4.2.1 The synoptic precipitation without typhoon impact
In comparison to the seasonal scale, the synoptic time-series variation of isotopic composition of precipitation is much more complex (Nlend et al., 2020). There are also significant differences between normal and typhoon-related precipitation, and these differences reflect the influence of meteorological parameters, physical processes, and moisture sources on isotopic composition of precipitation (Wolf et al., 2020; Zhang et al., 2015b). The existence of three different types of normal precipitation suggests that three different control mechanisms may exist. The δ18O value of type Ⅰ gradually increased with the decrease of atmospheric relative humidity and increase of temperature in the middle and late stages of precipitation (Fig. 3). Low humidity and high temperature promote re-evaporation of rain drops during precipitation (Peng et al., 2007). Thus, the δ18O value of type Ⅰ may mainly be controlled by re-evaporation. The δ18O value of type II gradually decreased with time, and the relative humidity still maintained a high level in the middle and late stages of precipitation, indicating that it was less affected by re-evaporation. The decrease of δ18O values is mainly controlled by the continuing equilibrium fractionation during condensation (Gedzelman and Lawrence, 1982). The atmospheric relative humidity of type III was maintained at a high level all the time, and the δ18O values show fluctuating variations, indicating that there might be a combination of influencing factors. In addition, differences in the moisture source may also have a significant effect on the isotope value variation of normal precipitation (Balagizi et al., 2018). Moisture of type Ⅰ mainly originated from inland and offshore areas of eastern China, which was a mixture of continental and oceanic air masses. However, there are differences in the source of moisture for different precipitation events at the same heights, for example, the moisture at 500 m height for the December 2018 precipitation event originated from the Bohai Sea and Yellow Sea (Fig. 7 a1), whereas that for the February 2019 precipitation event originated from central China (Fig. 7 a2), which may be an important reason for the difference in isotope values between the two precipitation events. Similarly, moisture of type II was mainly from oceanic air masses, but the source of moisture for the November 2018 precipitation event was a mixture from the East China Sea (West Pacific) and the Bay of Bengal (Fig. 7 b1), whereas that for the May 2019 precipitation event was a mixture from the East China Sea and the South China Sea (Fig. 7 b2), which led to the difference in isotope values between the two precipitation events. Moisture of type III was also from oceanic sources, mainly a mixture of the East and South China Seas, but the 1000 m height moisture for the June 11, 2019 precipitation event originated from the South China Sea whereas that for the June 23, 2019 precipitation event originated from the East China Sea (Fig. 7 c1 and c2). In summary, we suggest that local meteorological parameters during precipitation, which mainly control the re-evaporation process, are the main controlling factors for the variation patterns of δD and δ18O, whereas moisture sources control the overall isotope values of precipitation.
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