Vapor Isotope Research Articles

Water vapor stable isotope memory effects of common tubing materials

Abstract. Water molecules in vapor can exchange with gaseous water molecules sticking to surfaces of sampling tubing, and exchange rates are unique for each water isotopologue and tubing material. Therefore, water molecules on tubing walls take some time to reach isotopic equilibrium with a new vapor isotopic signal. This creates a memory effect that is observed as attenuation time for signal propagation in continuous stable water vapor isotope measurement systems. Tubing memory effects in δD and δ18O measurements can limit the ability to observe fast changes, and because δD and δ18O memory are not identical, this introduces transient deuterium excess (D-excess, defined as δD-8×δ18O) artifacts in time-varying observations. To our knowledge, a comprehensive performance comparison of commonly used tubing material water exchange properties in laser-based measurement systems has not been published. We compared how a large isotopic step change propagated through five commonly used tubing materials for water isotopic studies – perfluoroalkoxy (PFA), fluorinated ethylene propylene (FEP), polytetrafluoroethylene (PTFE), high-density polyethylene (HDPE), and copper – at two different temperatures and an airflow rate of 0.635 L min−1 through approximately 100 ft (30.5 m) of 1/4 in. (6.4 mm) outer diameter (o.d.) tubing. All commonly used tubing materials performed similarly to each other in terms of attenuation times, reaching δ18O-location-adjusted δD and δ18O 95 % completion in less than 45 s, with slight variations based on temperature. PFA does appear to perform slightly better than the other materials, although memory metric differences are small. A tubing material commonly used in the early 2000s but reported to have memory effects on δD, Dekabon, was also tested at ambient temperature and changing humidities. The Dekabon isotopic equilibrium was not reached until nearly an hour after source transition, much later than H2O mixing ratios equilibrated. Bev-A-Line XX (used in some soil O2 and CO2 gas studies) was also tested at ambient temperature, but it did not approach isotopic equilibrium until after nearly 6 h of testing. Therefore, we cannot recommend the use of Bev-A-Line XX or Dekabon in water vapor isotope applications. Source transition from heavy to light or from light to heavy affected isotopic transition speed only in experiments where H2O ppmv was changing. While a shorter tubing lengths and smaller inner diameters shorten the delay of signal propagation through the tubing, they did not greatly change the attenuation curves under these conditions for the current commonly used tubing materials tested. However, in Dekabon, attenuation curves were greatly extended with increased tubing length. Our results show that the commonly used plastic tubing materials tested were not inferior to copper in terms of isotopic memory under these conditions, and they are easier to work with and are less expensive than copper.

The Cumulative Effect of Wintertime Weather Systems on the Ocean Mixed‐Layer Stable Isotope Composition in the Iceland and Greenland Seas

AbstractThe Iceland and Greenland Seas are characterized by strong heat fluxes from the ocean to the atmosphere during wintertime. Here we characterize the atmospheric signal of this strong evaporation in terms of water vapor isotopes and investigate if such a signal can have a cumulative imprint on the ocean mixed‐layer. Observations include continuous water vapor isotope measurements, event‐based precipitation samples, and sea‐water samples taken at various depths from the research vessel Alliance during the Iceland‐Greenland Seas Project cruise in February and March 2018. In conjunction with a simulation from a regional, isotope‐enabled atmospheric model, we find that the predominant atmospheric isotope signature during predominant marine cold‐air outbreak conditions is −129.8 ± 16.6‰ for δ2H and −18.10 ± 2.87‰ for δ18O, with a d‐excess of 15.1 ± 7.9‰, indicating enhanced non‐equilibrium fractionation compared to the global average. During events of warm‐air intrusion from mid‐latitudes, near‐surface vapor becomes saturated and the vapor d‐excess approaches equilibrium or becomes negative. Similarly, precipitation d‐excess is lower and thus closer to equilibrium conditions during warm‐air intrusions. There are indications that an evaporation signal of waters exiting the Nordic Seas through Denmark Strait could be locally enhanced over seasons to years, as supported by simple model calculations. Our findings thus suggest that evaporation signals could be transferred into the ocean isotope composition in this region, potentially enabling mass‐balance constraints in isotope‐enabled coupled ocean‐atmosphere models.

