testing the feasibility of diminishing hail by seeding with silver iodide. The major conclusion is that no effect of seeding is detected at the 10% significance level. This is true regardless of whether hail or rainfall response variables are considered, which of two methods of obtaining daily values for the response variables over the target area is used, or what distribution, if any, is assumed for the variables. Even though the ratios of hailfall or rainfall on seed days to those on control days are generally greater than 1, the confidence intervals attached to these ratios are so large, because of the large natural variance in each response variable and the small sample sizes, that the true underlying seeding effects could in every case have ranged from substantial decreases to large increases. The large confidence intervals emphasize the necessity of large sample sizes, large experimental areas or effective covariates for obtaining definitive results in precipitation modification experiments.
Abstract
An extensive statistical analysis is made of the precipitation data collected during the randomized seeding experiment conducted by the National Hail Research Experiment during 1972-74, aimed at testing the feasibility of diminishing hail by seeding with silver iodide. The major conclusion is that no effect of seeding is detected at the 10% significance level. This is true regardless of whether hail or rainfall response variables are considered, which of two methods of obtaining daily values for the response variables over the target area is used, or what distribution, if any, is assumed for the variables. Even though the ratios of hailfall or rainfall on seed days to those on control days are generally greater than 1, the confidence intervals attached to these ratios are so large, because of the large natural variance in each response variable and the small sample sizes, that the true underlying seeding effects could in every case have ranged from substantial decreases to large increases. The large confidence intervals emphasize the necessity of large sample sizes, large experimental areas or effective covariates for obtaining definitive results in precipitation modification experiments.
Abstract
Much of the previous work which has led to the conclusion that coalescence is the dominant precipitation forming mechanism in cumulus clouds is reviewed. Observations in northeastern Colorado from several independent methods of investigation are summarized to show that in northeastern Colorado the ice (Bergeron–Findelsen) process is in all probability the dominant mechanism in spring and summer cumuli in their early and intermediate stages of development.
Results of microphysical observations coordinated with simultaneous radar observations are presented. The microphysical observations in clouds with observed effective reflectivities of up to 40 dBZ show that the observed reflectivities can be accounted for by measured ice particle sizes and concentrations. Liquid precipitation elements are not necessary and have been observed only rarely in these clouds except below the melting level.
Possible explanations of the differences between clouds in northeastern Colorado and those in other areas are discussed. The rarity of liquid precipitation particles coupled with the general inefficiency of the ice process at temperatures warmer than −10C suggests that there is potential for rainfall enhancement in the clouds in northeastern Colorado.
Abstract
Much of the previous work which has led to the conclusion that coalescence is the dominant precipitation forming mechanism in cumulus clouds is reviewed. Observations in northeastern Colorado from several independent methods of investigation are summarized to show that in northeastern Colorado the ice (Bergeron–Findelsen) process is in all probability the dominant mechanism in spring and summer cumuli in their early and intermediate stages of development.
Results of microphysical observations coordinated with simultaneous radar observations are presented. The microphysical observations in clouds with observed effective reflectivities of up to 40 dBZ show that the observed reflectivities can be accounted for by measured ice particle sizes and concentrations. Liquid precipitation elements are not necessary and have been observed only rarely in these clouds except below the melting level.
Possible explanations of the differences between clouds in northeastern Colorado and those in other areas are discussed. The rarity of liquid precipitation particles coupled with the general inefficiency of the ice process at temperatures warmer than −10C suggests that there is potential for rainfall enhancement in the clouds in northeastern Colorado.
Abstract
Electric-field measurements made in and near clouds during two airborne field programs are presented. Aircraft equipped with multiple electric-field mills and cloud physics sensors were flown near active convection and into thunderstorm anvil and debris clouds. The magnitude of the electric field was measured as a function of position with respect to the cloud edge to provide an observational basis for modifications to the lightning launch commit criteria (LLCC) used by the U.S. space program. These LLCC are used to reduce the risk that an ascending launch vehicle will trigger a lightning strike that could cause the loss of the mission or vehicle. Even with fields of tens of kV m−1 inside electrically active convective clouds, the fields external to these clouds decay to less than 3 kV m−1 within 15 km of cloud edge. Fields that exceed 3 kV m−1 were not found external to anvil and debris clouds.
Abstract
Electric-field measurements made in and near clouds during two airborne field programs are presented. Aircraft equipped with multiple electric-field mills and cloud physics sensors were flown near active convection and into thunderstorm anvil and debris clouds. The magnitude of the electric field was measured as a function of position with respect to the cloud edge to provide an observational basis for modifications to the lightning launch commit criteria (LLCC) used by the U.S. space program. These LLCC are used to reduce the risk that an ascending launch vehicle will trigger a lightning strike that could cause the loss of the mission or vehicle. Even with fields of tens of kV m−1 inside electrically active convective clouds, the fields external to these clouds decay to less than 3 kV m−1 within 15 km of cloud edge. Fields that exceed 3 kV m−1 were not found external to anvil and debris clouds.
