A23C-0299
A multi-year high-resolution climatology of the moisture sources of the European Alps: linking model results with observations of stable isotopes in precipitation
The water cycle of the European Alps is vitally important for large parts of Central Europe. A number of aspects of the hydrological cycle in that region are however not fully understood, in particular with respect to the origin of precipitation water. Better knowledge of the processes of the Alpine water cycle is also key to the interpretation of the numerous paleoclimate archives in the area, such as ice cores, peat bogs, and tree rings. This study presents a climatology of the moisture source regions of the Alpine mountain range, and initial attempts to create a link between model results and data from routine observations of stable isotopes in precipitation. A climatology of the Alpine moisture sources covering the years 1995-2002 was created, using a quantitative Lagrangian moisture source diagnostic based on backward trajectories, and ECMWF's ERA-40 reanalysis data. Calculations were performed on a 0.5° x 0.5° grid at 6-hourly time resolution. Monthly observational data of oxygen-18 and deuterium stable isotopes in precipitation were acquired from the Swiss National Network for the Observation of Isotopes in the Water Cycle (NISOT). The main moisture sources of the Alpine mountain range are the eastern North Atlantic, the western Mediterranean, and the central European land mass. However, a pronounced seasonal cycle is present in the in the moisture origin. Winter precipitation has a strong contribution from long-range transport of moisture originating in the North Atlantic, while during summer precipitation sources are considerably more local, and indicate recycling over the European continent. In addition, moisture sources at the northern and southern slope of the Alpine main crest show substantial differences, which also indicate differences in the frequency of extreme precipitation events. Precipitation stable isotope observations along a north-south transect in the western Alps generally confirm the influence of different moisture sources to precipitation in the Alps. Despite being limited by the temporal resolution and spatial extent of the stable isotope data, this study allows for a closer connection between the interpretation of stable isotopes in paleoarchives and physical climate processes.
A23C-0300
Environmental Significance of Oxygen Isotopes Variability in Precipitation and Cave Dripwater in Oregon
Accurate understanding and the correct interpretation of climate proxies such as the δ18O in calcite found in speleothems, requires detailed studies of modern precipitation at sites representative of specific cave systems. As part of an ongoing program at the Oregon Cave National Monument, (OCNM) in SW Oregon we have examined the spatial and temporal variability of δ18O in precipitation from three sites in Oregon that are part of the U.S. Network for Isotopes in Precipitation (USNIP), and the δ18O in cave dripwaters from OCNM . Each of the three USNIP sites (Alsea, Andrews LTER and Starkey) and the OCNM are representative of a different type of climate regime, ranging from maritime to desert conditions, but they are all affected by airmasses originating in the Pacific Ocean. We study the relationship between climate variables (temperature and precipitation) and δ18O for these sites, with particular focus on inter-annual and seasonal changes in the δ18O-climate relationships. We find that temperature is an important controlling factor of precipitation δ18O but the slope of temperature vs δ18O changes seasonally as well as inter-annually. We discuss the implications of our findings for paleoclimate studies using stable isotopes in the Pacific Northwest and we examine the transmission of the climate signal through the karst aquifer at OCNM as reflected in dripwater δ18O variability.