Integrating the controlled evaporation mixer with cavity ring-down spectroscopy for enhanced water vapor isotope calibration

Accurate measurement of water vapor isotopes (δ18O and δ2H) is fundamental for advancing our understanding of the hydrological cycle and improving hydrological model accuracy. This study introduces an innovative calibration methodology using a controlled evaporation mixer (CEM) for determining stable isotopic ratios in atmospheric water vapor via cavity ring-down spectroscopy. The CEM technique reliably produces a stable water vapor stream, crucial for enhancing the precision and accuracy of isotopic measurements. Its rapid adaptation to changes in water vapor concentration and compatibility with different water standards enhance calibration reliability. Demonstrated reproducibility in generating water vapor across a broad concentration range from 900 to over 25,000 ppmv, coupled with a substantial reduction in memory effects, makes this approach highly effective in both laboratory and field settings. This calibration advancement greatly enhances research capabilities for continuous atmospheric water vapor analysis, providing deeper insights into hydrological processes and atmospheric dynamics.

Skill of isotope-enabled climate models for daily surface water vapour in East Asia

The isotope-enabled general circulation models (GCM) have been widely applied to simulate the variability of stable isotopes in meteoric water at various time scales. The in-situ observations of water vapour isotopes are an important basis for assessing the performance of isotope-enabled GCMs, although they are still limited. Here we compiled the observations of near-surface water vapour isotopes on a daily scale at 17 stations in East Asia, and assessed the skill and the association between isotope error and meteorological errors on a daily scale. Generally, the spatial pattern and seasonal variability can be well simulated in the isotope-enabled GCMs. The models show better skill for warm and humid backgrounds, which also corresponds to the monsoonal regions with lower latitudes in East Asia. As spatial resolution is finer, the skill of models is better, which can be seen from the two GCMs. According to the correlation coefficient, the improvement of resolution is more obvious in summer than in winter, especially for IsoGSM. In addition, the correlation coefficient in winter is usually larger than that in summer. The daily modelling has good potential to investigate the daily or synoptic climate information in water isotopes. The findings are useful for understanding the applicability of isotope-enabled models in East Asia and the climate factors influencing the skill of isotope-enabled models on a daily basis.

Quantifying the below-cloud evaporation of raindrops using near-surface water vapour isotopes: Applications in humid and arid climates in East Asia

In global hydrological circulation, evaporation widely occurs from the land, the oceans, and other water surfaces. Compared to the evaporation from open water, the below-cloud evaporation of falling raindrops is more difficult to quantify. As an alternative to the traditional microphysical model, the difference in stable water isotopes between water vapour and precipitation provides a new perspective to estimate the raindrop mass loss. According to the recent observations of stable isotopes in near-surface water vapour and precipitation in five sampling stations from humid to arid climates in East Asia, we quantified the below-cloud evaporation of raindrops using both a microphysical model and an isotope inversion model. The results indicate that the isotope inversion model, relative to the microphysical model, usually underestimates the impact of below-cloud evaporation on precipitation, especially in arid inland. The sensitivity test of the two models to errors in climatic factors shows that the microphysical model was more sensitive to errors in temperature and relative humidity than the isotope inversion model. We also plot the ranges that the isotope inversion model has solutions under various meteorological and isotope inputs. The findings are useful for understanding the atmospheric processes below the cloud base and the comparability of different methods in quantifying below-cloud evaporation.

