Controlling factors of rainwater and water vapor isotopes at Bangalore, India: Constraints from observations in 2013 Indian monsoon

Abstract

Isotopic ratios of rainwaters are believed to decrease with the amount of rainfall. However, analyses of the isotopic composition of rainwater and water vapor samples collected from Bangalore during the monsoon period of 2013 fail to show any simple relationship with the local meteorological parameters whereas show good correlation with the regional integrated convective activity. The correlation is particularly high when the averaging is done over the preceding 8 to 15 days, showing the influence of mixing or residence time scale of atmospheric moisture. This observation emphasizes the role of regional atmospheric circulation driving the isotopic values. A comparison between observed isotope ratios in water vapor and rainwater with Isotope‐enabled Global Spectral Model shows discrepancies between the two. The observed values are relatively enriched, indicating a systematic bias in the model values. The higher observed values suggest underestimation of the evaporation in the model, which we estimate to be about 28 ± 15% on average. Simultaneous analyses of rainwater and water vapor isotopic composition again show definitive presence of raindrop evaporation (31 ± 14%). We also documented a distinct pattern of isotopic variation in six samples collected at Bangalore due to mixing of vapor from a cyclonic system in close proximity that originated from the Bay of Bengal. It seems that large‐scale isotopic depletion occurs during cyclones caused by Rayleigh fractionation due to massive rainout. These results demonstrate the power of rainwater and water vapor isotope monitoring to elucidate the genesis and dynamics of water recycling within synoptic‐scale monsoon systems.

Stable isotopes of atmospheric water vapour and precipitation in the northeast Qinghai‐Tibetan Plateau

“…The calculated parameter F varied from 56 to 99% during the study period. The parameter f ( f = 1‐ F , the fraction of raindrop reevaporation water) ranged from 1 to 43%, with an average of 16.4 ± 12.9%, which was consistent with the previous studies (Rahul et al, ; Worden et al, ). Worden et al () reported that 20–50% of rainfall evaporation contributed significantly to lower troposphere humidity near convective clouds.…”

“…A significant negative relationship between the reevaporation faction and rainfall d-excess is shown in Figure 6 (r= -0.667, p <.001), which supports the aforementioned hypothesis that re-evaporation below cloud influences the isotopic composition of rainfall during the descent process. A previous study by Rahul et al (2016) also reported similar findings, with a low coefficient between the re-evaporation fraction and rainfall d-excess.…”

“…Recent studies have indicated that temperature effect on water vapour isotopes was found during premonsoon periods without amount effect (He & Richards, ), but amount effect changed the vapour δ 18 O during the monsoon periods (Yin et al, ). Moreover, reevaporation of raindrops and isotopic exchange of raindrops with ambient moisture also greatly affect the variations of water vapour isotopes and thus increase the deviation from precipitation across seasons (Lekshmy, Midhun, & Ramesh, ; Rahul et al, ). Knowledge gaps still exist on the variations of water vapour isotopes across seasons, and the interaction between rain and vapour from Qinghai Lake, located in the Northeast Tibetan Plateau.…”

“…Stable isotopes (δ 18 O and δ 2 H) in precipitation provide important source signatures of different components of the hydrological cycle, including reevaporation (Salamalikis, Argiriou, & Dotsika, 2016;Wang, Zhang, Che, Zhu, & Liu, 2016), precipitation (Rahul, Ghosh, Bhattacharya, & Yoshimura, 2016;Wang, Zhang, Che, Chen, & Qiang, 2016), and moisture sources (Rangarajan, Laskar, Bhattacharya, Shen, & Liang, 2017;Strong, Sharp, & Gutzler, 2007). Global δ 18 O and δ 2 H levels in precipitation are monitored by the International Atomic Energy Agency to identify the relationship between atmospheric processes and isotopic signals in water vapour and precipitation (Angert, LEE, & Yakir, 2008;Jacob & Sonntag, 1991).…”

Supplementary material to &quot;Craig-Gordon model validation using observed meteorological parameters and measured stable isotope ratios in water vapor over the Southern Ocean&quot;

“…Atmospheric moisture, condensed inside the cold trap as ice, was allowed to melt at room temperature (≈ 15-20 • C) and then transferred into 5 ml polyethylene storage vials. ALl the samples were stored at 4 • C. A similar setup for water vapor sampling was presented in the earlier studies Rahul et al (2016Rahul et al ( , 2018.…”