A23C-0301
The relationship between the isotopic content of precipitation and the precipitation amount in tropical regions
General circulation models (GCMs) fitted with stable isotope schemes are widely used to interpret the isotope-climate relationship. However, previous studies have found that the spatial isotope/precipitation correlation simulated by GCMs is stronger and more widespread than the observed value. To understand the reason for this failure, we investigated the factors influencing the empirically well-known isotope/precipitation relationship, or precipitation amount effect, in the tropics using newly obtained daily precipitation isotope monitoring data over Asia. As in previous studies, we found an apparent correlation between the long-term monthly mean isotopic content and the corresponding precipitation amount for sub-tropical island stations. On a monthly time scale, the isotopic variability of precipitation for these stations was clearly related to the strength of the atmospheric convergence/divergence in the surrounding area. Hence, the rain-out process in this region strongly affected the isotopic content of the precipitation. This suggested that the isotopic variability of the source water for this precipitation was weak. However, for the other equatorial (Maritime Continent) or sub-tropical terrestrial (Indochina Peninsula) stations, we found only a weak isotope/precipitation relationship. At these stations, a relationship between the monthly isotopic content and the strength of atmospheric convergence/divergence was not apparent. It is likely that the isotopic content of vapor advected from remote areas by monsoon flow or strong moisture flux convergence varied strongly, causing a weak isotope/precipitation relationship. The fact that isotopic GCMs currently overestimate the isotope/precipitation correlation indicates that in some regions, the relative contribution of vapor from the local source has been overestimated, or the depletion of heavy isotopes during transport from the upstream source region has been underestimated. The evaluation of the amount effect using isotopic GCMs is useful not only to reconstruct paleoclimate conditions, but also to examine how GCMs can reproduce real atmospheric circulation over the tropics.
A23C-0302
Mapping Precipitation Patterns from the Stable Isotopic Composition of Surface Waters: Olympic Peninsula, Washington State
Available data indicate that large and persistent precipitation gradients are tied to topography at scales down to a few kilometers, but precipitation patterns in the majority of mountain ranges are poorly constrained at scales less than tens of kilometers. A lack of knowledge of precipitation patterns hampers efforts to understand the processes of orographic precipitation and identify the relationships between geomorphic evolution and climate. A new method for mapping precipitation using the stable isotopic composition of surface waters is tested in the Olympic Mountains of Washington State. Measured δD and δ18O of 97 samples of surface water are linearly related and nearly inseparable from the global meteoric water line. A linear orographic precipitation model extended to include in effects of isotopic fractionation via Rayleigh distillation predicts precipitation patterns and isotopic composition of surface water. Seven parameters relating to the climate and isotopic composition of source water are used. A constrained random search identifies the best-fitting parameter set. Confidence intervals for parameter values are defined and precipitation patterns are determined. Average errors for the best-fitting model are 4.8 permil in δD. The difference between the best fitting model and other models within the 95% confidence interval was less than 20%. An independent high-resolution precipitation climatology documents precipitation gradients similar in shape and magnitude to the model derived from surface water isotopic composition. This technique could be extended to other mountain ranges, providing an economical and fast assessment of precipitation patterns requiring minimal field work.
A23C-0303
The use of Stable Isotopes to Assess Climatic Controls on Groundwater Recharge in the Southern Sacramento Mountains, New Mexico
We used the stable isotopes of hydrogen and oxygen to relate the temporal variability of groundwater recharge to climatic conditions in the southern Sacramento Mountains as a part of a larger regional hydrogeologic study. The southern Sacramento Mountains are the primary recharge source not only to local aquifers, but also to the Lower Pecos River Basin, the Roswell Artesian aquifer and aquifers in the Salt Basin. Aquifers in the study area mainly consist of fractured limestone. In years prior to 2006, groundwater levels within the study area showed a steady decline. We observed a significant increase in regional groundwater levels and spring discharge during and shortly after the unusually wet 2006 monsoon season. We developed a local meteoric water line (LMWL) in δ18O vs. δD space based on precipitation samples collected from several different elevations over a period of two years. The stable isotopic compositions of streams during base flow conditions define an evaporation line with a slope of 5.5 that intersects the LMWL in the region that represents winter precipitation. Spring and well samples collected in 2003 and spring samples collected in 2008 exhibit isotopic compositions that plot near the evaporation line, indicating that groundwater recharge is largely snow melt that has subsequently undergone evaporation in local streams. After the unusually wet 2006 monsoon season, the isotopic compositions of springs sampled in fall of 2006 and wells sampled in spring of 2007 deviated from the evaporation line, plotting closer to the LMWL. This observed isotopic trend is thought to represent a large input of 2006 monsoon precipitation to the groundwater system via relatively short fracture-dominated flow paths. Stable isotope results indicate that while snow melt is probably the main source of groundwater recharge in the southern Sacramento Mountains, as exhibited by the 2003 and 2008 samples, above average summer precipitation events, such as in 2006, can also contribute to significant groundwater recharge.
