Comprehensive Study on Meso-Scale Precipitation System with Use of Stable Isotope Ratios of Water

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Analyses of oxygen isotopic composition of precipitation with several minutes intervals and meteorological observation by a polarimetric Doppler radar and GPS sondes were conducted during rainy season in 2004 at Okinawa. Large isotopic variation observed in event-to-event precipitation, in spite of the same vapor origins, implied that precipitation type (convective or stratiform) significantly affected isotopic values. Moreover, isotopic variation during convective precipitation was strongly affected by dryness in the middle troposphere before development of the convective system.Based on the intra-event analyses of isotopic variation, it was found that the development stage of convective systems strongly related to isotopic values. Isotopic values showed gradual decreasing trends in the developing stage, whereas they showed V-shape patterns in the dissipating stage of convective systems. These V-shape patterns could be caused by significant evaporation of falling raindrops.For quantit … More ative evaluation of isotopic change during raindrop falling, "isotopic exchange model of between raindrop and vapor for each size" was developed. As a result of sensitivity experiment, isotopic shifts were larger in smaller raindrops or at lower relative humidity. It could be demonstrated that relative humidity below cloud base was a significant controlling factor for the isotopic values of bulk(weighted mean)precipitation.Drop-size distribution of isotopic values in precipitation, which was expected from the model simulation, was observed by a raindrop collection method using liquid nitrogen and the isotope analysis for micro-liter of water samples. All observed results of three rainfall events at Nagoya in 2007 showed that higher hydrogen and oxygen isotopic values were found in smaller raindrops, which were consistent with the model calculation.Our results clearly indicate that the difference of cloud types and the evaporation effect during falling raindrops should be taken into account for the variation of isotopic composition of precipitation, not only the difference of vapor sources as discussed previously. Less