Deuterium Excess In Precipitation Research Articles

Moisture sources and climatic effects controlling precipitation stable isotope composition in a western Mediterranean island (Pianosa, Italy)

The Mediterranean basin is indicated as a hot spot of climate change, which is an area whose climate is especially responsive to variations. The insular environment is one of the most threatened by the current climate change, especially in terms of drought events, with serious consequences for water scarcity and water stress. This issue is even enhanced in small islands, whose ecosystems are among more sensitive to climatic changes and water availability. The stable isotope composition of hydrogen (δ2H) and oxygen (δ18O) in precipitation is globally recognized as a powerful natural tracer in the water cycle and represents the starting point to investigate hydrological processes. The understanding of the prevailing factors that drive the isotopic variability of precipitation in the Mediterranean is therefore essential to unravel the hydrological processes and to ensure proper and sustainable management of potentially vulnerable resources to climate change. Here, we discuss the results of multi-year isotopic monitoring in the period 2014–2021 of monthly precipitation collected on Pianosa Island (Italy), a small island located in the northern Tyrrhenian (western Mediterranean). The lower slope and intercept of the Local Meteoric Water Line of the island compared to the Global Meteoric Water Line indicated warmer and drier climatic conditions, suggesting the existence of sub-cloud evaporation processes of raindrops during precipitation, especially in summer. The mean δ18O of precipitation was lower with respect to other sites placed at higher elevation in this Mediterranean region, due to the lack of summer precipitation which were generally enriched in heavy isotopes. Temperature and amount effects may explain part of the δ18O variability observed at the monthly and seasonal scale. An HYSPLIT-based moisture uptake analysis indicated the area between the western Mediterranean basin, Italy, and the Adriatic Sea as the region that supplied most of the humidity associated with monthly precipitation samples on Pianosa Island. Less moisture was picked from the northwestern areas of Europe, the North Atlantic Ocean, the proximal Atlantic Ocean, the Iberian Peninsula and North Africa. Consistently with the rainout effect, the higher the moisture fraction picked from the more proximal regions, the more positive the δ18O of precipitation occurring on Pianosa Island; conversely, the higher the percentage of moisture sourced from more distal regions, the more negative the δ18O. A multiple linear model was proposed to predict the δ18O of monthly precipitation from temperature, precipitation amount and moisture origin data, which explained 45% of the δ18O variability. The deuterium excess variability on the island was partly controlled by the local climatic variables, whose effect potentially modifies the original d-excess signature imprinted at the moisture source. No relationship was found between the precipitation deuterium excess and moisture sources, suggesting that more attention should be paid when using the deuterium excess as a tracer of moisture origin, especially in the Mediterranean.

Is the deuterium excess in precipitation a reliable tracer of moisture sources and water resources fate in the western Mediterranean? New insights from Apuan Alps (Italy)

The deuterium excess is a function of the isotopic composition of oxygen and hydrogen in water. Traditionally, the d-excess is used worldwide to constrain the moisture sources of precipitation, as it mainly depends on the relative humidity and surface sea temperature at the evaporative sources, that control diffusion of the water molecules across the density gradient. A case in point is represented by precipitation deriving from vapor originating over the eastern Mediterranean Sea, characterized by the higher d-excess values (∼22‰) compared to the global average d-excess in precipitation of 10‰. Accordingly, d-excess values of ∼14‰ found in precipitation across the western Mediterranean are generally interpreted as the result of mixing of Atlantic and Mediterranean air masses. Nevertheless, several studies proved the d-excess values of precipitation evolves during transport of vapor and changes depending on local processes (e.g. sub-cloud evaporation of raindrops, continental moisture recycling, seeder-feeder mechanism), thus compromising the reliability of d-excess as a proxy of vapor source regions. With the aim to investigate the pattern of d-excess in precipitation at higher spatial and temporal resolution, and to evaluate its reliability for tracing the moisture source in the western Mediterranean, in this work we present the results of a study performed in the Apuan Alps (Tuscany, Italy), one of the rainiest areas in Italy. 19 single rain events were collected from 2020 to 2021 at two sites placed at different altitudes. Furthermore, monthly precipitations were collected at five sites at different elevations over different time intervals from 2019 to 2021. Event-based and monthly Local Meteoric Water Lines were computed at each site, indicating droplets evaporation beneath the clouds, especially at lower altitude sites. d-excess seasonally and spatially varied, with higher values registered at higher altitudes in winter and autumn, and lower values at lower altitudes in spring and summer. Large differences in d-excess were found among sites placed at different altitudes, and these differences ranged both at monthly and event scales. Mean and weighted mean d-excess vertical gradients of +0.73‰/100 m (r2 = 0.92) and +0.51‰/100 m (r2 = 0.95), respectively, were computed using 9 months in which monthly samples were collected at all five sites. This positive relationship between altitude and d-excess in precipitation is consistent with a process like sub-cloud evaporation that appears to be the most important process controlling the d-excess spatial variability in the mountainous areas. These findings evidence the importance of local processes acting on the d-excess and downsize the reliability of this parameter in tracing moisture sources in this area. Conversely, the d-excess vertical gradient, combined with the δ18O vertical gradient (i.e. altitude effect), could be used to identify the recharge areas of groundwater.