A23C-0304
Modelling the Isotopic Evolution of Precipitation Across Western Canada
The variability of synoptic circulation and associated shifts in vapour transport pathways that bring moisture from Pacific Ocean source regions to the Canadian Rocky Mountains govern both the amount and isotopic character of water storage in winter snowpacks. For this reason, it is important to understand the linkages between stable isotopes in precipitation and vapour trajectories in this region. Snowpit isotope data and snow accumulation records were obtained over the 2006-2007 winter season at two alpine field sites in the Canadian Rocky Mountains; the Haig Glacier (Kananaskis Country, AB) and the Opabin Glacier (Yoho National Park, B.C.). Individual accumulation events were identified in snowpit isotope stratigraphies, and additional water isotope samples from each event were collected along a transect in southern British Columbia. Seven secondary schools participated in this project, which allowed the near simultaneous collection of precipitation from each major storm system that crossed this region. Each major storm event over the sampling period was modelled using a coupled orographic-Rayleigh distillation model constrained with water isotope data from Vancouver. The orographic model enables us to explicitly represent the effects of topography on the distribution and intensity of precipitation along each storm trajectory. The output of this model are compared with Rayleigh curves generated using specific humidity data from the University of Wisconsin Nonhydrostatic Modeling System, and with a model that prescribes a linear decrease in temperature and pressure and neglects the effect of topography. The orographic-Rayleigh model is 64% better at predicting δ18O and 10% better at predicting deuterium excess in winter snowpacks in this region than the linear Rayleigh model.
A23C-0305
Regional Modeling of Stable Isotopes in Precipitation for the Present Greenland Climate.
The meso-scale climate model REMOiso fitted with isotope diagnostics has been run over the North
Atlantic area including Greenland. The model run was forced with SSTs and the upper wind field and covers
the period 1958-2001. We have compared the model output to observed temperature, precipitation, and
stable isotopes from high resolution ice cores and the GNIP network. The model captures the inter-annual
variability of temperature and precipitation very well, and also reproduces the west coast late summer/autumn
precipitation maximum as well as the contrasting east coast winter maximum. The model slightly
underestimates the accumulation for the center of the ice sheet as well as overestimating the temperature.
This warm bias is also seen in the isotopic values where the modelled annual mean δ18O is 4.5
‰ too high. However, compared to ice core and GNIP data the model does capture up to 30% of the
variability of the isotopes during winter.
Former studies have demonstrated the leading influence of the North Atlantic Oscillation (NAO) on
particularly Greenland's winter climate. We can reproduce this dominant role of the NAO with a strong
East/West contrast in the corresponding precipitation signal. The simulated water isotope pattern shows a
coherent signal in Central and North-West Greenland as it was shown before in observations. The NAO
explains nearly 20% of this leading pattern.
