Process-evaluation of tropospheric humidity simulated by general circulation models using water vapor isotopologues: 1. Comparison between models and observations

The goal of this study is to determine how H_2O and HDO measurements in water vapor can be used to detect and diagnose biases in the representation of processes controlling tropospheric humidity in atmospheric general circulation models (GCMs). We analyze a large number of isotopic data sets (four satellite, sixteen ground-based remote-sensing, five surface in situ and three aircraft data sets) that are sensitive to different altitudes throughout the free troposphere. Despite significant differences between data sets, we identify some observed HDO/H_2O characteristics that are robust across data sets and that can be used to evaluate models. We evaluate the isotopic GCM LMDZ, accounting for the effects of spatiotemporal sampling and instrument sensitivity. We find that LMDZ reproduces the spatial patterns in the lower and mid troposphere remarkably well. However, it underestimates the amplitude of seasonal variations in isotopic composition at all levels in the subtropics and in midlatitudes, and this bias is consistent across all data sets. LMDZ also underestimates the observed meridional isotopic gradient and the contrast between dry and convective tropical regions compared to satellite data sets. Comparison with six other isotope-enabled GCMs from the SWING2 project shows that biases exhibited by LMDZ are common to all models. The SWING2 GCMs show a very large spread in isotopic behavior that is not obviously related to that of humidity, suggesting water vapor isotopic measurements could be used to expose model shortcomings. In a companion paper, the isotopic differences between models are interpreted in terms of biases in the representation of processes controlling humidity

What controls precipitation δ18O in the southern Tibetan Plateau at seasonal and intra-seasonal scales?

Abstract HKT-ISTP 2013 B

Precipitation Water Stable Isotopes in the South Tibetan Plateau: Observations and Modeling

International audienceMeasurements of precipitation isotopic composition have been conducted on a daily basis for 1 yr at Bomi, in the southeast Tibetan Plateau, an area affected by the interaction of the southwest monsoon, the westerlies, and Tibetan high pressure systems, as well as at Lhasa, situated west of Bomi. The measured isotope signals are analyzed both on an event basis and on a seasonal scale using available meteorological information and airmass trajectories. The processes driving daily and seasonal isotopic variability are investigated using multidecadal climate simulations forced by twentieth-century boundary conditions and conducted with two different isotopic atmospheric general circulation models [the isotopic version of the Laboratoire de Meteorologie Dynamique GCM (LMDZiso) and the ECHAM4iso model]. Both models use specific nudging techniques to mimic observed atmospheric circulation fields. The models simulate a wet and cold bias on the Tibetan Plateau together with a dry bias in its southern part. A zoomed LMDZ simulation conducted with similar to 50-km local spatial resolution dramatically improves the simulation of isotopic compositions of precipitation on the Tibetan Plateau. Simulated water isotope fields are compared with new data and with previous observations, and regional differences in moisture origins are analyzed using back-trajectories. Here, the focus is on relationships between the water isotopes and climate variables on an event and seasonal scale and in terms of spatial and altitudinal isotopic gradients. Enhancing the spatial resolution is crucial for improving the simulation of the precipitation isotopic composition

Understanding the O-17 excess glacial-interglacial variations in Vostok precipitation

International audienceCombined measurements of delta O-18, delta O-17, and delta D in ice cores, leading to d excess and O-17 excess, are expected to provide new constraints on the water cycle and past climates. We explore different processes, both in the source regions and during the poleward transport, that could explain the O-17 excess increase by 20 per meg observed from the Last Glacial Maximum (LGM) to Early Holocene (EH) at the Vostok station. Using a single-column model over tropical and subtropical oceans, we show that the relative humidity at the surface is the main factor controlling O-17 excess in source regions. Then, using a Rayleigh-type model, we show that the O-17 excess signal from the source region is preserved in the polar snowfall, contrary to d excess. Evaporative recharge over mid and high latitudes and delta O-18 seasonality in polar regions can also affect the Vostok O-17 excess but cannot account for most of the 20 per meg deglacial increase from LGM to EH. On the other hand, a decrease of the relative humidity at the surface (rh(s)) by 8 to 22% would explain the observed change in O-17 excess. Such a change would not necessarily be incompatible with a nearly unchanged boundary layer relative humidity, if the surface thermodynamic disequilibrium decreased by 4 degrees C. Such a change in rh(s) would affect source and polar temperatures reconstructions from delta O-18 and d excess measurements, strengthening the interest of O-17 excess measurements to better constrain such changes

Asian monsoon hydrometeorology from TES and SCIAMACHY water vapor isotope measurements and LMDZ simulations: Implications for speleothem climate record interpretation

International audienceObservations show that heavy oxygen isotope composition in precipitation (delta O-18(p)) increases from coastal southeastern (SE) China to interior northwestern (NW) China during the wet season, contradicting expectations from simple Rayleigh distillation theory. Here we employ stable isotopes of precipitation and vapor from satellite measurements and climate model simulations to characterize the moisture processes that control Asian monsoon precipitation and relate these processes to speleothem paleoclimate records. We find that delta O-18(p) is low over SE China as a result of local and upstream condensation and that delta O-18(p) is high over NW China because of evaporative enrichment of O-18 as raindrops fall through dry air. We show that delta O-18(p) at cave sites over southern China is weakly correlated with upstream precipitation in the core of the Indian monsoon region rather than local precipitation, but it is well-correlated with the delta O-18(p) over large areas of southern and central China, consistent with coherent speleothem delta O-18(p) variations over different parts of China. Previous studies have documented high correlations between speleothem delta O-18(p) and millennial timescale climate forcings, and we suggest that the high correlation between insolation and speleothem delta O-18(p) in southern China reflects the variations of hydrologic processes over the Indian monsoon region on millennial and orbital timescales. The delta O-18(p) in the drier part (north of similar to 30 degrees N) of China, on the other hand, has consistently negative correlations with local precipitation and may capture local hydrologic processes related to changes in the extent of the Hadley circulation

