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Experimental studies of salt powder efficiency at convective cloud modification for precipitation enhancement

A.S. Drofa, V.G. Erankov, V.N. Ivanov, A.G. Shilin, G.F. Yaskevich
RPA Typhoon,Obninsk, Kaluga Region, 249038, Russia

The studies of an effect of convective clouds seeding with salt powder (NaCl) for precipitation enhancement are made in (Drofa et al., 2010). The experimental results supported by the results of numerical simulation showed that the use of the salt powder has numerous advantages in the effect of modification and in the consumption of the agent against hygroscopic particles generated by pyrotechnic flares (Cooper et al., 1997). The given paper presents the results of experimental studies of the efficiency of a polydisperse salt powder developed at RPA Typhoon (Obninsk, Russia) for stimulating precipitation from convective clouds.

Experimental studies of a salt powder seeding effect on a cloud medium were performed in the Big Cloud Chamber (BCC) of RPA Typhoon. The BCC is designed for modeling convective or stratified clouds and fogs of different origin under the conditions close to those in the real atmosphere. The chamber is a steel airtight cylinder of 18-m height and 15-m diameter. The total volume of the chamber is 3200 m3.

The process of convective cloud medium formation in the BCC is made through air expansion. For this external air is pumped into the chamber. As a result the air pressure in it is increased to the level of 1300 hPa. The air pressure decrease is made by opening holes in the upper part of the chamber. The air discharge from the chamber through the holes of different cross sections makes it possible to regulate the rate of pressure decrease in the chamber, thus setting a certain velocity of air updraft in the atmosphere during the formation of a convective cloud. Air temperature, pressure, humidity and the chamber wall temperatures are basic parameters for setting thermodynamic conditions in the BCC. These parameters are continuously measured during the experiments in the BCC. In the efforts described in this paper the pressure decrease in the BCC was made with the velocity of air mass updraft in the atmosphere of 1.3 m/s. Relative humidity of air in the chamber before the pressure drop is usually 90 96%. The air temperature is 22 25ºC.At such values of meteorological parameters a cloud medium in the BCC is formed in 2 3 min after the beginning of the pressure lowering. During ≈15 min the process of cloud medium formation in the BCC may be considered adequate to the process of convective cloud formation in the real atmosphere. The total lifetime of the cloud in the BCC is 40 50 min. At the chosen mode of air pressure lowering a cloud medium with the drop concentration from 1300 to 1900 cm-3 is formed. The liquid water content of the medium grows linearly with time and by 12 min it becomes equal to 0.8 g/m3. The largest drops in the background experiments reached the radii of 12 μm.

For measurements of cloud medium microstructure the BBC is equipped with the photoelectric particle analyzer developed at RPA Typhoon. This device is based on measurements of light scattered from particles at an angle of 90º. The operation principle of this device is in the analysis of amplitudes of pulses of light scattered from single particles at their flight through the measuring volume. For this, the calculated dependences of scattered light intensity on a drop size are used. The range of measured drop radii with this meter is within 1 50 μm. The number of drop size measuring channels is 360. The photoelectric meter allows one to obtain cloud drop spectra at the time of data accumulation from 1 s at the medium flow rate through the measuring volume over 2.5 cm3/s. The particle concentrations are determined from the pulse frequency of light scattered from every particle. The particles size distribution function is constructed from the analyzer data. The integral parameters of the size distribution function such as mass concentrations and effective radii of the particles are also calculated.

The agent under study is a specially prepared powder of NaCl with aerosil (SiO2) used as an anticaking admixture. The measurements of microstructure of the dry salt particles under this study were made in the BCC at the air relative humidity of about 40%. After the salt powder was introduced into the chamber with the pneumatic atomizing nozzle the aerosol was uniformly mixed inside the chamber and measured there. To study the microstructure of the salt powders made a laser and TV aerosol analyzers developed at RPA Typhoon were used.

A TV-analyzer was used for the analysis of larger in size particle distributions. This device analyzes the particles coming into the microscope field of view, the sizes of which are then measured. The range of measured particles radii is from 1 to 50 μm. The number of channels is 200. As the results of TV-analyzer measurements show, the particles with radii over 15 μm are not observed in the samples of the salt powder under study. The analysis of particle images registered with the TV-analyzer showed that the particles with the radii over 6 μm rather frequently have the form of conglomerates of large salt particles with finer salt particles or aerosil stuck to them. The laser analyzer is designed for measurements of solid aerosol particles microstructure. It is based on measurements of light scattered from aerosol particles in the range of scattering angles from 60 to 120. A continuously operating laser at the 0.63 μm wavelength serves as a light source in this analyzer. The laser analyzer operation characteristic is calculated with the account for the geometric peculiarities of its optical scheme. The aerosol under study is supplied via a thin capillary into the region of laser beam focusing. The flow rate of air passing through the analyzer volume is 1 cm3/s. The laser analyzer makes it possible to measure particle size distributions within the particle radii of 0.1 5 μm. The number of particle size measurement channels is 120.

