Importance of rain evaporation and continental convection in the tropical water cycle

Water isotope ratios reflect convection intensity rather than rain type proportions in the pantropics

Against the traditional view, a recently published theory argued that isotope ratios are higher in convective precipitation but lower in stratiform precipitation and proposed that isotope ratios reflect rain type proportions. This theory has been widely cited despite some early reservations. Whether the theory represents a faithful reflection of signals of water isotope ratios remains unclear. Here, we reassess its validity from different timescales and broader observations from the pantropics. Unexpectedly, our findings contradict the theory on daily, monthly, and even annual timescales. Pantropical precipitation isotope ratios remain strongly correlated to convection intensity but are independent of rain type proportions because stratiform precipitation isotope ratios cover a large range of values. We find that the theory has many serious weaknesses related to preferential data selection and suggest that new theories need to be validated at more locations on different timescales before gaining widespread acceptance.

Tropical mountain ice core δ18O: A Goldilocks indicator for global temperature change

Very high tropical alpine ice cores provide a distinct paleoclimate record for climate changes in the middle and upper troposphere. However, the climatic interpretation of a key proxy, the stable water oxygen isotopic ratio in ice cores (δ18Oice), remains an outstanding problem. Here, combining proxy records with climate models, modern satellite measurements, and radiative-convective equilibrium theory, we show that the tropical δ18Oice is an indicator of the temperature of the middle and upper troposphere, with a glacial cooling of -7.35° ± 1.1°C (66% CI). Moreover, it severs as a "Goldilocks-type" indicator of global mean surface temperature change, providing the first estimate of glacial stage cooling that is independent of marine proxies as -5.9° ± 1.2°C. Combined with all estimations available gives the maximum likelihood estimate of glacial cooling as -5.85° ± 0.51°C.

Atmospheric-moisture-induced polyacrylate hydrogels for hybrid passive cooling

Heat stress is being exacerbated by global warming, jeopardizing human and social sustainability. As a result, reliable and energy-efficient cooling methods are highly sought-after. Here, we report a polyacrylate film fabricated by self-moisture-absorbing hygroscopic hydrogel for efficient hybrid passive cooling. Using one of the lowest-cost industrial materials (e.g., sodium polyacrylate), we demonstrate radiative cooling by reducing solar heating with high solar reflectance (0.93) while maximizing thermal emission with high mid-infrared emittance (0.99). Importantly, the manufacturing process utilizes only atmospheric moisture and requires no additional chemicals or energy consumption, making it a completely green process. Under sunlight illumination of 800 W m-2, the surface temperature of the film was reduced by 5 °C under a partly cloudy sky observed at Buffalo, NY. Combined with its hygroscopic feature, this film can simultaneously introduce evaporative cooling that is independent of access to the clear sky. The hybrid passive cooling approach is projected to decrease global carbon emissions by 118.4 billion kg/year compared to current air-conditioning facilities powered by electricity. Given its low-cost raw materials and excellent molding feature, the film can be manufactured through simple and cost-effective roll-to-roll processes, making it suitable for future building construction and personal thermal management needs.

The role of continental evapotranspiration on water vapour isotopic variability in the troposphere

Rainforests play an important role in hydrological and carbon cycles, both at regional and global scales. They pump large quantities of moisture from the soil to the atmosphere and are major rainfall hotspots of the world. Satellite-observed stable water isotope ratios have played an essential role in determining sources of moisture in the atmosphere. Satellites provide information about the processes involving vapour transport in different zones of the world, identifying sources of rainfall and distinguishing moisture transport in monsoonal systems. This paper focuses on major rainforests of the world (Southern Amazon, Congo and Northeast India) to understand the role of continental evapotranspiration in influencing tropospheric water vapour. We have used satellite measurements of 1H2H16O/1H216O from Atmospheric InfraRed Sounder (AIRS), evapotranspiration (ET), solar-induced fluorescence (SIF), precipitation (P), atmospheric reanalysis-derived moisture flux convergence (MFC) and wind to discern the role of ET in influencing water vapour isotopes. A global map of the correlation between δ2Hv and ET-P flux indicates that densely vegetated regions in the tropics show the highest positive correlation (r > 0.5). Using mixing models and observations of specific humidity and isotopic ratio over these forested regions, we discern the source of moisture in pre-wet and wet seasons.

A century and a half precipitation oxygen isoscape for China generated using data fusion and bias correction

The precipitation oxygen isotopic composition is a useful environmental tracer for climatic and hydrological studies. However, accurate and high-resolution precipitation oxygen isoscapes are currently lacking in China. In this study, a precipitation oxygen isoscape in China for a period of 148 years is built by integrating observed and iGCMs-simulated isotope compositions using an optimal hybrid approach of three data fusion and two bias correction methods. The temporal and spatial resolutions of the isoscape are monthly and 50-60 km, respectively. Results show that the Convolutional Neural Networks (CNN) fusion method performs the best (correlation coefficient larger than 0.95 and root mean square error smaller than 1‰), and the other two data fusion methods perform slightly better than the bias correction methods. Thus, the isoscape is generated by using the CNN fusion method for the common 1969-2007 period and by using the bias correction methods for remaining years. The generated isoscape, which shows similar spatio-temporal distributions to observations, is reliable and useful for providing strong support for tracking atmospheric and hydrological processes.

New insights into diffusive kinetic fractionation during liquid condensation under supersaturated environment: an alternative approach for isotope tagging of ground-level water vapour

Stable water isotopes in ground-level vapour are key to estimating water exchange between geospheres. Their sampling, however, is limited to laser-absorption spectrometers and satellite observations, having inherent shortcomings. This study investigates diffusive kinetic fractionation during liquid condensation under supersaturated environment, providing a cost-effective, reliable way of sampling ground-level vapour isotopes (¹⁸O, ²H). Experiments were undertaken at three locations in India with 'liquid' samples collected from condensation of ambient air at 0°C. Simultaneously, pristine 'vapour' was sampled via cryogenic-trapping using liquid nitrogen-alcohol slush at -78°C. The 'liquid' condensed under supersaturation was progressively more depleted in ¹⁸O, and less enriched in ²H than expected under equilibrium fractionation, with an increasing degree of supersaturation expressed as saturation index (S_i). This study revealed: (1) S_i, molecular density, Rh, T together control the extent of isotopic kinetic fractionation. (2) The presence of diffusive concentration gradient inhibits the flow of heavier isotopes during liquid condensation. (3) The stochastic nature of the process cannot be explained using a physics-based model alone. The artificial neural network model is hence deployed to sample δ¹⁸O (δ ²H) within -0.24 ± 1.79‰ (0.53 ± 11.23 ‰) of true value. (4) The approach can be extended to ground-validate isotope-enabled general circulation models and satellite observations.

