Search Results
You are looking at 1 - 10 of 27 items for :
- Author or Editor: S. Twomey x
- Article x
- Refine by Access: All Content x
Abstract
Numerical integrations were made of the statistical equations describing the evolution of droplet distributions by coalescence. The results confirmed that increasing the droplet concentration, for the same liquid water content, greatly adds to the difficulty attending rain formation by coalescence. The spread of the initial distribution was not, on the other hand, an important parameter.
The computed times for rain formation were less than a half-hour when the cloud contained 50 droplets cm−3 and 1 gm m−3 liquid water. When compared with observations these times are a little too long, but rain which forms more quickly can be explained by more rapid growth in regions of higher liquid water content.
Abstract
Numerical integrations were made of the statistical equations describing the evolution of droplet distributions by coalescence. The results confirmed that increasing the droplet concentration, for the same liquid water content, greatly adds to the difficulty attending rain formation by coalescence. The spread of the initial distribution was not, on the other hand, an important parameter.
The computed times for rain formation were less than a half-hour when the cloud contained 50 droplets cm−3 and 1 gm m−3 liquid water. When compared with observations these times are a little too long, but rain which forms more quickly can be explained by more rapid growth in regions of higher liquid water content.
Abstract
Several years ago Telford pointed out that the simplest coalescence model, in which a small group of droplets grew with unit collection efficiency by collecting droplets of half their volume, did not remain bimodal but that the statistical fluctuations in the discrete coalescence events caused many sizes to be evolved from the original two. A few droplets per million grew much more rapidly than the average; Telford was thereby able to shorten considerably the time necessary for rain formation in warm clouds.
Results have been obtained by numerical solution of the integro-differential equation which describes the time variation of a droplet distribution. Hocking's collection efficiencies were used. The computations show that Telford's mechanism is equally, if not more, effective when the initial distribution is continuous.
Abstract
Several years ago Telford pointed out that the simplest coalescence model, in which a small group of droplets grew with unit collection efficiency by collecting droplets of half their volume, did not remain bimodal but that the statistical fluctuations in the discrete coalescence events caused many sizes to be evolved from the original two. A few droplets per million grew much more rapidly than the average; Telford was thereby able to shorten considerably the time necessary for rain formation in warm clouds.
Results have been obtained by numerical solution of the integro-differential equation which describes the time variation of a droplet distribution. Hocking's collection efficiencies were used. The computations show that Telford's mechanism is equally, if not more, effective when the initial distribution is continuous.
Abstract
The size of cloud nuclei acting at 0.75% supersaturation was estimated by using varying flow rates through Nuclepore filters to discriminate between different sizes. The nuclei, sampled in clean continental and maritime air at Robertson, N.S.W., were found to be small, not much greater than the theoretical minimum radius of ∼1−6 cm permitted by nucleation theory.
This result agrees quite well with previous estimates of cloud-nucleus size from experiments carried out at Chesapeake Bay, Md. It is concluded that the atmospheric residence time of cloud nuclei cannot be more than a few days and that they must be composed entirely or partly of a water-soluble material since an insoluble particle of such a small size could not nucleate condensation at supersaturations of the order of 1% or less.
Abstract
The size of cloud nuclei acting at 0.75% supersaturation was estimated by using varying flow rates through Nuclepore filters to discriminate between different sizes. The nuclei, sampled in clean continental and maritime air at Robertson, N.S.W., were found to be small, not much greater than the theoretical minimum radius of ∼1−6 cm permitted by nucleation theory.
This result agrees quite well with previous estimates of cloud-nucleus size from experiments carried out at Chesapeake Bay, Md. It is concluded that the atmospheric residence time of cloud nuclei cannot be more than a few days and that they must be composed entirely or partly of a water-soluble material since an insoluble particle of such a small size could not nucleate condensation at supersaturations of the order of 1% or less.
Abstract
When non-absorbing scatterers (e.g., cloud drops) are added in an absorbing layer (e.g., a dust layer), the optical paths of the radiation will be greatly changed if the scattering component is optically thick. The absorption will thus be altered. Results are given of numerical computations to determine the effect of non-absorbing cloud drops on the absorption of radiation by a dust layer. Absorption is found to be decreased for low angles of incidence and increased for higher angles of incidence.
Abstract
When non-absorbing scatterers (e.g., cloud drops) are added in an absorbing layer (e.g., a dust layer), the optical paths of the radiation will be greatly changed if the scattering component is optically thick. The absorption will thus be altered. Results are given of numerical computations to determine the effect of non-absorbing cloud drops on the absorption of radiation by a dust layer. Absorption is found to be decreased for low angles of incidence and increased for higher angles of incidence.
Abstract
Natural cloud nuclei in both maritime and continental air masses in southeastern Australia (Robertson, N.S.W.) show a high volatility when heated. Few natural nuclei withstand the high temperatures which can be applied to sodium chloride and other sodium salts. Most nuclei are therefore something other than sodium chloride or sea salt—probably ammonium sulfate. The concentration of non-volatile nuclei which could be sea salt rarely exceeded 10 cm−3.