A23C-0306
Assessing the Simulated Hydrological Cycle in GCMs through Isotopes and an Observational-Based Regression Model
The isotopic composition of precipitation (herein after denoted as δ) is widely used for both hydrology and climate variability studies. Mapping out the spatial distribution of δ values has been done by several studies using regressions. Isotope equipped General Circulation Models (GCMs) provide another approach in predicting the spatial distribution of δ values. In this study, regressions are performed on both the Global Network for Isotopes in Precipitation (GNIP) observational records and three GCMs to examine how well the models capture the balance of local and non-local (advective) controls. This type of analysis provides a measure of which processes give rise to model errors, and thus expands on simple model/observation comparisons. In particular, the models have large errors over the high-latitudes, where predicted δ values are not depleted enough; a regression analysis provides insight into why the models perform poorly in these regions. The regression used here incorporates temperature, precipitation, latitude and altitude as predictors. The regression is performed on the GNIP station observations as well as the GCMs. The GCMs examined here are MUGCM, ECHAM, and GISS, using simulated δ, temperature, and percipitation fields. The regression bias, associated with processes not captured by the local conditions (both observed and simulated), is defined as ε = δR - δO where δO is the observed or simulated value and δR is the regression result. The observational-based regression has large-magnitude biases over certain locations. For instance, the regression predicts the δ values to be too low over the Southern Oceans and the Arctic Ocean north of Scandinavia. Over most of Canada and Alaska, the regression predicts δ values that are not depleted enough. These large biases are a result of non-local processes (such as vapor advection) that play an important role in determining the observed δ value in these regions. Many of the biases that appeared in the observational-based regression model also show up in the GCM- based regression. However, the magnitudes and extent are much larger for the GCM-based regressions. For example, GCM-based regressions have large biases throughout most of the high northern-latitudes, whereas the observational-based regression only has large biases in western Canada. The same type of result also occurs in the southern hemisphere. These results suggest that GCM errors could be a result of over-emphasizing the role of non-local processes, such as improper vapor transport.
A23C-0307
Evapotranspiration and its effects on atmospheric water isotope balance in forests
An understanding of atmospheric water vapor content and its isotopic composition is important if we are to be
able to model future water vapor dynamics and its potential feedbacks on future climate change. Here we
investigated water isotope fractionation that accompanies surface evapotranspiration, providing analytical
solutions to mechanistically interpret isotopic variations of water vapor in forest air. Water vapor observed at
three heights over 3 consecutive days in an old-growth coniferous forest in the Pacific Northwest, USA,
suggests two general patterns. First, a stratified structure of 2H/1H and 18O/16O values
of water vapor exists where most positive values were consistently observed above the canopy (60 m), lowest
values were often observed near the forest floor (0.5 m) and observed values at 10 m generally fell in
between. Second, this vertical profile diminished when atmospheric entrainment from the air aloft prevailed,
causing an increase in average 2H/1H but a decrease in average 18O/16O values of
water vapor during the day. Observed differences between 0.5m and 60m range between 2-6‰ for
δ18O and 20-40‰ for δ2H at night, but are usually less than 1.5‰ for
δ18O and 7‰ for δ2H during the day. We develop a canopy H2O isotope
model that successfully simulates principal features of the time evolution of stable hydrogen and oxygen
isotope ratios of water vapor. Here we provide modeling and observational evidence that detects the material
conservation of water vapor and its stable isotope contents in forest air, which has implications to larger-
scale predictions of precipitation across the terrestrial landscape.
http://www.sci.sdsu.edu/biomet/pmwiki.php/PmWiki/Home
A23C-0308
The Atmospheric Water Cycle Above a Tropical Closed Lake (Lake Ihotry, Southwestern Madagascar)
We present an indirect approach based on a lake isotope mass balance model to infer the seasonal evolution of the isotope composition of atmospheric water vapour, and to quantify the atmospheric water cycle above a lake. Starting from the lake water balance previously established, the isotope mass balance allows to calculate the isotopic composition of the moisture evaporated from the lake surface (deltaE). The composition of the ambient atmospheric vapour above the lake (deltaAL) is then derived from the Craig-Gordon model, applied at a daily time-step during a 8-months dry season. The ambient atmospheric vapour (deltaAL) is found to be clearly influenced by the locally evaporated vapour (deltaE). During the rainfall season, an atmospheric mass balance is used to estimate the average composition of the regional atmospheric pool (deltaAR). During the dry season, the atmospheric mass balance is calculated at a daily time-step and shows that the contribution of the evaporated vapour to the ambient moisture varies between 20% and 50%, following the evolution of the evaporation rate. Moreover, despite similar delta18O values, we have found a clear contrast between the composition of regional rainfall and on-lake rainfall, the latter being sampled near the lake shore. While the regional precipitation presents a low value of deuterium excess (d=4.3‰) attributed to an evaporation process in the atmosphere, the local precipitation shows a high value of deuterium excess (d=16.1‰) attributed to the recycling of local vapour. The comparison between rainfall amounts from two precipitation stations shows that the recycled moisture contributes at least to 16% of on-lake precipitation. In the context of climate change, this recycling process may enhance the regional response of water balance, especially in semi-arid tropical areas, where the availability of surface water for evaporation is highly variable. This work proposes a valuable complement to direct water vapour measurements, in yielding the long-term evolution of the atmospheric vapour composition with spatially averaged values and smoothed temporal variations and is applicable to other remote regions with little background data and limited resources.
