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.
NPO-20895
This Brief includes a Technical Support Package (TSP).
Remote Sensing of Electric Fields in Clouds
(reference NPO-20895) is currently available for download from the TSP library.
Don't have an account?
Overview
The document presents a technical support package from NASA detailing a novel method for remotely sensing electric fields in clouds, particularly those containing ice crystals. The work, attributed to inventor Steven J. Walter and associated with the Jet Propulsion Laboratory (JPL), outlines a unique approach that exploits the relationship between the polarization-dependent brightness of radar or radiometric signals and the average orientation of ice crystals, which is influenced by electric fields.
The motivation for this research arose from a need to improve lightning forecasting methods at NASA's Kennedy Space Center. Existing commercial devices were deemed insufficient, prompting the exploration of new techniques. The document highlights that traditional methods, such as 183 GHz millimeter-wave observations, may not provide optimal sensitivity for measuring electric fields. Instead, the proposed method suggests using millimeter-wavelength radar or submillimeter-wavelength radiometers to retrieve information about the bulk orientation of ice crystals, which can be indicative of the electric field strength within storm clouds.
The technical concept involves measuring the scattering parameters, specifically the Stokes parameters, at an angle relative to the crystal's inertial axes. This measurement can reveal differences in radar reflectivity and radiometric brightness temperatures between vertical and horizontal polarizations, which are influenced by the orientation of the ice crystals. By analyzing these differences, researchers can infer the electric field strength within the cloud.
The document emphasizes that this method is technically feasible and could significantly enhance the reliability of lightning forecasts, which is crucial for pre-launch operations at NASA. It also notes that the technology has not yet been commercialized or used for its intended purpose, indicating that further development and validation are needed.
In summary, this NASA technical support package outlines an innovative approach to remote sensing of electric fields in clouds, leveraging advanced radar and radiometric techniques to improve our understanding of atmospheric phenomena and enhance safety in aerospace operations. The research represents a significant advancement in meteorological technology, with potential applications in both scientific research and practical forecasting.