Contribution of fluorescent primary biological aerosol particles to low-level Arctic cloud residuals

Abstract. Mixed-phase clouds (MPCs) are key players in the Arctic climate system due to their role in modulating solar and terrestrial radiation. Such radiative interactions rely, among other factors, on the ice content of MPCs, which is regulated by the availability of ice-nucleating particles (INPs). While it appears that INPs are associated with the presence of primary biological aerosol particles (PBAPs) in the Arctic, the nuances of the processes and patterns of INPs and their association with clouds and moisture sources have not been resolved. Here, we investigated for a full year the abundance of and variability in fluorescent PBAPs (fPBAPs) within cloud residuals, directly sampled by a multiparameter bioaerosol spectrometer coupled to a ground-based counterflow virtual impactor inlet at the Zeppelin Observatory (475 m a.s.l.) in Ny-Ålesund, Svalbard. fPBAP concentrations (10−3–10−2 L−1) and contributions to coarse-mode cloud residuals (0.1 to 1 in every 103 particles) were found to be close to those expected for high-temperature INPs. Transmission electron microscopy confirmed the presence of PBAPs, most likely bacteria, within one cloud residual sample. Seasonally, our results reveal an elevated presence of fPBAPs within cloud residuals in summer. Parallel water vapor isotope measurements point towards a link between summer clouds and regionally sourced air masses. Low-level MPCs were predominantly observed at the beginning and end of summer, and one explanation for their presence is the existence of high-temperature INPs. In this study, we present direct observational evidence that fPBAPs may play an important role in determining the phase of low-level Arctic clouds. These findings have potential implications for the future description of sources of ice nuclei given ongoing changes in the hydrological and biogeochemical cycles that will influence the PBAP flux in and towards the Arctic.

Dual-path coupling V-shaped structure off-axis integrated cavity output spectroscopy (V-OA-ICOS) for water vapor stable isotope detection at 3.66 μm

Residual cavity mode noise and weak output signal are the main factors that restrict the detection performance of off-axis integrated cavity output spectroscopy (OA-ICOS). In order to overcome the above drawbacks, a novel V-shaped off-axis integrated cavity output spectroscopy (V-OA-ICOS) device was demonstrated, and the dual-path coupling method was further proposed. The dual-path coupling V-OA-ICOS device can not only suppress the residual cavity mode noise but also enhance the intensity of the output signal. The V-shaped resonator cavity ensures a more uniform and square-shaped spot pattern distribution on the surface of the highly reflective mirrors, significantly reducing spots overlap. The dual-path coupling method can re-couple the laser reflected by the front cavity mirror, thereby increasing the total energy and mode density of the cavity. The spectrum of water vapor stable isotopes HD16O and H216O at 3.66 μm was measured to illustrate the performance of the presented approach, the dual-path coupling V-OA-ICOS increases the signal-to-noise ratio (SNR) by a factor ∼ 2.1 compared to single path V-OA-ICOS and the signal intensity by a factor of ∼ 2. The δD variation was obtained by measuring the outdoor water vapor isotope using this device for three consecutive days. The variation range of δD is −220‰ ∼ −120‰. This work proposes a new cavity structure and optical re-coupling method, which offers a novel approach to enhance the performance of OA-ICOS devices.

Is the Isotopic Composition of Precipitation a Robust Indicator for Reconstructions of Past Tropical Cyclones Frequency? A Case Study on Réunion Island From Rain and Water Vapor Isotopic Observations

AbstractBased on a 6‐year long record (2014–2020) of the isotopic composition of rain (δ18Op) at Réunion Island (55°E, 22°S), in the South‐West Indian Ocean, this study shows that the annual isotopic composition of precipitation in this region is strongly controlled by the number of cyclones, the number of best‐track days, and the proportion of cyclonic rain during the year. Our results support the use of δ18Op in annual‐resolved tropical climate archives as a reliable proxy of past cyclone frequency. The influence of the proportion of cyclonic rain on the annual isotopic composition arises from the systematically more depleted precipitation and water vapor during cyclonic events than during less organized convective systems. The analysis of the daily to hourly isotopic composition of water vapor (δ18Ov) during low‐pressure systems and the reproduction of daily δ18Ov observations by AGCMs with a global medium to coarse resolution (LMDZ‐iso and ECHAM6‐wiso) suggest that during cyclonic periods the stronger depletion mainly arises from both enhanced large‐scale precipitation and water vapor‐rain interactions under humid conditions.