Range-resolved detection of boundary layer stable water vapor isotopologues using a ground-based 1.98 µm differential absorption LIDAR

This paper presents a first demonstration of range-resolved differential absorption LIDAR (DIAL) measurements of the water vapor main isotopologue H₂ ¹⁶O and the less abundant semi-heavy water isotopologue HD¹⁶O with the aim of determining the isotopic ratio. The presented Water Vapor and Isotope Lidar (WaVIL) instrument is based on a parametric laser source emitting nanosecond pulses at 1.98 µm and a direct-detection receiver utilizing a commercial InGaAs PIN photodiode. Vertical profiles of H₂ ¹⁶O and HD¹⁶O were acquired in the planetary boundary layer in the suburban Paris region up to a range of 1.5 km. For time averaging over 25 min, the achieved precision in the retrieved water vapor mixing ratio is 0.1 g kg^-1 (2.5% relative error) at 0.4 km above ground level (a.g.l.) and 0.6 g kg^-1 (20%) at 1 km a.g.l. for 150 m range bins along the LIDAR line of sight. For HD¹⁶O, weaker absorption has to be balanced with coarser vertical resolution (600 m range bins) in order to achieve similar relative precision. From the DIAL measurements of H₂ ¹⁶O and HD¹⁶O, the isotopic abundance δD was estimated as -51‰ at 0.4 km above the ground and -119‰ in the upper part of the boundary layer at 1.3 km a.g.l. Random and systematic errors are discussed in the form of an error budget, which shows that further instrumental improvements are required on the challenging path towards DIAL-profiling of the isotopic abundance with range resolution and precision suitable for water cycle studies.

Inverse altitude effect disputes the theoretical foundation of stable isotope paleoaltimetry

Stable isotope paleoaltimetry that reconstructs paleoelevation requires stable isotope (δD or δ¹⁸O) values to follow the altitude effect. Some studies found that the δD or δ¹⁸O values of surface isotopic carriers in some regions increase with increasing altitude, which is defined as an “inverse altitude effect” (IAE). The IAE directly contradicts the basic theory of stable isotope paleoaltimetry. However, the causes of the IAE remain unclear. Here, we explore the mechanisms of the IAE from an atmospheric circulation perspective using δD in water vapor on a global scale. We find that two processes cause the IAE: (1) the supply of moisture with higher isotopic values from distant source regions, and (2) intense lateral mixing between the lower and mid-troposphere along the moisture transport pathway. Therefore, we caution that the influences of those two processes need careful consideration for different mountain uplift stages before using stable isotope palaeoaltimetry.

The “inverse altitude effect” (IEA) directly contradicts the basic theory of stable isotope paleoaltimetry. This study explores the causes of the IAE from an atmospheric circulation perspective using δD in water vapor on the global scale.

Pleistocene drivers of Northwest African hydroclimate and vegetation

Savanna ecosystems were the landscapes for human evolution and are vital to modern Sub-Saharan African food security, yet the fundamental drivers of climate and ecology in these ecosystems remain unclear. Here we generate plant-wax isotope and dust flux records to explore the mechanistic drivers of the Northwest African monsoon, and to assess ecosystem responses to changes in monsoon rainfall and atmospheric pCO₂. We show that monsoon rainfall is controlled by low-latitude insolation gradients and that while increases in precipitation are associated with expansion of grasslands into desert landscapes, changes in pCO₂ predominantly drive the C₃/C₄ composition of savanna ecosystems.

Isotopic constraints confirm the significant role of microbial nitrogen oxides emissions from the land and ocean environment

Nitrogen oxides (NO_x, the sum of nitric oxide (NO) and N dioxide (NO₂)) emissions and deposition have increased markedly over the past several decades, resulting in many adverse outcomes in both terrestrial and oceanic environments. However, because the microbial NO_x emissions have been substantially underestimated on the land and unconstrained in the ocean, the global microbial NO_x emissions and their importance relative to the known fossil-fuel NO_x emissions remain unclear. Here we complied data on stable N isotopes of nitrate in atmospheric particulates over the land and ocean to ground-truth estimates of NO_x emissions worldwide. By considering the N isotope effect of NO_x transformations to particulate nitrate combined with dominant NO_x emissions in the land (coal combustion, oil combustion, biomass burning and microbial N cycle) and ocean (oil combustion, microbial N cycle), we demonstrated that microbial NO_x emissions account for 24 ± 4%, 58 ± 3% and 31 ± 12% in the land, ocean and global environment, respectively. Corresponding amounts of microbial NO_x emissions in the land (13.6 ± 4.7 Tg N yr⁻¹), ocean (8.8 ± 1.5 Tg N yr⁻¹) and globe (22.5 ± 4.7 Tg N yr⁻¹) are about 0.5, 1.4 and 0.6 times on average those of fossil-fuel NO_x emissions in these sectors. Our findings provide empirical constraints on model predictions, revealing significant contributions of the microbial N cycle to regional NO_x emissions into the atmospheric system, which is critical information for mitigating strategies, budgeting N deposition and evaluating the effects of atmospheric NO_x loading on the world.

Amazonian terrestrial water balance inferred from satellite-observed water vapor isotopes

Atmospheric humidity and soil moisture in the Amazon forest are tightly coupled to the region’s water balance, or the difference between two moisture fluxes, evapotranspiration minus precipitation (ET-P). However, large and poorly characterized uncertainties in both fluxes, and in their difference, make it challenging to evaluate spatiotemporal variations of water balance and its dependence on ET or P. Here, we show that satellite observations of the HDO/H₂O ratio of water vapor are sensitive to spatiotemporal variations of ET-P over the Amazon. When calibrated by basin-scale and mass-balance estimates of ET-P derived from terrestrial water storage and river discharge measurements, the isotopic data demonstrate that rainfall controls wet Amazon water balance variability, but ET becomes important in regulating water balance and its variability in the dry Amazon. Changes in the drivers of ET, such as above ground biomass, could therefore have a larger impact on soil moisture and humidity in the dry (southern and eastern) Amazon relative to the wet Amazon.

The evolution of the Amazon forest is tightly coupled to its terrestrial water balance. Here, the authors show that forest biomass changes in the Amazon are a driver of the spatiotemporal variation of evapotranspiration, and such changes could have a larger impact on water availability in the dry regions (southern, eastern) of the Amazon.

Magnetic-actuated "capillary container" for versatile three-dimensional fluid interface manipulation

Fluid interfaces are omnipresent in nature. Engineering the fluid interface is essential to study interfacial processes for basic research and industrial applications. However, it remains challenging to precisely control the fluid interface because of its fluidity and instability. Here, we proposed a magnetic-actuated "capillary container" to realize three-dimensional (3D) fluid interface creation and programmable dynamic manipulation. By wettability modification, 3D fluid interfaces with predesigned sizes and geometries can be constructed in air, water, and oils. Multiple motion modes were realized by adjusting the container's structure and magnetic field. Besides, we demonstrated its feasibility in various fluids by performing selective fluid collection and chemical reaction manipulations. The container can also be encapsulated with an interfacial gelation reaction. Using this process, diverse free-standing 3D membranes were produced, and the dynamic release of riboflavin (vitamin B₂) was studied. This versatile capillary container will provide a promising platform for open microfluidics, interfacial chemistry, and biomedical engineering.