The results of dry salt particles microstructure measurements made with the laser analyzer are given in Figure 1. The Figure shows the salt particles size distributions for the salt powder developed at RPA Typhoon. Here also shown are the salt particles size distributions for the salt powder presented in [] and the spectra of background aerosol. The measurements were performed at similar mass concentrations (about 0.4 mg/m3) of salt powders introduced into the BCC. The spectra are obtained during the time of data accumulation of about 10 min. The spectrum of salt powder particles was obtained by subtraction of background aerosol spectrum from the aerosol spectrum in the BCC after the introduction of the salt powder. The spectrums of salt particles, as is seen from Fig. 1, is rather well approximated by the distribution like f(r)=a r-ν exp(-(r/ro)2), (1) where a is the normalizing factor. The values of parameters ν and r0 for the salt powder presented in (Drofa et al., 2010) are: ν = 1.5 and r0 = 5 μm. For the powder of SI RPA Typhoon: ν=0.5 and ro = 5.3 μm. From comparison of the parameters of the spectra studied it follows that the powder in (Drofa et al., 2010) differs in a larger content of finely-dispersed particles. For comparison, the particle size distribution of the South African hygroscopic flares (Cooper et al., 1997) is given in Figure 1 at a mass concentration equal to the salt powder concentration of 0.4 mg/m3. Figure 1 reveals that the South African flares contain a very high number of particles with r<1 μm as compared to the salt powder. At the same time, the particles with r>1 μm are much less in number. The present paper gives the measurement results of changes of the cloud microstructure in the cloud chamber at the introduction of the salt powder and the estimations of the impact of the salt powder based on the observed changes of the cloud microstructure. The experiments in the BCC on cloud medium modification by the salt powder were performed according to the following method. The pumping of the chamber up to the initial high pressure was made. The salt particles were injected into it by a pneumatic atomizing nozzle. With the help of fans the aerosol was uniformly mixed inside the chamber. The salt particles were introduced into the chamber at a relative humidity of about 96%. By this, the conditions of particles introduction into the subcloud layer of a convective cloud were simulated. The modification effect was assessed from the results of comparing the parameters of the cloud medium with the corresponding parameters during the background experiment (without the introduction of salt particles). Figure 2 gives the results of cloud drop spectrum measurements at different time steps at the introduction of the salt powder into the BCC with mass concentration of 0.67 mg/m3. As is seen from Figure, the introduction of salt particles manifests itself in the large-drop cloud spectrum fraction in the tail of drop distribution the drops formed on salt particles appear. Maximum radius of drops observed in this case reached 16 μm, that is significantly higher than that in the background experiment. Fine-droplet spectrum range did not practically change as compared to the spectra obtained in the background experiment. The spectrum maximum in this range is formed on the background condensation nuclei.

Figure 2 shows a drop spectrum at the initial moment of cloud medium formation. These drops are covered with water salt particles not-dissolved completely in water. Temporal variations of cloud drop spectra at the introduction into the BCC of the salt powder with the mass concentration of 8.7 mg/m3 are shown in Figure 3. As is seen from the Figure, at such significant concentrations of particles introduced a bimodal character of cloud spectrum is clearly defined.

The fine-droplet spectrum mode is formed on the background aerosol (particles), and the large-drop fraction on salt particles. The spectrum maximum of drops formed on the background aerosol is created later than in the background experiment. This effect can be explained by the fact that at the introduction of a great number of salt particles water vapor supersaturation in the cloud medium decreases, thus resulting in a slower growth of drops formed on background condensation nuclei. The sizes of the largest drops here are considerably greater than in the background aerosol. But due to sedimentation, they fall on the floor of the BCC, and in the spectra measured after the 4th minute of the cloud medium formation large drops with the radii more than 20 μm are not observed. A necessary condition for obtaining the positive effect at modification with hygroscopic particles (additional precipitation from clouds) is a growth of cloud drops caused by the introduction of particles as far as the enlargement of cloud drops is the main factor of stimulating the gravitational-induced coagulation in clouds with subsequent precipitation formation.

The changes in cloud drop sizes in the modification experiments were estimated by us from the changes in their concentrations against drop concentrations in the background experiment. Under similar experimental conditions (the same liquid water contents of the media) the relationship Nb/N=(r/rb)3 is fulfilled, where r and rb, N and Nb are effective drop radii and their number concentrations at modification and in the background experiment, correspondingly. Here, the value of Nb/N characterizes a change in cloud drop sizes under modification. The effect of modification can be considered positive at Nb/N>1 (i.e. when the cloud drops become larger). The value of Nb/N may serve an estimate of the effect of cloud modification with hygroscopic particles (Drofa, 2006). The more the value of Nb/N is realized in the subcloud layer, the greater precipitation amounts can be obtained due to modification.