Quantifying the contribution of evaporation from Lake Taihu to precipitation with an isotope-based method

ABSTRACT Moisture recycling plays a crucial role in regional hydrological budgets. The isotopic composition of precipitation has long been considered as a good tracer to investigate moisture recycling. This study quantifies the moisture recycling fractions (f r) in the Lake Taihu region using spatial variations of deuterium excess in precipitation (d P) and surface water vapour flux (d E). Results show that d P at a site downwind of the lake was higher than that at an upwind site, indicating the influence of lake moisture recycling. Spatial variations in d P after sub-cloud evaporation corrections were 2.3, 1.4 and 3.2 ‰, and d E values were 27.4, 32.3 and 31.4 ‰ for the first winter monsoon, the summer monsoon and the second winter monsoon, respectively. Moisture recycling fractions were 0.48 ± 0.13, 0.07 ± 0.03 and 0.38 ± 0.05 for the three monsoon periods, respectively. Both using the lake parameterization kinetic fractionation factors or neglecting sub-cloud evaporation would decrease f r, and the former has a larger influence on the f r calculation. The larger f r in the winter monsoon periods was mainly caused by lower specific humidity of airmasses but comparable moisture uptake along their trajectories compared to the summer monsoon period.

Isotopic composition of precipitation and groundwater onshore of the Rio del Rey Basin, southwest Cameroon: local meteoric lines and recharge

The link between rainfall and groundwater recharge in the Rio del Rey Basin, which is of socio-economic importance to Cameroon, is poorly understood. Accordingly, the stable isotopes in monthly rainfall from January to December 2012 (in Lobe and Mundemba) and 52 surface water and groundwater samples were investigated. High values of δ18O and δD were recorded in the dry period (February to March), and the least values of δ18O and δD were observed in the wet period (September). This indicates that different condensation processes primarily influenced stable isotopes in rainfall as a function of the difference in moisture sources. The relationship between δD and δ18O defined the Lobe meteoric water line as δD = 7.97 δ18O + 12.48 and Mundemba water line as δD = 7.75 δ18O + 10.79. The similarity of their slopes to the global meteoric line suggests that the isotopic composition of investigated rains was not significantly affected by evaporation during precipitation. The ranges in deuterium-excess of precipitation from 5.8 to 16.56‰ suggest the source of vapour is from the Atlantic Ocean. The groundwater isotope values (ranging from −3.81 to −2.52‰ for δ18O) plotted close to and along the GMWL, showing that its isotopic composition is of meteoric origin under rapid recharge conditions. The isotopic similarity between groundwater and June–August rains suggests a significant recharge during this period.