Understanding water and energy fluxes in the Amazonia: Lessons from an observation-model intercomparison

Tropical forests are an important part of global water and energy cycles, but the mechanisms that drive seasonality of their land-atmosphere exchanges have proven challenging to capture in models. Here, we (1) report the seasonality of fluxes of latent heat (LE), sensible heat (H), and outgoing short and longwave radiation at four diverse tropical forest sites across Amazonia-along the equator from the Caxiuanã and Tapajós National Forests in the eastern Amazon to a forest near Manaus, and from the equatorial zone to the southern forest in Reserva Jaru; (2) investigate how vegetation and climate influence these fluxes; and (3) evaluate land surface model performance by comparing simulations to observations. We found that previously identified failure of models to capture observed dry-season increases in evapotranspiration (ET) was associated with model overestimations of (1) magnitude and seasonality of Bowen ratios (relative to aseasonal observations in which sensible was only 20%-30% of the latent heat flux) indicating model exaggerated water limitation, (2) canopy emissivity and reflectance (albedo was only 10%-15% of incoming solar radiation, compared to 0.15%-0.22% simulated), and (3) vegetation temperatures (due to underestimation of dry-season ET and associated cooling). These partially compensating model-observation discrepancies (e.g., higher temperatures expected from excess Bowen ratios were partially ameliorated by brighter leaves and more interception/evaporation) significantly biased seasonal model estimates of net radiation (R_n ), the key driver of water and energy fluxes (LE ~ 0.6 R_n and H ~ 0.15 R_n ), though these biases varied among sites and models. A better representation of energy-related parameters associated with dynamic phenology (e.g., leaf optical properties, canopy interception, and skin temperature) could improve simulations and benchmarking of current vegetation-atmosphere exchange and reduce uncertainty of regional and global biogeochemical models.

Isotopic dynamics of precipitation and its regional and local drivers in a plateau inland lake basin, Southwest China

Shrinkage of plateau lakes under climate strength has drawn growing attention. Because of its intricate implication to hydro-meteorological condition and climate system, stable isotopes in precipitation (e.g. δ²H_p and δ¹⁸O_p) provide us a powerful tool to understand the climate-hydrologic dynamics in shrinking lakes. However, how the regional atmospheric circulation, moisture sources and local fractionation processes drive isotopic variability from temporal to spatial scale has rarely been reported for remote plateau lakes. Hence, we collected a total of 98 rainfall samples at the south and the north shores of Chenghai lake, Yunnan-Guizhou Plateau to study the potential driving forces of precipitation isotope variability during the wet season of 2019. Based on backward trajectories of air masses obtained from HYSPLIT model, 68% of moisture came from δ¹⁸O depleted ocean (Indian Ocean, Bay of Bengal, South China Sea and Pacific Ocean), and the rainout process promoted the isotopic depletion when moisture arrived at the study basin. Evapotranspiration increased the heavy isotope ratios in precipitation originated from continents (northern China inland and western continents). The temporal dynamics of δ¹⁸O_p and δ²H_p were in phase with the convection activities intensity underlined the influence from large-scale atmospheric circulation. Local meteorological factors played a secondary role in isotope variability. Precipitation amount-effect strongly affected isotope ratios while mild anti-temperature effect was observed at daily scale. Interestingly, the rainfall isotope ratios showed different mechanisms in govern at lake south shore and north shore, with a distance of 19 km in between. This south-to-north difference can be explained by either lower 1.03% sub-evaporation in the south shore or 7% of recycled moisture contributing to precipitation in the north shore. Our findings discover the driving forces for δ¹⁸O_p variation and provide solid interpretations for hydro-climate change in Southwest China.

Rain Evaporation, Snow Melt, and Entrainment at the Heart of Water Vapor Isotopic Variations in the Tropical Troposphere, According to Large-Eddy Simulations and a Two-Column Model

We aim at developing a simple model as an interpretative framework for the water vapor isotopic variations in the tropical troposphere over the ocean. We use large-eddy simulations of disorganized convection in radiative-convective equilibrium to justify the underlying assumptions of this simple model, to constrain its input parameters and to evaluate its results. We also aim at interpreting the depletion of the water vapor isotopic composition in the lower and midtroposphere as precipitation increases, which is a salient feature in tropical oceanic observations. This feature constitutes a stringent test on the relevance of our interpretative framework. Previous studies, based on observations or on models with parameterized convection, have highlighted the roles of deep convective and mesoscale downdrafts, rain evaporation, rain-vapor diffusive exchanges, and mixing processes. The interpretative framework that we develop, valid in case of disorganized convection, is a two-column model representing the net ascent in clouds and the net descent in the environment. We show that the mechanisms for depleting the troposphere as the precipitation rate increases all stem from the higher tropospheric relative humidity. First, when the relative humidity is larger, less snow sublimates before melting and a smaller fraction of rain evaporates. Both effects lead to more depleted rain evaporation and eventually more depleted water vapor. This mechanism dominates in regimes of large-scale ascent. Second, the entrainment of dry air into clouds reduces the vertical isotopic gradient and limits the depletion of tropospheric water vapor. This mechanism dominates in regimes of large-scale descent.

Evaporation of Sessile Droplets of Polyelectrolyte/Surfactant Mixtures on Silicon Wafers

The wetting and evaporation behavior of droplets of aqueous solutions of mixtures of poly(diallyldimethylammonium chloride) solution, PDADMAC, with two different anionic surfactants, sodium laureth sulfate, SLES, and sodium N-lauroyl N-methyl taurate, SLMT, were studied in terms of the changes of the contact angle θ and contact length L of sessile droplets of the mixtures on silicon wafers at a temperature of 25 °C and different relative humidities in the range of 30–90%. The advancing contact angle θa was found to depend on the surfactant concentration, independent of the relative humidity, with the mixtures containing SLES presenting improved wetting behaviors. Furthermore, a constant droplet contact angle was not observed during evaporation due to pinning of the droplet at the coffee-ring that was formed. The kinetics for the first evaporation stage of the mixture were independent of the relative humidity, with the evaporation behavior being well described in terms of the universal law for evaporation.

Generating Electricity from Natural Evaporation Using PVDF Thin Films Incorporating Nanocomposite Materials

Natural evaporation has recently come under consideration as a viable source of renewable energy. Demonstrations of the validity of the concept have been reported for devices incorporating carbon-based nanocomposite materials. In this study, we investigated the possibility of using polymer thin films to generate electricity from natural evaporation. We considered a polymeric system based on polyvinylidene fluoride (PVDF). Porous PVDF films were created by incorporating a variety of nanocomposite materials into the polymer structure through a simple mixing procedure. Three nanocomposite materials were considered: carbon nanotubes, graphene oxide, and silica. The evaporation-induced electricity generation was confirmed experimentally under various ambient conditions. Among the nanocomposite materials considered, mesoporous silica (SBA-15) was found to outperform the other two materials in terms of open-circuit voltage, and graphene oxide generated the highest short-circuit current. It was found that the nanocomposite material content in the PVDF film plays an important role: on the one hand, if particles are too few in number, the number of channels will be insufficient to support a strong capillary flow; on the other hand, an excessive number of particles will suppress the flow due to excessive water absorption underneath the surface. We show that the device can be modeled as a simple circuit powered by a current source with excellent agreement between the theoretical predictions and experimental data.

