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Abstract
The electrical charge transferred to highly insulated spheres by the diffusion of ions from ionized moving air is directly measured. The equilibrium charge is proportional to the logarithm of the ratio of the positive and negative ion conductivities, and depends upon the velocity of the sphere relative to its ionized environment. The measurements agree with theory.
Abstract
The electrical charge transferred to highly insulated spheres by the diffusion of ions from ionized moving air is directly measured. The equilibrium charge is proportional to the logarithm of the ratio of the positive and negative ion conductivities, and depends upon the velocity of the sphere relative to its ionized environment. The measurements agree with theory.
Abstract
Measurements in a giant expansion chamber show that the size of newly formed cloud droplets depends critically upon the cleanliness of the processed air. Droplets formed in ordinary air are small, but droplets large enough to precipitate are immediately formed by condensation whenever the condensation-nuclei density is sufficiently reduced. The product of mean drop mass and the activated nuclei density approximates the available free water per unit volume. Cloud droplets formed from polluted surface air are usually too small to precipitate, but large droplets formed overhead by condensation in sufficiently clean air may fall through the polluted cloud and initiate rain through association processes. Since the activated nuclei density is normally observed to decrease with increasing altitude, the probability of generating droplets sufficiently large to initiate rain increases as the vertical development of a cloud system increases. The population densities of large cloud droplets normally observed near the tops of precipitating clouds may be explained in terms of an overlying parcel of cooled air that is initially relatively free of nuclei.
Since pollution is swept out of the atmosphere by diffusion onto cloud droplets, and by droplet movement, it is suggested that periods of general cloudiness and precipitation reduce the original nuclei density and permit the subsequently formed droplets to grow still larger, thus increasing the probability of appreciable precipitation. The rain producing cycle is, therefore, equipped with a feed-back or regenerative mechanism which normally proceeds, in a given mass of air, until the air is appreciably desiccated.
Condensation nuclei as well as water vapor normally accumulate simultaneously in fair weather. The presence of nuclei may delay the initiation of precipitation until sufficient vertical instability can be established to lift or cool the relatively clean overlying layers. The precipitation cycle may then be re-established. The investigation shows that rain may be formed directly from the vapor in clean air, without the production of clouds.
Abstract
Measurements in a giant expansion chamber show that the size of newly formed cloud droplets depends critically upon the cleanliness of the processed air. Droplets formed in ordinary air are small, but droplets large enough to precipitate are immediately formed by condensation whenever the condensation-nuclei density is sufficiently reduced. The product of mean drop mass and the activated nuclei density approximates the available free water per unit volume. Cloud droplets formed from polluted surface air are usually too small to precipitate, but large droplets formed overhead by condensation in sufficiently clean air may fall through the polluted cloud and initiate rain through association processes. Since the activated nuclei density is normally observed to decrease with increasing altitude, the probability of generating droplets sufficiently large to initiate rain increases as the vertical development of a cloud system increases. The population densities of large cloud droplets normally observed near the tops of precipitating clouds may be explained in terms of an overlying parcel of cooled air that is initially relatively free of nuclei.
Since pollution is swept out of the atmosphere by diffusion onto cloud droplets, and by droplet movement, it is suggested that periods of general cloudiness and precipitation reduce the original nuclei density and permit the subsequently formed droplets to grow still larger, thus increasing the probability of appreciable precipitation. The rain producing cycle is, therefore, equipped with a feed-back or regenerative mechanism which normally proceeds, in a given mass of air, until the air is appreciably desiccated.
Condensation nuclei as well as water vapor normally accumulate simultaneously in fair weather. The presence of nuclei may delay the initiation of precipitation until sufficient vertical instability can be established to lift or cool the relatively clean overlying layers. The precipitation cycle may then be re-established. The investigation shows that rain may be formed directly from the vapor in clean air, without the production of clouds.
Abstract
The diurnal variations in tropical cloudiness and tropospheric winds during the Australian Monsoon Experiment (AMEX) Phase II are documented and compared to those observed elsewhere. A diurnal variation in tropical cloudiness was found to be a consistent feature of both disturbed and undisturbed conditions. The tropical cloudiness, as inferred from satellite and radar data, averaged over the entire period of AMEX Phase II, was at a maximum during the morning over the ocean and during the late afternoon over the Arnhem Land region of northern Australia. The diurnal variation in high cloud, as measured by satellite was 3:2 over the ocean and 4:1 over Arnhem Land. Radar data indicated a 1 0: 1 variation in convection over Arnhem Land, a 2:1 variation over the neighboring ocean and a 3:2 variation in the stratiform echoes over both Ambem land and the neighboring mean.
Interaction between local circulations and the large scale flow was found to he associated with the observed diurnal variations in tropical cloudiness. The large scale monsoon circulation exhibited a diurnal oscillation with maxima in both the low-level easterly and equatorial westerly flow during the early morning. Variations in the vertical motion fields were in phase with the inferred cloudiness changes, but the midlevel maximum in vertical motion did not correspond with the strongest boundary layer convergence. The precise timing upward vertical motion over oceanic regions within the primary AMEX domain and the less reliably observed region to the north of Australia varied according to the degree of convective activity; consistent features were a maximum in vertical motion at 0830 LST during disturbed conditions and an 0230 LST maximum during suppressed conditions.
