Electric Field Orientation of Radar Chaff: Implications for Dual-Polarimetric Radar Observations

The effect of thunderstorm electric fields (10³ to ~6 x 10⁵ V/m) on the orientation of ice crystals is now well established by dual-polarimetric radar observations. The theory developed by Weinheimer and Few (1987) and previously used to interpret these observations has been extended to conductive chaff fibers, whose aspect ratios are 10-100 times greater than values for typical ice crystals. Theory shows that the electric torque acting on an aspherical conductor increased markedly with the aspect ratio. These calculations show that the aerodynamic and electrical torques acting on the chaff fiber can be matched with an imposed electric field of ~200 V/m. This value is of the same order as the vertical fair weather electric field in the lower troposphere. Dual-polarimetric radar observations of chaff in fair weather conditions showing large negative values (~-8 dB) of differential reflectivity and substantially larger inferred fall speeds (~1 m/s) than are expected for horizontally oriented chaff (~0.2 m/s) are both consistent with the vertical alignment of chaff fibers in the naturally occurring electric field of the atmosphere.