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.
[Insert Figure 7]