The results of cloud drop concentration measurements obtained at different number concentrations of salt particles introduced are given in Figure 4. As is seen from the Figure, with increasing the concentration of particles introduced, the effect of cloud drop enlargement grows practically linearly. From Figure 4 it is also seen that the effect of cloud drop enlargement at seeding with the salt powder of RPA Typhoon is considerably higher than the effect the effect from the powder presented in (Drofa et al., 2010) to obtain a similar effect of modification considerably smaller number of salt particles are needed. It is explained by the presence of a significant portion of fine-disperse particles on which small cloud droplets not contributing much in the enlargement of cloud drops in their whole population are formed.

For comparison, the calculation data of cloud drop concentration at seeding with hygroscopic particles from the South Africa pyrotechnic compound are give in Figure 4. The calculations were made with the method proposed in (Drofa, 2006) for the conditions corresponding to the conditions of cloud medium formation during experiments in the BCC. As is shown in (Drofa, 2006), maximum effect of modification with particles having narrow size distributions is achieved at certain concentrations of particles introduced.

Their values are determined by atmospheric conditions and the relation of parameters characterizing atmospheric aerosol and the hygroscopic particles introduced. For the conditions realized during the experiments in the BCC the calculation results give the estimate of Nb/N = 1.42. This value is obtained at the concentration of particles of 300 cm-3. The mass concentration of the pyrotechnic compound introduced in this case is W≈0.05 mg/m3. At higher or lower concentrations the value of Nb/N decreases. At mass concentrations W>0.14 mg/m3 the effect of overseeding appears, i.e. at the introduction of a great number of such particles drop concentration in the cloud grows and the drops average size decreases. Just this is the reason for decreasing the intensity of precipitation formation in a cloud under overseeding. The seeding with the salt powder, as is seen from Figure 4, does not lead to the effect of overseeding at rather high mass concentrations of introduced particles.

As the above-mentioned data show, with salt powders, large drops with the radii over 20 μm are not observed because of their sedimentation onto the BCC floor in the cloud drop spectra obtained during the experiments with salt powders. At the same time, the data on the number of such drops may be rather a significant estimate of the modification effect because the largest drops in the cloud spectrum determine the intensity of coagulation processes for precipitation formation in the cloud. For experimental assessing the number of large drops falling from the cloud medium during experiments the measurements of sediment drops were performed. For this, a platform with the area of 0.5 m2 was installed on the BCC floor. The mass of this platform during the experiment in the BCC was constantly controlled by an electronic analytical balance.

Thus, during the experiment the mass of water collected on the platform due to falling drops was measured. The measurement results of the mass of water collected during experiments in the BCC are shown in Figure 5. Here shown is the temporal

trend of changes in the mass of water accumulated in the experiments with the introduction of salt powders and in the background experiments. As is seen from Figure 5, precipitation of large cloud drops in the background experiments begins to reveal itself after the 6-th min of cloud medium formation in the chamber. Vertical segments in the graph (background measurements) show the scatter of measurement data in separate experiments. By the 16-th min of experiments from 0.9 to 1.3 g of water were collected. In the experiments with salt powders, as one can see in the Figure, the process of drop settling begins earlier. The mass of accumulated water increases with increasing the mass of the salt powder introduced into the BCC. The data of accumulated water measurements may serve as a comparative estimate of the modification effect at testing different hygroscopic agents. From Figure 5 it is seen that at similar amounts of powders introduced the mass of collected water at seeding with the powder of RPA Typhoon is significantly greater than in the case of the powder presented in (Drofa et al., 2010). So it can be concluded that the effect of large drop formation caused by the salt powder of PRA

Thus the results of experimental studies show that the introduction of the salt powder into a cloud medium under formation results in the formation in the large-drop spectrum tail of an additional number of large drops and in the broadening of drop spectrum. This result is a positive factor for stimulating coagulation processes in clouds and precipitation formation. No effect on the fine-drop spectrum formed on the background condensation nuclei is seen at rather considerable concentrations of the powder introduced. 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. With an increase o the mass concentration of the particles introduced this effect increases. This effect is also a positive factor for stimulating the transformation of cloud drops into rain drops. At rather high concentrations of the salt powder the effect of overseeding (at which instead of drop enlargement the reduction of their sizes and an increase of cloud drop concentration occurs) is not observed.

References

  • Cooper W.A., Bruintjes R.T., Mather G.K., Calculations pertaining to hygroscopic seeding with flares. J. Appl. Meteorol., 36, 14491469, 1997.
  • Drofa A.S., Formation of cloud microstructure: the role of hygroscopic particles. Atmos. Ocean. Phys., 42, 326-336., 2006.
  • Drofa A. S., Ivanov V. N., Rosenfeld D., Shilin A.G., Studying an Effect of Salt Powder Seeding Used for Precipitation Enhancement from Convective Clouds. Atmos. Chem. Phys., 10, 8011-8023, 2010.