Temporal and Spatial Variations of Hydrometeor Orientations in Thunderstorms

Abstract

Changing orientations of hydrometeors due to rapidly changing electric fields in thunderstorms were observed by the 11-cm polarimetric Doppler radar that was operated by the Geophysics Directorate of Phillips Laboratory in Sudbury, Massachusetts. The radar transmitted signals of right circular polarization and received signals of right and left circular polarization in a dual-channel receiver. The effects of electric fields appear as differential attenuation and differential phase shift in propagation that are derivable from the cross-covariance of the two received signals. The data were used initially to investigate time dependence of these effects at the altitudes where they were strongest. The analysis has been extended to examine the spatial structure and temporal variation of the observed effects through most of the vertical extent of the storms. The results illustrate the combined role of hydrometeor microphysics and the electric field in producing the observed differential phase shifts and apparent canting angles. In particular, at heights above about 7 km it was found not only that the differential propagation effects are large through a depth of a few kilometers, but also that the change of propagation effects coincident with lightning often occurs throughout a similar depth. At heights of 4?5 km the propagation effects are of smaller magnitude, are nearly constant in time, and suggest the presence of large, aerodynamically oriented hydrometeors.