The Earth's Electrical Environment (1986)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

MODELS OF THE DEVELOPMENT OF THE ELECTRICAL STRUCTURE OF CLOUDS 132 microphysical development of the liquid and solid phases in the cloud and all the possible electrical processes that are operating. These requirements are not yet attaintable with our current state of knowledge and available computers. The number of processes involved and the large range of scales (from the molecular level to the dynamic scale—1000 m) cannot all be included in a single model. Therefore, attempts have been made to deal with the problem of the development of the electrical structure of clouds by emphasizing some processes and ignoring others. A few models, for instance, simulate the microphysical processes only, neglecting the macroscale dynamics altogether, whereas others have gone to the other extreme and simulate the dynamics in detail while simplifying the microphysics dramatically. To fill in the gap, some modelers have tried to deal with both the microphysics and the dynamics with sacrifices at both ends of the scale. We will review some of these models, try to establish a common denominator from their results and conclusions, and draw attention to some unanswered points that need further work. GENERAL REQUIREMENTS FROM ELECTRICAL MODELS OF THUNDERCLOUDS The validity of any thunderstorm model is determined by its ability to simulate observed features. Owing to the large natural variability of the various processes in thunderclouds, it is difficult to find a ''typical" storm with which all models could be compared. It is possible, however, to list some common observed features to use as general criteria for such comparisons. The following summary by Mason (1971) of the basic thunderstorm observations still appears to be valid: 1. The average duration of precipitation and electrical activity from a single cell is about 30 min. 2. The average electric moment destroyed in a lightning flash is about 100 C km, the corresponding charge being 20 to 30 C. 3. In a large, extensive cumulonimbus, this charge is separated in a volume bounded approximately by the – 5°C and – 40°C levels and has an average radius of perhaps 2 to 3 km. 4. The negative charge resides at altitudes just above the – 5°C isotherm. Krehbiel et al. (1979) observed that the negative charge transferred by lightning originates from regions between – 10°C and – 17°C, independent of the height above ground and regardless of the geographical location of the thunderstorm. The main positive charge is situated several kilometers higher. Another subsidiary small positive charge may also exist near cloud base, centered at or below the 0°C level. 5. The charge-separation processes are closely associated with the development of precipitation, probably in the form of soft hail (particles containing both liquid water and ice). 6. Sufficient charge must be separated to supply the first lightning flash within 12 to 20 min of the appearance of precipitation particles of radar-detectable size (d ≤ 200 µm). MECHANISMS OF CHARGE SEPARATION For space-charge centers to build up in clouds, charge must be separated first in the microscale, and then larger- scale processes can act to separate the opposite charges in space. When accomplished, this dual-scale process leads to the buildup of a space-charge distribution similar to that in Figure 10.1. In thunderclouds the charge separated on a microscale by particle interactions is subsequently separated on a macroscale with the help of convection and gravitational settling. Convection plays a role in cloud particle growth by forcing the condensation of water vapor until the particles are large enough to coalesce. Some of the interactions between cloud particles, particularly those followed by rebounding, may result in charge separation (as will be discussed later). These charges are then separated by differential terminal settling velocities. The larger particles, which carry predominantly one Figure 10.1 A schematic of the main space-charge distribution and currents in a thundercloud.