A proposed method for remote sensing of the electric field in a cloud that contains ice crystals would exploit the relationship between (1) the polarization-dependent radiometric or radar brightness of the cloud and (2) the average or bulk orientation of the crystals as affected by the electric field. The proposed method would complement other methods now used to measure natural electric fields in efforts to forecast lightning. A major advantage of the proposed method is that a few ground-based and/or airborne instruments could quickly survey a fairly large region of the sky.
In a nonelectrified cloud, the average orientation of ice crystals tends to be horizontal because it is aerodynamically stable. On the other hand, atmospheric electric fields, have vertical gradients that tend to electrically polarize the crystals, causing the orientation of their long axes to be aligned vertically. Hence the bulk orientation of ice crystals in a cloud is a balance between the electric and aerodynamic effects.
In the proposed method, one would observe a cloud by use of millimeter-wavelength radar, taking separate simultaneous measurements of the radar reflectivity in horizontal and vertical polar-izations. Alternatively, one could measure the millimeter- or submillimeter-wavelength radiometric brightness temperature in both horizontal and vertical polarizations. The reason for doing so is that the bulk radar reflectivity or radiometric brightness temperature of the ice crystals depends on the scattering cross-section of the crystal. Since the long axis of the crystals has a greater cross-section than the short axis, the difference in radar reflectivity or atmospheric brightness at the two polarizations is sensitive to the bulk orientation of the crystals. In principle, it should be possible to invert the measurement data to retrieve information on the bulk orientation of the crystals and thus on the electric field.
This work was done by Steven J. Walter of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Physical Sciences category.