A23C-0309
Water vapor isotopes measurements at Mauna Loa, Hawaii: Comparison of laser spectroscopy and remote sensing with traditional methods, and the need for ongoing monitoring
The isotopic composition of water vapor (2H/1H and 18O/16 ratios) provides unique
information on the transport pathways that link the water sources to regional sinks, and thus proves useful in
understanding the large scale humidity budgets. Recent advances in measurement technology allow the
monitoring of water vapor isotope composition in ways which has can revolutionize investigations of
atmospheric hydrology. Traditional measurement of isotopic composition requires trapping of samples with
either large volume vacuum flasks or by trapping liquid samples with cryogens for later analyses using mass
spectrometry, and are laborious and seldom span more than just short dedicated observational periods. On
the other hand, laser absorption spectroscopy can provide almost continuous and autonomous in situ
measurements of isotope abundances with precision almost that of traditional mass spectrometry, and
observations from spacecraft can make almost daily maps of the global isotope distributions. In October of
2008 three laser based spectrometers were deployed at the Mauna Loa Laboratory in Hawaii to make
continuous measurement of the 2H and 18O abundance of free tropospheric water vapor. These
results are compared with traditional measurements and with measurements from two satellite platforms.
While providing field validation of the new methodologies, the data show variability which captures the
transport processes in the region. The data are used to characterize the role of large scale mixing of dry air,
the influence of the boundary layer and the importance of moist convection in controlling the low humidity of
subtropical air near Hawaii. Although the record is short, it demonstrates the usefulness of using robust
isotope measurements to understand the budgets of the most important greenhouse gas. This work
motivates establishing a continuous record of isotopes measurement at baseline sites, like Mauna Loa, such
that the changes in water cycle can be understood and monitored as climate changes.
http://atoc.colorado.edu/~dcn/HAVAIKI
A23C-0310
An optical feedback cavity enhanced absorption spectrometer for atmospheric moisture isotope ratio measurements
Measurements of the isotopic composition of water vapor in the upper troposphere and lower stratosphere are vital in testing the various hypotheses of stratospheric aridity, the relative importance of large scale and convective transport processes, and the role of cirrus anvil clouds. Close to the surface, the isotopic composition of moisture is crucial in understanding the hydrological cycle and its link to the carbon cycle through the exchange of oxygen atoms in the ecosystem. Isotope measurements directly on the vapor compartment of the hydrological cycle represent also the missing link in the validation of global circulation modelling efforts. In order to address these scientific issues, we have developed an ultra-sensitive near-infrared spectrometer to measure the water deuterium and oxygen isotope ratios in-situ, with a high temporal resolution (spectra are recorded at ~0.1 s time resolution, but are generally averaged for between 1 and 100 seconds). The instrument was designed for use on high-altitude airborne platforms, but has also been applied to tropospheric moisture measurements. Here we report on the laboratory calibration of the device, and show results of airborne campaigns, as well as long term continuous monitoring of near-surface atmospheric moisture. The measurement precision depends on the volume mixing ratio, and reaches values of 0.15, 0.4, and 1.3 per mil for O-18, O-17, and D, respectively, at a mixing ratio of 2500 ppmv and an averaging time of 20 s.
