Contributions of Atmospheric Transport and Rain–Vapor Exchange to Near-Surface Water Vapor in the Zhanjiang Mangrove Reserve, Southern China: An Isotopic Perspective

Abstract

Coastal mangroves are increasingly recognized as valuable natural resources and important sites of water and carbon exchange. In this study, we examine atmospheric water cycling in the boundary layer above a coastal mangrove forest in southern China. We collected site observations of isotopic ratios in water vapor and precipitation along with core meteorological variables during July 2017. Our evaluation of these data highlights the influences of large-scale atmospheric transport and rain–vapor exchange in the boundary layer water budget. Rain–vapor exchange takes different forms for different types of rainfall events. The evolution of isotopic ratios in water vapor suggests that substantial rain recycling occurs during the passage of large-scale organized convective systems, but that this process is much weaker during rainfall associated with less organized events of local origin. We further examine the influences of large-scale transport during the observation period using a Lagrangian trajectory-based moisture source analysis. More than half (63%) of the boundary layer moisture during the study period traced back to the South China Sea, consistent with prevailing southerly to southwesterly flow. Other important moisture sources included mainland Southeast Asia and the Indian Ocean, local land areas (e.g., Hainan Island and the Leizhou Peninsula), and the Pacific Ocean. Together, these five regions contributed more than 90% of the water vapor. The most pronounced changes in isotopic content due to large-scale transport during the study period were related to the passage of Tropical Storm Talas. The outer rain bands of this tropical cyclone passed over the measurement site on 15–17 July, causing a sharp reduction in the heavy isotopic content of boundary layer water vapor and a substantial increase in deuterium excess. These changes are consistent with extensive isotopic distillation and rain–vapor exchange in downdrafts associated with the intense convective systems produced by this storm.

Ocean air masses from the East Asian monsoon dominate the land‐surface atmospheric water cycles in the coastal areas of Liaodong Bay, Northeast China

“…Lai et al (2018) examined the atmospheric water cycling above a coastal mangrove forest in Southern China. They also found that the substantial increase of atmospheric water vapour d‐excess is related to the passage of a tropical typhoon, “Talas” (Lai et al, 2018). This variation in air mass and monsoon activity also caused inter‐annual differences in isotopes between 2019 and 2020.…”

“…The theoretical basis of this application is that the d‐excess of water vapour is closely linked to the environmental conditions during the kinetic fractionation processes (Araguas‐Araguas et al, 2000; Craig, 1961). Hence, the strong sensitivity of d‐excess to local atmosphere relative humidity (RH) can place better constraints on the water transport processes than the analyses of water vapour δ 2 H and δ 18 O alone (Lai et al, 2018; Wei & Lee, 2019). Previous work has traced both the ocean moisture sources (Dahinden et al, 2021; Gonzalez et al, 2016; Salamalikis et al, 2015) and continental moisture recycling (Aemisegger et al, 2014; Fiorella et al, 2019).…”

“…These studies confirm the great potential of in situ water vapour δ vapor measurements to reconstruct the complex hydrological processes. However, among the available data on atmospheric water vapour δ vapor , the data from coastal wetlands are scarce (Delattre et al, 2015; Lai et al, 2018). For example, Delattre et al (2015) showed that in a Mediterranean coastal wetland that exposed to high evaporation the diurnal fluctuation of atmospheric water vapour δ vapor is driven by local evaporation, vertical air mass redistribution and dew formation.…”

Controls on stable isotopic characteristics of water vapour over Thailand

“…Ingraham and Criss (1993) have noted that the water vapour isotopic composition and the exchange rate can change due to different geographical areas and weather conditions. Furthermore, precipitation–water vapour exchange takes different forms for different types of rainfall events (Lai et al, 2018).…”

“…Most values that fall along the linear regression line reflect the equilibrium isotopic exchange, whereas the points that deviate may reflect secondary evaporation (Figure 10). This deviation indicates that equilibration between boundary layer vapour and rainfall was incomplete and further suggests that non‐equilibrium isotopic fractionation may have occurred through partial evaporation of the raindrops (Lai et al, 2018).…”