Controls on stable isotopic characteristics of water vapour over Thailand

Abstract

AbstractThis study examined the weekly water vapour isotopic composition (δ18Ov) in Thailand. The water vapour was cryogenically collected from eight sites across the country. Two observational samples were collected over one 24‐h period each week (a daytime and a night‐time sample), from September 2013 to September 2014. The primary aim was to investigate the environmental factors influencing water vapour isotopes. The results revealed differences in water vapour isotopic values between day and night samples. Three periods of depleted δ18Ov were associated with large‐scale convective systems in September, December, and May. The statistical relationship between the climate variables and water vapour isotopes indicated that the amount of precipitation and relative humidity were the primary controls on both diurnal and seasonal isotopic variability. The temperature did not affect the δ18Ov, mainly because the atmospheric processes are a function of vertical convection rather than temperature in tropical regions. The water vapour deuterium excess (d‐excess) showed greater variability in 2013 than in 2014. The d‐excess variation reflected the differences in convection occurring in the day and night. In addition, the vapour phase data were combined with the local meteoric water line to identify the local water vapour line and the interaction between the isotopic composition of water vapour and liquid water. The water vapour isotopic patterns paralleled the precipitation isotopes on rainy days because of equilibrium isotopic exchange. Water vapour and precipitation were isotopically similar under low humidity but showed greater differences from each other under wetter conditions. The study results provide insight into water vapour isotopic characteristics in tropical regions and constrain the role of large‐scale atmospheric processes relative to isotopic variability of water vapour in Thailand and nearby countries.