3.2 The range and evolution of stable isotopes in a synoptic time-series of precipitation with and without typhoon
In this study, a total of 8 normal precipitation (without typhoon impact) and 3 typhoon-related precipitation synoptic time-series variations were determined from the samples (Fig. 3 and 4).
[Insert Figure 3]
The mean values of δD and δ18O during normal precipitation showed significant seasonal differences, with samples collected in spring (March and May) and winter (December and February) being significantly higher than that in summer (June) and autumn (November). In contrast, the mean stable isotopic values of samples collected during typhoon-related precipitation in summer (August) were significantly lower than that in autumn (November).
The synoptic time-series variations of δD and δ18O of normal precipitation can be divided into three types. The δD and δ18O values of type Ⅰ first decreased and then increased with the continuation of the precipitation process, while the D-excess first increased and then decreased, and the lower isotope value corresponded to the times with a higher atmospheric relative humidity (Fig.3). This pattern appeared four times during the study period, in December 2018, February 2019, March 2019, and May 2019, respectively. Among them, the precipitation process in March 2019 showed fluctuations in isotope values due to the long duration of precipitation and short interruptions in the precipitation process. The δD and δ18O values of type Ⅱ gradually decreased with the continuation of the precipitation process until the end of the precipitation, whereas the D-excess increased and then decreased. This type appeared twice during the study period in November 2018 and May 2019, respectively. The δD and δ18O values of type Ⅲ fluctuated continuously with the precipitation process, and the D-excess fluctuated accordingly, while the atmospheric relative humidity remained stable without obvious changes during the whole precipitation process. This type appeared two times during the study period, both occurring in June 2019.
[Insert Figure 4]
The time-wise variations of isotopes were similar under different typhoon-related precipitation processes, which can be divided into three stages: the initial stage of precipitation was associated with the highest isotopes, the steady stage of precipitation with relatively lower isotope values, and the end stage of precipitation with relatively higher isotope values again but slightly lower than that during the initial stage, showing a generally U-shaped pattern. It should be noted that in the time-series of the tropical depression precipitation (Fig. 4 a), two sub- U-shape patterns can be discerned within the overall U-shape, which may be due to the second landing of the tropical depression in the Taiwan Strait. In addition, the variation of δD and δ18O showed an opposite trend to the variation of atmospheric relative humidity, reflecting the significant effect of evaporation.