A23C-0311
Wavelength-Scanned Cavity Ring Down Spectroscopy: Opening new doors for tracing water isotopes in the hydrosphere, biosphere and atmosphere.
Stable isotopes of water are proven indicators, tracers and recorders of processes that affect the hydrologic cycle. Measurements of stable isotopes (δD and δ18O) in water are typically done with IRMS systems in laboratories that preclude real-time field use. In practice, this limits the spatial and temporal density of samples that can be taken, and thus also limits the utility of isotopes as a tool to validate models of water transport. A prominent example of this problem is isotopes in atmospheric water vapor. While the isotopic concentration of water vapor in the free troposphere is known to contain information about global moisture transport, mixing processes, and evaporation and transpiration fluxes, measurements of water vapor are labor intensive and difficult to make in the field, because of reliance upon off-line cryogenic extraction methods and traditional laboratory IRMS. This has seriously limited their utility in validating models, as well in tracking and quantifying processes in nature. A new measurement technique, based on Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS), has recently become available to enable high-throughput, stable isotope ratio measurements in water and water vapor. The instruments are small and portable enough to enable field use. Lab tests analyzing both known and unknown waters have shown that the reproducibility of the system is competitive with IRMS systems. The typical precision of this technique is conservatively <0.8 permil for δD and <0.1 permil for δ18O (2 sigma), and is sufficient to allow accurate determination of deuterium excess. Both liquid water and water vapor measurements are calibrated by injecting known liquid waters in the field. Here we show an example of water vapor isotopes measured in the field at Woods Hole, Massachusetts (~20m amsl). The temporal resolution is 15 minutes, allowing one to see remarkable variability masked by the traditional sampling which collects vapor over long time periods. We examine the data in the context of mixing models of air. Similar field based, high frequency sampling of liquid water, for example stream flow is also possible, as are real time analyses of ice cores.
A23C-0312
Real-time measurements of the stable isotopes in water vapour by Fourier Transform Infrared spectroscopy
The stable isotopes in water vapour are strongly influenced by atmospheric hydrological processes. For this reason, high-resolution time series of the stable isotopes in water vapour offer a continuous record of the processes related to atmospheric waters. Until recently the collection of high-resolution time series have been limited due to the lack of instrumentation capable of collecting large continuous datasets. We have developed a FTIR spectrometer that can be used to collect high resolution time series of both δ18O and δ2H in water vapour. The performance of this instrument is similar to that of the other available stable isotope instrumentation. Measurement precision for a 20 minute measurement is of the order of 0.4 and 1 per mille for δ18O and δ2H, respectively. Measurement precision can be improved beyond this if consecutive measurements are averaged. The instrument is currently deployed in Sydney, Australia, where measurements are being collected to interpret the effect of synoptic scale weather patterns on the stable isotope values of water vapour. Here a snapshot of these measurements and results from some of the calibration procedures are shown to illustrate the instruments capabilities. The instrument is also capable of simultaneously measuring the atmospheric mixing ratio of a number of climatically sensitive trace gases including, CO2, CH4, CO and N2O.
A23C-0313
A study of the evolution of the stable isotope signature of water vapour during the passage of synoptic low pressure systems over Sydney, Australia
The stable isotopic composition of water vapour can provide a tracer of the evapotranspiration, precipitation and atmospheric mixing that has occurred along the trajectory of an air mass. Incorporation of stable water isotopes into weather and climate prediction models therefore has the potential to aid in their evaluation and improvement, but this requires the availability of continuous high-resolution time series of quality isotope data. We present an analysis of a one-year time series of hourly water vapour stable isotope measurements, collected using Fourier Transform Infrared spectroscopy at a site near Sydney, Australia. The analysis investigates large variations in the time series of water vapour δ2H and δ18O values and their inter-relationship, corresponding to shifts in meteorological conditions associated with the passage of synoptic-scale weather systems. To help understand the variations in the observed signals, the stable isotope and meteorological data has been augmented with measurements of atmospheric radon concentrations and back trajectory analyses. The combination of these different indicators provides an insight into the synoptic scale processes and source regions that influence the stable isotopic composition of water vapour in the atmosphere.