Hydroclimate footprint of pan-Asian monsoon water isotope during the last deglaciation

Oxygen isotope speleothem records exhibit coherent variability over the pan-Asian summer monsoon (AM) region. The hydroclimatic representation of these oxygen isotope records for the AM, however, has remained poorly understood. Here, combining an isotope-enabled Earth system model in transient experiments with proxy records, we show that the widespread AM δ18Oc signal during the last deglaciation (20 to 11 thousand years ago) is accompanied by a continental-scale, coherent hydroclimate footprint, with spatially opposite signs in rainfall. This footprint is generated as a dynamically coherent response of the AM system primarily to meltwater forcing and secondarily to insolation forcing and is further reinforced by atmospheric teleconnection. Hence, widespread δ18Op depletion in the AM region is accompanied by a northward migration of the westerly jet and enhanced southwesterly monsoon wind, as well as increased rainfall from South Asia (India) to northern China but decreased rainfall in southeast China.

Vapor isotopic evidence for the worsening of winter air quality by anthropogenic combustion-derived water

Water vapor emitted from anthropogenic combustion for winter heating in northern China may exacerbate air pollution. This hypothesis is of considerable scientific and environmental interest. We conducted a multiyear sampling campaign of air vapor isotope compositions and associated atmospheric data from the city of Xi’an, located in an enclosed basin in northwestern China. We found that the fraction of combustion-derived water vapor increases with increasing relative humidity and with the concentration of particulate matter with an aerodynamic diameter less than 2.5 μm in polluted conditions based on field observation, isotopic analysis, and numerical simulation. Our results demonstrated that combustion-derived water is nontrivial when considering energy policy for improving air quality.

Anthropogenic combustion-derived water (CDW) may accumulate in an airshed due to stagnant air, which may further enhance the formation of secondary aerosols and worsen air quality. Here we collected three-winter-season, hourly resolution, water-vapor stable H and O isotope compositions together with atmospheric physical and chemical data from the city of Xi’an, located in the Guanzhong Basin (GZB) in northwestern China, to elucidate the role of CDW in particulate pollution. Based on our experimentally determined water vapor isotope composition of the CDW for individual and weighted fuels in the basin, we found that CDW constitutes 6.2% of the atmospheric moisture on average and its fraction is positively correlated with [PM_2.5] (concentration of particulate matter with an aerodynamic diameter less than 2.5 μm) as well as relative humidity during the periods of rising [PM_2.5]. Our modeling results showed that CDW added additional average 4.6 μg m⁻³ PM_2.5 during severely polluted conditions in the GZB, which corresponded to an average 5.1% of local anthropogenic [PM_2.5] (average at ∼91.0 μg m⁻³). Our result is consistent with the proposed positive feedback between the relative humidity and a moisture sensitive air-pollution condition, alerting to the nontrivial role of CDW when considering change of energy structure such as a massive coal-to-gas switch in household heating in winter.

Mechanical Forcing of Convection by Cold Pools: Collisions and Energy Scaling

Forced mechanical lifting through cold pool gust fronts can trigger new convection and, as previous work highlights, is enhanced when cold pools collide. However, as shown by conceptual models, the organization of the convective cloud field emerging from two versus three colliding cold pools differs strongly. In idealized dry large-eddy simulations we therefore compare collisions between two and three cold pools. The triggering likelihood is quantified in terms of the cumulative vertical mass flux of boundary layer air and the instantaneous updraft strength, generated at the cold pool gust fronts. We find that cold pool expansion can be well described by initial potential energy alone. Cold pool expansion monotonically slows but shows an abrupt transition between an axisymmetric and a broken-symmetric state mirrored by a sudden drop in expansion speed. We characterize these two dynamic regimes by two distinct power law exponents and explain the transition by the onset of "lobe-and-cleft" instabilities at the cold pool head. Two-cold pool collisions produce the strongest instantaneous updrafts in the lower boundary layer, which we expect to be important in environments with strong convective inhibition. Three-cold pool collisions generate weaker but deeper updrafts and the strongest cumulative mass flux and are thus predicted to induce the largest midlevel moistening, which has been identified as a precursor for the transition from shallow to deep convection. Combined, our findings may help decipher the role of cold pools in spatially organizing convection and precipitation.

Seasonality of moisture supplies to precipitation over the Third Pole: a stable water isotopic perspective

This study integrated isotopic composition in precipitation at 50 stations on and around the Tibetan Plateau (TP) and demonstrated the distinct seasonality of isotopic composition in precipitation across the study period. The potential effect of water vapor isotopes on precipitation isotopes is studied by comparing the station precipitation data with extensive isotopic patterns in atmospheric water vapor, revealing the close linkage between the two. The analysis of contemporary water vapor transport and potential helps confirm the different mechanisms behind precipitation isotopic compositions in different areas, as the southern TP is more closely related to large-scale atmospheric circulation such as local Hadley and summer monsoon circulations during other seasons than winter, while the northern TP is subject to the westerly prevalence and advective moisture supply and precipitation processes. The new data presented in this manuscript also enrich the current dataset for the study of precipitation isotopes in this region and together provide a valuable database for verification of the isotope-integrated general circulation model and explanation of related physical processes.

What Controls the Water Vapor Isotopic Composition Near the Surface of Tropical Oceans? Results From an Analytical Model Constrained by Large-Eddy Simulations

The goal of this study is to understand the mechanisms controlling the isotopic composition of the water vapor near the surface of tropical oceans, at the scale of about a hundred kilometers and a month. In the tropics, it has long been observed that the isotopic compositions of rain and vapor near the surface are more depleted when the precipitation rate is high. This is called the "amount effect." Previous studies, based on observations or models with parameterized convection, have highlighted the roles of deep convective and mesoscale downdrafts and rain evaporation. But the relative importance of these processes has never been quantified. We hypothesize that it can be quantified using an analytical model constrained by large-eddy simulations. Results from large-eddy simulations confirm that the classical amount effect can be simulated only if precipitation rate changes result from changes in the large-scale circulation. We find that the main process depleting the water vapor compared to the equilibrium with the ocean is the fact that updrafts stem from areas where the water vapor is more enriched. The main process responsible for the amount effect is the fact that when the large-scale ascent increases, isotopic vertical gradients are steeper, so that updrafts and downdrafts deplete the subcloud layer more efficiently.

