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Studying an effect of salt powder seeding used for precipitation enhancement from convective clouds
 A. S. Drofa1, V. N. Ivanov1, D. Rosenfeld2, and A. G. Shilin1 Complete text

1 Institute of Experimental Meteorology, Research and Production Association “Typhoon”, Obninsk, Russia
2 Institute of Earth Sciences, The Hebrew University of Jerusalem, Israel

Received: 10 February 2010 – Accepted: 13 April 2010 – Published: 23 April 2010
Correspondence to: D. Rosenfeld (daniel.rosenfeld@huji.ac.il)
Published by Copernicus Publications on behalf of the European Geosciences Union.


Abstract

The experimental and theoretical studies of cloud microstructure modification with the ”optimal” salt powder for obtaining additional precipitation amounts from convective clouds are performed. The results of experiments carried out in the cloud chamber at the conditions corresponding to the formation of convective clouds have shown 5 that the introduction of the salt powder before a cloud medium is formed in the chamber results in the formation on the large-drop “tail” of additional large drops. In this case seeding with the salt powder leads to enlargement of the whole population of cloud drops and to a decrease of their total concentration as compared to the background cloud medium.
These results are the positive factors for stimulating coagulation processes in clouds and for subsequent formation of precipitation in them. An overseeding effect, which is characterized by increased droplet concentration and decreased droplet size, was not observed even at high salt powder concentrations.
The results of numerical simulations have shown that the transformation of cloud drop spectra induced by the introduction of the salt powder results in more intense coagulation processes in clouds as compared to the case of cloud modification with hygroscopic particles with relatively narrow particle size distributions, the South African hygroscopic particles from flares being an example of such distributions. The calculation results obtained with a one-dimensional model of a warm convective cloud demonstrated that the effect of salt powder on clouds (total amounts of additional precipitation) is significantly higher than the effect caused by the use of hygroscopic particles with narrow particle size distributions at comparable consumptions of seeding agents. Here we show that seeding at rather low consumption rate of the salt powder precipitation can be obtained from otherwise non precipitating warm convective clouds.


Conclusions

The results of experiments carried out in the cloud chamber at the conditions corresponding to the formation of convective clouds have shown that:

  • The introduction of the salt powder before a cloud medium is formed results in the formation on the large-drop “tail” and in the broadening of drop size spectrum. This result is a positive factor for stimulating coagulation processes in clouds and for subsequent formation of precipitation in them.
  • No impact is observed on the fine-droplet spectrum fraction formed on background condensation nuclei even at moderate amounts of the powder introduced.
  • Seeding with the salt powder leads also to enlargement of the whole population of cloud drops and to a decrease of their total concentration as compared to the background cloud medium. With the introduction of increasing mass concentration of salt particles, this effect increases practically linearly. This factor also leads to a positive modification effect for stimulating the conversion of cloud water into rain drops.
  • At the introduction of very high concentration of the powder no “overseeding” is observed, i.e., increase of drop concentration along with reduction of their size is not observed.
The results of numerical simulations of cloud medium formation at the initial stage of condensation have shown that:
  • The introduction of the salt powder into the convective subcloud layer leads to the appearance of a large-drop “tail” in the cloud drop size distribution. The shape of the spectrum in the large-drop region is determined by the salt powder particles spectrum.
  • The shape of the spectrum of drops formed on background condensation nuclei does not change at rather high concentrations of the powder introduced. This means that the introduction of the salt powder does not change much the conditions of the formation of cloud drops on the background aerosol particles, except for at very high mass concentration. This result is confirmed by the experimental data obtained in the cloud chamber.
  • As the analysis of numerical simulation results shows, the transformation of cloud drop spectra induced by the introduction of the salt powder results in much more intense coagulation processes in clouds as compared to the case of cloud modification with particles from hygroscopic flare at the same mass concentration.
  • The salt powder results also in much more intense coagulation also with respect to hygroscopic particles having 1- μm very narrow particle size distribution of the same mass concentration.
The calculated rainfall amounts of the numerical simulations with a 1-dimensional numerical model of a warm convective cloud have shown that:
  • The effect of the salt powder on clouds (total amounts of additional rain) is significantly higher than that caused by the use of hygroscopic flares at comparable consumptions of seeding agents (of the order of 10 kg/km2).
  • At the consumptions of the salt powders over 20 kg/km2 rainfall can be obtained from otherwise no-precipitating clouds with thicknesses of 2.5<H<3.5 km. At the consumptions of about 50 kg/km2 of the powder the maximum effect of modification – maximum precipitation amounts – is realized. A further increase of the amounts of the salt powder introduced into a cloud does not result in significant additional precipitation amounts. Owing to a fairly simplified description of cloud development in the present 1-dimentional model, the consumptions of the salt powders indicated above should be regarded as estimates. For more accurate calculations it is necessary to use more realistic 2 or 3-dimensional cloud model with full ice microphysics and dynamic feedbacks to the precipitation forming processes.
In summary, the experimental data and the results of numerical simulations presented demonstrate the great promises arising from the use of the salt powder studied for obtaining additional precipitation amounts from convective clouds when accelerating the coagulation results in additional rainfall on the ground. Thus it is proposed to recommend using this salt powder in the seeding experiments in the natural atmospheric conditions.