Linking Lagrangian model simulations with stable water isotope measurements in Arctic weather systems.

2021 ◽  
Author(s):  
Aina Johannessen ◽  
Alena Dekhtyareva ◽  
Andrew Seidl ◽  
Harald Sodemann
Keyword(s):  
Water Cycle ◽  
Lagrangian Model ◽  
Water Isotopes ◽  
Stable Water Isotopes ◽  
Atmospheric Water ◽  
Model Simulations ◽  
Evaporation Source ◽  
Isotope Measurements ◽  
Water Isotope ◽  
Eulerian Models

<p>Transport of water from an evaporation source towards a precipitation sink is the essence of the atmospheric water cycle. However, there are significant challenges with the representation of the atmospheric water cycle in models. For example, incomplete representation of sub-grid scale processes like evaporation, mixing or precipitation can lead to substantial model errors. Here we investigate the combined use of Lagrangian and Eulerian models and in-situ observations of stable water isotopes to reduce such sources of model error. The atmospheric water cycle in the Nordic Seas during cold air outbreaks (CAOs) is confined to a limited area, and thus may be used as a natural laboratory for hydrometeorological studies. We apply Lagrangian and Eulerian models together with observations taken during the ISLAS2020 field campaign in the Arctic in spring 2020 for characterising source-sink relationships in the water cycle. During the field campaign, we observed an alternating sequence of cold air outbreaks (CAO) and warm air intrusions (WAI) over the key measurement sites of Svalbard and northern Norway. Thereby, meteorological and stable water isotope measurements have been performed at multiple sites both upstream and downstream of the CAOs and WAIs. The Lagrangian model FLEXPART has been run with the input data from the regional convection-permitting numerical weather prediction model AROME Arctic at 2.5 km resolution to investigate transport patterns. The combination of observations and model simulations allows us to quantify the connection between source and sink for different weather systems, as well as the link between large-scale transport and stable water isotopes. Findings will lead to a better understanding of processes in the water cycle and the degree of conservation of isotopic signals during transport. This study may also serve as a guideline on how to evaluate the performance of Lagrangian transport models using stable water isotope measurements, and on how to detect constraints for quantifying the transport route and evaporation source from stable water isotope measurements for future work, including an aircraft campaign planned in 2021.</p>

scholarly journals Exploring water cycle dynamics by sampling multiple stable water isotope pools in a developed landscape in Germany

2016 ◽  
Vol 20 (9) ◽  
pp. 3873-3894 ◽  
Author(s):  
Natalie Orlowski ◽  
Philipp Kraft ◽  
Jakob Pferdmenges ◽  
Lutz Breuer
Keyword(s):  
Land Use ◽  
Soil Water ◽  
Hydrological Model ◽  
Water Cycle ◽  
Water Isotopes ◽  
Stable Water Isotopes ◽  
Age Dynamics ◽  
Isotopic Signatures ◽  
Spatially Distributed ◽  
Water Isotope

Abstract. A dual stable water isotope (δ2H and δ18O) study was conducted in the developed (managed) landscape of the Schwingbach catchment (Germany). The 2-year weekly to biweekly measurements of precipitation, stream, and groundwater isotopes revealed that surface and groundwater are isotopically disconnected from the annual precipitation cycle but showed bidirectional interactions between each other. Apparently, snowmelt played a fundamental role for groundwater recharge explaining the observed differences to precipitation δ values. A spatially distributed snapshot sampling of soil water isotopes at two soil depths at 52 sampling points across different land uses (arable land, forest, and grassland) revealed that topsoil isotopic signatures were similar to the precipitation input signal. Preferential water flow paths occurred under forested soils, explaining the isotopic similarities between top- and subsoil isotopic signatures. Due to human-impacted agricultural land use (tilling and compression) of arable and grassland soils, water delivery to the deeper soil layers was reduced, resulting in significant different isotopic signatures. However, the land use influence became less pronounced with depth and soil water approached groundwater δ values. Seasonally tracing stable water isotopes through soil profiles showed that the influence of new percolating soil water decreased with depth as no remarkable seasonality in soil isotopic signatures was obvious at depths > 0.9 m and constant values were observed through space and time. Since classic isotope evaluation methods such as transfer-function-based mean transit time calculations did not provide a good fit between the observed and calculated data, we established a hydrological model to estimate spatially distributed groundwater ages and flow directions within the Vollnkirchener Bach subcatchment. Our model revealed that complex age dynamics exist within the subcatchment and that much of the runoff must has been stored for much longer than event water (average water age is 16 years). Tracing stable water isotopes through the water cycle in combination with our hydrological model was valuable for determining interactions between different water cycle components and unravelling age dynamics within the study area. This knowledge can further improve catchment-specific process understanding of developed, human-impacted landscapes.

