Hans R. Pruppacher

The Role of Natural and Anthropogenic Pollutants in Cloud and Precipitation Formation

Since the resurgence of interest in cloud and precipitation physics, stimulated by the demonstration in 1946 of the possibility of cloud modification by artificial nucleation, an ever-increasing amount of research has been reported in the literature on the mechanisms which underlie the formation of clouds and precipitation in the atmosphere. This research has essentially been carried out along two lines of study: studies of the cloud microphysical processes including the phase change of water substance in the atmosphere and the growth of the formed cloud particles to precipitation particles, and studies of the cloud macrophysical processes including all those dynamic processes which govern the formation, growth, dimensions, shape, organization, lifetime, and vigor of the cloud as a whole. In recent years, it has been realized that the cloud dynamical and cloud microphysical processes have to be viewed in a combined manner for a complete understanding of the formation of clouds and precipitation. Thus, much work is currently being done with this aim in mind.

Equilibrium Between Water Vapor, Water, Aqueous Solutions, and Ice in Bulk

In this chapter, we shall discuss the equilibrium thermodynamics of and between the bulk phases of water, ice and aqueous solutions. In addition to providing useful information on the behavior of water substance, this material, with surface effects included, will also serve as a basis for our later discussion on the phase changes which lead to cloud particle formation.

The Atmospheric Aerosol

An understanding of the cloud forming processes in the atmosphere requires knowledge of the physical and chemical characteristics of the atmospheric- aerosol. In discussing this gaseous suspension of solid and liquid particles, it is customary to include all gases except water vapor, and all solid and liquid particles except hydrometeors, i.e., cloud and raindrops, and ice particles. In the present chapter we shall present a brief discussion of the characteristics of the gaseous constituents followed by a more detailed description of the main physical and chemical characteristics of the aerosol particles. For background on the subjects covered, the reader is referred to the texts of Rasool (1973), Butcher and Charlson (1972), Hidy (1972), McCormac (1971), Matthews et al. (1971), Hidy and Brock (1970), Singer (1970), Stern (1968). Davies (1966), Fuchs (1964), Green and Lane (1964), Junge (1963a), Cadle (1961, 1966); to the survey reports SMIC (1971), SCEF (1970), and Robinson and Robbins (1968, 1969, 1971); and to the survey articles by Junge (1969a, 1971. 1972a,b,c, 1974). Most of the data on which this chapter is based are derived from these sources.

Diffusion Growth and Evaporation of Water Drops and Ice Crystals

Immediately following their formation through heterogeneous nucleation, cloud particles proceed to grow by the process of vapor diffusion. Later on they may also experience growth by the mechanisms of collision and subsequent coalescence or sticking. In the present chapter we shall describe the individual and collective growth (and evaporation) of cloud particles by vapor diffusion; collisional growth (and breakup) are considered in Chapters 15 and 16.

Hydrodynamics of Single Cloud and Precipitation Particles

Once formed, cloud particles immediately begin to move under the action of gravity and frictional forces, the latter arising from their motion relative to the air. Some fraction of these particles will undergo complex hydrodynamic interactions causing some to collide. The particles will experience growth if the collision results in a permanent union. However, most of the time most cloud particles will simply fall with negligible interaction. It is this basic mode of isolated motion that we address ourselves to in this chapter. Furthermore, for simplicity we shall defer to Chapter 17 the consideration of the complicating influence of electrical forces.

The Electrical State of the Atmosphere and its Effects on Cloud Microphysics

The subject of cloud electricity is quite massive and complex in its own right, and has in addition many controversial aspects. Consequently, we shall make no attempt here to provide a comprehensive treatment of the entire subject; rather, we shall restrict ourselves primarily to summaries of some observed electrical properties of clouds and the particles contained within them, and to a consideration of the effects these electrical properties have on various microphysical processes. To a lesser extent we shall also consider concurrently the question of how the various types of clouds and cloud particles acquire their characteristic states of electrification. The topics of lightning, current budgets, and measurement techniques are omitted altogether.

Mechanics of the Atmospheric Aerosol

The main purpose of this chapter is to outline the basic dynamical behavior of aerosols. We shall therefore discuss the phenomena of Brownian motion, diffusion, sedimentation, and coagulation of aerosol particles, including some effects of turbulence and the so-called ‘phoretic’ forces. (Possible electrical influences will be discussed in Chapter 17.) We shall also extend and/or apply the various formulations to the problems of evaluating mechanisms which remove aerosol particles from the atmosphere, and to the problems of explaining some observed features of the size distributions of atmospheric aerosol particles. This, in turn, will provide much of the mathematical framework which we shall subsequently apply to the study of the individual or collective growth of cloud particles by diffusion (Chapter 13), and by collision and coalescence (Chapters 15 and 16).

Microphysics of Ice Particle-Drop Interactions

In Chapter 13 we discussed the growth of ice particles by vapor diffusion from drops to ice crystals, and in Chapter 14 we described ice particle growth by the accretion of supercooled drops, and the formation of snowflakes by the process of ice crystal aggregation. In this chapter we shall look closer at the microphysics of ice particle-drop interactions, and also at the physics of melting and freezing of individual cloud particles.

Cloud Particle Interactions-Collision, Coalescence, and Breakup

In Chapter 10 we discussed the behavior of isolated cloud particles in some detail. Now we shall consider their hydrodynamic interactions, with a view to providing a quantitative assessment of the processes of particle growth by collision and coalescence, and of collisional breakup. We shall first treat the collision problem for drops of radii less than about 500μm which, in accordance with our previous description of drop distortion in Section 10.3.2, may be regarded as rigid spheres (at least when falling in isolation). This will be followed by a somewhat briefer discussion of the phenomena of drop coalescence and breakup. Finally, we shall consider water drop-ice crystal and ice crystal-ice crystal interactions, which lead to the formation of graupel, hail, and snow particles.

Growth of Cloud Drops by Collision and Coalescence

As we have seen in Chapter 1, it has long been established that the presence of ice is not necessary for precipitation formation in tropical cumuli. Also, radar observations of clouds outside the tropics have shown that the formation of echoes, indicating the presence of precipitation, can occur at temperatures warmer than 0°C. In such cases the Wegener-Bergeron-Findeisen mechanism of precipitation formation (Section 13.3.1) is absent, and the flow of water up the spectrum from small droplets to rain must occur by the process of collision and coalescence. This is also often referred to as the collection process, or as the ‘warm rain’ process. The latter designation is somewhat inappropriate, since collection growth occurs also in clouds colder than 0°C. For example. Braham (1964) found evidence of collection growth of supercooled drops in summer cumuli over the central U.S.