Isotopic equilibrium between raindrops and water vapor during the onset and the termination of the 2005–2006 wet season in the Bolivian Andes

The isotopic equilibrium state between precipitation and low-level water vapor is a common assumption in numerous paleoclimate and atmospheric studies based on water stable isotopes. However, the paucity of field observations limits the validation of this assumption. This study examines the isotopic equilibrium state from event-based precipitation and daily near-surface water vapor samples collected during the onset and the termination of the 2005–2006 wet season in the Bolivian Andes (Zongo valley, 16°09′S, 68°07′W). Our observations show that the observed isotopic composition of precipitation (δDp) deviates from the theoretical isotopic composition of precipitation at equilibrium with water vapor (δDp_eq). Disequilibriums (ΔDp_eq = δDp - δDp_eq) are mostly negative (73%), indicating that precipitation is more depleted than a condensate that would have been formed from surface water vapor, and half of them are between −10 and + 10‰. They are significantly correlated to δDp (r2 = 0.30, n = 70, p < 0.001) suggesting that controls on δDp also impact ΔDp_eq. Although equilibrium state does not prevail at the individual rain event scale, a strong relationship is observed between δDp and δDp_eq over the whole period of field samplings (r2 = 0.86, n = 70, p < 0.001). The review of possible causes to explain the disequilibriums shows that below-cloud rain evaporation and diffusive exchanges are little involved. Other local processes such as rain type, condensation conditions and surface water recycling appear as better candidates to explain ΔDp_eq. Lastly, we explore how local processes affect δDp. We show that large-scale dynamic along air masses history is dominant (nearly 80%) to explain δDp whereas local effects are dominant to explain deuterium excess in precipitation. In consequence, we conclude that δDp is a correct candidate to examine and to reconstruct large-scale atmospheric processes from past to present time scales.

Modeling Insights into Precipitation Deuterium Excess as an Indicator of Raindrop Evaporation in Lanzhou, China

The deuterium excess in precipitation is an effective indicator to assess the existence of sub-cloud evaporation of raindrops. Based on the synchronous measurements of stable isotopes of hydrogen and oxygen (δ2H and δ18O) in precipitation for several sites in Lanzhou, western China, spanning for approximately four years, the variations of deuterium excess between the ground and the cloud base are evaluated by using a one-box Stewart model. The deuterium excess difference below the cloud base during summer (−17.82‰ in Anning, −11.76‰ in Yuzhong, −21.18‰ in Gaolan and −12.41‰ in Yongdeng) is greater than that in other seasons, and difference in winter is weak due to the low temperature. The variations of deuterium excess in precipitation due to below-cloud evaporation are examined for each sampling site and year. The results are useful to understand the modification of raindrop isotope composition below the cloud base at a city scale, and the quantitative methods provide a case study for a semi-arid region at the monsoon margin.

Deuterium Excess in Precipitation Reveals Water Vapor Source in the Monsoon Margin Sites in Northwest China

The deuterium excess (d) in precipitation, determined by the stable hydrogen and oxygen isotopes (δ2H and δ18O), is a widely applied parameter in tracing the water vapor source. Based on the multiple-year observations of stable water isotopes in precipitation at four stations in the Lanzhou city, Northwest China, we analyzed the variations in deuterium excess in precipitation at the Asian monsoon margin region. The mean value of deuterium excess at the study region is 11.0‰ in the dry season and 8.0‰ in the wet season. The d value in precipitation negatively correlates with air temperature and vapor pressure. The low d value during the wet season reflects the monsoon moisture transported from long distances. During the dry season, the continental air masses correspond to the higher d value in precipitation. The moisture regimes based on reanalysis data are generally consistent with the findings using a stable isotopic approach, and the monsoon moisture is highlighted in summer precipitation at these monsoon margin sites.

Spatial distribution and interannual trends of δ18O, δ2H, and deuterium excess in precipitation across North-Eastern Italy

