MEASUREMENTS OF ICE-FORMING RATES PRODUCED BY SUPERSONIC FLOW AND INVESTIGATION OF MECHANISMS OF ICE CRYSTAL GENERATION

The ice forming rates p_i produced by supersonic flow have been measured in supercooled fog suspended in a cold chamber at temperatures of 0 to-12℃.It has been shown that the ice-forming rates are 10^(11)to 10^(12)crystals per gram of air at Mach numbers of 1.1 to 1.84 and total pressures below 6 atm,and that the ice-forming rates slowly increase with increasing Mach numbers and total pressures and with decreasing ambient temperatures.These results are theoretically consistent with the rules for expansion in supersonic flow. In review of mechanisms of ice crystal generation,authors propose that the homogeneous nucleation-freezing (condensation-freezing)of water vapor in supersonic flow be the dominant mechanism.In our opinion,supersonic flow, for artificial precipitation,could be expected to provide an economical,non-polluting and convenient technology,and suitable for the long-distance operation.The data obtained from our experiments indicate that it is feasible to develop a new technique for weather modification.

On Mechanisms of Nucleation of Ice Crystals by Aerodynamic Cooling

The investigation of mechanisms of nucleation of ice crystals by aerodynamic cooling produced by supersonic airflow is carried out. Three processes are considered to be the principal causes for aerodynamic cooling and nucleation of ice crystals. They are f adiabatic cooling in supersonic airflow. cooling at the cores of vortices around the edge of airflow and entrainment of ambient stationary air into supersonic airflow. It is thcrmodynamically confirmed that the temperature lowering in supersonic flow depends on the Mach number M there and stagnant pressure Po at a certain stagnant temperature To The temperature will decrease by more than 6℃ asM increases by 0.1. The influence of Po on cooling is shown through the variation of mass flow rates, which increase with Po Experiments in laboratory have shown that ice forming rate Pi produced by supersonic airflow increases from 1011 to 1012/g as M increases from 1.10 to 1.84 at Po=5 and 6 atm, and Pi increases from 4.3×1011 to 10.3×1012/g as the mass flow rate increases from 3.5 to 5.7 g/s and Po increases from 1.5 to 5.0 atm at M=1.80 and To=25℃. In field experiments the ice concentrations of 50 to 200 per liter in about 2000 m3 were measured when air of about 0.5 g were spurted at a Mach number of M=1.80 into supercooled fog with temperatures between -0.5℃ and -4.6℃.These results are compatible with the prediction of aerodynamics.The snapshot taken in experiments represents the detailed structures of vortex motion around a supersonic airflow.

ON ICE FORMING NUCLEI

Cloud condensation nuclei(CCN)constitute a reservoir of latent ice-forming nuclei(IFN)active by conden sation-followed-by-freezing and by sorption.Two classes of aerosol particles active as IFN by sorption were found.The first produces IFN concentrations which are dependent on temperature only.The IFN concen- tration consecutively increases with lowering of temperature and does not respond to the increasing partial water vapor pressure.The second produces IFN concentrations which are dependent on partial water vapor pressure only,The IFN concentration consecutively decreases with decreasing of the partial water vapor pres- sure that takes place with the lowering temperature. Sulfates(mostly ammonium sulfate)constitute an integral part of IFN active by condensation-followed-by- freezing and by sorption.Backward isentropic air parcel trajectories linked polluted air rich in sulfates and clean air with high and low concentrations of IFN active by condensation-followed-by-freezing at-19.7℃ and S_w=0.1%. The experiments were performed in a dynamic filter processing chamber with aerosol particles collected on membrane filters.