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Abstract
The fragmentation of freezing water droplets in natural clouds has been postulated by several workers, and this phenomenon has been observed in numerous laboratory investigations. However, the profound effect that environmental conditions can have on fragmentation has not been fully appreciated. In the first part of this paper the factors that might affect the freezing behavior and fragmentation of a water drop are discussed, and, where possible, are analyzed in detail.
In the second part of the paper results are presented of laboratory experiments on the freezing of suspended water drops 1 mm in diameter. Drops nucleated in air under equilibrium conditions were never observed to shatter and only one drop in ten ejected an ice splinter. The shattering and large splinter counts from suspended drops nucleated in air which have been reported by other workers are attributed to the contamination of the drops by carbon dioxide and nucleation under non-equilibrium conditions. Drops frozen in hydrogen shattered frequently if the temperature was lower than −9C. Drops frozen in helium at −10 to −12C shattered on occasions. In a mixture of air and carbon dioxide the shattering behavior was very dependent on the concentration of carbon dioxide. Large numbers of ice splinters were detected only if a drop shattered.
Abstract
The fragmentation of freezing water droplets in natural clouds has been postulated by several workers, and this phenomenon has been observed in numerous laboratory investigations. However, the profound effect that environmental conditions can have on fragmentation has not been fully appreciated. In the first part of this paper the factors that might affect the freezing behavior and fragmentation of a water drop are discussed, and, where possible, are analyzed in detail.
In the second part of the paper results are presented of laboratory experiments on the freezing of suspended water drops 1 mm in diameter. Drops nucleated in air under equilibrium conditions were never observed to shatter and only one drop in ten ejected an ice splinter. The shattering and large splinter counts from suspended drops nucleated in air which have been reported by other workers are attributed to the contamination of the drops by carbon dioxide and nucleation under non-equilibrium conditions. Drops frozen in hydrogen shattered frequently if the temperature was lower than −9C. Drops frozen in helium at −10 to −12C shattered on occasions. In a mixture of air and carbon dioxide the shattering behavior was very dependent on the concentration of carbon dioxide. Large numbers of ice splinters were detected only if a drop shattered.
Abstract
An electrostatic cloud droplet sizing device (electrostatic disdrometer) originally developed by Keily and Millen has been tested, modified extensively, and calibrated in our laboratory. The investigations have shown that soon after entry into the probe orifice, the incoming droplet is broken into many fragments. These impact and splash on an electrode raised to a 510 V potential. Measured pulses for a given droplet size give a reproducible calibration curve.
Airborne tests of the probe have shown it to operate reliably with minimal maintenance. Comparisons were made between values of the liquid water content measured by the electrostatic disdrometer and by the Johnson-Williams hot-wire, liquid-water-content meter and between the droplet size distributions measured by the disdrometer and by impaction slide replicas. The comparisons were satisfactory within the limits of instrument measuring and sampling errors and actual variations in the droplets spectra resulting from the separation of the instruments on the aircraft during the tests.
Abstract
An electrostatic cloud droplet sizing device (electrostatic disdrometer) originally developed by Keily and Millen has been tested, modified extensively, and calibrated in our laboratory. The investigations have shown that soon after entry into the probe orifice, the incoming droplet is broken into many fragments. These impact and splash on an electrode raised to a 510 V potential. Measured pulses for a given droplet size give a reproducible calibration curve.
Airborne tests of the probe have shown it to operate reliably with minimal maintenance. Comparisons were made between values of the liquid water content measured by the electrostatic disdrometer and by the Johnson-Williams hot-wire, liquid-water-content meter and between the droplet size distributions measured by the disdrometer and by impaction slide replicas. The comparisons were satisfactory within the limits of instrument measuring and sampling errors and actual variations in the droplets spectra resulting from the separation of the instruments on the aircraft during the tests.
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
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
The response of the CSIRO liquid water content (LWC) device to water drops of different sizes has been investigated in a wind tunnel. Two series of experiments were conducted. The first compared the probe-measured LWC of sprays with different median volume diameters (MVD) to the LWC computed through water mass conservation considerations; the second series compared the probe LWC to that computed from the droplet spectra measured by Particle Measuring Systems' (PMS) probes. In the first series of experiments, the response was found to decrease from 100% for a water spray with an MVD of ∼ 20 μm to about 50% for a spray with an MVD of ∼ 150–200 μm. From these results, we expect that the King Probe can be used without correction for measuring the LWC of droplet distributions with MVDs less than about 40 μm. As the MVD increases, there will be a gradually diminishing response, which for MVDs of greater than 100 μm will require substantial correction.
The second series of experiments produced physically unreasonable results, suggesting that the size calibration of the PMS Forward Scattering Spectrometer Probe needs to be reevalualed. These results also indicate that a correction is required for the PMS Two-Dimensional Grey Optical Array Imaging Probe.
Abstract
The response of the CSIRO liquid water content (LWC) device to water drops of different sizes has been investigated in a wind tunnel. Two series of experiments were conducted. The first compared the probe-measured LWC of sprays with different median volume diameters (MVD) to the LWC computed through water mass conservation considerations; the second series compared the probe LWC to that computed from the droplet spectra measured by Particle Measuring Systems' (PMS) probes. In the first series of experiments, the response was found to decrease from 100% for a water spray with an MVD of ∼ 20 μm to about 50% for a spray with an MVD of ∼ 150–200 μm. From these results, we expect that the King Probe can be used without correction for measuring the LWC of droplet distributions with MVDs less than about 40 μm. As the MVD increases, there will be a gradually diminishing response, which for MVDs of greater than 100 μm will require substantial correction.