Daily precipitation isotope variation in Midwestern United States: Implication for hydroclimate and moisture source

Stable isotopes of oxygen (δ¹⁸O) and hydrogen (δD) in precipitation can be used as dual conservative tracers in the hydrologic cycle and help to understand hydrological and atmospheric processes. Although long-term monthly precipitation global isotope datasets are available in some locations, currently there are limited daily precipitation isotope data, particularly in the Midwest region of the USA. In this study we report a daily precipitation δ¹⁸O and δD dataset from March 2014-December 2017 in Dayton, Ohio, the USA. The daily δ¹⁸O and δD vary from -28.0 to 0.4‰, and -214.0 to 9.0‰ respectively. The data exhibit strong seasonality with lower δ¹⁸O and δD values in the winter and higher values in the summer. The precipitation isotopic values are mainly controlled by temperature, and show no correlation with precipitation amount and relative humidity. However, δ¹⁸O-temperature relationship varies among different seasons. The correlation is the strongest in winter (R² = 0.56), weaker in spring (R² = 0.28) and fall (R² = 0.24), and almost non-existent in summer (R² = 0.1). The slope values also vary with highest value in winter (0.68‰/^OC) and much smaller in other seasons. The HYSPLIT back trajectory analyses show that Pacific, Gulf of Mexico, Arctic and Continental moistures are the main sources for southwestern Ohio with different seasonal contributions. The isotopic compositions of precipitation from different sources show small intra-season variations but large seasonal variability. Our daily-resolved dataset provides new insights into the main controls on the isotopic composition of precipitation and its seasonal variations, which could help to understand atmospheric processes and enable their proper use in interpretation of paleoclimate proxies, particularly those with seasonal bias.

Earth's water reservoirs in a changing climate

Progress towards achieving a quantitative understanding of the exchanges of water between Earth's main water reservoirs is reviewed with emphasis on advances accrued from the latest advances in Earth Observation from space. These exchanges of water between the reservoirs are a result of processes that are at the core of important physical Earth-system feedbacks, which fundamentally control the response of Earth's climate to the greenhouse gas forcing it is now experiencing, and are therefore vital to understanding the future evolution of Earth's climate. The changing nature of global mean sea level (GMSL) is the context for discussion of these exchanges. Different sources of satellite observations that are used to quantify ice mass loss and water storage over continents, how water can be tracked to its source using water isotope information and how the waters in different reservoirs influence the fluxes of water between reservoirs are described. The profound influence of Earth's hydrological cycle, including human influences on it, on the rate of GMSL rise is emphasized. The many intricate ways water cycle processes influence water exchanges between reservoirs and thus sea-level rise, including disproportionate influences by the tiniest water reservoirs, are emphasized.

Stable Isotopes of Water and Nitrate for the Identification of Groundwater Flowpaths: A Review

Nitrate contamination in stream water and groundwater is a serious environmental problem that arises in areas of high agricultural activities or high population density. It is therefore important to identify the source and flowpath of nitrate in water bodies. In recent decades, the dual isotope analysis (δ15N and δ18O) of nitrate has been widely applied to track contamination sources by taking advantage of the difference in nitrogen and oxygen isotope ratios for different sources. However, transformation processes of nitrogen compounds can change the isotopic composition of nitrate due to the various redox processes in the environment, which often makes it difficult to identify contaminant sources. To compensate for this, the stable water isotope of the H2O itself can be used to interpret the complex hydrological and hydrochemical processes for the movement of nitrate contaminants. Therefore, the present study aims at understanding the fundamental background of stable water and nitrate isotope analysis, including isotope fractionation, analytical methods such as nitrate concentration from samples, instrumentation, and the typical ranges of δ15N and δ18O from various nitrate sources. In addition, we discuss hydrograph separation using the oxygen and hydrogen isotopes of water in combination with the nitrogen and oxygen isotopes of nitrate to understand the relative contributions of precipitation and groundwater to stream water. This study will assist in understanding the groundwater flowpaths as well as tracking the sources of nitrate contamination using the stable isotope analysis in combination with nitrate and water.

Relating Moisture Transport to Stable Water Vapor Isotopic Variations of Ambient Wintertime along the Western Coast of Korea

Atmospheric water vapor transfers energy, causes meteorological phenomena and can be modified by climate change in the western coast region of Korea. In Korea, previous studies have utilized precipitation isotopic compositions in the water cycle for correlations with climate variables, but there are few studies using water vapor isotopes. In this study, water vapor was directly collected by a cryogenic method, analyzed for its isotopic compositions, and used to trace the origin and history of water vapor in the western coastal region of Korea during the winter of 2015/2016. Our analysis of paired mixing ratios with water vapor isotopes can explain the mechanism of water vapor isotopic fractionation and the extent of the mixing of two different air masses. We confirm the correlation between water vapor isotopes and meteorological parameters such as temperature, relative humidity, and specific humidity. The main water vapor in winter was derived from the continental polar region of northern Asia and showed an enrichment of 10 per mil (δ18O) through the evaporation of the Yellow Sea. Our results demonstrate the utility of using ground-based isotope observations as a complementary resource for constraining isotope-enabled Global Circulation Model in future investigations of atmospheric water cycles. These measurements are expected to support climate studies (speleothem) in the west coast region of Korea.

Onset of summer monsoon in Northeast India is preceded by enhanced transpiration

Variations in isotopic composition of water vapor in the atmosphere is an important indicator of the processes within the hydrological cycle. Isotopic signature of water vapor and precipitation can be helpful in partitioning evaporation and transpiration fluxes. It is well known that transpiration from forested regions supplies a significant amount of vapor to the atmosphere in monsoon and post-monsoon seasons. Here, we utilize observations from Tropospheric Emission Spectrometer (TES), Atmospheric Infra-Red Sounder (AIRS) and simulation models to ascertain that transpiration is dominant in the forests of Northeast India (NE) during pre-monsoon season. Our results show an increase in δD of 78.0 ± 7.1‰ and in specific humidity of 3.1 ± 0.2 g kg-1 during the pre-monsoon months of April-May compared to January-February. In the monsoon months of July-August, δD reduces by 53.0 ± 6.5‰ albeit the specific humidity increases by 3.4 ± 0.2 g kg-1. Using joint observations of specific humidity and isotope ratio in lower troposphere, we discern the moisture sources over NE India in pre-monsoon and monsoon seasons and posit the role of transpiration in continental recycling during pre-monsoon season.

Stable Isotope Reveals Tap Water Source under Different Water Supply Modes in the Eastern Margin of the Qinghai–Tibet Plateau

Based on 1260 tap water samples gathered monthly and 136 surface water samples collected seasonally in the eastern margin of the Qinghai–Tibet Plateau, the local tap water line, the basic spatiotemporal characteristics of tap water isotopes, and their indication for water source under different water supply modes were discussed, linking the local tap water supply and water source information. A new tap water isotopes data set based on dense sampling sites was established, which was reliable for the analysis of tap water isotope features, tap water supply management, and tap water sources. The main conclusions are: (1) The local tap water lines in Gannan and Longnan are δ2H = (7.06 ± 0.17) δ18O + (3.24 ± 1.75) (r2 = 0.81, p < 0.01) and δ2H = (5.66 ± 0.09) δ18O + (−8.12 ± 0.82) (r2 = 0.82, p < 0.01), respectively. (2) The annual mean δ2H and δ18O in tap water show an increasing trend from southwest to northeast. The seasonal differences of δ2H and δ18O in tap water in Gannan and Longnan are small. (3) The correlation of tap water isotopes with those in main source water is high, while that of isotopes in tap water with those in non-water source is low. Under the central water supply mode by local tap water company, tap water isotopes in Gannan where groundwater is the direct water source show weak connection with those in surface water and precipitation, and those in tap water in Longnan with surface water as main source water reveal good connection with isotopes in surface water. Under mixed water supply modes, tap water isotopes indicate that surface water is the main tap water source in Gannan and Longnan with multiple water sources.

