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- Author or Editor: L. F. Radke x
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Abstract
An airborne instrument which provides real-time measurements of the concentrations of ice particles in excess of a certain size has been developed. The instrument utilizes a beam of polarized light and the birefringent nature of ice to detect ice particles but to exclude the counting of water drops no matter how large. Theoretical considerations are discussed which show that the direct use of the birefringence property of ice provides a much larger signal than is possible when only the light scattered from particles is detected. The design, testing and field evaluation of the prototype instrument are discussed.
Abstract
An airborne instrument which provides real-time measurements of the concentrations of ice particles in excess of a certain size has been developed. The instrument utilizes a beam of polarized light and the birefringent nature of ice to detect ice particles but to exclude the counting of water drops no matter how large. Theoretical considerations are discussed which show that the direct use of the birefringence property of ice provides a much larger signal than is possible when only the light scattered from particles is detected. The design, testing and field evaluation of the prototype instrument are discussed.
Abstract
The original automatic cloud condensation nucleus counter developed by Radke and Hobbs has been improved by reducing the size and weight of the instrument and increasing the sampling rate.
In comparison tests with four other counters, the improved automatic counter was found to give counts within ±50% of the mean of all five counters.
Abstract
The original automatic cloud condensation nucleus counter developed by Radke and Hobbs has been improved by reducing the size and weight of the instrument and increasing the sampling rate.
In comparison tests with four other counters, the improved automatic counter was found to give counts within ±50% of the mean of all five counters.
Abstract
Abstract
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.
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
The concentrations of cloud condensation nuclei (CCN) in the plumes from coal-fired electric power plants are generally about 2 to 5 times greater than in the ambient air unaffected by the plumes. However, if the ambient air is very clean, the concentrations of CCN in a coal power plant plume can be up to ∼80 times greater than in the ambient air. The rates of production of CCN due to gas-to-particle (g-to-p) conversion in the plume from one of the plants studied were measured on different occasions to be ∼2 × 1015 and ∼5 × 1013 CCN h−1 per mole of SO2. The maximum current of CCN to be expected in the plume from a coal power plant is ∼1017 CCN s−1. After a travel time of ∼1 h, most of the CCN in power plant plumes have been produced by g-to-p conversion rather than emitted directly from the stack.
The concentrations of ice nuclei in the plumes did not differ significantly from those in the ambient air.
The materials in a plume may be transported rapidly in the vertical if the plume is entrained into a convective cloud. The plume may cause a lowering in the altitude of the cloud base, but any effects that the plume may have on the drop size distribution in a convective cloud are often less than the natural variations. By contrast, in stratiform clouds a plume can produce marked increases in the concentration of small drops (∼10–20 μm diameter) and in the total concentrations of drops
Abstract
The concentrations of cloud condensation nuclei (CCN) in the plumes from coal-fired electric power plants are generally about 2 to 5 times greater than in the ambient air unaffected by the plumes. However, if the ambient air is very clean, the concentrations of CCN in a coal power plant plume can be up to ∼80 times greater than in the ambient air. The rates of production of CCN due to gas-to-particle (g-to-p) conversion in the plume from one of the plants studied were measured on different occasions to be ∼2 × 1015 and ∼5 × 1013 CCN h−1 per mole of SO2. The maximum current of CCN to be expected in the plume from a coal power plant is ∼1017 CCN s−1. After a travel time of ∼1 h, most of the CCN in power plant plumes have been produced by g-to-p conversion rather than emitted directly from the stack.
The concentrations of ice nuclei in the plumes did not differ significantly from those in the ambient air.
The materials in a plume may be transported rapidly in the vertical if the plume is entrained into a convective cloud. The plume may cause a lowering in the altitude of the cloud base, but any effects that the plume may have on the drop size distribution in a convective cloud are often less than the natural variations. By contrast, in stratiform clouds a plume can produce marked increases in the concentration of small drops (∼10–20 μm diameter) and in the total concentrations of drops
Abstract
Measurements of the concentrations of cloud condensation nuclei (CCN) in the air in Washington State have shown that pulp and paper mills, and certain other industries, are prolific sources of CCN. The rate of production of CCN from large paper mills can be as high as 1019 sec−1. Direct observations show that clouds often form downwind of these mills and, in some cases, these clouds produce precipitable particles very efficiently.