NE Italy presents high-quantity, high-quality and easily exploitable groundwater resources that are seriously threatened by anthropogenic pressures. This study analyzes the oxygen and hydrogen isotopic composition of 2250 precipitation samples collected in 36 sites across the north-easternmost region of Italy, Friuli Venezia Giulia, between 1984 and 2015. This is an unprecedented dataset for North Italy with a high density of sampling sites and a decadal temporal extension. A series of both routine and original chemometric approaches were applied to investigate the temporal and spatial variability of the isotopic composition through relationships with geographical and weather variables. New statistical approaches were presented to model the seasonal and spatial patterns of isotopic composition as well as to summarize the large amount of isotopic data. Significant gradients of δ18O and δ2H were detected in the area due to the peculiar orography and climate of the region; the amplitude of the monthly patterns also presented similar gradients. The deuterium excess did not present a clear seasonality, but higher values were found in autumn. The deuterium excess-to-δ18O ratio exhibited typical patterns throughout the region when grouping the sites for altitude and continentality; in winter, large differences of δ18O were detected among groups, but deuterium excess remained almost unchanged. The inter-site correlations were moderately high across all the territory even for deseasonalized data. The local meteoric water line (using all the single samples δ2H=7.8·δ18O+8.9) was also estimated at annual and seasonal basis, evidencing the presence of spatial gradients according to the orographic and weather characteristics of the region. Statistically significant increasing interannual trends (0.23–0.87‰/y) were detected in 11 sites for deuterium excess; the presence of these trends was linked to local processes. Two multiple linear regression models were applied to reconstruct the isotopic composition of precipitation at a regional scale. The stepwise approach returned the best results with root mean square errors in the 0.5-1‰, 3.5–8.2‰ and 0.9–1.5‰ intervals for δ18O, δ2H, and deuterium excess, respectively. The deuterium excess was not modelled in winter, where no relationships were found with geographic variables.

Expanding the Isotopic Toolbox to Track Monarch Butterfly (Danaus plexippus) Origins and Migration: On the Utility of Stable Oxygen Isotope (δ18O) Measurements

The measurement of naturally occurring stable hydrogen (δ2H) and carbon (δ13C) isotopes in wings of the eastern North American monarch butterflies (Danaus plexippus) have proven useful to infer natal origins of individuals overwintering in Mexico. This approach has provided a breakthrough for monarch conservation because it is the only viable means of inferring origins at continental scales. Recently, routine simultaneous analyses of tissue δ2H and δ18O of organic materials has emerged leading to questions of whether the dual measurement of these isotopes could be used to more accurately infer spatial origins even though the two isotopes are expected to be coupled due to the meteoric relationship. Such refinement would potentially increase the accuracy of isotopic assignment of wintering monarchs to natal origin. We measured a sample of 150 known natal-origin monarchs from throughout their eastern range simultaneously for both δ2H and δ18O wing values. Wing δ2H and δ18O values were correlated (r2=0.42). We found that wing δ2H values were more closely correlated with amount-weighted growing season average precipitation δ2H values predicted for natal sites (r2=0.61) compared to the relationship between wing δ18O values and amount-weighted growing season average precipitation δ18O values (r2=0.30). This suggests that monarch wing δ2H values will be generally more useful in natal assignments than δ18O values. Spatial information related to the use of deuterium excess in environmental waters was similarly found to be not useful when applied to monarch wings likely due to the considerable variance in wing δ18O values. Nonetheless, we recommend further testing of monarch wing δ2H and δ18O values from known natal sites with an emphasis on field data across a strong gradient in precipitation deuterium excess.

On the link between the North Atlantic storm track and precipitation deuterium excess in Reykjavik

This study establishes a robust process‐based dynamical link between the variability of the deuterium excess (d) in monthly winter precipitation in Reykjavik and the frequency of strong large‐scale ocean evaporation (SLOE) events in the subpolar North Atlantic. SLOE events are induced by cold advection in the rear of extratropical cyclones. The link between SLOE and d emerges from the fact that a substantial fraction of moisture that precipitates in Reykjavik in winter originates from SLOE events. It is shown that positive anomalies of monthly d co‐occur with positive anomalies in the frequency of SLOE events at the moisture source of Reykjavik precipitation. An intensification and a northeastwards shift of the North Atlantic storm track acts as a driver for the enhanced SLOE frequency in months with positive anomalies in d. This link also implies that in the subpolar North Atlantic, variations of d in monthly (or seasonal) precipitation reveal information about the storm track dynamics in the vicinity of Iceland. Potential implications of this finding for the interpretation of d as a proxy for the location and intensity of storm tracks are discussed.