Abstract
The diurnal variations in tropical cloudiness and tropospheric winds during the Australian Monsoon Experiment (AMEX) Phase II are documented and compared to those observed elsewhere. A diurnal variation in tropical cloudiness was found to be a consistent feature of both disturbed and undisturbed conditions. The tropical cloudiness, as inferred from satellite and radar data, averaged over the entire period of AMEX Phase II, was at a maximum during the morning over the ocean and during the late afternoon over the Arnhem Land region of northern Australia. The diurnal variation in high cloud, as measured by satellite was 3:2 over the ocean and 4:1 over Arnhem Land. Radar data indicated a 1 0: 1 variation in convection over Arnhem Land, a 2:1 variation over the neighboring ocean and a 3:2 variation in the stratiform echoes over both Ambem land and the neighboring mean.
Interaction between local circulations and the large scale flow was found to he associated with the observed diurnal variations in tropical cloudiness. The large scale monsoon circulation exhibited a diurnal oscillation with maxima in both the low-level easterly and equatorial westerly flow during the early morning. Variations in the vertical motion fields were in phase with the inferred cloudiness changes, but the midlevel maximum in vertical motion did not correspond with the strongest boundary layer convergence. The precise timing upward vertical motion over oceanic regions within the primary AMEX domain and the less reliably observed region to the north of Australia varied according to the degree of convective activity; consistent features were a maximum in vertical motion at 0830 LST during disturbed conditions and an 0230 LST maximum during suppressed conditions.
Abstract
Vertical-section radar observations of precipitation in the winter of 1951–1952 have been related to upper-air data. On three of the 22 days, there was very little signal and practically no pattern observed. It is possible that these are cases, such as described recently by Wexler and Austin (1953), in which the snow crystals grow gradually while descending through cloud-free air, at a saturation intermediate between that of ice and water.
On the remaining 19 days, trail patterns were observed. On 13 of them, well-defined snow trails, usually with generating elements visible, were detected. Upper-air analyses showed the air aloft to be stable on these days of good pattern. On six days, the records contained only parts of trails, the whole pattern being less well-defined; the air aloft was unstable on these days. Since the well-defined patterns occur on stable days, instability is not the initiating mechanism; rather, the presence of instability confuses the pattern.
For the 13 stable days, the top of the trail echo was, on the average, near the top of a main middle cloud, the top of this cloud being defined as the height above which radiosonde data show the relative humidity to drop sharply. The average cloud-top height for these 13 days was some 1300 ft above a frontal surface, the average frontal-surface height over Montreal being about 11,000 ft.
The results of this study indicate the presence of a relatively shallow “active layer” of stratiform cloud, straddling a frontal surface, in which snow-generating elements of considerable lifetime exist.
Abstract
Vertical-section radar observations of precipitation in the winter of 1951–1952 have been related to upper-air data. On three of the 22 days, there was very little signal and practically no pattern observed. It is possible that these are cases, such as described recently by Wexler and Austin (1953), in which the snow crystals grow gradually while descending through cloud-free air, at a saturation intermediate between that of ice and water.
On the remaining 19 days, trail patterns were observed. On 13 of them, well-defined snow trails, usually with generating elements visible, were detected. Upper-air analyses showed the air aloft to be stable on these days of good pattern. On six days, the records contained only parts of trails, the whole pattern being less well-defined; the air aloft was unstable on these days. Since the well-defined patterns occur on stable days, instability is not the initiating mechanism; rather, the presence of instability confuses the pattern.
For the 13 stable days, the top of the trail echo was, on the average, near the top of a main middle cloud, the top of this cloud being defined as the height above which radiosonde data show the relative humidity to drop sharply. The average cloud-top height for these 13 days was some 1300 ft above a frontal surface, the average frontal-surface height over Montreal being about 11,000 ft.
The results of this study indicate the presence of a relatively shallow “active layer” of stratiform cloud, straddling a frontal surface, in which snow-generating elements of considerable lifetime exist.
Abstract
Line integral techniques are used to calculate vertically integrated heat and moisture budgets over the Gulf of Carpentaria during Phase II of the Australian Monsoon Experiment (AMEX). The budget area is an array of six radiosondes in a monsoon environment, and the calculations are performed every 6 hours over a period of 33 days.
During convective outbreaks the integrated heating and drying of the large scale by the cumulonimbus activity has a magnitude of the order of 10°C day−1. The heat and moisture sources are dominated by the flux divergence terms, which account for over 90% of the variance. The observed warming is as large as ±1°C day−1 but is diurnally dominated and does not correspond to the latent heat release. The integrated moisture convergence has a high correlation with latent heat release but not with the measured moisture storage. The convective heat source is also highly correlated with middle tropospheric vertical velocity.
Mean budgets are presented for each of the four diurnal observation times. Also, budgets were run with each station, in turn, excluded from the sonde array to determine sensitivity of the results to the data network.
Abstract
Line integral techniques are used to calculate vertically integrated heat and moisture budgets over the Gulf of Carpentaria during Phase II of the Australian Monsoon Experiment (AMEX). The budget area is an array of six radiosondes in a monsoon environment, and the calculations are performed every 6 hours over a period of 33 days.
During convective outbreaks the integrated heating and drying of the large scale by the cumulonimbus activity has a magnitude of the order of 10°C day−1. The heat and moisture sources are dominated by the flux divergence terms, which account for over 90% of the variance. The observed warming is as large as ±1°C day−1 but is diurnally dominated and does not correspond to the latent heat release. The integrated moisture convergence has a high correlation with latent heat release but not with the measured moisture storage. The convective heat source is also highly correlated with middle tropospheric vertical velocity.
Mean budgets are presented for each of the four diurnal observation times. Also, budgets were run with each station, in turn, excluded from the sonde array to determine sensitivity of the results to the data network.