scholarly journals Exploring water cycle dynamics through sampling multitude stable water isotope pools in a small developed landscape of Germany

2015 ◽  
Vol 12 (2) ◽  
pp. 1809-1853 ◽  
Author(s):  
N. Orlowski ◽  
P. Kraft ◽  
L. Breuer
Keyword(s):  
Land Use ◽  
Soil Water ◽  
Agricultural Land ◽  
Water Cycle ◽  
Mean Transit Time ◽  
Arable Land ◽  
Water Isotopes ◽  
Stable Water Isotopes ◽  
Isotopic Signatures ◽  
Water Isotope

Abstract. Conducting a dual stable water isotope (δ2H and δ18O) study in the developed landscape of the Schwingbach catchment (Germany) helped to unravel connectivity and disconnectivity between the different water cycle components. The two-year weekly to biweekly measurements of precipitation, stream, and groundwater isotopes revealed that surface and groundwater are decoupled from the annual precipitation cycle but showed bidirectional interactions between each other. Seasonal variations based on temperature effects were observed in the precipitation signal but neither reflected in stream nor in groundwater isotopic signatures. Apparently, snowmelt played a fundamental role for groundwater recharge explaining the observed differences to precipitation δ-values. A spatially distributed snapshot sampling of soil water isotopes in two soil depths at 52 sampling points across different land uses (arable land, forest, and grassland) revealed that top soil isotopic signatures were similar to the precipitation input signal. Preferential water flow paths occurred under forested soils explaining the isotopic similarities between top and subsoil isotopic signatures. Due to human-impacted agricultural land use (tilling and compression) of arable and grassland soils, water delivery to the deeper soil layers was reduced, resulting in significant different isotopic signatures. However, the land use influence smoothed out with depth and soil water approached groundwater δ-values. Seasonally tracing stable water isotopes through soil profiles showed that the influence of new percolating soil water decreased with depth as no remarkable seasonality in soil isotopic signatures was obvious at depth > 0.9 m and constant values were observed through space and time. Little variation in individual isotope time series of stream and groundwater restricted the use of classical isotope hydrology techniques e.g. mean transit time estimation or hydrograph separation. Still, tracing stable water isotopes through the water cycle was valuable for determining interactions between different water cycle components and gaining catchment specific process understanding in a developed, human-impacted landscape.

Identification of source-sink relationships in southern Africa by stable water isotopes analysis and Lagrangian moisture source diagnostics

2020 ◽  
Author(s):  
Marielle Geppert ◽  
Stephan Pfahl ◽  
Ulrich Struck ◽  
Ingo Kirchner ◽  
Elisha Shemang ◽  
...  
Keyword(s):  
Isotopic Composition ◽  
Southern Africa ◽  
Specific Humidity ◽  
Water Isotopes ◽  
Stable Water Isotopes ◽  
Moisture Source ◽  
Moisture Sources ◽  
Transport Patterns ◽  
Isotope Measurements ◽  
Water Isotope

<p>Many palaeoclimate reconstructions are based on the fact that stable water isotopes are conserved in different highly resolved paleo-archives such as ice cores or calcium carbonates. Stable water isotopes are tracers of moisture in the atmosphere because they record information about evaporation and condensation processes during the transport of air parcels. These processes cause isotopic fractionation that leads to isotopic enrichment or depletion. The isotopic composition of precipitation is strongly correlated with altitude above sea level, distance to the coast and local surface air temperature. Knowledge on the source and transport of moisture is thus crucial for the interpretation of stable isotopes in precipitation and in palaeo-archives.<br>Studies analysing the linkage between stable water isotope measurements and moisture sources in southern Africa are scarce. Yet, as changes in the transport pattern can influence precipitation patterns and amounts, in a semi-arid region like southern Africa that is threatened by droughts, this knowledge is of particular interest. Thus, the aims of this study are (1) to reveal the principal moisture source areas and transport routes of specific target areas in southern Africa, (2) to assess the influence of different transport patterns on the isotopic composition of precipitation and by this (3) to create a modern analogue for palaeoclimate studies in this region.<br>About 200 water samples, mainly from headwaters of rivers, but also from precipitation events, springs and lakes, were collected throughout southern Africa and the stable water isotope composition (δ<sup>2</sup>H and δ<sup>18</sup>O) was analysed. To detect moisture sources for this set of isotope measurements, backward air parcel trajectories were calculated from the sample location, using the LAGRANTO tool based on ERA5 reanalysis data. Variations in specific humidity along the trajectories were then used to detect moisture uptake.<br>The analysis reveals main transport patterns related to the Intertropical Convergence Zone and easterly winds as well as the effects of topographical forcing, which is, for example, very pronounced above Lesotho. The results provide detailed insights into the relationships between atmospheric circulation and δ<sup>2</sup>H and δ<sup>18</sup>O values of precipitation over southern Africa, which is a prerequisite for the interpretation of isotopic records that are used for palaeoclimatic reconstructions.</p>