A comparison of precipitation and streamflow records in Washington for the period 1929–46 with those for 1947–66 shows that a number of areas have had a mean annual precipitation during the second period more than 30% greater than that in the first period. In nearly all cases these areas are in the vicinity of large industrial sources of CCN and it is suggested that the higher precipitation in recent years is a consequence of the increased numbers of very efficient CCN emitted into the atmosphere by these sources.
Abstract
Measurements of the concentrations of cloud condensation nuclei (CCN) in the air in Washington State have shown that pulp and paper mills, and certain other industries, are prolific sources of CCN. The rate of production of CCN from large paper mills can be as high as 1019 sec−1. Direct observations show that clouds often form downwind of these mills and, in some cases, these clouds produce precipitable particles very efficiently.
A comparison of precipitation and streamflow records in Washington for the period 1929–46 with those for 1947–66 shows that a number of areas have had a mean annual precipitation during the second period more than 30% greater than that in the first period. In nearly all cases these areas are in the vicinity of large industrial sources of CCN and it is suggested that the higher precipitation in recent years is a consequence of the increased numbers of very efficient CCN emitted into the atmosphere by these sources.
Abstract
No abstract available.
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No abstract available.
Abstract
Since the Ptarmigan flights in the 1950s, the springtime visibility reduction in the Arctic has been identified with pollution aerosol. However, observed values of the dry aerosol extinction coefficient are too small to explain the observed visibility reductions. Water uptake by the aerosol appears to be an important component of the Arctic turbidity. Furthermore, the presence of lower-tropospheric ice crystals may have a dominant effect on the visibility in the Arctic. Data obtained from a series of meteorological, chemical, and cloud microphysical measurements made by the University of Washington research aircraft during April 1983 and 1986 are used as input for a visibility model. The model calculates the water uptake by the dry aerosols as a function of relative humidity and determines the single-scattering properties of the aerosols and the ice crystals. Path radiances are calculated using an extension to the delta-Eddington approximation introduced by Hering. Modeled visual ranges indicate that aerosols alone cannot account for the low visibilities. In certain cases, the inclusion of ice crystals along with the aerosols can produce visual ranges which are as low as those observed. A comparison between visual ranges obtained using the model and estimated using Koschmieder's equation showed that Koschmieder's equation will generally underestimate the visual range.
Abstract
Since the Ptarmigan flights in the 1950s, the springtime visibility reduction in the Arctic has been identified with pollution aerosol. However, observed values of the dry aerosol extinction coefficient are too small to explain the observed visibility reductions. Water uptake by the aerosol appears to be an important component of the Arctic turbidity. Furthermore, the presence of lower-tropospheric ice crystals may have a dominant effect on the visibility in the Arctic. Data obtained from a series of meteorological, chemical, and cloud microphysical measurements made by the University of Washington research aircraft during April 1983 and 1986 are used as input for a visibility model. The model calculates the water uptake by the dry aerosols as a function of relative humidity and determines the single-scattering properties of the aerosols and the ice crystals. Path radiances are calculated using an extension to the delta-Eddington approximation introduced by Hering. Modeled visual ranges indicate that aerosols alone cannot account for the low visibilities. In certain cases, the inclusion of ice crystals along with the aerosols can produce visual ranges which are as low as those observed. A comparison between visual ranges obtained using the model and estimated using Koschmieder's equation showed that Koschmieder's equation will generally underestimate the visual range.
Abstract
Results from numerical simulations of the Colorado Front Range downslope windstorm of 9 December 1992 are presented. Although this case was not characterized by severe surface winds, the event caused extreme clear-air turbulence (CAT) aloft, as indicated by the severe structural damage experienced by a DC-8 cargo jet at 9.7 km above mean sea level over the mountains. Detailed measurements from the National Oceanic and Atmospheric Administration/Environmental Research Laboratories/Environmental Technology Laboratory Doppler lidar and wind profilers operating on that day and from the Defense Meteorological Satellite Program satellite allow for a uniquely rich comparison between the simulations and observations.
Four levels of grid refinement were used in the model. The outer domain used National Centers for Environmental Prediction data for initial and boundary conditions. The finest grid used 200 m in all three dimensions over a 48 km by 48 km section. The range of resolution and domain coverage were sufficient to resolve the abundant variety of dynamics associated with a time-evolving windstorm forced during a frontal passage. This full range of resolution and model complexity was essential in this case. Many aspects of this windstorm are inherently three-dimensional and are not represented in idealized models using either 2D or so-called 2D–3D dynamics.
