was used to confirm that the background subtracted atmospheric ␥ radiation ͑GRR͒ was directly related to precipitation rates. An anomalous increase in GRR ͑AGR͒ shortly after lightning strokes several times that expected from only the observed precipitation was concurrently observed at two sites separated by a distance of 220 m. The increase in activity in the detector environment subsequently decayed with a half-life of about 50 min. A model, which assumes the presence of an ensemble of radioisotopes produced by proton and photon-induced nuclear reactions in the atmosphere, fits the observed 50 min ͑33 min͒ decay half-life without ͑with͒ rain with correlation coefficients of 0.98. The possibilities that energetic protons and to a lesser extent brehmsstrahlung resulting from ions accelerated in electric fields during positive lightning produce this delayed activity and that AGR may be used as a remote monitor of lightning as well as the relative intensity of integrated lightning activity are discussed herein.

A model correctly predicting the observed atmospheric γ radiation from radon progeny adsorbed onto the surface of rain and snow enables monitoring of precipitation rates to better than 0.3 mm/h. This model predicts that the surface (volume) deposition of radon progeny yields γ-ray flux rates proportional to the 2/5 (3/5) power of rain rates, respectively. The γ-ray rates observed were proportional to the 0.45 power for rain, but for snow which does not form spherical droplets a linear relationship was observed with proportionality constant dependent upon snow type. Atmospheric γ radiation was measured at one site in Mitaka, Japan since 1999 and more recently at another site a distance 220 m from the first using 12.9 cm3 NaI detectors. Correlation coefficients ranging from 0.85 to 0.95 and improving with elevation were obtained when data were fit with predominantly surface adsorption model and an age correction factor for the 30 min half-life of dominant radon progeny. Measurement of atmospheric γ radiation from radon progeny naturally adsorbed by rain at 20 m above ground (moderate building height) may serve as a reliable, live monitor of precipitation rates limited only by counting statistics. The regional applicability of this technique has been confirmed and will most likely have global and remote applicability.

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正会員 博(工) 国土交通省 国土技術政策総合研究所 河川研究部（〒305-0804 茨城県つくば市旭1）2 正会員 修(工) 国土交通省 国土技術政策総合研究所 河川研究部（〒305-0804 茨城県つくば市旭1）The Ministry of Land, Infrastructure, Transport and Tourism (MLIT) built the rainfall observation network composed of 38 X-band dual polarimetric radars called XRAIN. The results of verifying the accuracy of the XRAIN rainfall, attenuation correction caused underestimation of radar rainfall distant from radar, and that underestimation caused underestimation of the compositing rainfall close to radar. The method of attenuation correction was modified to correct for attenuation of the almost all intensity of rainfall. The compositing rainfall processing was modified to add the weighting function related to the distance from radar. Modifying of attenuation correction and compositing rainfall processing improved underestimation of XRAIN rainfall.