scholarly journals A novel approach to process brittle ice for continuous flow analysis of stable water isotopes

2018 ◽  
Vol 64 (244) ◽  
pp. 289-299 ◽  
Author(s):  
REBECCA L. PYNE ◽  
ELIZABETH D. KELLER ◽  
SILVIA CANESSA ◽  
NANCY A. N. BERTLER ◽  
ALEX R. PYNE ◽  
...  
Keyword(s):  
Continuous Flow ◽  
Ice Core ◽  
Flow Analysis ◽  
Ice Cores ◽  
Water Isotopes ◽  
Stable Water Isotopes ◽  
Climate History ◽  
Continuous Flow Analysis ◽  
Isotope Measurements ◽  
Water Isotope

ABSTRACTBrittle ice, which occurs in all intermediate-depth and deep ice cores retrieved from high-latitude regions, presents a challenge for high-resolution measurements of water isotopes, gases, ions and other quantities conducted with continuous flow analysis (CFA). We present a novel method of preserving brittle ice for CFA stable water isotope measurements using data from a new ice core recovered by the Roosevelt Island Climate Evolution (RICE) project. Modest modification of the drilling technique and the accommodation of non-horizontal fractures (‘slanted breaks’) in processing led to a substantial improvement in the percentage of brittle ice analyzed with CFA (87.8%). Whereas traditional processing methods remove entire fragmented pieces of ice, our method allowed the incorporation of a total of 3 m of ice (1% of the 261 m of brittle ice and ~1300 years of climate history) that otherwise would not have been available for CFA. Using the RICE stable water isotope CFA dataset, we demonstrate the effect of slanted breaks and analyze the resulting smoothing of the data with real and simulated examples. Our results suggest that retaining slanted breaks are a promising technique for preserving brittle ice material for CFA stable water isotope measurements.

scholarly journals Stable water isotope tracing through hydrological models for disentangling runoff generation processes at the hillslope scale

2014 ◽  
Vol 18 (10) ◽  
pp. 4113-4127 ◽  
Author(s):  
D. Windhorst ◽  
P. Kraft ◽  
E. Timbe ◽  
H.-G. Frede ◽  
L. Breuer
Keyword(s):  
Water Isotopes ◽  
Richards Equation ◽  
Advective Transport ◽  
Stable Water Isotopes ◽  
Runoff Generation ◽  
Modeling Framework ◽  
Mixing Processes ◽  
Study Region ◽  
Modeling Experiment ◽  
Water Isotope

Abstract. Hillslopes are the dominant landscape components where incoming precipitation becomes groundwater, streamflow or atmospheric water vapor. However, directly observing flux partitioning in the soil is almost impossible. Hydrological hillslope models are therefore being used to investigate the processes involved. Here we report on a modeling experiment using the Catchment Modeling Framework (CMF) where measured stable water isotopes in vertical soil profiles along a tropical mountainous grassland hillslope transect are traced through the model to resolve potential mixing processes. CMF simulates advective transport of stable water isotopes 18O and 2H based on the Richards equation within a fully distributed 2-D representation of the hillslope. The model successfully replicates the observed temporal pattern of soil water isotope profiles (R2 0.84 and Nash–Sutcliffe efficiency (NSE) 0.42). Predicted flows are in good agreement with previous studies. We highlight the importance of groundwater recharge and shallow lateral subsurface flow, accounting for 50 and 16% of the total flow leaving the system, respectively. Surface runoff is negligible despite the steep slopes in the Ecuadorian study region.

