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T. Hiron and A. I. Flossmann

Abstract

Even though ice formation mechanisms in clouds probably obey all the same thermodynamic principles, the associated mechanical and thermal energy transfers differ with respect to the exact pathway and the associated phases. Consequently, heterogeneous ice nucleation parameterizations play an important role in cloud modeling.

The 1.5D bin-resolved microphysics Detailed Scavenging Model (DESCAM) was used to assess the role of the parameterizations for different ice initiation processes. Homogeneous nucleation, deposition freezing, contact freezing, immersion freezing, and condensation freezing were treated explicitly, and their impacts alone and in competition with each other on cloud microphysics and precipitation were studied. The role of efficiently ice-nucleating bacteria on cloud evolution was addressed, as well as means to consider different chemical natures of ice nucleation particles.

For the conditions studied, it was found that deposition and contact freezing only played a negligible role with respect to the other ice-nucleating mechanisms. Homogeneous freezing and classical immersion freezing showed a similar behavior. Both freezing rates increase with increasing drop age (i.e., size). This suggests a possibility for regrouping processes in future parameterized cloud models. Condensation freezing parameterization, however, acts at much warmer temperatures in clouds and for much smaller drops. The associated release of latent heat at lower altitudes caused significantly different cloud dynamics with respect to homogeneous/immersion freezing. This suggests that, in future parameterized models, the condensation freezing process needs particular attention, as well as the fact that ice-forming nuclei (IN) are a subset of aerosol particles that are depleted and replenished like the rest of the population.

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A. I. Flossmann and H. R. Pruppacher

Abstract

Our model for the scavenging of aerosol particles has been coupled with the two-dimensional form of the convective cloud model of Clark and Collaborators. The combined model was then used to simulate a convective warm cloud for the meteorological situation which existed at 1100 LST 12 July 1985 over Hawaii; assuming an aerosol size distribution of maritime number concentration and of mixed composition with (NH4)2SO4 as the soluble compound. A shallow model cloud developed 26 min after the onset of convection leading to moderate rain which began after 45 min and ended after 60 min. Various parameters which characterize the dynamics and micophysics of the cloud, as well as the scavenging mechanism taking place inside and below the cloud were computed during the cloud development. The computation showed that: 1) the scavenged aerosol mass became redistributed inside the cloud water as the cloud grew, whereby the main aerosol mass scavenged always remained associated with the main water mass in the cloud; 2) in-cloud scavenging of aerosol particles was mainly controlled by nucleation while impaction scavenging played a negligible role; 3) below-cloud scavenging, which is caused by impaction scavenging, contributed only 5% to the overall particle scavenging and contributed about 40% to the aerosol mass in the rain on the ground; and 4) the sulfur concentrations inside the rain water were found to be reasonable as compared to observations available in literature, considering that the present model does not yet include the effects of SO2 scavenging.

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A. I. Flossmann and H. R. Pruppacher

Abstract

No abstract available

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R. R. Alheit, A. I. Flossmann, and H. R. Pruppacher

Abstract

A theoretical model has been formulated which allows the study of the effects of an ice phase on the removal of atmospheric aerosol particles by nucleation and impaction scavenging in a convective cloud. This microphysical model—although in principle applicable to higher dimensional cloud dynamic models—was tested by using a simple parcel model with entrainment as the dynamic framework. The present model has been applied to and numerically evaluated for a convective cloud in which the cloud particles grow via vapor deposition, collision, and coalescence and riming. The computations were carried out for a rural-background aerosol of given particle size distribution. Two different chemical compositions of the aerosol particles and two different modes of ice initiation were considered. Our study shows for in-cloud scavenging (i) scavenging of aerosol particles by drop nucleation dominates impaction scavenging by drops as well as all other scavenging mechanisms; (ii) scavenging of aerosol particles by nucleation of snow crystals via drop freezing or any other nucleation mechanism dominates impaction scavenging by snow crystals; (iii) impaction scavenging of aerosol particles by snow crystals is, for the conditions studied, the least efficient scavenging mechanism; (iv) scavenging of aerosol particles by riming of graupel is an extremely efficient process due to the prominent scavenging of aerosol particles by nucleation of drops and the efficient uptake of drops by graupel; (v) the transfer of aerosol mass into the ice phase by riming of graupel and by freezing of drops dominates all other transfer mechanisms, (vi) inside mixed ice-water clouds the aerosol mass becomes redistributed in such a manner that the main aerosol mass is associated with the main graupel mass if riming is the dominant process of precipitation formation, and with the main water mass if collision and coalescence of drops in the dominant process of precipitation formation.

