[1] Isotope ratios of precipitation and water vapor were observed during the passage of Typhoon Shanshan at Ishigaki Island, southwestern Japan, on 15-16 September 2006. Such high-resolution isotopic observations allow for qualitative understanding of atmospheric moisture cycling; they revealed that isotope ratios of both the precipitation and water vapor decreased radially inward in the cyclone's outer region; anomalously high isotope ratios appeared in the cyclone's inner region; and d-excess tended to decrease in the cyclone's inner region. In the cyclone's outer region, the water vapor was isotopically depleted due to the rainout effect which involves both condensation efficiency as reflected in inwardly increasing cloud thickness and isotopic exchange between falling droplets and the ambient water vapor. In contrast, water vapor in the cyclone's inner region was isotopically enriched due to weak rainout effect in conjunction with intensive isotopic recharge from the sea spray and sea surface with heavy isotope ratios. Since water vapor mainly acts as a source of precipitation, the isotope ratios of precipitation also had systematic variation. A unique circumstance is the intensity of isotopic exchange with almost saturated surface air and high winds, causing anomalously high isotope ratios and low d-excess values in the cyclone's inner region.

Spatial and temporal variations of the isotopic composition of precipitation were investigated to better understand their controlling factors. Precipitation was collected from six locations in Hokkaido, Japan, and event-based analyses were conducted for a period from March 2010 to February 2013. Relatively low δ values and a high d-excess for annual averages were observed at three sites located along the Japan Sea compared to the three sites at Pacific Ocean side.Lower δ values in spring and fall and higher d-excess in winter were observed for the region along the Japan Sea. In total, 264 precipitation events were identified. Precipitation originated predominantly from low-pressure system (LPS) events, which were classified as northwest (LPS-NW) and southeast (LPS-SE) events according to the routes of the low-pressure center, that passed northwest and southeast of Hokkaido, respectively. LPS-SE events showed lower δ 18 O than LPS-NW events, which is attributable to the lower δ 18 O of water vapor resulting from heavy rainfalls in the upstream region of the LPS air mass trajectories over the Pacific Ocean. This phenomenon observed in Hokkaido can be found in other midlatitude coastal regions and applied for hydrological, atmospheric, and paleoclimate studies. A characteristic spatial pattern was found in LPS-NW events, in which lower δ 18 O was observed on the Japan Sea side than on the Pacific Ocean side in each season. This is likely due to the location of the sampling sites and their distance from the LPS: Precipitation with lower δ 18 O in the region along the Japan Sea occurs in a well-developed cloud system near the low-pressure center in cold and warm sectors of LPS, whereas precipitation with higher δ 18 O on the Pacific side mainly occurs in a warm sector away from the low-pressure center. Air mass from the north does not always cause low δ in precipitation, and the precipitation process in the upstream region is another important factor controlling the isotopic composition of precipitation, other than the local temperature and precipitation amount. KEYWORDS air mass trajectory, atmospheric water vapor, Hokkaido, precipitation event, route of low-pressure system, stable isotopes of water

Deciduous forest covers vast areas of permafrost under severe dry climate in eastern Siberia. Understanding the water cycle in this forest ecosystem is quite important for climate projection. In this study, diurnal variations in isotopic compositions of atmospheric water vapour were observed in eastern Siberia with isotope analyses of precipitation, sap water of larch trees, soil water, and water in surface organic layer during the late summer periods of 2006, 2007, and 2008. In these years, the soil moisture content was considerably high due to unusually large amounts of summer rainfall and winter snowfall. The observed sap water d 18 O ranged from À17.9% to À13.3%, which was close to that of summer precipitation and soil water in the shallow layer, and represents that of transpired water vapour. On sunny days, as the air temperature and mixing ratio rose from predawn to morning, the atmospheric water vapour d 18 O increased by 1% to 5% and then decreased by about 2% from morning to afternoon with the mixing ratio. On cloudy days, by contrast, the afternoon decrease in d 18 O and the mixing ratio was not observed. These results show that water vapour that transpired from plants, with higher d 18 O than the atmospheric water vapour, contributes to the increase in d 18 O in the morning, whereas water vapour in the free atmosphere, with lower d 18 O, contributes to the decrease in the afternoon on sunny days. The observed results reveal the significance of transpired water vapour, with relatively high d 18 O, in the water cycle on a short diurnal time scale and confirm the importance of the recycling of precipitation through transpiration in continental forest environments such as the eastern Siberian taiga. Figure 4. Diurnal variations in d-excess and d 18 O of atmospheric water vapour (d 18 O V ), VPD, atmospheric mixing ratio of water vapour (Q) and air temperature (T) at 6 m height (dotted line) and 24 m height (solid line), and wind speed (thin solid line) and direction (cross) at 32 m height, observed on

Isotopic composition of atmospheric water vapour (δ18OV) was observed at a larch forest near Yakutsk in eastern Siberia during the late summer periods of 2006, 2007 and 2008. The δ18OV [and deuterium excess (d‐excess)] values observed in 2006 and 2008 positively (and negatively) correlated with mixing ratio of atmospheric water vapour, whereas, in 2007 when soil was extremely wet and resulted in limitation of plant transpiration, neither correlation was found between mixing ratio and δ18OV nor d‐excess. Observed results were also compared with components of atmospheric water balance calculated for a 500 × 500 km region; however, neither specific relationship between δ18OV and horizontal advection (direction) nor evapotranspiration was observed. On the other hand, obviously low δ18OV and high d‐excess values were found with low mixing ratio after removal of water vapour from the atmosphere because of the process of rainout in 2006 and 2008. Assuming the δ18OV under this condition to be a background, and also assuming the δ18O of sap water in larch trees as transpired water vapour, contribution of transpiration to the atmospheric water vapour was calculated. Fraction of transpired water vapour to the atmospheric water vapour was nearly 0·8 in maximum when plant transpiration was active under warm condition. Our isotope data confirm the importance of recycling of water through transpiration of forest plants in taiga to the hydrologic cycle in eastern Siberia. Copyright © 2013 John Wiley & Sons, Ltd.

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