Formation Process of Spatial and Temporal Distribution of Stable Isotopic Composition in Precipitation over The Slope of Mt. Tsukuba

Abstract

In order to investigate the spatial and temporal variation of stable isotopic composition in precipitation on the mountain slope, rain water samples were collected with high temporal resolution at four different altitudes on a southern slope of Mt.Tsukuba, Ibaraki Prefecture, from August through December, 2004.
The altitude effect of the observed rainfall events ranged from -0.33 to -0.10 ‰/100 m. Because the altitude effect was observed in every rainfall event regardless of dominant wind directions, the only Rayleigh model cannot explain the altitude effect in this study area. The altitude effect was also clear in the δ¹⁸O profile of water vapor before condensation estimated by the Rayleigh model.
The altitude effect showed the temporal change that the effect is clear at the onset of the rainfall event, and gradually becomes unclear around the rainfall peak, then it becomes clear again after the rainfall peak.
The temporal change might proceed by the temporal change of ascending current in the cloud caused by the latent heat's of the rain drop coagulation. It is possible that a superior ascending current would be caused by a large amount of drop coagulating in the cloud during the rainfall peak, consequently an ascending current might decrease isotopic gradient in the atmospheric water vapor, and altitude effect gets weakened.