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Daily precipitation samples were collected at four sites in Mount Yulong and Mount Meili regions of southeastern Tibetan Plateau and analyzed for isotopic composition. Combined with water vapor isotopic composition derived from satellites data (Infrared Atmospheric Sounding Interferometer, Tropospheric Emission Spectrometer, and Greenhouse gases Observing Satellite), the factors controlling the variability in the isotopic composition of precipitation are investigated at the synoptic, intraseasonal, and seasonal time scales. At the seasonal scale, the isotope composition in precipitation is controlled by processes along air mass trajectories affecting the water vapor at the large scale (>200 km), especially upstream deep convection and air mass origin, and by local processes transforming the large-scale water vapor isotopic composition into the precipitation composition, especially rain evaporation and local circulation. At the intraseasonal and synoptic scales, the isotope composition in precipitation is mainly controlled by these local processes. The shorter the time scale of isotopic variations, the smaller the spatial imprint of these variations. The fact that different factors control the isotopic variability at different time scales calls for caution when applying relationships observed at these time scales to interpret isotopic variations at paleoclimatic time scales. While general circulation models successfully capture the isotopic variability and its controlling factors at the seasonal scale, it fails to simulate them at shorter time scales, because it fails to simulate the local processes.

Plain Language Summary Water molecules can be light (one oxygen atom and two hydrogen atoms) or heavy (one hydrogen atom is replaced by a deuterium atom). These different molecules are called water isotopes. The isotopic composition of precipitation, when recorded in ice cores, can provide information about past climate. In southeastern TP ice cores, the isotopic records have been interpreted as reflecting past variations in temperature,or as reflecting past variations in monsoon strength. This illustrates that we need to better understand what processes control the isotopic composition of precipitation. We investigate the processes at the seasonal (>1 month), intraseasonal (10-30 days), and synoptic (<10 days) time scales. We use precipitation samples collected in the southeastern TP and satellite retrievals of water vapor isotope. We show that storm activity along air mass trajectories is an important factor controlling isotopic variations at the seasonal scale. But local processes, such as the evaporation of rain drops or local winds, are also important, especially at shorter time scales. The fact that the processes control the isotopic composition of precipitation depend on the time scale calls for caution when applying the factors observed in present-day variability to interpret isotopic ice core records at very long time scales.