Atmospheric HDO Abundance Measurements in the Tibetan Plateau Based on Laser Heterodyne Radiometer

The Tibet Plateau is known as the “third pole” of the world, and its environmental change profoundly impacts East Asia and even the global climate. HDO is the stable isotope of water vapor, which acts as an ideal tracer for studying the water cycle, and which is commonly used for atmospheric circulation and climatic studies. To monitor the water vapor isotopic abundance in the Tibetan Plateau, a portable laser heterodyne radiometer was operated in Golmud in August 2019. The radiometer utilizes a narrow-linewidth 3.66 μm distributed feedback interband cascade laser as the local oscillator, the heterodyne module is been optimized and the radiometer performs with high resolution and stability in obtaining spectral data. Furthermore, the absorption spectra of atmospheric HDO and H2O are obtained, and the retrieval method for water vapor isotopic abundance is discussed. The optimal estimation method is adopted to retrieve the density of HDO and H2O. The average column density of H2O was 1.22 g/cm2, and the HDO/H2O ratio in Golmud was 178 ± 15 × 10−6 during the observation. For a better understanding of the retrieval, the retrieval errors are analyzed and compared. The results indicate that the smoothing error is significantly higher than the measurement error in this work. The backward trajectory analysis of atmospheric transport is used to investigate the relationship between water vapor density and atmospheric motion. The results indicate that the variation of H2O column density and HDO/H2O ratio have a relationship with atmospheric movements.

Water isotopic characterisation of the cloud–circulation coupling in the North Atlantic trades – Part 2: The imprint of the atmospheric circulation at different scales

Abstract. Water vapour isotopes reflect the history of moist atmospheric processes encountered by the vapour since evaporating from the ocean, offering potential insights into the controls of shallow trade-wind cumuli. Given that these clouds, particularly their amount at the cloud base level, play an important role in the global radiative budget, improving our understanding of the hydrological cycle associated with them is crucial. This study examines the variability of water vapour isotopes at cloud base in the winter trades near Barbados and explores its connection to the atmospheric circulations ultimately governing cloud fraction. The analyses are based on nested COSMOiso simulations with explicit convection during the EUREC4A (Elucidating the role of clouds-circulation coupling in climate) field campaign. It is shown that the contrasting isotope and humidity characteristics in clear-sky versus cloudy environments at cloud base emerge due to vertical transport on timescales of 4 to 14 h associated with local, convective circulations. In addition, the cloud base isotopes are sensitive to variations in the large-scale circulation on timescales of 4 to 6 d, which shows on average a Hadley-type subsidence but occasionally much stronger descent related to extratropical dry intrusions. This investigation, based on high-resolution isotope-enabled simulations in combination with trajectory analyses, reveals how dynamical processes at different timescales act in concert to produce the observed humidity variations at the base of trade-wind cumuli.

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

AbstractLong‐term atmospheric water vapour hydrogen (δ2H), oxygen (δ18O) and deuterium excess (d‐excess) can provide unique insights into the land‐atmosphere coupling processes. The in‐situ measurements of atmospheric water vapour δ2H, δ18O and d‐excess were conducted above a reed wetland of Liaodong Bay (2019–2020). We found significant inter‐annual variations in atmospheric water vapour isotopes between the two growing (May–September) seasons. The δ2H, δ18O and d‐excess of atmospheric water vapour exhibited different seasonal and diurnal cycles concerning the vertical measurement heights, especially in 2019. The isotopic differences of atmospheric water vapour among vertical measurement heights were more evident in the daytime. Rainfall events directly impacted the diurnal patterns of water vapour isotopes, and the influences depended on rainfall intensities. However, only weak correlations existed between water vapour isotopes and local meteorological factors (R2 = 0.01–0.16, p < 0.001), such as water vapour concentration (w), Relative Humidity (RH) and surface air temperature (Ta). Based on the back‐air trajectory analyses, the spatial–temporal dynamics of atmospheric water vapour isotopes are highly synchronized with monsoon activities. Different water vapour sources influence the water vapour isotope in this region and the higher d‐excess value is related to the intense convection brought by the monsoon. High‐resolution measurements of atmospheric water vapour isotopes will improve our understanding of the hydrological cycles in coastal areas.

