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- Author or Editor: P. V. Hobbs x
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Abstract
A previously derived theoretical expression, supported by experimental observations, for the rate of sintering between two ice spheres is applied to the case of adhesion between ice particles down to temperatures of −40C. Appreciable bonding is shown to take place between the particles within 10 seconds of contact even at −40C. The results predicted by the theory are found to be in quantitative agreement with the degree of sintering observed in aggregates of ice particles from natural ice fogs.
Abstract
A previously derived theoretical expression, supported by experimental observations, for the rate of sintering between two ice spheres is applied to the case of adhesion between ice particles down to temperatures of −40C. Appreciable bonding is shown to take place between the particles within 10 seconds of contact even at −40C. The results predicted by the theory are found to be in quantitative agreement with the degree of sintering observed in aggregates of ice particles from natural ice fogs.
Abstract
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Abstract
The organization and structure of a narrow cold-frontal rainband (NCFR) on the small mesoscale and the microscale have been investigated through quantitative radar reflectivity, Doppler radar observations, airborne observations and surface measurements. The NCFR was composed of small mesoscale regions of heavy precipitation called “precipitation cores” (PCs) oriented at an angle to the synoptic-scale cold front; in horizontal cross section the PCs were roughly elliptical in shape. Areas of lighter precipitation called “gap regions” (GRs) separated the PCs. The PCs were so oriented that their loading edges were regions of strong low-level convergence.
The weather associated with the passage of a PC resembled that of a squall-line gust front, with concurrent windshifts and pressure checks occurring ∼5 min before heavy precipitation and a fall in temperature. The changes in surface weather that accompanied the passage of a GR were more variable but tended to be less marked than for PCs. Thus, the sequence of weather experienced by a ground station can be markedly affected by its position with respect to the small mesoscale structure of the cold front.
The loading edges of the PCs were generally marked by relatively strong updrafts and high liquid water contents. Ice particle concentrations were high, particularly in the upper regions of the updrafts and in the downdraft regions of the PCs. Considerable ice enhancement, probably due to ice splinter production during riming, was present in these two regions. Riming was the dominant mechanism for the growth of precipitation in the PCs.
Several aspects of the small mesoscale structure of cold fronts are reminiscent of features seen with gravity currents. Also, the velocity of motion predicted by gravity current theory is in good agreement with the observed motion of the cold front.
We now visualize a cold front on the small mesoscale as a series of parallel line segments, each passing through the long axes of a PC, connected by kinks in the GRs. In the vicinity of a kink, the circulation can form a meso-low which, in extreme cases, may be a preferred region for the development of tornadoes and downbursts.
Abstract
The organization and structure of a narrow cold-frontal rainband (NCFR) on the small mesoscale and the microscale have been investigated through quantitative radar reflectivity, Doppler radar observations, airborne observations and surface measurements. The NCFR was composed of small mesoscale regions of heavy precipitation called “precipitation cores” (PCs) oriented at an angle to the synoptic-scale cold front; in horizontal cross section the PCs were roughly elliptical in shape. Areas of lighter precipitation called “gap regions” (GRs) separated the PCs. The PCs were so oriented that their loading edges were regions of strong low-level convergence.
The weather associated with the passage of a PC resembled that of a squall-line gust front, with concurrent windshifts and pressure checks occurring ∼5 min before heavy precipitation and a fall in temperature. The changes in surface weather that accompanied the passage of a GR were more variable but tended to be less marked than for PCs. Thus, the sequence of weather experienced by a ground station can be markedly affected by its position with respect to the small mesoscale structure of the cold front.
The loading edges of the PCs were generally marked by relatively strong updrafts and high liquid water contents. Ice particle concentrations were high, particularly in the upper regions of the updrafts and in the downdraft regions of the PCs. Considerable ice enhancement, probably due to ice splinter production during riming, was present in these two regions. Riming was the dominant mechanism for the growth of precipitation in the PCs.
Several aspects of the small mesoscale structure of cold fronts are reminiscent of features seen with gravity currents. Also, the velocity of motion predicted by gravity current theory is in good agreement with the observed motion of the cold front.
We now visualize a cold front on the small mesoscale as a series of parallel line segments, each passing through the long axes of a PC, connected by kinks in the GRs. In the vicinity of a kink, the circulation can form a meso-low which, in extreme cases, may be a preferred region for the development of tornadoes and downbursts.
Abstract
A cloud condensation nuclei counter has been developed in which the concentration of water droplets which form on cloud condensation nuclei in a large thermal diffusion chamber is determined electronically by measuring the light scattering coefficient of the cloud. The counter operates completely automatically and may be used to determine the concentration of cloud condensation nuclei in the air (active at a given supersaturation) at any desired interval of time down to a minimum of about 2 min. The counts obtained by this method are in good agreement with direct visual counting of droplets in the thermal diffusion chamber.