The influence of the synoptic regime on stable water isotopes in precipitation at Dome C, East Antarctica

Abstract. The correct derivation of paleotemperatures from ice cores requires exact knowledge of all processes involved before and after the deposition of snow and the subsequent formation of ice. At the Antarctic deep ice core drilling site Dome C, a unique data set of daily precipitation amount, type, and stable water isotope ratios is available that enables us to study in detail atmospheric processes that influence the stable water isotope ratio of precipitation. Meteorological data from both automatic weather station and a mesoscale atmospheric model were used to investigate how different atmospheric flow patterns determine the precipitation parameters. A classification of synoptic situations that cause precipitation at Dome C was established and, together with back-trajectory calculations, was utilized to estimate moisture source areas. With the resulting source area conditions (wind speed, sea surface temperature, and relative humidity) as input, the precipitation stable isotopic composition was modeled using the so-called Mixed Cloud Isotope Model (MCIM). The model generally underestimates the depletion of 18O in precipitation, which was not improved by using condensation temperature rather than inversion temperature. Contrary to the assumption widely used in ice core studies, a more northern moisture source does not necessarily mean stronger isotopic fractionation. This is due to the fact that snowfall events at Dome C are often associated with warm air advection due to amplification of planetary waves, which considerably increases the site temperature and thus reduces the temperature difference between source area and deposition site. In addition, no correlation was found between relative humidity at the moisture source and the deuterium excess in precipitation. The significant difference in the isotopic signal of hoarfrost and diamond dust was shown to disappear after removal of seasonality. This study confirms the results of an earlier study carried out at Dome Fuji with a shorter data set using the same methods.

Characteristics of Stable Isotope in Precipitation and Its Relationship with ENSO in Shanghai

A total of 70 precipitation samples were collected from August 2014 to August 2015 in Shanghai and the oxygen and hydrogen isotope of precipitation were analyzed. The relationship between the δ18O and deuterium excess of precipitation and ENSO events was addressed. The results showed that δD and δ18O of precipitation in Shanghai were higher in winter and spring than in summer and fall. The slope of meteoric water line with these data was a little smaller than the global meteoric water line. The main reason could be the unbalanced secondary evaporation in the process of precipitation. The correlation between the δ18O in precipitation and temperature and precipitation varied in different time scales. On a seasonal scale, this work revealed that there was no significant temperature effect in winter, but significant precipitation effect was found in the annual scales, which was obviously affected by the atmospheric circulation. δ18O values and deuterium excess in precipitation recorded the La Niña and El Niño conversion process. During the La Niña event, the δ18O and d values were more negative than those in the El Niño event.

Variations of Stable Isotope in Precipitation and Its Atmospheric Circulation Effect in Chongqing

Variations of hydrogen and oxygen in stable isotopes and deuterium excess in precipitation as well as their relationship with precipitation,temperature and ENSO (El Niño/La Niño-Southern Oscilation) were analyzed by using 100 daily precipitation stable isotope data from April to October, 2015. The result showed that:① The equation of local meteoric water line (LMWL) was established:δD=8.28δ18O+12.34, the intercept and slope of which were greater than the global meteoric water 1ine (GMWL) but was similar to most of the southern China monsoon region.② Both stable isotopes and deuterium excess in precipitation indicated obvious seasonal variation in the monsoon region, high in summer and low in winter. This was attributed to the difference in moisture sources in different seasons. ③ In addition, no significant correlation was detected between δ18O values in precipitation with temperature and precipitation (r=0.03; r=0.12). But it was sensitive to atmospheric circulation effect, and was positively correlated with ENSO.δ18O values in precipitation and deuterium excess in precipitation clearly recorded the La Niño and El Niño conversion process between 2014 and 2015. During the El Niño event, the δ18O and d values became obvious on the high side, and the changes were the opposite during La Niño.

