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N. H. Fletcher

Abstract

The concept of active nucleation sites is examined in the light of standard nucleation theory and it is suggested that these sites are probably small re-entrant corners or jogs in growth steps on the surface of the nucleating particle. The nucleation behavior of a spherical particle with a conical pit on its surface is examined in detail and the activity of a population of particles having a log-normal distribution of such pits is worked out. The parameters involved in the theory can be evaluated within acceptable limits and the resulting curves exhibit the phenomena observed experimentally. Generalized activity curves are given in parametric form for more detailed comparison with experiment.

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N. H. Fletcher

Abstract

The ice nucleation behavior of smokes consisting of silver iodide, together with a soluble component such as potassium iodide, is considered theoretically. It is proposed, when released into a natural cloud, that the smoke particles take up water to form solution droplets in each of which is suspended a small particle of silver iodide produced by hydrolysis of the mixed salt. If the temperature is low enough, this silver iodide particle may nucleate the freezing of the solution droplet. This process is treated thermodynamically, and the nucleation activity of the smoke particles is calculated as a function of size. It is found that the behavior is insensitive to the nature and exact concentration of the soluble component. Theoretical curves for the activity of such a mixed smoke are derived and show good agreement with experiment. This treatment supersedes the author's 1959 calculation based on the assumption of sublimation activity.

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N. H. Fletcher

Abstract

A general theory of the action of small aerosol particles as ice-crystal nuclei is developed and its predictions compared with experiment. It is found possible to decide under what conditions a given particle will act as a sublimation or as a freezing nucleus, and it is concluded that silver iodide generally behaves as a sublimation nucleus. The size effect is used to explain the activity distribution of silver iodide smoke, and this in turn explains the time lag observed in the nucleation of ice crystals by this smoke.

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N. H. Fletcher

Abstract

The photo de-activation of silver iodide as an ice-crystal nucleus is ascribed to the production of photolytic silver at trapping centers in the volume and on the surface of the nucleating particle. Silver produced at the surface raises the free energy of the interface between the nucleus and ice and so reduces the activity of the particle. The rate of de-activation is found to depend upon the size of the particle, and thus the size distribution of particles in a smoke has a strong influence upon its decay behavior. Calculations on the basis of this theory explain the wide variations in the decay rate reported in different experiments.

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N. H. Fletcher

Abstract

The ice-forming ability of silver-iodide smoke depends upon the size of the smoke particles in a way which can be calculated. From this relation, it is possible to find the maximum number of nuclei produced, per gram of silver iodide, which are active at a given temperature. Comparison of the performance of such an ideal generator with that of experimental burners shows that optimum performance is being approached in some cases.

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N. H. Fletcher

Abstract

No abstract.

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N. H. Fletcher

Abstract

No abstract available.

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W. D. King
and
N. H. Fletcher

Abstract

Thermoelastic theory is used to calculate thermal stresses in ice crystals of idealized shapes when a small area on one surface is warmed to 0°C, in simulation of riming of the crystal by a cloud droplet. It is shown that a typical riming event involving a 20 μm diameter droplet impinging on a 500 μm plate or column would need to occur at a temperature colder than −35°C before fracture could be expected. For a given ice crystal size, the thermal stresses increase with droplet diameter, and thin plates rimed by large droplets are the most sensitive to thermal shock, but still need to be colder than −10°C before fracture would occur. It is consequently concluded that the thermal shock mechanism is unlikely to be responsible for the proliferation of ice crystals in clouds at temperatures as warm as −4°C.

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W. D. King
and
N. H. Fletcher

Abstract

Thermal shock tests were conducted on large numbers of ice spheres and plates, all of macroscopic size. The thermal shock was applied by cooling the specimens to the desired temperature, and then rapidly warming part of one surface by bringing water in contact with it. The spheres had a median cracking temperature of −16°C, and comparison with thermoelastic theory yielded tensile strength values for ice in the range 20–30 bars. Initiation of cracking in thick plates was a function of the temperature and of the ratio a/b (ratio of radius of warmed area to that of the cylindrical plate). For a/b=0.6, −20°C was the critical temperature, but for a/b≤0.2, which is a more appropriate scaling factor in terms of riming of cloud particles, the samples had to be colder than −35°C before any cracks appeared. None of the samples fragmented or separated. Because the experimentally applied temperature changes were more severe than would be experienced by rimed ice crystals in clouds, it is concluded that thermal shock is unlikely to be an important ice multiplication mechanism at −5°C.

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J. K. Fletcher
,
C. A. Diop
,
E. Adefisan
,
M. A. Ahiataku
,
S. O. Ansah
,
C. E. Birch
,
H. L. Burns
,
S. J. Clarke
,
J. Gacheru
,
T. D. James
,
C. K. Ngetich Tuikong
,
D. Koros
,
V. S. Indasi
,
B. L. Lamptey
,
K. A. Lawal
,
D. J. Parker
,
A. J. Roberts
,
T. H. M. Stein
,
E. Visman
,
J. Warner
,
B. J. Woodhams
,
L. H. Youds
,
V. O. Ajayi
,
E. N. Bosire
,
C. Cafaro
,
C. A. T. Camara
,
B. Chanzu
,
C. Dione
,
W. Gitau
,
D. Groves
,
J. Groves
,
P. G. Hill
,
I. Ishiyaku
,
C. M. Klein
,
J. H. Marsham
,
B. K. Mutai
,
P. N. Ndiaye
,
M. Osei
,
T. I. Popoola
,
J. Talib
,
C. M. Taylor
, and
D. Walker

Abstract

Testbeds have become integral to advancing the transfer of knowledge and capabilities from research to operational weather forecasting in many parts of the world. The first high-impact weather testbed in tropical Africa was recently carried out through the African Science for Weather Information and Forecasting Techniques (SWIFT) program, with participation from researchers and forecasters from Senegal, Ghana, Nigeria, Kenya, the United Kingdom, and international and pan-African organizations. The testbed aims were to trial new forecasting and nowcasting products with operational forecasters, to inform future research, and to act as a template for future testbeds in the tropics. The African SWIFT testbed integrated users and researchers throughout the process to facilitate development of impact-based forecasting methods and new research ideas driven both by operations and user input. The new products are primarily satellite-based nowcasting systems and ensemble forecasts at global and regional convection-permitting scales. Neither of these was used operationally in the participating African countries prior to the testbed. The testbed received constructive, positive feedback via intense user interaction including fishery, agriculture, aviation, and electricity sectors. After the testbed, a final set of recommended standard operating procedures for satellite-based nowcasting in tropical Africa have been produced. The testbed brought the attention of funding agencies and organizational directors to the immediate benefit of improved forecasts. Delivering the testbed strengthened the partnership between each country’s participating university and weather forecasting agency and internationally, which is key to ensuring the longevity of the testbed outcomes.

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