Stable Isotope Ratios in Tap Water of a Riverside City in a Semi-Arid Climate: An Application to Water Source Determination

Stable isotopes (e.g., δ2H and δ18O) in tap water are important tools to understand the local climate or environment background, water sources and the state of regional water supply. Based on 242 tap water samples, 35 precipitation samples and 24 surface water samples gathered in the urban area of Lanzhou, the basic spatiotemporal characteristics of isotopes in tap water, their connection with isotopes in other water bodies and change during the process from raw water to tap water are discussed in detail, combining the information of local tap water supply and water source. It can provide reliable help for understanding the isotope characteristics of local tap water, regional water supply management and determination of tap water source of in a small area. Except for the establishment of a new data set of isotopes in tap water with complete time series and uniform spatial distribution of sampling sites, other results show that: (1) The Local Tap Water Line (LTWL) of Lanzhou is δ2H = (6.03 ± 0.57) δ18O + (−8.63 ± 5.44) (r2 = 0.41, p < 0.01). (2) For seasonal variations, δ2H and δ18O in tap water both are higher in autumn and lower in spring. The diurnal and daily variations of isotopes in tap water are not large. As for spatial variations, the monthly mean values of δ2H and δ18O in tap water at each sampling site show little difference. The isotopes in tap water collected from one single sampling site can be considered as a representative for isotopes in tap water in the area with a single tap water source. (3) Isotopes in tap water show weak connection with precipitation isotopes, but exhibit good connection (consistent seasonal variation, similar numerical range, small numerical difference and high correlation) with isotopes in surface water, which is the direct water source. Isotopes in water change little from raw water to tap water. Isotopic composition of tap water in Lanzhou can be used as a representative of isotopes in surface water.

Revised paleoaltimetry data show low Tibetan Plateau elevation during the Eocene

Paleotopographic reconstructions of the Tibetan Plateau based on stable isotope paleoaltimetry methods conclude that most of the Plateau's current elevation was already reached by the Eocene, ~40 million years ago. However, changes in atmospheric and hydrological dynamics affect oxygen stable isotopes in precipitation and may thus bias such reconstructions. We used an isotope-equipped general circulation model to assess the influence of changing Eocene paleogeography and climate on paleoelevation estimates. Our simulations indicate that stable isotope paleoaltimetry methods are not applicable in Eocene Asia because of a combination of increased convective precipitation, mixture of air masses, and widespread aridity. Rather, a model-data comparison suggests that the Tibetan Plateau only reached low to moderate (less than 3000 meters) elevations during the Eocene, reconciling oxygen isotope data with other proxies.

A Review on the Methods for Observing the Substance and Energy Exchange between Atmosphere Boundary Layer and Free Troposphere

Atmosphere boundary layer (ABL or BL) acts as a pivotal part in the climate by regulating the vertical exchange of moisture, aerosol, trace gases and energy between the earth surface and free troposphere (FT). However, compared with research on the exchange between earth surface and ABL, there are fewer researches on the exchange between ABL and FT, especially when it comes to the quantitative measurement of vertical exchange flux between them. In this paper, a number of various methodologies for investigating the exchange of the substance and energy between ABL and FT are reviewed as follows: (1) methods to obtain entrainment rate, which include method by investigating the height of inversion layer, method of flux-jump, estimating with dataset from the ASTEX Lagrangian Experiments and method of using satellite observations and Microwave Imager; (2) mass budget method, which can yield quantitative measurements of exchange flux between ABL and FT; (3) qualitative measurements: method based on Rayleigh distillation and mixing processes, methods of ground-based remote sensing and airborne tracer-tracer relationship/ratio method.

Hydrological interactions between oases and water vapor transportation in the Tarim Basin, northwestern China

The spatial evolution of the oasis cold-wet effects in Tarim Basin, northwestern China, are explored quantitatively by using the China Meteorological Forcing Dataset, with spatial and temporal resolution of 0.1 degree and 3 hours, respectively, and the hydrological interactions between the oases and water vapor transfer are discussed to uncover the influence of oases on regional precipitation. The results reveal that the annual precipitation exhibits an increasing trend from desert toward oasis at all four oases. However, the cold effect is not the dominating factor increasing precipitation, since the maximum increasing rate of precipitation is accompanied by the minimum decreasing rate of temperature. Indeed, water vapor transportation is more important than the cold effect. The maximum promotion of precipitation is observed in western basin, where the water vapor transfer follows the gradients of decreasing temperature and increasing humidity. Conversely, the minimum promotion in southwestern basin results from water vapor transportation following the temperature increasing. Therefore, the transfer of water vapor and its interactions with local surface conditions determine the precipitation in oasis areas. Understanding these processes is crucial to exploring the formation and spatial layout of oases, which is helpful for preventing desertification and protecting the fragile oasis ecosystem.

Isotopic study of intraseasonal variations of plant transpiration: an alternative means to characterise the dry phases of monsoon

The isotopic characteristics of plant transpired water are strongly controlled by soil evaporation process, primarily by relative humidity. The monsoon system is characterised by large variability of several atmospheric parameters; the primary one being the rainfall, which in turn, modulates the relative humidity. Due to the strong dependency of transpiration on relative humidity, it is expected that this process would vary in accordance with the active and break periods of the monsoon season, which are known to produce cycles of humid and relatively dry phases during a monsoon season. To study the transpiration process, an experiment was conducted wherein rainwater and transpired water were collected from a few plants and analyzed for their isotopic ratios during the summer monsoon seasons of 2016 and 2017. The difference between the isotopic characteristics of the transpired water and rain water is expected to be nominally positive, however, a large variability was observed. This difference is found to be high (low) during the reduced (enhanced) humidity conditions and varies in tandem with the break and active phases of the monsoon season. This characteristic feature may thus be used to delineate the dry and wet phases of monsoon on local to regional scale.

