Characteristic changes in cross-polarization discrimination due to thunderclouds on satellite-to-ground path

Abstract

It is known that ice crystals in rain clouds induce significant depolarization of microwaves and millimeter waves on satellite-to-ground path as well as raindrops. In thunderstorm events, moreover, the cross-polarization discrimination (XPD) and cross-polar phase often show very rapid changes with time scale of a second or less. These characteristic changes are usually explained by alignment of ice crystals in thunderclouds due to aerodynamic-gravitational and/or electrostatic forces. However, very little is yet known about mechanisms for producing this type of depolarization, such as direction and degree of the alignment of ice crystals or relationship between aerodynamic-gravitational and electrostatic forces. In this study, the characteristic XPD changes are investigated using the Japan's CS-3 beacon signal (19.45GHz, right-hand circular polarization) which was observed every second in the thunderstorm events. These observations of Ka-band satellite signal radiowave have been conducted in our university for the past four years from 1990 to 1993. We have obtained more than 200 examples of rapid changes in twelve thunderstorm events during this observational period. The XPD changes are compared with time and location of cloud-to-ground (CG) lightning strokes which were recorded around our earth station by the Lightning Location and Protection System (LLPS) of a nearby electric power company. The effects of the ice crystal alignment due to electrostatic force and convective air flow are then discussed in the light of recent thunderstorm electrification theories.