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A. I. Flossmann, H. R. Pruppacher, and J. H. Topalian

Abstract

A theoretical model has been formulated which allows the processes which control the wet deposition of atmospheric aerosol particles and pollutant gases to be included in cloud dynamic models. The cloud considered in the model was allowed to grow by condensation and collision–coalescence, to remove aerosol particles by nucleation and impaction scavenging, and to remove pollutant gases by convective diffusion. The model was tested by using a simple air-parcel model as the dynamic framework. In this form the model was used to determine the fate of ammonium sulfate [(NH4)2SO4] particles and sulfur dioxide (SO2) gas as they became scavenged by cloud and precipitation drops. Special emphasis was placed on determining 1) the evolution with time of the mass of total sulfur as S(IV) and S(V1) inside the drops, 2) the evolution with time of the acidity of the cloud water as a function of various oxidation rates and as a function of drop size, 3) the relative importance of sulfur scavenging from SO2 as compared to sulfur scavenging from (NH4)2SO4 particles, and 4) the effect of cloud drop evaporation on the aerosol particle size distribution in the air.

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A. I. Flossmann, W. D. Hall, and H. R. Pruppacher

Abstract

A theoretical model is formulated which allows the processes that control the wet deposition of atmospheric pollutants to be included in cloud dynamic models. The model considers the condensation process and the collision-coalescence process which, coupled together, control the fate of atmospheric aerosol particles removed by clouds and precipitation through nucleation scavenging and impaction scavenging. The model was tested by substituting a simple parcel model for the dynamic framework. In this form the model was used to determine the time evolution of the aerosol particle mass scavenged by drops as well as the aerosol particle mass left unactivated in air as “drop-interstitial” aerosol. In the present computation all aerosol particles are assumed to have the same composition. Our study shows for inside cloud scavenging: 1) collision and coalescencence causes among the various drop size categories a redistribution of the scavenged aerosol particles in such a manner that the main aerosol particle mass is always associated with the main water mass, thus ensuring that if a cloud reaches the precipitation stage it will also return to the ground the main aerosol particle mass scavenged by the cloud; 2) although the main aerosol particle mass is contained in the large drops, the mass mixing ratio of the captured aerosol in the cloud water is larger inside smaller drops than inside larger drops, implying that smaller drops are more contaminated than larger ones; 3) through nucleation scavenging the total number concentration of aerosol particles is predicted to become reduced by 48 to 94% depending on the composition of the particles, the reduction being mainly confined to aerosol particles larger than 0.1 μm in radius. This implies that a drop interstitial aerosol exists that consists of a particle population reduced in number concentration by up to 94% and reduced in mass by several orders of magnitude, as compared to the particle concentration outside the cloud. 4) Although the aerosol particle mass scavenged by impaction scavenging cannot completely be neglected in accounting for the total amount of aerosol particle mass scavenged by clouded it is smaller by several orders of magnitude than the aerosol particle mass removed by nucleation scavenging.

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Petra S. Respondek, A. I. Flossmann, R. R. Alheit, and H. R. Pruppacher

Abstract

The effects of an ice phase on the wet deposition of aerosol particles was studied by means of the authors’ 2D cloud dynamics model with spectral microphysics applied to the Cooperative Convective Precipitation Experiment in Miles City, Montana, on 19 July 1981. The cloud macrostructure as well as the cloud microstructure simulated by the model was found to agree well with observations. Although no on-site observations were available with respect to the chemical composition of the cloud and rain water, the values predicted by the model compared well with typical nearby measurements. The following conclusions can be derived from the model computations: (1) In confirmation of the authors' previous findings, derived from a parcel model, it was found that inside mixed ice-water clouds the aerosol mass becomes redistributed in such a way that the main aerosol mass is always associated with the main water or ice mass. (2) Since riming was the dominant growth mechanism of the hydrometeors in the cloud considered, the main aerosol mass–originally associated with the cloud drops via nucleation scavenging–became part of the graupel by riming. (3) In confirmation of earlier results for “warm” clouds, the scavenging efficiency of the cloud was found to be given within a few percent by the precipitation efficiency of the cloud system. (4) By purposely inhibiting ice nucleation but otherwise keeping all dynamic, thermodynamic, and microphysical input parameters the same, it could be shown that the changes in the microphysical structure of the cloud, which significantly altered both the time rainfall began and the rainfall duration, also significantly altered the wet deposition of chemical species. A careful consideration of the ice phase in cloud chemical modeling is therefore required.

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