Abstract
A cloud condensation nuclei counter has been developed in which the concentration of water droplets which form on cloud condensation nuclei in a large thermal diffusion chamber is determined electronically by measuring the light scattering coefficient of the cloud. The counter operates completely automatically and may be used to determine the concentration of cloud condensation nuclei in the air (active at a given supersaturation) at any desired interval of time down to a minimum of about 2 min. The counts obtained by this method are in good agreement with direct visual counting of droplets in the thermal diffusion chamber.
Abstract
Measurements have been made of the electric charge acquired by an ice sphere as the result of whirling it through various types of natural cloud and snowfall. The sphere received an appreciable charge only if ice were present in the air. In the case of an ice crystal cloud or a fall of snow crystals, the charge on the sphere was generally negative provided that the air temperature was −4C or less. Simultaneous measurements of the charge on the ice sphere and the number of ice crystals colliding with a sphere of similar size yielded a value for the charge on the sphere of −2 × 10−3 esu per ice crystal collision. When the air temperature was above −4C, the charge on the sphere was erratic but was found to have the same sign as the charge on the particles in the air. If graupel particles were present the ice sphere always received a large positive charge.
The presence of supercooled droplets in the air caused a considerable reduction in the charge accumulated on the ice sphere. When the sphere was whirled through a cloud consisting entirely of supercooled droplets negligible charge was separated. This result is in disagreement with the laboratory measurements of Latham and Mason, and casts doubt on the mechanism of thunderstorm electrification proposed by these workers.
Abstract
Measurements have been made of the electric charge acquired by an ice sphere as the result of whirling it through various types of natural cloud and snowfall. The sphere received an appreciable charge only if ice were present in the air. In the case of an ice crystal cloud or a fall of snow crystals, the charge on the sphere was generally negative provided that the air temperature was −4C or less. Simultaneous measurements of the charge on the ice sphere and the number of ice crystals colliding with a sphere of similar size yielded a value for the charge on the sphere of −2 × 10−3 esu per ice crystal collision. When the air temperature was above −4C, the charge on the sphere was erratic but was found to have the same sign as the charge on the particles in the air. If graupel particles were present the ice sphere always received a large positive charge.
The presence of supercooled droplets in the air caused a considerable reduction in the charge accumulated on the ice sphere. When the sphere was whirled through a cloud consisting entirely of supercooled droplets negligible charge was separated. This result is in disagreement with the laboratory measurements of Latham and Mason, and casts doubt on the mechanism of thunderstorm electrification proposed by these workers.
Abstract
Simultaneous measurements have been made of the concentrations of cloud condensation nuclei, sodium-containing particles, Aitken nuclei, and the magnitude of the light scattering coefficient of the air, for a period of two months in the Olympic Mountains of Washington State.
Large short-term changes in the magnitudes of these four quantities were found to be related to variations in the local meteorological conditions. The most striking changes occurred with the build up and the evaporation of cumulus clouds upwind of the measuring site. The results indicate that growing clouds absorb (and also probably generate) large numbers of particulates, and that these particulates are released when the clouds dissipate. Precipitation also caused significant reductions in the concentrations of particulates in the air.
Longer period variations in particulate concentrations were associated with the diurnal convective cycle and changes in air mass. Continental air contained higher concentrations of cloud condensation nuclei and Aitken nuclei than maritime air, but the Pacific Ocean appeared to be the principal source of sodium-containing particles. However, even in maritime air the measured concentrations of sodium-containing particles were always less than about 1% of the concentrations of cloud condensation nuclei active at 1% supersaturation.
Abstract
Simultaneous measurements have been made of the concentrations of cloud condensation nuclei, sodium-containing particles, Aitken nuclei, and the magnitude of the light scattering coefficient of the air, for a period of two months in the Olympic Mountains of Washington State.
Large short-term changes in the magnitudes of these four quantities were found to be related to variations in the local meteorological conditions. The most striking changes occurred with the build up and the evaporation of cumulus clouds upwind of the measuring site. The results indicate that growing clouds absorb (and also probably generate) large numbers of particulates, and that these particulates are released when the clouds dissipate. Precipitation also caused significant reductions in the concentrations of particulates in the air.
Longer period variations in particulate concentrations were associated with the diurnal convective cycle and changes in air mass. Continental air contained higher concentrations of cloud condensation nuclei and Aitken nuclei than maritime air, but the Pacific Ocean appeared to be the principal source of sodium-containing particles. However, even in maritime air the measured concentrations of sodium-containing particles were always less than about 1% of the concentrations of cloud condensation nuclei active at 1% supersaturation.
