Radar Observations of Changing Orientations of Hydrometeors in Thunderstorms

摘要

Changing orientations of hydrometeors due to rapidly changing electric fields inthunderstorms 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 at heights of 7-11 km in thunderstorms appear as differential phase shifts in the propagation medium due to highly oriented ice particles. These effects are evident in rangewise profiles of the cross-covariance amplitude ratio derived from the two received signals. Some of the observations show specific differential phase shifts up to 1.6 degree per km in range intervals of a few kilometers and up to 0.8 degree per km in range intervals up to 18 km with a distinct tendency of increasing phase shift prior to an occurrence of lightning. Many occurrences of lightning were accompanied by sudden increases or decreases of the phase shift, indicative of corresponding changes in the magnitude of the electric field, or by sudden changes in the orientation of the cross-covariance amplitude ratio, indicative of sudden changes of the canting angle of the propagation medium. Following such a sudden change, the propagation medium usually returned to its prior state in a time interval between several seconds and several tens of seconds, depending on the electrical activity of the storm. These results support the possibility of characterizing the electric field in clouds by radar measurements. (AN).