Stable isotopes in water vapor and rainwater over Indian sector of Southern Ocean and estimation of fraction of recycled moisture

Stable Hydrogen and Oxygen isotopic composition of water vapor, rainwater and surface seawater show a distinct trend across the latitude over the Southern Indian Ocean. Our observations on isotopic composition of surface seawater, water vapor and rainwater across a transect covering the tropical Indian Ocean to the regions of the Southern Ocean showed a strong latitudinal dependency; characterized by the zonal process of evaporation and precipitation. The sampling points were spread across diverse zones of SST, wind speed and rainfall regimes. The observed physical parameters such as sea surface temperature, wind speed and relative humidity over the oceanic regions were used in a box model calculation across the latitudes to predict the isotopic composition of water vapor under equilibrium and kinetic conditions, and compared with results from isotope enabled global spectral model. Further, we obtained the average fraction of recycled moisture across the oceanic transect latitudes as 13.4 ± 7.7%. The values of recycled fraction were maximum at the vicinity of the Inter Tropical Convergence Zone (ITCZ), while the minimum values were recorded over the region of subsidence and evaporation, at the Northern and Southern latitudes of the ITCZ. These estimates are consistent with the earlier reported recyling values.

Negative soil moisture-precipitation feedback in dry and wet regions

Soil moisture-precipitation (SM-P) feedback significantly influences the terrestrial water and energy cycles. However, the sign of the feedback and the associated physical mechanism have been debated, leaving a research gap regarding global water and climate changes. Based on Koster’s framework, we estimate SM-P feedback using satellite remote sensing and ground observation data sets. Methodologically, the sign of the feedback is identified by the correlation between monthly soil moisture and next-month precipitation. The physical mechanism is investigated through coupling precipitation and soil moisture (P-SM), soil moisture ad evapotranspiration (SM-E) and evapotranspiration and precipitation (E-P) correlations. Our results demonstrate that although positive SM-P feedback is predominant over land, non-negligible negative feedback occurs in dry and wet regions. Specifically, 43.75% and 40.16% of the negative feedback occurs in the arid and humid climate zones. Physically, negative SM-P feedback depends on the SM-E correlation. In dry regions, evapotranspiration change is soil moisture limited. In wet regions, evapotranspiration change is energy limited. We conclude that the complex SM-E correlation results in negative SM-P feedback in dry and wet regions, and the cause varies based on the environmental and climatic conditions.

Emerging Technologies and Synergies for Airborne and Space-Based Measurements of Water Vapor Profiles

A deeper understanding of how clouds will respond to a warming climate is one of the outstanding challenges in climate science. Uncertainties in the response of clouds, and particularly shallow clouds, have been identified as the dominant source of the discrepancy in model estimates of equilibrium climate sensitivity. As the community gains a deeper understanding of the many processes involved, there is a growing appreciation of the critical role played by fluctuations in water vapor and the coupling of water vapor and atmospheric circulations. Reduction of uncertainties in cloud-climate feedbacks and convection initiation as well as improved understanding of processes governing these effects will result from profiling of water vapor in the lower troposphere with improved accuracy and vertical resolution compared to existing airborne and space-based measurements. This paper highlights new technologies and improved measurement approaches for measuring lower tropospheric water vapor and their expected added value to current observations. Those include differential absorption lidar and radar, microwave occultation between low-Earth orbiters, and hyperspectral microwave remote sensing. Each methodology is briefly explained, and measurement capabilities as well as the current technological readiness for aircraft and satellite implementation are specified. Potential synergies between the technologies are discussed, actual examples hereof are given, and future perspectives are explored. Based on technical maturity and the foreseen near-mid-term development path of the various discussed measurement approaches, we find that improved measurements of water vapor throughout the troposphere would greatly benefit from the combination of differential absorption lidar focusing on the lower troposphere with passive remote sensors constraining the upper-tropospheric humidity.

Steady Method for the Analysis of Evaporation Dynamics

Droplet evaporation is an important phenomenon governing many man-made and natural processes. Characterizing the rate of evaporation with high accuracy has attracted the attention of numerous scientists over the past century. Traditionally, researchers have studied evaporation by observing the change in the droplet size in a given time interval. However, the transient nature coupled with the significant mass-transfer-governed gas dynamics occurring at the droplet three-phase contact line makes the classical method crude. Furthermore, the intricate balance played by the internal and external flows, evaporation kinetics, thermocapillarity, binary-mixture dynamics, curvature, and moving contact lines makes the decoupling of these processes impossible with classical transient methods. Here, we present a method to measure the rate of evaporation of spatially and temporally steady droplets. By utilizing a piezoelectric dispenser to feed microscale droplets (R ≈ 9 μm) to a larger evaporating droplet at a prescribed frequency, we can both create variable-sized droplets on any surface and study their evaporation rate by modulating the piezoelectric droplet addition frequency. Using our steady technique, we studied water evaporation of droplets having base radii ranging from 20 to 250 μm on surfaces of different functionalities (45° ≤ θ_a,app ≤ 162°, where θ_a,app is the apparent advancing contact angle). We benchmarked our technique with the classical unsteady method, showing an improvement of 140% in evaporation rate measurement accuracy. Our work not only characterizes the evaporation dynamics on functional surfaces but also provides an experimental platform to finally enable the decoupling of the complex physics governing the ubiquitous droplet evaporation process.

Potential for natural evaporation as a reliable renewable energy resource

About 50% of the solar energy absorbed at the Earth’s surface drives evaporation, fueling the water cycle that affects various renewable energy resources, such as wind and hydropower. Recent advances demonstrate our nascent ability to convert evaporation energy into work, yet there is little understanding about the potential of this resource. Here we study the energy available from natural evaporation to predict the potential of this ubiquitous resource. We find that natural evaporation from open water surfaces could provide power densities comparable to current wind and solar technologies while cutting evaporative water losses by nearly half. We estimate up to 325 GW of power is potentially available in the United States. Strikingly, water’s large heat capacity is sufficient to control power output by storing excess energy when demand is low, thus reducing intermittency and improving reliability. Our findings motivate the improvement of materials and devices that convert energy from evaporation.

The evaporation of water represents an alternative source of renewable energy. Building on previous models of evaporation, Cavusoglu et al. show that the power available from this natural resource is comparable to wind and solar power, yet it does not suffer as much from varying weather conditions.

Rainforest-initiated wet season onset over the southern Amazon

Although it is well established that transpiration contributes much of the water for rainfall over Amazonia, it remains unclear whether transpiration helps to drive or merely responds to the seasonal cycle of rainfall. Here, we use multiple independent satellite datasets to show that rainforest transpiration enables an increase of shallow convection that moistens and destabilizes the atmosphere during the initial stages of the dry-to-wet season transition. This shallow convection moisture pump (SCMP) preconditions the atmosphere at the regional scale for a rapid increase in rain-bearing deep convection, which in turn drives moisture convergence and wet season onset 2-3 mo before the arrival of the Intertropical Convergence Zone (ITCZ). Aerosols produced by late dry season biomass burning may alter the efficiency of the SCMP. Our results highlight the mechanisms by which interactions among land surface processes, atmospheric convection, and biomass burning may alter the timing of wet season onset and provide a mechanistic framework for understanding how deforestation extends the dry season and enhances regional vulnerability to drought.

