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ATMOSPHERIC ELECTRICITY IN THE PLANETARY BOUNDARY LAYER 154 Figure 11.3 Simple electrode effect in nonturbulent air with constant volume ionization rate of 10 ion pair cmâ3 secâ 1 over a plain surface (from Hoppel, 1967). Turbulent Transport of Electrical Properties Most aspects of the structure of the PBL are dominated by the effects of turbulence (Haugen, 1973; Wyngaard, 1980), and electrical processes are no exception. Turbulent mixing prevents the buildup of radioactive emanations in shallow layers near the ground except under very stable conditions, disperses aerosols over a greater depth increasing the columnar resistance, and redistributes space charge, producing convection currents. The almost continual state of turbulent motion in the atmosphere is caused by the combined influences of drag, heating, and evaporation from the underlying surface. It is only in cases of extremely low wind speed and strong surface cooling that laminar flow may be found, and even then only for short periods and over limited areas. Drag generates turbulence through shear instability, which transfers kinetic energy from the mean flow to the turbulence. The energy goes into eddies on the scale of the mean velocity gradient, which is strongest near the surface, and tends to produce turbulence with a local integral scale comparable to the height above the surface. Both heating and evaporation from the surface tend to produce an unstable density gradient. The structure of the turbulence produced by buoyant instability can be quite different from that of shear-generated turbulence because warm, moist parcels starting near the surface accelerate as they rise through an unstable environment. The resulting eddies tend to be elongated vertically and to have a size scale determined by the thickness of the entire PBL. The turbulent kinetic energy injected into the flow by these two mechanisms is not dissipated at the scales where it is produced. Instead, energy cascades down to ever smaller eddy sizes and is dissipated primarily at scales smaller than a centimeter and results in a wide range of eddy sizes. These eddies efficiently mix passive contaminants like ions, aerosols, space charge, and radioactive gases. The turbulent transport of space charge is equivalent to an electrical convection current through the atmosphere. It is evident from Figure 11.3 that turbulent diffusion would disperse the space charge resulting from the imbalance of positive and negative ions in the lower layers, producing a net upward flow of charge (opposing the downward conduction current). Electric field fluctuations are also caused by turbulent movement of this space charge. To specify the average electric field, the instantaneous field must therefore be averaged over an interval of time longer than the period of the largest eddy. Figure 11.4 shows an electric-field profile over the ocean in the tradewinds off the coast of Barbados and illustrates the increase in vertical extend caused by turbulence. Each circle represents an average of 50 to 300 measurements of 10-sec duration, and the bars are the standard deviations. The large deviations illustrate the variability in the instantaneous field and the necessity of averaging to obtain a meaningful profile in the PBL. The variations are greatest where the gradient is steepest and are much less at a height of 160 m above the surface. The solid and dotted lines are the result of numerical modeling where Ï is a parameter related to the strength of turbulent mixing. It is obvious from this discussion that any treatment of atmospheric electricity in the PBL must include the effects of turbulence. This variability with time and position at low altitudes demonstrates the danger inherent in balloon soundings used to obtain integrated ionospheric potential. Neither spatial nor temporal averaging is possible during the rapid ascent and only a coarse vertical resolution is available where fields are largest and most variable.
