Effects of Large-scale Moisture Transport and Mesoscale Processes on Precipitation Isotope Ratios Observed at Sumatera, Indonesia

Isotopic and meteorological observations in November 2006 on the west coast of Sumatera, Indonesia during the intense observation period of the Hydrometeorological ARray for Intraseasonal Variation-Monsoon AUto-monitoring (HARIMAU2006), revealed the impacts of large-scale moisture transport and mesoscale processes on precipitation isotope ratios. Intraseasonal changes in the precipitation delta H-2 in November had large variability ranging from +10 to -65 per mil, as a result of the changes in the large-scale moisture transport associated with the intraseasonal oscillation with a time-scale of 10-15 day over Sumatera. The isotopic composition of precipitation was independent from difference in precipitation type (convective or stratiform precipitation). An isotope circulation model reproduced the observed isotopic changes, supporting that the isotopic effect of large-scale moisture transport was the main contributor to intraseasonal isotopic changes. In high-frequency samples taken over a shorter time scale, isotopic variability was related to event type classified by the analysis of radar observations, although the isotopic effects of mesoscale processes on the isotopic averages of each precipitation event were almost masked by the isotopic effect of large-scale moisture transport. The precipitation delta H-2 accompanying the well-organized convection type decreased significantly by about 20 per mil. Drastic changes in isotope ratios could be described by the Rayleigh distillation process. Isotope ratios of precipitation gently decreased and subsequently increased in the unorganized convection type since the water vapor in surrounding convectively rising air isotopically enriched the remaining low-isotope water vapor advected from the precedent clouds. Isotope ratios in the stratiform precipitation remained steady, possibly attributable to the homogeneous moisture of stratiform clouds.