scholarly journals Stable water isotopes as a tool to investigate tropospheric moisture transport pathways over the eastern subtropical North Atlantic

2020 ◽  
Author(s):  
Fabienne Dahinden ◽  
Franziska Aemisegger ◽  
Sabine Barthlott ◽  
Emanuel Christner ◽  
Christoph Dyroff ◽  
...  
Keyword(s):  
North Atlantic ◽  
Moisture Transport ◽  
Water Cycle ◽  
Phase Changes ◽  
Water Isotopes ◽  
Stable Water Isotopes ◽  
Physical Processes ◽  
Air Masses ◽  
Saharan Air Layer ◽  
Sahel Region

<div> <div> <div> <p>The subtropical atmospheric water cycle is a key component in the climate system. Free-tropospheric humidity and low-level cloud cover over the subtropical oceans strongly affect the global radiative balance via the greenhouse and albedo effects. However, the complex interaction of dynamical processes controlling the subtropical tropospheric moisture budget is still not fully understood. Stable water isotopes have proven to be highly useful to investigate the physical mechanisms involved in the atmospheric water cycle. These natural tracers of water phase changes capture the moist diabatic history experienced by air parcels. Additionally, due to the distinct fingerprints of air masses with different origin, the isotopic composition of water vapor can further provide information about atmospheric processes that do not involve phase changes, for instance, turbulent mixing or large-scale water vapor transport. To enhance the understanding of the mechanisms controlling the subtropical tropospheric humidity, we performed dedicated high-resolution simulations with the isotope-enabled regional weather and climate prediction model COSMOiso. Comparison with ground-based remote sensing (Fourier transform infrared spectroscopy) and aircraft-based in situ isotope observations from the project MUSICA enables us to evaluate and constrain the representation of relevant physical processes in the model.</p> <p>Our simulations confirm the current state of knowledge about the contrasting moisture transport conditions over the eastern subtropical North Atlantic, resulting from an interplay between humid, isotopically enriched air primarily coming from Africa on the one hand and dry, depleted air mainly originating from the upper-level extratropical North Atlantic on the other hand. Additionally, we show that North African air masses that are affected by the Saharan heat low (SHL) and air masses which come from the Sahel region further south are associated with a distinct isotope signature. This difference is mainly due to the fact that air masses from the Sahel region have experienced moist convection and cloud processing, whereas the Saharan air layer is a well-mixed air mass with a more homogenous isotope composition. We systematically assess the dynamical drivers behind these contrasting conditions. In particular, we investigate the importance of the SHL dynamics on moistening the free troposphere over the eastern subtropical North Atlantic. In summer, the SHL induces low-level convergence of air masses from different sources, which are then convectively lifted to higher altitudes and are eventually transported within the Saharan air layer across the North Atlantic, where they mix with dry, descending free tropospheric air. Detailed analysis of isotopic signals along kinematic back- trajectories of different air masses arriving over the Canary Islands allows to disentangle governing physical processes and relevant moisture sources that affect the free tropospheric humidity. The adopted Lagrangian isotope perspective notably enhances our understanding of air mass mixing and offers a sound interpretation of the free tropospheric humidity and isotopic variability on time scales of hours to days in contrasting atmospheric conditions over the eastern subtropical North Atlantic.</p> </div> </div> </div>

scholarly journals Stable water isotope tracing through hydrological models for disentangling runoff generation processes at the hillslope scale

2014 ◽  
Vol 11 (5) ◽  
pp. 5179-5216 ◽  
Author(s):  
D. Windhorst ◽  
P. Kraft ◽  
E. Timbe ◽  
H.-G. Frede ◽  
L. Breuer
Keyword(s):  
Water Isotopes ◽  
Richards Equation ◽  
Advective Transport ◽  
Stable Water Isotopes ◽  
Runoff Generation ◽  
Modeling Framework ◽  
Mixing Processes ◽  
Study Region ◽  
Modeling Experiment ◽  
Water Isotope