A23C-0314
A global view on near-surface deuterated water vapour - First results from SCIAMACHY onboard ENVISAT
Water vapour is by far the most important greenhouse gas in the atmosphere and an accurate knowledge of hydrological cycles and their feedback mechanisms is therefore indispensable for reliable climate predictions. The relative abundance of HDO provides a deeper insight into hydrological cycles as evaporation and condensation processes deplete heavy water in the gas phase. Only recently, global measurements of HDO depletions in the middle to lower troposphere were performed by the Tropospheric Emission Spectrometer (TES) aboard the Aura spacecraft. Global measurements of the isotopic composition of near-surface water vapor are so far missing. The SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY) instrument aboard the European Space Agency (ESA)'s environmental research satellite ENVISAT records the intensity of solar radiation, reflected from the Earth surface or the atmosphere, at moderate spectral resolution between 240 and 2390 nm. Its potential to simultaneously retrieve HDO and H2O total columns with high sensitivity toward the surface has so far not been exploited. Here, we present first retrievals of the near-global relative deuterated water vapor distribution from SCIAMACHY. Large scale features such as the latitudinal effect or continental gradients in North-America can be nicely observed. Even small scale features such as relatively high HDO abundances above the Red Sea can be observed. Comparisons with ground-based Fourier Transform measurements (FTS) indicate that also retrievals at high latitude sites such as Ny Alesund (79deg N) are feasible. We will present near-global measurements from SCIAMACHY, including long-term means showing pronounced large-scale as well as small-scale features. Further, we report on large seasonal variations, being higher than those observed by TES. For selected stations in tropical, mid and high-latitude sites, we show comparisons with ground-based direct sun FTS measurements.
A23C-0315
Comparison between Tropospheric Emission Spectrometer (TES) observations and the NCAR CAM isotope model
Water vapor isotopes are widely used for understanding the global hydrological cycle because they are tracers of evaporation or condensation processes and also can identify moisture sources. New observations of water vapor isotope from the Tropospheric Emission Spectrometer (TES) on NASA's Aura spacecraft in the middle troposphere have been used to identity tropospheric moisture sources, their transport pathways and efficiencies in which vapor is converted to precipitation and back again. Here, we compare the TES observations of water vapor isotopes to the simulated results from Community Atmospheric Model (CAM) with water isotope enabled. Our objective is to examine the moisture transport characteristics within the CAM model, and to use this insight to help studies of global water cycle, paleoclimate, and the response of precipitation to climate change. TES data utilized in this study are from December 2005 to August 2007. Two seasonal data are created from data retrievals from December to February (DJF) and June to August (JJA) dates, respectively. The global distribution of the TES water vapor isotope is consistent with the CAM results. Both water vapor isotope results show the expected spatial distribution ("latitude effect") and a hemispheric symmetry (larger variations over northern hemisphere than over southern hemisphere during JJA and vice versa during DJF). This agreement indicates that the large-scale transport of moisture from the equator and sub tropics to polar regions is well explained by the CAM results. However, significant differences exist in the mid-latitude Pacific Ocean and over regions of strong convection, such as summer time Africa, India, and Asia. These differences point towards improving our understanding of advective and convective moisture transport into the troposphere over Asia and the warm pool and the Pacific.
A23C-0316
Isotopic Variations of Water Vapor in Regions of Deep Convection
Interpretation of many long-term climate records depends on interpreting variations in the isotopic composition of deposited precipitation. However, it can be unclear as to whether these isotopic variations are due to changes in moisture source, transport, convection, precipitation efficiency, or temperature. In this study we use new observations of HDO and H2O in the free troposphere from the Aura Tropospheric Emission Spectrometer to examine the relationship between deep convection and excess isotopic depletion in the observed vapor. We observe that increased rainfall within regions of deep convection produces vapor that is more humid and more depleted than background vapor. However, subsidence associated with deep convection can also bring dry and isotopically depleted vapor from the upper troposphere to lower altitudes. Both of these processes can influence precipitation amounts and its isotopic composition. In particular we examine these processes over regions where long-term climate records have been extracted such as Asia and Borneo in order to better relate the isotopic variations of the climate records to specific climate processes.