Contrasting Seasonal Isotopic Signatures of Near‐Surface Atmospheric Water Vapor in the Central Arctic During the MOSAiC Campaign

AbstractThe Arctic is experiencing unprecedented moistening which is expected to have far‐reaching impact on global climate and weather patterns. However, it remains unclear whether this newly sourced moisture originates locally from ice‐free ocean regions or is advected from lower latitudes. In this study, we use water vapor isotope measurements in combination with trajectory‐based diagnostics and an isotope‐enabled atmosphere general circulation model, to assess seasonal shifts in moisture sources and transport pathways in the Arctic. Continuous measurements of near‐surface vapor, δ18O, and δD were performed onboard RV Polarstern during the Multidisciplinary drifting Observatory for the Study of Arctic Climate expedition from October 2019 to September 2020. Combining this isotope data set with meteorological observations reveals that the spatiotemporal evolution of δ18O mimics changes in local temperature and humidity at synoptic to seasonal time scales, while corresponding d‐excess changes suggest a seasonal shift in the origin of moisture. Simulation results from the particle dispersion model FLEXPART support these findings, indicating that summer moisture originates from nearby open ocean, while winter moisture comes from more remote sources with longer residence time over sea‐ice. Results from a nudged ECHAM6‐wiso simulation also indicate that evaporative processes from the ocean surface reproduce summer isotope values, but are insufficient to explain measured winter isotope values. Our study provides the first isotopic characterization of Central Arctic moisture over the course of an entire year, helping to differentiate the influence of local processes versus large‐scale vapor transport on Arctic moistening. Future process‐based investigations should focus on assessing the non‐equilibrium isotopic fractionation during airmass transformation over sea‐ice.

A flexible device to produce a gas stream with a precisely controlled water vapour mixing ratio and isotope composition based on microdrop dispensing technology

Abstract. ​​​​​​​Here we describe a versatile device to produce a gas stream with a precisely controlled water vapour mixing ratio and stable water isotope composition based on microdrop dispensing technology. To produce a moist airstream, the microdrop dispensing technology ejects micrometre-size water droplets that completely evaporate into a stream of carrier gas heated to 60 ∘C. By precisely controlling the contribution of water standards from two dispenser heads into a carrier gas stream, the device allows one to set the air–vapour stream to any isotope ratio between two endmember waters. We show that the Allan deviation of the water vapour mixing ratio is 10 ppmv over more than 24 h and reaches 0.004 ‰ for δ18O and 0.02 ‰ for δ2H for a flow rate of 40 sccm. Tests with flow rates from 40–250 sccm show an increase of the Allan deviation with higher flow rates. Tests with mixing standard water from two dispenser heads show a linear mixing across a range of water vapour mixing ratios from 1000 to 20 000 ppmv. In addition to presenting the design and several performance characteristics of the new system, we describe two application examples. First, we utilise the device to determine the water vapour mixing ratio–isotope ratio dependency, a common artefact of water vapour isotope spectrometers. Second, we utilise the device to provide a constant background stream of moist air for fluid inclusion water isotope analysis in calcite samples from stalagmites. The observed flexibility and precision of the device underpins its usefulness and potential for a wide range of applications in atmospheric water vapour isotope measurements. Future developments could focus on reducing the number of manual interventions needed to clear dispenser heads from gas bubbles and the provision of a water vapour stream at flow rates of up to several litres per minute.