Abstract. Hillslopes are the dominant landscape components where incoming precipitation is transferred to become groundwater, streamflow or atmospheric water vapor. However, directly observing flux partitioning in the soil is almost impossible. Hydrological hillslope models are therefore being used to investigate the involved processes. Here we report on a modeling experiment using the Catchment Modeling Framework (CMF) where measured stable water isotopes in vertical soil profiles along a tropical mountainous grassland hillslope transect are traced through the model to resolve potential mixing processes. CMF simulates advective transport of stable water isotopes 18O and 2H based on the Richards equation within a fully distributed 2-D representation of the hillslope. The model successfully replicates the observed temporal pattern of soil water isotope profiles (R2 0.84 and NSE 0.42). Predicted flows are in good agreement with previous studies. We highlight the importance of groundwater recharge and shallow lateral subsurface flow, accounting for 50% and 16% of the total flow leaving the system, respectively. Surface runoff is negligible despite the steep slopes in the Ecuadorian study region.

scholarly journals A new interpretative framework for below-cloud effects on stable water isotopes in vapour and rain

2019 ◽  
Vol 19 (2) ◽  
pp. 747-765 ◽  
Author(s):  
Pascal Graf ◽  
Heini Wernli ◽  
Stephan Pfahl ◽  
Harald Sodemann
Keyword(s):  
Phase Space ◽  
Isotopic Composition ◽  
Ambient Air ◽  
Water Isotopes ◽  
Cloud Base ◽  
Stable Water Isotopes ◽  
Near Surface ◽  
Frontal Passage ◽  
Isotope Measurements ◽  
Interpretative Framework

Abstract. Raindrops interact with water vapour in ambient air while sedimenting from the cloud base to the ground. They constantly exchange water molecules with the environment and, in sub-saturated air, they evaporate partially or entirely. The latter of these below-cloud processes is important for predicting the resulting surface rainfall amount. It also influences the boundary layer profiles of temperature and moisture through evaporative latent cooling and humidity changes. However, despite its importance, it is very difficult to quantify this process from observations. Stable water isotopes provide such information, as they are influenced by both rain evaporation and equilibration (i.e. the exchange of isotopes between raindrops and ambient air). This study elucidates this option by introducing a novel interpretative framework for stable water isotope measurements performed simultaneously at high temporal resolution in both near-surface vapour and rain. We refer to this viewing device as the ΔδΔd-diagram, which shows the isotopic composition (δ2H, d-excess) of equilibrium vapour from precipitation samples relative to the ambient vapour. It is shown that this diagram facilitates the diagnosis of below-cloud processes and their effects on the isotopic composition of vapour and rain since equilibration and evaporation lead to different pathways in the two-dimensional phase space of the ΔδΔd-diagram, as investigated with a series of sensitivity experiments with an idealized below-cloud interaction model. The analysis of isotope measurements for a specific cold front in central Europe shows that below-cloud processes lead to distinct and temporally variable imprints on the isotope signal in surface rain. The influence of evaporation on this signal is particularly strong during periods with a weak precipitation rate. After the frontal passage, the near-surface atmospheric layer is characterized by higher relative humidity, which leads to weaker below-cloud evaporation. Additionally, a lower melting layer after the frontal passage reduces time for exchange between vapour and rain and leads to weaker equilibration. Measurements from four cold frontal events reveal a surprisingly similar slope of ΔdΔδ=-0.30 in the phase space, indicating a potentially characteristic signature of below-cloud processes for this type of rain event.

scholarly journals On the discrepancy of HCl processing in the core of the wintertime polar vortices

2018 ◽  
Vol 18 (12) ◽  
pp. 8647-8666 ◽  
Author(s):  
Jens-Uwe Grooß ◽  
Rolf Müller ◽  
Reinhold Spang ◽  
Ines Tritscher ◽  
Tobias Wegner ◽  
...  
Keyword(s):  
Cosmic Rays ◽  
Polar Vortex ◽  
Galactic Cosmic Rays ◽  
Lagrangian Model ◽  
The Arctic ◽  
Late Winter ◽  
Model Simulations ◽  
Ozone Loss ◽  
The Antarctic ◽  
Eulerian Models

