Microphysical processes

The overall rainfall rate and amoimt depend on these microphysical processes and even more greatly on the initial amount of water vapor present, and on the vertical motions that transport water upward, cool the air, and cause supersaturation to occur in the first place. Thus the delivery of water to the Earth s surface as one step in the hydrologic cycle is controlled by both microphysical and meteorologic processes. The global average precipitation amounts to about 75 cm/yr or 750 L/(m yr). 

In these discussions we will thus use the following explicit definition of a chemical measurement in the atmosphere the collection of a definable atmospheric phase as well as the determination of a specific chemical moiety with definable precision and accuracy. This definition is required since most atmospheric pollutants are not inert gaseous and aerosol species with atmospheric concentrations determined by source strength and physical dispersion processes alone. Instead they may undergo gas-phase, liquid-phase, or surface-mediated conversions (some reversible) and, in certain cases, mass transfer between phases may be kinetically limited. Analytical methods for chemical species in the atmosphere must transcend these complications from chemical transformations and microphysical processes in order to be useful adjuncts to atmospheric chemistry studies. 

The impact of secondary aerosols on indirect radiative forcing is the most variable and is the least understood [3]. The reasons why the indirect effect of secondary aerosols is so difficult to describe is that it depends upon [1] (1) a series of molecular-microphysical processes that connect aerosol nucleation to cloud condensation nuclei to cloud drops and then ultimately to cloud albedo and (2) complex cloud-scale dynamics on scales of 100-1000 km involve a consistent matching of multiple spatial and time scales and are extremely difficult to parameterize and incorporate in climate models. Nucleation changes aerosol particle concentrations that cause changes in cloud droplet concentrations, which in turn, alter cloud albedo. Thus, macro-scale cloud properties that influence indirect forcing result from both micro-scale and large-scale dynamics. To date, the micro-scale chemical physics has not received the appropriate attention. 

Estimating the "indirect effect" is difficult because the mechanisms underlying the chain of microphysical processes connecfing emissions with cloud albedo are uncertain—twice as uncertain as the direct effect as mentioned previously. Clouds form when an air parcel is cooled fhrough vertical liffing and the vapor becomes... 

The current version of GEM-AQ has five size-resolved aerosols types, viz. sea salt, sulphate, black carbon, organic carbon, and dust. The microphysical processes which describe formation and transformation of aerosols are calculated by a sectional aerosol module (Gong et al. 2003). The particle mass is distributed into 12 logarithmically spaced bins from 0.005 to 10.24 pm radius. This size distribution leads to an additional 60 advected tracers. The following aerosol processes are accounted for in the aerosol module nucleation, condensation, coagulation, sedimentation and dry deposition, in-cloud oxidation of SO2, in-cloud scavenging, and below-cloud scavenging by rain and snow. 

K.C. Young, Microphysical Processes in Clouds, Oxford University Press, New York, 1993. 

This paper describes the development of a chamber facility designed to investigate a range of atmospheric aerosol processes driven by their potential to affect radiative forcing. Development of a suite of models investigating coupled chemical and microphysical processes has informed the definition of a preliminary experimental programme. The chamber construction is underway and the experiments are phased to follow the chamber development and completion. 

The different microphysical processes—the field action and the various binary collision processes—in which the electrons are involved in a weakly ionized plasma lead to a complex redistribution of the electrons in their phase space, i.e., their combined coordinate and velocity space. 

Such complete studies of electron kinetic problems allow the essential non-equilibrimn properties of the electron component to be revealed and a deeper understanding of the interplay between the various microphysical processes involved in the kinetics of the electrons to be gained. In particular, this point has been illustrated by some examples concerning the temporal and spatial relaxation of the electrons and the electron response to temporal and spatial pulselike disturbances of the electric field. 

We stated that not all clouds precipitate. Indeed, from only a very small proportion of clouds does precipitation actually reach the ground surface below. The basic problem is that cloud water droplets or ice particles are frequently too small to fall from the cloud base or to survive on the way to the ground because they evaporate. Whereas a cloud droplet is on average 8 pm in diameter, a rain drop is between 500 and 5000 pm (0.5-5 mm) this means that a small rain drop is as large in volume as 240 000 cloud drops. Assuming 240 cloud droplets cm (cf Table 2.25), there is only one rain drop in 1 L of air. Several microphysical processes occur in clouds depending on temperature, vertical resolution, dynamic and other parameters that result in growth of a particle (Fig. 2.39) and different precipitation forms (Table 2.26). 

In air, permanent solid particles (atmospheric aerosols) occur either from primary sources out of the atmosphere or from gas-to-particle conversion within the atmosphere. All trace matter shows a high variability in concentration because of chemical and microphysical processes. Liquid particles (cloud, fog and rain droplets), however, are not permanent in air and form and exist only under specific physicochemical conditions (the presence of condensation nuclei and water vapor saturation). The transition from molecules to droplets comprises many steps ... 

Feingold G, Chuang PY (2002) Analysis of the influence of film-foiming compounds on droplet growth implications for cloud microphysical processes and climate. J Atmos Sci 59 2006-2018... 

In Figure 10, we show the interrelationship between the three microphysical processes of nucleation, condensation, and coagulation, which result in the precipitation and growth of particles during RESS. As described below, arrows 1-6 illustrate how a given process (at the head of the arrow) affects a second process (at its tail). Arrow 1 Only when nucleation has produced a sufficient number of particles does condensation become an effective precipitation process. 

Figure 10 The three microphysical processes that constitute particle formation and growth. Arrows 1-6 indicate the impact of one process on another. Changes in particle size distribution caused by a given pair of processes are also shown.

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