Sub-cloud rain evaporation in the North Atlantic winter trade winds derived by pairing isotopic data with a bin-resolved microphysical model

Abstract. Sub-cloud rain evaporation in the trade wind region significantly influences the boundary layer mass and energy budgets. Parameterizing it is, however, difficult due to the sparsity of well-resolved rain observations and the challenges of sampling short-lived marine cumulus clouds. In this study, sub-cloud rain evaporation is analyzed using a steady-state, one-dimensional model that simulates changes in drop sizes, relative humidity, and rain isotopic composition. The model is initialized with relative humidity, raindrop size distributions, and water vapor isotope ratios (e.g., δDv, δ18Ov) sampled by the NOAA P3 aircraft during the Atlantic Tradewind Ocean–Atmosphere Mesoscale Interaction Campaign (ATOMIC), which was part of the larger EUREC4A (ElUcidating the RolE of Clouds–Circulation Coupling in ClimAte) field program. The modeled surface precipitation isotope ratios closely match the observations from EUREC4A ground-based and ship-based platforms, lending credibility to our model. The model suggests that 63 % of the rain mass evaporates in the sub-cloud layer across 22 P3 cases. The vertical distribution of the evaporated rain flux is top heavy for a narrow (σ) raindrop size distribution (RSD) centered over a small geometric mean diameter (Dg) at the cloud base. A top-heavy profile has a higher rain-evaporated fraction (REF) and larger changes in the rain deuterium excess (d=δD-8×δ18O) between the cloud base and the surface than a bottom-heavy profile, which results from a wider RSD with larger Dg. The modeled REF and change in d are also more strongly influenced by cloud base Dg and σ rather than the concentration of raindrops. The model results are accurate as long as the variations in the relative humidity conditions are accounted for. Relative humidity alone, however, is a poor indicator of sub-cloud rain evaporation. Overall, our analysis indicates the intricate dependence of sub-cloud rain evaporation on both thermodynamic and microphysical processes in the trade wind region.

Satellite-Based Distribution of Inverse Altitude Effect of Global Water Vapor Isotopes: Potential Influences on Isotopes in Climate Proxies

The widely-distributed altitude effect of stable isotopes in meteoric water, i.e., the negative correlation between stable hydrogen (or oxygen) isotope compositions and altitude, is the theoretical basis of isotope paleoaltimetry in climate proxies. However, as many recent local observations have indicated, the inverse altitude effect (IAE) in meteoric water does exist, and the regime controlling IAE is still unclear on a global scale. Based on a remote sensing product of the Infrared Atmospheric Sounding Interferometer (IASI), we examined the global frequency of IAE in water vapor isotopes, and the possible influences on isotopes in precipitation and climate proxies. According to the satellite-based δD values in water vapor at 2950 m and 4220 m above sea level, frequent IAEs are observed on a daily scale in North Africa, West and Central Asia, and North America, and IAEs are more likely to occur during the daytime than during the nighttime. We also converted water vapor δD to precipitation δD via equilibrium fractionation and then analyzed the potential presence of IAE in precipitation, which is more associated with climate proxies, and found that the spatial and temporal patterns of water vapor can be transferred to the precipitation. In addition, different thresholds of δD difference were also tested to understand the impact of random errors. The potential uncertainty of the changing isotope and altitude gradient should be considered in paleo-altitude reconstructions.

A set of methods to evaluate the below-cloud evaporation effect on local precipitation isotopic composition: a case study for Xi'an, China