Both the timing and location of wave breaking compared well with observations. The model also reproduced cross-stream wavelike perturbations in the jet stream that compared well with the orientation and spacing of cloud bands observed by satellite and lidar. Model results also show that the observed CAT derives from interactions between these wavelike jet stream disturbances and mountain-forced internal gravity waves. Due to the nearly east–west orientation of the jet stream, these two interacting wave modes were orthogonal to each other. Thermal gradients associated with the intense jet stream undulations generated horizontal vortex tubes (HVTs) aligned with the mean flow. These HVTs remained aloft while they propagated downstream at about the elevation of the aircraft incident, and evidence for such a vortex was seen by the lidar. The model and observations suggest that one of these intense vortices may have caused the aircraft incident.
Reports of strong surface gusts were intermittent along the Front Range during the period of this study. The model showed that interactions between the gravity waves and flow-aligned jet stream undulations result in isolated occurrences of strong surface gusts in line with observations. The simulations show that strong shears on the upper and bottom surfaces of the jet stream combine to provide an episodic “downburst of turbulence.” In the present case, the perturbations of the jet stream provide a funnel-shaped shear zone aligned with the mean flow that acts as a guide for the downward transport of turbulence resulting from breaking gravity waves. The physical picture for the upper levels is similar to the surface gusts described by Clark and Farley resulting from vortex tilting. The CAT feeding into this funnel came from all surfaces of the jet stream with more than half originating from the vertically inclined shear zones on the bottom side of the jet stream. Visually the downburst of turbulence looks similar to a rain shaft plummeting to the surface and propagating out over the plains leaving relatively quiescent conditions behind.
Abstract
Results from numerical simulations of the Colorado Front Range downslope windstorm of 9 December 1992 are presented. Although this case was not characterized by severe surface winds, the event caused extreme clear-air turbulence (CAT) aloft, as indicated by the severe structural damage experienced by a DC-8 cargo jet at 9.7 km above mean sea level over the mountains. Detailed measurements from the National Oceanic and Atmospheric Administration/Environmental Research Laboratories/Environmental Technology Laboratory Doppler lidar and wind profilers operating on that day and from the Defense Meteorological Satellite Program satellite allow for a uniquely rich comparison between the simulations and observations.
Four levels of grid refinement were used in the model. The outer domain used National Centers for Environmental Prediction data for initial and boundary conditions. The finest grid used 200 m in all three dimensions over a 48 km by 48 km section. The range of resolution and domain coverage were sufficient to resolve the abundant variety of dynamics associated with a time-evolving windstorm forced during a frontal passage. This full range of resolution and model complexity was essential in this case. Many aspects of this windstorm are inherently three-dimensional and are not represented in idealized models using either 2D or so-called 2D–3D dynamics.
Both the timing and location of wave breaking compared well with observations. The model also reproduced cross-stream wavelike perturbations in the jet stream that compared well with the orientation and spacing of cloud bands observed by satellite and lidar. Model results also show that the observed CAT derives from interactions between these wavelike jet stream disturbances and mountain-forced internal gravity waves. Due to the nearly east–west orientation of the jet stream, these two interacting wave modes were orthogonal to each other. Thermal gradients associated with the intense jet stream undulations generated horizontal vortex tubes (HVTs) aligned with the mean flow. These HVTs remained aloft while they propagated downstream at about the elevation of the aircraft incident, and evidence for such a vortex was seen by the lidar. The model and observations suggest that one of these intense vortices may have caused the aircraft incident.
Reports of strong surface gusts were intermittent along the Front Range during the period of this study. The model showed that interactions between the gravity waves and flow-aligned jet stream undulations result in isolated occurrences of strong surface gusts in line with observations. The simulations show that strong shears on the upper and bottom surfaces of the jet stream combine to provide an episodic “downburst of turbulence.” In the present case, the perturbations of the jet stream provide a funnel-shaped shear zone aligned with the mean flow that acts as a guide for the downward transport of turbulence resulting from breaking gravity waves. The physical picture for the upper levels is similar to the surface gusts described by Clark and Farley resulting from vortex tilting. The CAT feeding into this funnel came from all surfaces of the jet stream with more than half originating from the vertically inclined shear zones on the bottom side of the jet stream. Visually the downburst of turbulence looks similar to a rain shaft plummeting to the surface and propagating out over the plains leaving relatively quiescent conditions behind.
