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CHARGING MECHANISMS IN CLOUDS AND THUNDERSTORMS 124 red to a soft hail particle (R = 1 mm) by an ice particle (r = 50 Âµm) is â 0.006 pC using F = â 970 (determined previously for this experiment). Since contact charging happens much more rapidly than discharging of net charge between particles by conduction, a buildup of charge can readily occur in multiple collisions. A charge of 20 pC could accumulate under favorable conditions with ice crystal concentrations of 100 per liter after 3000 collisions (or about 6 minutes). This amount is comparable with negative charges found on ice precipitation in highly electrified regions of thunderstorms (e.g., see Latham, 1981). Thus, interface charging appears to be capable of microscale charge separation in amounts that can account for a major aspect of thunderstorm electrification. However, the wide range and sign variation for the interface factor (and charge) found in laboratory studies seems to be at odds with observations of the predominantly negative charge center for thunderstorms. It is apparent that additional laboratory measurements along with more detailed field observations are required to sort out the discrepancy. The relation of the interface factor (F) to temperature, liquid-water content, and impurities must be known before interface charging can be reliably applied to the variety of conditions in the hail stage. This completes our detailed discussion of microscale charge separation. We have gone from diffusion charging in the cloud stage to the more complex mechanisms involving precipitation in the rain and hail stages. In the following section we consider the relative importance of these mechanisms, and in particular, we evaluate their contributions by comparison with observations of clouds and thunderstorms. EVALUATION The requirements for a satisfactory explanation of charge separation in clouds and thunderstorms are fairly well known (e.g., Mason, 1972; Latham, 1981; Takahashi, 1982). Any theory must be capable of explaining microscale and cloud-scale charge separation on a suitable time scale. For a fairly complete assessment of charge-separation mechanisms, we need to take into account the evolution of charges and fields, and their interactions, in at least two dimensions. Such an evaluation is well beyond the scope of this paper (see Chapter 10, this volume, on cloud modeling.) What we can do here is reiterate some of the pronounced strengths and weaknesses for the various charging mechanisms. We can also indicate where model studies would be helpful in quantifying our gross conclusions and point to areas in need of further laboratory or field research. In making our evaluation we consider the major requirements for an adequate theory of charge separation in three stages: (1) the cloud stage for small cumulus clouds that contain only cloud droplets and drizzle drops; (2) the rain stage for larger cumulus clouds that contain raindrops formed by accretion of cloud droplets; and (3) the hail stage for the upper regions of large cumulus clouds where precipitation (notably, soft hail) is formed by accretion of supercooled droplets. Cloud Stage Electrification is generally weak in the cloud stage. Ion mechanisms dominate because of the absence of precipitation and their associated charge-separation mechanisms. Charges have been observed to range from about 1 to 20 electrons for small cloud droplets with a normal distribution centered near zero charge. Values of the average charge magnitude are indicated in Figure 9.6 by region C1 from measurements in stratocumulus clouds (Phillips and Kinzer, 1958). The observations agree with a Boltzmann equilibrium (line c) after the Figure 9.6 Average charge magnitude for cloud and precipitation particles. Regions C1 and C2 show cloud stage (after Phillips and Kinzer, 1958; Gunn, 1957), regions R1 and R2 show rain stage for shallow and deep convection (after Takahashi, 1973a, 1978), and regions H1 and H2 show hail stage (after Takahashi, 1973a; Latham, 1981). Lines labeled c, r, and h are from equations given by lower inset. Lines 1 through 6 have R 2 E dependence on charge (see upper inset for corresponding mechanisms and fields).