Abstract. When hydrometeors fall from an in-cloud saturated environment toward the ground, especially in arid and semiarid regions, below-cloud processes may heavily alter the isotopic composition of precipitation through equilibrium and non-equilibrium fractionations. If these below-cloud processes are not correctly identified, they can lead to misinterpretation of the precipitation isotopic signal. To correctly understand the environmental information recorded in the precipitation isotopes, qualitatively analyzing the below-cloud processes and quantitatively calculating the below-cloud evaporation effect are two important steps. Here, based on 2 years of synchronous observations of precipitation and water vapor isotopes in Xi'an, China, we compiled a set of effective methods to systematically evaluate the below-cloud evaporation effect on local precipitation isotopic composition. The ΔdΔδ diagram is a tool to effectively diagnose below-cloud processes, such as equilibration or evaporation, because the isotopic differences (δ2H; d-excess) between the precipitation-equilibrated vapor and the observed vapor show different pathways. By using the ΔdΔδ diagram, our data show that evaporation is the major below-cloud process in Xi'an, while snowfall samples retain the initial cloud signal because they are less impacted by the isotopic exchange between vapor and solid phases. Then, we chose two methods to quantitatively characterize the influence of below-cloud evaporation on local precipitation isotopic composition. One is based on the raindrop's mass change during its falling (hereafter referred to as method 1), and the other is dependent on the variations in precipitation isotopic composition from the cloud base to the ground (hereafter referred to as method 2). By comparison, we found that there are no significant differences between the two methods in evaluating the evaporation effect on δ2Hp, except for snowfall events. The slope of the evaporation in proportion to the variation in δ2H (Fi/Δδ2H) is slightly larger in method 1 (1.0 ‰ %−1) than in method 2 (0.9 ‰ %−1). Additionally, both methods indicate that the evaporation effect is weak in autumn and heavy in spring. Through a sensitivity test, we found that in two methods, relative humidity is the most sensitive parameter, while the temperature shows different effects on the two methods. Therefore, we concluded that both methods are suited to the investigation of the below-cloud evaporation effect, while in method 2, other below-cloud processes, such as supersaturation, can still be included. By applying method 2, the diagnosis of below-cloud processes and the understanding of their effects on the precipitation isotopic composition will be improved.

Fate of herbicides in cropped lysimeters: 1. Influence of different processes and model structure on vadose zone flow

AbstractUnderstanding transport and fate processes in the subsurface is of fundamental importance to identify the leaching potentials of herbicides or other compounds to groundwater resources. HYDRUS‐1D was used to simulate water flow and solute transport in arable land lysimeters. Simulations were compared to observed drainage rates and stable water isotopes (δ18O) in the drainage. Four different model setups were investigated and statistically evaluated for their model performance to identify dominant processes for water flow characterization in the vadose zone under similar cultivation management and climatic conditions. The studied lysimeters contain soil cores dominated by sandy gravel (Ly1) and clayey sandy silt (Ly2), both cropped with maize located in Wielenbach, Germany. First, a single‐porosity setup was chosen. For Ly1, modeling results were satisfactory, but for Ly2, the damping observed in the isotope signature of the drainage could not be fully covered. By considering immobile water with a dual‐porosity setup for Ly2, model performance improved. This could be due to a higher fraction of fine pores in Ly2 available for water storage, leading to mixing processes of isotopically enriched summer precipitation and lighter winter water. Accounting for isotopic evaporation fractionation processes in both model setups did not lead to improved model performance. Consequentially, the difference in soil hydraulic properties between the two lysimeters seems to impact water flow processes. Knowledge of such differences is crucial to prevent contamination and mitigate potential risks to soil and groundwater.

Quantifying Raindrop Evaporation Deficit in General Circulation Models from Observed and Model Rain Isotope Ratios on the West Coast of India

