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ATMOSPHERIC ELECTRICITY IN THE PLANETARY BOUNDARY LAYER 159 Figure 11.12 Two profiles of convection current density computed from aircraft measurements of field and conductivity (from Kraakevik, 1958). Analogously, the increase in conductivity at the termination of the fog event may also precede actual dissipation. A typical data recording through a fog event is seen in Figure 11.13. This phenomenon has been reported by many observers (Dolezalek, 1963), but it has not as yet received an adequate physical analysis. Similarly, there are effects associated with surf and waterfalls wherein some charge separation is produced by mechanical breakup of the water surfaces (Blanchard, 1963). For example, the positive space charge produced by breaking surf can lead to an appreciably larger electric field on shore than outside the surf zone during onshore winds. In contrast to the positive charge produced along ocean coasts, sufficient negative charge has been observed along the shore of a freshwater body (Lake Superior) during heavy surf to reverse the fair-weather electric field (Gathman and Hoppel, 1970). Negative charge is also observed from waterfalls. In some cases there are also strong local effects associated with smoke plumes and with volcanic eruptions. Again there is a separation of charge, which then diffuses away from the source. MODELING AND THEORY The complicated dependencies of the local electrical variables on the ionization profile, aerosol concentrations, turbulent structure of the PBL, and temporal variations in the global electrical circuit make it dangerous to trust intuitive notions when interpreting measurements made in the PBL. Increased insight into the meaning of the observations is obtained by modeling various physical mechanisms mathematically. Some of the more important results of these theoretical efforts follow. The electric field tends to be nearly vertical in fair weather, and the meteorological structure of the PBL usually changes slowly in comparison to the electrical relaxation time. This has led naturally to the assumption of a quasi- steady, horizontally homogeneous mean state and to one-dimensional, time-independent models of the mean electrical structure. These assumptions imply that the conduction-and convection-current densities Figure 11.13 Atmospheric-electric fog effectâa typical example of successful forecasts of both on set and dissipation (from Serbu and Trent, 1958).