Influence of Below-Cloud Evaporation on Deuterium Excess in Precipitation of Arid Central Asia and Its Meteorological Controls

Abstract The deuterium excess is a second-order parameter linking water-stable oxygen and hydrogen isotopes and has been widely used in hydrological studies. The deuterium excess in precipitation is greatly influenced by below-cloud evaporation through unsaturated air, especially in an arid climate. Based on an observation network of isotopes in precipitation of arid central Asia, the difference in deuterium excess from cloud base to ground was calculated for each sampling site. The difference on the southern slope of the Tian Shan is generally larger than that on the northern slope, and the difference during the summer months is greater than that during the winter months. Generally, an increase of 1% in evaporation of raindrops causes deuterium excess to decrease by approximately 1‰. Under conditions of low air temperature, high relative humidity, heavy precipitation, and large raindrop diameter, a good linear correlation is exhibited between evaporation proportion and difference in deuterium excess, and a linear regression slope of &amp;lt;1‰ %−1 can be seen; in contrast, under conditions of high air temperature, low relative humidity, light precipitation, and small raindrop diameter, the linear relationship is relatively weak, and the slope is much larger than 1‰ %−1. A sensitivity analysis under different climate scenarios indicates that, if air temperature has increased by 5°C, deuterium excess difference decreases by 0.3‰–4.0‰ for each site; if relative humidity increases by 10%, deuterium excess difference increases by 1.1‰–10.3‰.

Paired stable isotopologues in precipitation and vapor: A case study of the amount effect within western tropical Pacific storms

AbstractUnderstanding controls on the stable isotopic composition of precipitation and vapor in the West Pacific Warm Pool is vital for accurate representation of convective processes in models and correct interpretation of isotope‐based paleoclimate proxies, yet a lack of direct observational evidence precludes the utility of these isotopic tracers. Results from a measurement campaign at Manus Island, Papua New Guinea from 28 April to 8 May 2013 demonstrate variability in the stable isotopic composition (δD and δ18O) of precipitation and vapor in individual precipitation events and over a 10 day period. Isotope ratios in water vapor and precipitation progressively increased throughout the period of measurement, coincident with a transition from high to low regional convective activity. Vapor isotope ratios approached equilibrium with seawater during the quiescent period and likely reflected downwind advection of distilled vapor and re‐evaporation of rainfall during the period of regional convection. On a 5 min timescale across individual storms, isotope ratios in precipitation were strongly correlated with isotope ratios in surface vapor. However, individual precipitation isotope ratios were not strongly correlated with surface meteorological data, including precipitation rate, in all storms. Yet across all events, precipitation deuterium excess was negatively correlated with surface temperature, sea level pressure, and cloud base height and positively correlated with precipitation rate and relative humidity. Paired surface precipitation and vapor isotope ratios indicate condensation at boundary layer temperatures. The ratio of these paired values decreased with increasing precipitation rate during some precipitation events, suggesting rain re‐evaporation and precipitation in equilibrium with an isotopically distinct upper level moisture source. Results from the short campaign support the interpretation that isotope ratios in precipitation and vapor in the western tropical Pacific are indicators of regional convective intensity at the timescale of days to weeks. However, a nonstationary relationship between rain rate and stable isotope ratios in precipitation during individual convective events suggests that condensation, rain evaporation, moisture recycling, and regional moisture convergence do not always yield an amount effect relationship on intraevent timescales.

Deuterium excess variations of rainfall events in a coastal area of South Australia and its relationship with synoptic weather systems and atmospheric moisture sources

In this study, isotopic compositions of monthly (Global Network of Isotopes in Precipitation), event, and intraevent rain samples are used to examine the relationship between precipitation deuterium excess, the type of synoptic weather systems, and associated moisture directions in a coastal area of South Australia. The results indicate that both synoptic weather systems and associated atmospheric moisture sources influence deuterium excess values in precipitation. Rain events caused by frontal systems tend to have moisture sources from the Indian Ocean to the south of Australia. They usually have deuterium excess values of 15‰ to 25‰, depending on the moisture source direction. Rain events caused by synoptic low‐pressure and trough systems tend to have inland moisture sources, and have a deuterium excess of 10‰ to 15‰. In addition to weather systems and associated moisture sources, subcloud processes alter the deuterium excess in the resulting precipitation, which is an effect that is more significant during summer when it is warm and dry. Together, these factors contribute to the seasonal variability of deuterium excess in the study area. Deuterium excess of winter frontal precipitation, resulting from minimal subcloud evaporation, is useful to infer the moisture source direction. In other seasons, deuterium excess in precipitation is more likely altered by subcloud evaporation. Nevertheless, intraevent samples in the middle of a frontal event that has experienced minimal subcloud evaporation are useful to estimate cloud deuterium excess. The results also suggest that an abrupt change in dominant precipitation weather patterns occurs between January and February, characterized by a sudden decrease in δ18O and deuterium excess.