Raindrop evaporation is an important sub-cloud process that modifies rainfall amount and rainwater isotope values. Earlier studies have shown that various general circulation models (GCMs) do not incorporate this process properly during the simulation of water isotope ratios (oxygen and hydrogen). Our recent study has demonstrated that an inadequate estimation of this process for the Indian Summer Monsoon (ISM) results in significant biases (model-observed values) in the simulation of various GCMs on a monthly scale. However, a quantitative estimation was lacking. The magnitude of raindrop evaporation depends upon ambient humidity and temperature, which vary considerably during the ISM. Consequently, the isotope biases would also vary over various time scales. The present study aims to investigate the magnitude of the monthly scale variation in raindrop evaporation in the simulations and its causal connection with the corresponding variation in isotope biases. Towards this, we compare an 11-year-long (1997–2007) dataset of rain isotope ratios (both oxygen and hydrogen) from an Indian station, Kozhikode (Kerala), obtained under the Global Network of Isotopes in Precipitation (GNIP) programme of the International Atomic Energy Agency (IAEA) with the corresponding outputs of two isotope-enabled nudged GCMs—ISOGSM and LMDZ4. The raindrop evaporation fractions are estimated for 44 ISM months (June–September) of the study period using the Stewart (1975) formalism. Using a simple condensation–accretion model based on equilibrium fractionation from vapour, obtained from two adopted vapour isotope profiles, we estimate the liquid water isotope ratios at the cloud base. Considering this water as the initial rain, the raindrop evaporation fractions are estimated using the observed oxygen and hydrogen isotope ratios of Kozhikode surface rain samples. The estimated fractions show strong positive correlations with the isotope biases (R2 = 0.60 and 0.66). This suggests that lower estimates of raindrop evaporation could be responsible for the rain isotope biases in these two GCMs.

The premise of temperature effect playing in regions with complex moisture sources – Evidence from high-resolution water vapor isotopes

Atmospheric precipitation has widely taken part in the formation and accumulation of many geological archives. In these processes, the isotope fractionations are tightly related to the meteorological factors, such as temperature and precipitation amount. Therefore, precipitation isotopes are usually regarded as an effective proxy to reconstruct the paleoclimate and paleoenvironmental changes. However, with the deepening of research, it is found that the temperature effect is not suitable for all regions and time scales, especially in regions with complex moisture sources. Moreover, the applicable mechanism of the temperature effect is still unclear. Here, we chose Xi’an as our study site, because it is located in the transition zone of the East Asian Summer Monsoon (EASM) and the westerlies with relatively complex water vapor sources. Through conducting a three-year, high-resolution, relatively continuous water vapor isotopic composition measurements in Xi’an, we defined the duration of the EASM here, which normally starts in June and is over in September. By separating the δ18Op into monsoon and non-monsoon seasons, we found a significant temperature effect in the non-monsoon seasons, with a correlation coefficient of 0.54, while no temperature effect is observed in the monsoon seasons and the whole year. Our water vapor isotopic results suggested that the establishment of temperature effect of precipitation isotopes should follow two prerequisites: 1. single water vapor source; 2. large temperature gradient. Our results specify the applicable conditions of temperature effect and potentially help us to better use precipitation isotopes to understand the paleo-temperature variations in different regions.

Characteristics of water vapor isotopes and moisture sources for short-duration heavy rainfall events in Nanjing, eastern China

Short-duration heavy rainfall causes severe urban flooding, threatening urban security and socio-economic development. The lower reaches of the Yangtze River are one of the regions with the highest frequency of short-duration heavy rainfalls in China. In this study, hourly stable isotope compositions in water vapor (δ18Ov and d-excessv) are analyzed for nine short-duration heavy rainfall events during the summer monsoon season (June to September) from 2013 to 2021 in Nanjing, eastern China. The circulation patterns that lead to these events can be divided into four types: tropical cyclone, low-pressure vortex, cold front, and western North Pacific subtropical high. During these events, δ18Ov is enriched, ranging from −18.8 ‰ to −13.7 ‰ with a pre-storm increasing trend. This is largely caused by strongly isolated meso- and small-scale convections and the close proximity of oceanic moisture sources. The d-excessv shows three different variation patterns during rainfall events: increasing, decreasing, and irregular fluctuations. They each correspond to the increasing contribution of terrestrial moisture from eastern China, proximal oceanic moisture from China’s offshore waters (including the South China Sea, East China Sea, and Yellow Sea), and the mixing influence of these two moisture sources. Multiple patterns of the d-excessv variations reflect both importance of terrestrial moisture from eastern China and oceanic moisture from China’s offshore waters for summer short-duration heavy rainfall events in eastern China.