Heterogeneous Drop Freezing in the Immersion Mode: Model Calculations Considering Soluble and Insoluble Particles in the Drops

Abstract

A method is presented to consider the influence of both soluble and insoluble aerosol particles on drop freezing in the immersion mode in cloud models. Most atmospheric aerosol particles contain soluble and insoluble materials. Thus, a more realistic description of drop freezing should account for both factors. In general, salt particles depress the freezing point according to the salt concentration, while insoluble particles raise the freezing temperature. Based on laboratory experiments, a semiempirical equation was derived to calculate the median freezing temperature as a function of the drop size and for different insoluble particles. The freezing point depression was determined using the activity coefficients of the salt solutions. The results obtained with the freezing model for drops containing soluble and insoluble particles are consistent with experimental results. An equation for the freezing rate was derived and incorporated into an adiabatic air parcel model with detailed sectional microphysics. Model simulations were carried out to compare the present approach to the Bigg approach for drop freezing, which is often employed in cloud models. The results indicated that the Bigg equation describes drop freezing well in the immersion mode for a "mean" insoluble particle; however, the presented equations consider the significantly different freezing characteristics of various ice nuclei. Therefore, drop freezing in the immersion mode can be described for defined aerosol particle distributions as a function of the fractions of the different insoluble particles to the total aerosol particles.