SIMULATIONS OF STABLE ISOTOPIC FRACTIONATION IN MIXED CLOUD IN MIDDLE LATITUDES——A CASE OF THE PRECIPITATION AT ÜRÜMQI

The introduced mathematical model takes into account the action of the kinetic fractionation effect in super saturation environment at ice surface as liquid and solid phases coexist in mixed cloud. The kinetic fractionation effect is controlled by the super saturation ratio Si at ice surface in the mixed cloud. The greater the Si,the stronger the kinetic fractionation effect. The kinetic fractionation action makes the synthetic fractionation factor decreased and the change of the δ18O with temperature flatted,compared to that at equilibrium state. The δ18O values in the wet adiabatic cooling process are greater than in the isobaric cooling process under the same temperature,showing that the fractionation rate of the latter is faster than that of the former. The change rate of the δ18O against temperature in the wet adiabatic cooling process is smaller than in the isobaric cooling process under same humidity. Compared to the δ18O in the liquid phase in the equilibrium process(Si=1.0),the δ18O in the solid phase is of a smaller change rate for greater Si and a greater one for smaller Si.With increasing Si,the kinetic fractionation effect is strengthened,and perhaps ae,which means that the δ18O in the solid phase will maintain unchanged or increase with decreasing temperature. The liquid water contents in the cloud are of heavy δ18O values because the isotopic memories from the previous condensation process are preserved in the liquid water. Consequently, the curves of the δ18O against temperature have Iow slopes,compared to ones in which the liquid-water contents in the cloud are not considered. The more the liquid-water contents in the cloud,the lower the slope. The contribution of the liquid-water contents in the cloud is only to change the magnitude of stable isotopic ratio in water, but not the variation trend with temperature.