Dual-Polarization Rain Observation by X- and Ka-band Dual-Wavelength Radar

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  • X-バンドとKa一バンドの2周波レーダによる降雨の偏波観測

Abstract

Rain observations by vertically polarized and horizontally polarized radiowaves, using a dual-wavelength radar with X-band and Ka-band radiowaves, were performed. Though the polarization switching is slow, a feature of the radar, that the beams at the X-band and Ka-band are the same, makes it possible to compare directly the ZDRs at both frequencies. The so-called ZDRs were mea-sured and the difference between those in the X-band and Ka-band was studied. ZDRs in the X-band increase with rainfall rate, which is as expected. ZDRS in the Ka-band slightly decrease with rainfall rates at a longer observation range, while ZDRs increase slightly with rainfall rates at a short ob-servation range. These facts can be explained by two causes. One is the polarization characteristic of raindrops in the Ka-band. Unlike in the X-band, the Ka-band radar cross sections of raindrops do not always increase as the diameter of a raindrop increases, which results in a weak increase of ZDRs with rainfall rates at short ranges. The other is the differential rain attenuation for vertically and horizontally polarized radiowaves. The rain attenuation for horizontally polarized radiowaves is larger than that for the vertically polarized radiowaves. For a long observation range, the differential rain attenuation is large enough to compensate for the difference of the radar cross sections of rain.

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