A23C-0317
Optimal Estimates of Regional Moisture Exchange Using Isotopic Ratios of Water Vapor From the Tropospheric Emission Spectrometer
Further understanding of the variations in the seasonal sources of mid-tropospheric moisture, including the contributions from surface evaporation and the re-evaporation of falling rain, can provide refined knowledge of the global hydrological cycle. Stable water isotope measurements from the Tropospheric Emission Spectrometer (TES) are useful in this regard since isotopic fractionations occurring during evaporation and condensation give rise to measurable variations in the isotopic composition of water vapor, which can be used to estimate the strength of regional hydrologic processes. The present study employs an optimized Lagrangian isotopic exchange model to estimate the regional supply of water vapor from surface evaporation and rainfall evaporation, the loss of moisture via precipitation, and the regional isotopic signatures of the source water for the 500-825 hPa layer. Given the advection pathways formed from NCEP/NCAR Reanalysis wind fields, the Lagrangrian model uniquely constrains these moisture exchange processes by employing the changes in isotopic composition found en route over a one to three day time frame. The estimates show vigorous moisture exchange occurring over land surfaces in monsoonal regions, where the timescale for complete replacement of the moisture in the parcels by the evaporative supply of boundary layer moisture is found to be on the order of two to four days during the seasonal monsoons. Conversely, over areas of oceanic upwelling in the subtropics, our estimates reveal a much slower timescale for moisture replacement on the order of ten to fifteen days. The timescale for the complete loss of initial moisture in the parcels via precipitation varies from one or two days in the mid-latitudes to three or four days in the tropics. The estimates of the isotopic composition of the source waters found from the model show that recently evaporated oceanic water enters the 500-825 hPa layer primarily over land surfaces during the seasonal monsoons, as well as over the western Pacific warm pool. Additionally, rainfall recycling is found to be an important part of the hydrology directly upstream of the monsoonal regions of northern Australia, the Amazon Basin, and southeastern Asia. Using the isotopic measurements from TES, this study provides unique diagnostics of mechanisms that control the seasonal distribution of water vapor.
A23C-0318
Study on data assimilation of vapor isotope with the ensemble Kalman filter
Since the first isotopic AGCM came out in 1980s, it has been dreamed of "Isotope Reanalysis" for better
analyses of global vapor/precipitation fields in high spatial and temporal resolution, but it has not been
realized in the same way as the common Reanalysis products. There were three main reasons for this
incapability: 1) Too few observation data available 2) Low accuracy of the model and 3) No appropriate
assimilation algorithm available. However, the situation has been changed: 1) vapor isotopes have recently
become observable in a spatially and temporally high resolution by a remote sensing technique (e.g.,
Worden et al., 2007); 2) the recent isotopic AGCM also has shown nice reproducibility of precipitation
isotopes when the large scale circulation fields are constrained (Yoshimura et al., 2008); and 3) the
Ensemble Kalman Filter approach, which is practically the most appropriate for data assimilation of the
isotopic AGCM, became feasible in terms of computational cost (e.g., Miyoshi and Yamane, 2007).
Furthermore, the nudged simulation, which was carried out prior to the realization of the dream, showed a
significant correlation (R=0.55) in vapor dD compared with the TES observations, implying large possibility of
improvement in modeled vapor/precipitation isotopes by having isotopic information assimilated. This study
therefore aims to make the first "Isotope Reanalysis" by applying LETKF and TES data to IsoGSM, and some
experimental results are going to be presented.
http://meteora.ucsd.edu/~kyoshimura/IsoGSM1