Abstract. More than 3 decades after the discovery of the ozone hole, the processes involved in its formation are believed to be understood in great detail. Current state-of-the-art models can reproduce the observed chemical composition in the springtime polar stratosphere, especially regarding the quantification of halogen-catalysed ozone loss. However, we report here on a discrepancy between simulations and observations during the less-well-studied period of the onset of chlorine activation. During this period, which in the Antarctic is between May and July, model simulations significantly overestimate HCl, one of the key chemical species, inside the polar vortex during polar night. This HCl discrepancy is also observed in the Arctic. The discrepancy exists in different models to varying extents; here, we discuss three independent ones, the Chemical Lagrangian Model of the Stratosphere (CLaMS) as well as the Eulerian models SD-WACCM (the specified dynamics version of the Whole Atmosphere Community Climate Model) and TOMCAT/SLIMCAT. The HCl discrepancy points to some unknown process in the formulation of stratospheric chemistry that is currently not represented in the models. We characterise the HCl discrepancy in space and time for the Lagrangian chemistry–transport model CLaMS, in which HCl in the polar vortex core stays about constant from June to August in the Antarctic, while the observations indicate a continuous HCl decrease over this period. The somewhat smaller discrepancies in the Eulerian models SD-WACCM and TOMCAT/SLIMCAT are also presented. Numerical diffusion in the transport scheme of the Eulerian models is identified to be a likely cause for the inter-model differences. Although the missing process has not yet been identified, we investigate different hypotheses on the basis of the characteristics of the discrepancy. An underestimated HCl uptake into the polar stratospheric cloud (PSC) particles that consist mainly of H2O and HNO3 cannot explain it due to the temperature correlation of the discrepancy. Also, a direct photolysis of particulate HNO3 does not resolve the discrepancy since it would also cause changes in chlorine chemistry in late winter which are not observed. The ionisation caused by galactic cosmic rays provides an additional NOx and HOx source that can explain only about 20 % of the discrepancy. However, the model simulations show that a hypothetical decomposition of particulate HNO3 by some other process not dependent on the solar elevation, e.g. involving galactic cosmic rays, may be a possible mechanism to resolve the HCl discrepancy. Since the discrepancy reported here occurs during the beginning of the chlorine activation period, where the ozone loss rates are small, there is only a minor impact of about 2 % on the overall ozone column loss over the course of Antarctic winter and spring.

Tracing dew and fog water inputs into temperate grassland using stable water isotopes

2020 ◽  
Author(s):  
Yafei Li ◽  
Andreas Riedl ◽  
Franziska Aemisegger ◽  
Nina Buchmann ◽  
Werner Eugster
Keyword(s):  
Isotopic Composition ◽  
Water Vapour ◽  
Temperate Grassland ◽  
Water Isotopes ◽  
Stable Water Isotopes ◽  
Atmospheric Water ◽  
Radiation Fog ◽  
Atmospheric Water Vapour ◽  
Leaf Surfaces ◽  
Equilibrium Fractionation

<p>Dew and fog have proven to be essential water sources for plants across many Earth’s ecosystems. Under climate change with more frequent no-rain days expected in summer, the inputs of dew and fog on short-statured temperate grassland species are expected to become more important as an additional water source. In 2018, Switzerland experienced the driest April to August period of the last five decades. Our research using stable water isotopes investigates how dew and ground radiation fog affected Swiss grasslands in the extreme summer 2018 based on three intensive observation nights of dew and fog. Focusing on an intensively managed grassland located at a valley bottom close to Chamau (CH-CHA) in Switzerland, we measured the isotopic composition (δ<sup>2</sup>H and δ<sup>18</sup>O) of near-surface atmospheric water vapour with a cavity ring-down spectrometer (Picarro L2130-i), and the isotopic composition of the droplets on the leaf surface. Combining the water isotopes with eddy-covariance and meteorological measurements, we analysed the isotope dynamics and fractionation during these three dew and fog nights. Our results indicated that during dew and fog formation, water vapour δ<sup>2</sup>H and δ<sup>18</sup>O gradually decreased under saturated and even slightly supersaturated conditions, but fluctuated under unsaturated conditions. The isotopes of the sampled droplets on the leaf surfaces deviated from the expected isotopic composition based on water vapour under the equilibrium fractionation assumption. During dew and ground radiation fog nights, condensed water was a mix from two sources, atmospheric water vapour and vapour flux from the ground to the foliage. Condensation processes were accompanied by the evaporation from leaf surfaces and the diffusion in the supersaturated layer above the leaf surfaces. This caused non-equilibrium fractionation of water isotopes, seen in the fluctuation of water vapour isotopes and in the observed deviation in the sampled droplets from equilibrium liquids of water vapour. Thus, the isotopic approach is complementary to the often employed micro-lysimetric approach and helps to understand the dynamics and the sources of water vapour during dew and ground radiation fog formation.</p>

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