Abstract
Natural cloud nuclei in both maritime and continental air masses in southeastern Australia (Robertson, N.S.W.) show a high volatility when heated. Few natural nuclei withstand the high temperatures which can be applied to sodium chloride and other sodium salts. Most nuclei are therefore something other than sodium chloride or sea salt—probably ammonium sulfate. The concentration of non-volatile nuclei which could be sea salt rarely exceeded 10 cm−3.
Abstract
The composition of the larger (> 10−12 g) hygroscopic particles in the atmosphere was investigated, with use of a phase-transition method. Particles were collected on stretched spider webs exposed from an aircraft, and samples were taken in both maritime and continental air in different meteorological and geographical situations. It was found that the great majority of hygroscopic particles was composed of sea salt, sometimes in combination with insoluble material. A chemical test on individual particles indicated that no appreciable decrease in chloride content or replacement of chlorides by carbonates took place. Only occasionally were large soluble particles, other than sea salt, detected.
Abstract
The composition of the larger (> 10−12 g) hygroscopic particles in the atmosphere was investigated, with use of a phase-transition method. Particles were collected on stretched spider webs exposed from an aircraft, and samples were taken in both maritime and continental air in different meteorological and geographical situations. It was found that the great majority of hygroscopic particles was composed of sea salt, sometimes in combination with insoluble material. A chemical test on individual particles indicated that no appreciable decrease in chloride content or replacement of chlorides by carbonates took place. Only occasionally were large soluble particles, other than sea salt, detected.
Abstract
Maritime air was followed from the coast, and measurements of seal-salt nucleus distributions were carried out. It was found that the concentrations encountered in air which had been over land for a considerable time ranged from very low values to values approaching those usually found in maritime air. It also seemed that convective cloud formation or precipitation rapidly lowered the salt concentration. In the absence of such factors, no appreciable diminution in total concentration occurred; vertical mixing, however, often gave rise to elevated salt concentrations at higher levels. Very low concentrations were found above post-frontal subsidence inversions over land, in air streams which had recently come from over the ocean.
Abstract
Maritime air was followed from the coast, and measurements of seal-salt nucleus distributions were carried out. It was found that the concentrations encountered in air which had been over land for a considerable time ranged from very low values to values approaching those usually found in maritime air. It also seemed that convective cloud formation or precipitation rapidly lowered the salt concentration. In the absence of such factors, no appreciable diminution in total concentration occurred; vertical mixing, however, often gave rise to elevated salt concentrations at higher levels. Very low concentrations were found above post-frontal subsidence inversions over land, in air streams which had recently come from over the ocean.
Abstract
The application of nonlinear iterafive algorithms to two-dimensional tomographic reconstructions is discttszed and a number of numerical examples are given, using as an illustrative basis reconstruction of the spatial distribution of liquid water in clouds from measurements of microwave attenuation. (The method, however, is not restricted to that specific problem and appears to be especially suitable for inversions involving a large number of unknowns.)
Abstract
The application of nonlinear iterafive algorithms to two-dimensional tomographic reconstructions is discttszed and a number of numerical examples are given, using as an illustrative basis reconstruction of the spatial distribution of liquid water in clouds from measurements of microwave attenuation. (The method, however, is not restricted to that specific problem and appears to be especially suitable for inversions involving a large number of unknowns.)
Abstract
Using Nuclepore filters at a sequence of flow rates, to discriminate according to particle size, and a suitable mathematical inversion procedure, particle size distributions have been obtained for surface atmospheric aerosol samples. Measurements in relatively unpolluted air at Robertson, N.S.W., Australia, yield a fairly reproducible size distribution, with a maximum always close to 10−6 cm radius; measurements in more polluted air at The University of Arizona have led to very similar size distributions.
Abstract
Using Nuclepore filters at a sequence of flow rates, to discriminate according to particle size, and a suitable mathematical inversion procedure, particle size distributions have been obtained for surface atmospheric aerosol samples. Measurements in relatively unpolluted air at Robertson, N.S.W., Australia, yield a fairly reproducible size distribution, with a maximum always close to 10−6 cm radius; measurements in more polluted air at The University of Arizona have led to very similar size distributions.
Abstract
Using published data for water vapor absorption and for absorption by liquid (or ice) water, the absorption of solar radiation by clouds was computed for several representative cloud models. Absorption was found to approach 20% of the solar flux for the more absorbing and thicker clouds. There were systematic differences between continental and maritime clouds, the latter absorbing more for the same cloud thickness—an effect produced by the greater absorption efficiency of the larger maritime drops.
Abstract
Using published data for water vapor absorption and for absorption by liquid (or ice) water, the absorption of solar radiation by clouds was computed for several representative cloud models. Absorption was found to approach 20% of the solar flux for the more absorbing and thicker clouds. There were systematic differences between continental and maritime clouds, the latter absorbing more for the same cloud thickness—an effect produced by the greater absorption efficiency of the larger maritime drops.