The Fossil Record of Plant Physiology and Development—What Leaves Can Tell Us

Plants provide unmatched opportunities to evaluate long debated evolutionary patterns in terms of the detailed biology of the fossil organisms. Leaves serve here as an example of how those advantages can be exploited. Over the history of vascular plants, three important transitions in leaf evolution—the origin of laminate leaves, the progressive loss of seed plant morphological diversity, and the evolution of more angiosperm-like leaves—also represent major shifts in leaf development and physiology. These transitions often occurred in parallel in different lineages, such as the evolution of marginal growth in each of at least four independent origins of laminate leaves during the Devonian and Carboniferous. Each also entailed dramatic reorganizations of leaf hydraulics. For example, the length of the finest distributary vein order varies from up to tens of centimeters down to hundreds of microns in successive groups of dominant seed plants. Angiosperms impose an additional trend upon these patterns with the evolution of their uniquely high vein densities. Vein density strongly influences and can provide a proxy for other physiological characteristics, such as assimilation and transpiration rates. The large increase in transpiration capacity accompanying the evolution of angiosperm leaf traits may even play an important role in feeding precipitation and thereby altering local climate.

Stable isotopes in atmospheric water vapor and applications to the hydrologic cycle

The measurement and simulation of water vapor isotopic composition has matured rapidly over the last decade, with long-term datasets and comprehensive modeling capabilities now available. Theories for water vapor isotopic composition have been developed by extending the theories that have been used for the isotopic composition of precipitation to include a more nuanced understanding of evaporation, large-scale mixing, deep convection, and kinetic fractionation. The technologies for in-situ and remote sensing measurements of water vapor isotopic composition have developed especially rapidly over the last decade, with discrete water vapor sampling methods, based on mass spectroscopy, giving way to laser spectroscopic methods and satellite- and ground-based infrared absorption techniques. The simulation of water vapor isotopic composition has evolved from General Circulation Model (GCM) methods for simulating precipitation isotopic composition to sophisticated isotope-enabled microphysics schemes using higher-order moments for water- and ice-size distributions. The incorporation of isotopes into GCMs has enabled more detailed diagnostics of the water cycle and has led to improvements in its simulation. The combination of improved measurement and modeling of water vapor isotopic composition opens the door to new advances in our understanding of the atmospheric water cycle, in processes ranging from the marine boundary layer, through deep convection and tropospheric mixing, and into the water cycle of the stratosphere. Finally, studies of the processes governing modern water vapor isotopic composition provide an improved framework for the interpretation of paleoclimate proxy records of the hydrological cycle.

Isotopic modeling of the sub-cloud evaporation effect in precipitation

In dry and warm environments sub-cloud evaporation influences the falling raindrops modifying their final stable isotopic content. During their descent from the cloud base towards the ground surface, through the unsaturated atmosphere, hydrometeors are subjected to evaporation whereas the kinetic fractionation results to less depleted or enriched isotopic signatures compared to the initial isotopic composition of the raindrops at cloud base. Nowadays the development of Generalized Climate Models (GCMs) that include isotopic content calculation modules are of great interest for the isotopic tracing of the global hydrological cycle. Therefore the accurate description of the underlying processes affecting stable isotopic content can improve the performance of iso-GCMs. The aim of this study is to model the sub-cloud evaporation effect using a) mixing and b) numerical isotope evaporation models. The isotope-mixing evaporation model simulates the isotopic enrichment (difference between the ground and the cloud base isotopic composition of raindrops) in terms of raindrop size, ambient temperature and relative humidity (RH) at ground level. The isotopic enrichment (Δδ) varies linearly with the evaporated raindrops mass fraction of the raindrop resulting to higher values at drier atmospheres and for smaller raindrops. The relationship between Δδ and RH is described by a 'heat capacity' model providing high correlation coefficients for both isotopes (R(2)>80%) indicating that RH is an ideal indicator of the sub-cloud evaporation effect. Vertical distribution of stable isotopes in falling raindrops is also investigated using a numerical isotope-evaporation model. Temperature and humidity dependence of the vertical isotopic variation is clearly described by the numerical isotopic model showing an increase in the isotopic values with increasing temperature and decreasing RH. At an almost saturated atmosphere (RH=95%) sub-cloud evaporation is negligible and the isotopic composition hardly changes even at high temperatures while at drier and warm conditions the enrichment of (18)Ο reaches up to 20‰, depending on the raindrop size and the initial meteorological conditions.

Vapor hydrogen and oxygen isotopes reflect water of combustion in the urban atmosphere

Anthropogenic modification of the water cycle involves a diversity of processes, many of which have been studied intensively using models and observations. Effective tools for measuring the contribution and fate of combustion-derived water vapor in the atmosphere are lacking, however, and this flux has received relatively little attention. We provide theoretical estimates and a first set of measurements demonstrating that water of combustion is characterized by a distinctive combination of H and O isotope ratios. We show that during periods of relatively low humidity and/or atmospheric stagnation, this isotopic signature can be used to quantify the concentration of water of combustion in the atmospheric boundary layer over Salt Lake City. Combustion-derived vapor concentrations vary between periods of atmospheric stratification and mixing, both on multiday and diurnal timescales, and respond over periods of hours to variations in surface emissions. Our estimates suggest that up to 13% of the boundary layer vapor during the period of study was derived from combustion sources, and both the temporal pattern and magnitude of this contribution were closely reproduced by an independent atmospheric model forced with a fossil fuel emissions data product. Our findings suggest potential for water vapor isotope ratio measurements to be used in conjunction with other tracers to refine the apportionment of urban emissions, and imply that water vapor emissions associated with combustion may be a significant component of the water budget of the urban boundary layer, with potential implications for urban climate, ecohydrology, and photochemistry.

Variations in stable hydrogen and oxygen isotopes in atmospheric water vapor in the marine boundary layer across a wide latitude range

The newly-developed cavity ring-down laser absorption spectroscopy analyzer with special calibration protocols has enabled the direct measurement of atmospheric vapor isotopes at high spatial and temporal resolution. This paper presents real-time hydrogen and oxygen stable isotope data for atmospheric water vapor above the sea surface, over a wide range of latitudes spanning from 38°N to 69°S. Our results showed relatively higher values of δ(18)O and δ(2)H in the subtropical regions than those in the tropical and high latitude regions, and also a notable decreasing trend in the Antarctic coastal region. By combining the hydrogen and oxygen isotope data with meteoric water line and backward trajectory model analysis, we explored the kinetic fractionation caused by subsiding air masses and related saturated vapor pressure in the subtropics, and the evaporation-driven kinetic fractionation in the Antarctic region. Simultaneous observations of meteorological and marine variables were used to interpret the isotopic composition characteristics and influential factors, indicating that d-excess is negatively correlated with humidity across a wide range of latitudes and weather conditions worldwide. Coincident with previous studies, d-excess is also positively correlated with sea surface temperature and air temperature (Tair), with greater sensitivity to Tair. Thus, atmospheric vapor isotopes measured with high accuracy and good spatial-temporal resolution could act as informative tracers for exploring the water cycle at different regional scales. Such monitoring efforts should be undertaken over a longer time period and in different regions of the world.