The second series of experiments produced physically unreasonable results, suggesting that the size calibration of the PMS Forward Scattering Spectrometer Probe needs to be reevalualed. These results also indicate that a correction is required for the PMS Two-Dimensional Grey Optical Array Imaging Probe.
Abstract
The results of a heat flux comparison experiment carried out at Hay, New South Wales, Australia, during May 1966 using a sonic anemometer thermometer (SAT), Fluxatron and Evapotron are reported. The instruments agree with each other to within a factor of 2 for individual runs. The large fluctuations from run to run of the individual estimates are mainly caused by the fact that the Eulerian point average does not provide an adequate statistical sample of the heat flux. This point is illustrated by the non-stationary behavior of the instantaneous product of vertical wind and temperature. As auxiliary results, values of σ w /u * and σ T /T * have been obtained which are somewhat higher than, but in general agreement with, observations reported by Mordukhovich and Zwang.
Abstract
The results of a heat flux comparison experiment carried out at Hay, New South Wales, Australia, during May 1966 using a sonic anemometer thermometer (SAT), Fluxatron and Evapotron are reported. The instruments agree with each other to within a factor of 2 for individual runs. The large fluctuations from run to run of the individual estimates are mainly caused by the fact that the Eulerian point average does not provide an adequate statistical sample of the heat flux. This point is illustrated by the non-stationary behavior of the instantaneous product of vertical wind and temperature. As auxiliary results, values of σ w /u * and σ T /T * have been obtained which are somewhat higher than, but in general agreement with, observations reported by Mordukhovich and Zwang.
Abstract
Wet wind tunnel tests have been Performed on several versions of the CSIRO probe designed for the airborne measurement of liquid water content. Four different controller units and 17 different Probe sensors (including half-size and shielded versions) were tested. Even with tests conducted under extreme conditions, differences in response for all units were always less than 15%, except for the shielded units which needed to be operated at least 60°C better than the unshielded ones to yield the same output. Probe measurements with the unshielded sensors were typically within 5% and always within 10% of the tunnel values which were determined from icing cylinder measurements. The short length probes performed equally as well as the standard length ones and have certain operational advantages, such as Water robustness and the ability to operate at higher liquid water contents for a given supply voltage. Changing airspeed from 15 to 100 m s−1 and ambient temperature from −28°C to +10°C produced no measurable effect on any probe response, whereas grossly overdamping the probes via incorrect offset voltages could reduce the apparent output sensitivity by as much as 50%.
Abstract
Wet wind tunnel tests have been Performed on several versions of the CSIRO probe designed for the airborne measurement of liquid water content. Four different controller units and 17 different Probe sensors (including half-size and shielded versions) were tested. Even with tests conducted under extreme conditions, differences in response for all units were always less than 15%, except for the shielded units which needed to be operated at least 60°C better than the unshielded ones to yield the same output. Probe measurements with the unshielded sensors were typically within 5% and always within 10% of the tunnel values which were determined from icing cylinder measurements. The short length probes performed equally as well as the standard length ones and have certain operational advantages, such as Water robustness and the ability to operate at higher liquid water contents for a given supply voltage. Changing airspeed from 15 to 100 m s−1 and ambient temperature from −28°C to +10°C produced no measurable effect on any probe response, whereas grossly overdamping the probes via incorrect offset voltages could reduce the apparent output sensitivity by as much as 50%.
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.
Abstract
Experiments have been conducted to determine the effects, within cumulus clouds, of seeding with silver iodide; such effects are determined by correlating the presence of ice particles with the presence of silver iodide. This correlation is made possible by the use of a sailplane, which has the capability of remaining in the cloud updraft and the seeded plume for relatively long periods of time. The correlation method provided a very sensitive test for distinguishing seeding effects from natural ice development. Case studies using this technique are discussed.
Measurements in one small cloud having a base at −12°C showed concentrations at −15°C in the plume of up to 400 ice crystals per liter about 4 min after seeding. Aggregation of stellar dendrites occurred for particles as small as 400–500 μm diameter and about half of the available crystals were in aggregates after about 15 min. The output efficiency of the TB-1 pyrotechnic flares used in the experiment is estimated to be at least 1011 ice particles per gram of silver iodide in the temperature range −12 to −17°C which is close to previously reported values. Significant nucleation of ice by silver iodide occurred for at least 20 min after seeding. Linear ice crystal growth rates of stellar dendrites of the order of 0.15 mm min−1 were determined from these observations.
Abstract
Experiments have been conducted to determine the effects, within cumulus clouds, of seeding with silver iodide; such effects are determined by correlating the presence of ice particles with the presence of silver iodide. This correlation is made possible by the use of a sailplane, which has the capability of remaining in the cloud updraft and the seeded plume for relatively long periods of time. The correlation method provided a very sensitive test for distinguishing seeding effects from natural ice development. Case studies using this technique are discussed.
Measurements in one small cloud having a base at −12°C showed concentrations at −15°C in the plume of up to 400 ice crystals per liter about 4 min after seeding. Aggregation of stellar dendrites occurred for particles as small as 400–500 μm diameter and about half of the available crystals were in aggregates after about 15 min. The output efficiency of the TB-1 pyrotechnic flares used in the experiment is estimated to be at least 1011 ice particles per gram of silver iodide in the temperature range −12 to −17°C which is close to previously reported values. Significant nucleation of ice by silver iodide occurred for at least 20 min after seeding. Linear ice crystal growth rates of stellar dendrites of the order of 0.15 mm min−1 were determined from these observations.