Observation and modeling of stable water isotopes as diagnostics of rainfall dynamics over southeastern Australia

A unified approach of observation and modeling was applied to the investigation of three circulation types that typically bring rain to southeastern Australia. Observations from the Melbourne University Network of Isotopes in Precipitation of high‐resolution variations in the ratios of 18O and 2H were collected for (1) mixed frontal, (2) convective, and (3) stratiform precipitation events. Isotopic content of precipitation varied over both high and low frequencies because of influences from local variations in rain intensity and rainout by large‐scale precipitation. Deuterium excess showed a weak relationship with rainfall amount on intraevent time scales but was stronger under convective rainfall conditions. As a supplement to the observations, a version of the National Center for Atmospheric Research Community Atmosphere Model running an isotope hydrology scheme simulated the mixed frontal and stratiform events by nudging with reanalyses. The simulations represented well the evolution of vapor profiles of 18O and deuterium excess. Trajectories for the mixed frontal case illustrated the structure of the vapor profiles, revealing a convergence of air masses from different source regions. Deuterium excess in precipitation was represented less accurately by the model, indicating a possible shortcoming in the parameterization of postcondensation processes in the general circulation model. By combining observations and modeling in this way, detail of the structure and history of the events was provided that would be unavailable from the sampling of precipitation alone.

Interpolating the isotopic composition of modern meteoric precipitation

An accurate representation of the spatial distribution of stable hydrogen and oxygen isotopes in modern precipitation is required for many hydrological, paleoclimate, and ecological applications. No standardized method for achieving such a representation exists, and potential errors associated with previously employed methods are not understood. Using resampling, we test the accuracy of interpolated δD and δ18O estimates made using four methods. Prediction error for all methods is strongly related to number of data and will likely decline with the addition of new data. The best method lowers estimation error by 10–15% relative to others tested and gives an average error, using all available data, ≈2.5% of the global range. We present and interpret global maps of interpolated δD, δ18O, and deuterium excess in precipitation and the 95% confidence intervals for these values created using the optimal method. These depict global and regional patterns, make evident the robustness of interpolated isotopic patterns, and highlight target areas for future precipitation sampling.

Generalized view of source-region effects on δD and deuterium excess of ice-sheet precipitation

AbstractIt is generally recognized at present that ice-core deuterium excess measurements are potentially useful for reconstructions of vapor source-region temperature and humidity history, and that such measurements provide a method for correcting isotopic paleothermometers for effects of source temperature variations. Here we use a zonally averaged vapor-transport and isotopic-distillation model to show that deuterium excess of precipitation on the ice sheets is sensitive to a wide variety of source-region climate changes in addition to those changes of temperature and humidity that affect the composition of evaporate. Moreover, it is demonstrated that this wide variety of source-region changes all cause anticorrelated changes in deuterium excess with δD and δ18O over the ice sheets, suggesting that deuteriu 14m excess is a generally more useful tool for correcting isotopic thermometers than is currently recognized.

Earth surface evaporative process: A case study from the Great Lakes region of the United States based on deuterium excess in precipitation

The “deuterium excess” parameter in rain, defined as d = δD − 8 δ 18O, is a powerful geophysical tool which can be used to estimate secondary processes that influence atmospheric vapor content in the evaporation-condensation cycle in nature. The d-excess has been calculated, based on the systematic analysis of the stable oxygen and hydrogen isotope ratios, in precipitation samples from Kalamazoo, southwest Michigan. The results show that moisture derived from the evaporation of Lake Michigan contributes between 9–16% during the summer months to the atmospheric vapor load in the region. Admittedly, this estimate is an upper value since in our model we have assumed that transpiration from leaf surfaces is minimal compared to the evaporation from Lake Michigan. It is therefore obvious that large freshwater systems need to be regarded as a potentially significant source of atmospheric vapor load. Isotopic investigations such as this will enable to attain quantitative estimates of this important aspect, a knowledge of which, will enhance the usefulness of or perhaps even be essential in calculations based on many atmospheric circulation models.