The Influence of Convective Aggregation on the Stable Isotopic Composition of Water Vapor

Remote sensing datasets of water vapor isotopic composition are used along with objective measures of convective aggregation to better understand the impact of convective aggregation on the atmospheric hydrologic cycle in the global tropics (30°N to 30°S) for the period 2015–2020. When convection is unaggregated, vertical velocity profiles are top-heavy, mixing ratios increase and water vapor δD decreases as the mean precipitation rate increases, consistent with partial hydrometeor evaporation below anvils into a relatively humid atmospheric column. Aggregated convection is associated with bottom-heavy vertical velocity profiles and a positive correlation between mixing ratio and δD, a result that is consistent with isotopic enrichment from detrainment of shallow convection near the observation level. Intermediate degrees of aggregation do not display significant variation in δD with mixing ratio or precipitation rate. Convective aggregation provides a useful paradigm for understanding the relationships between mixing ratio and isotopic composition across a range of convective settings. The results presented here may have utility for a variety of applications including the interpretation of paleoclimate archives and the evaluation of numerical simulations of convection

High regional climate sensitivity over continental China constrained by glacial-recent changes in temperature and the hydrological cycle

The East Asian monsoon is one of Earth’s most significant climatic phenomena, and numerous paleoclimate archives have revealed that it exhibits variations on orbital and suborbital time scales. Quantitative constraints on the climate changes associated with these past variations are limited, yet are needed to constrain sensitivity of the region to changes in greenhouse gas levels. Here, we show central China is a region that experienced a much larger temperature change since the Last Glacial Maximum than typically simulated by climate models. We applied clumped isotope thermometry to carbonates from the central Chinese Loess Plateau to reconstruct temperature and water isotope shifts from the Last Glacial Maximum to present. We find a summertime temperature change of 6–7 °C that is reproduced by climate model simulations presented here. Proxy data reveal evidence for a shift to lighter isotopic composition of meteoric waters in glacial times, which is also captured by our model. Analysis of model outputs suggests that glacial cooling over continental China is significantly amplified by the influence of stationary waves, which, in turn, are enhanced by continental ice sheets. These results not only support high regional climate sensitivity in Central China but highlight the fundamental role of planetary-scale atmospheric dynamics in the sensitivity of regional climates to continental glaciation, changing greenhouse gas levels, and insolation

Middle Miocene Climate and Stable Oxygen Isotopes in Europe Based on Numerical Modeling

The Middle Miocene (15.99–11.65 Ma) of Europe witnessed major climatic, environmental, and vegetational change, yet we are lacking detailed reconstructions of Middle Miocene temperature and precipitation patterns over Europe. Here, we use a high-resolution (∼0.75°) isotope-enabled general circulation model (ECHAM5-wiso) with time-specific boundary conditions to investigate changes in temperature, precipitation, and δ18O in precipitation (δ18Op). Experiments were designed with variable elevation configurations of the European Alps and different atmospheric CO2 levels to examine the influence of Alpine elevation and global climate forcing on regional climate and δ18Op patterns. Modeling results are in agreement with available paleobotanical temperature data and with low-resolution Middle Miocene experiments of the Miocene Model Intercomparison Project (MioMIP1). However, simulated precipitation rates are 300–500 mm/yr lower in the Middle Miocene than for pre-industrial times for central Europe. This result is consistent with precipitation estimates from herpetological fossil assemblages, but contradicts precipitation estimates from paleobotanical data. We attribute the Middle Miocene precipitation change in Europe to shifts in large-scale pressure patterns in the North Atlantic and over Europe and associated changes in wind direction and humidity. We suggest that global climate forcing contributed to a maximum δ18Op change of ∼2‰ over high elevation (Alps) and ∼1‰ over low elevation regions. In contrast, we observe a maximum modeled δ18Op decrease of 8‰ across the Alpine orogen due to Alpine topography. However, the elevation-δ18Op lapse rate shallows in the Middle Miocene, leading to a possible underestimation of paleotopography when using present-day δ18Op—elevation relationships data for stable isotope paleoaltimetry studies

Isotopic exchange on the diurnal scale between near-surface snow and lower atmospheric water vapor at Kohnen station, East Antarctica

Quantifying the magnitude of post-depositional processes affecting the isotopic composition of surface snow is essential for a more accurate interpretation of ice core data. To achieve this, high temporal resolution measurements of both lower atmospheric water vapor and surface snow iso- topic composition are required. This study presents contin- uous measurements of water vapor isotopes performed in East Antarctica (Kohnen station) from December 2013 to January 2014 using a laser spectrometer. Observations have been compared with the outputs of two atmospheric gen- eral circulation models (AGCMs) equipped with water va- por isotopes: ECHAM5-wiso and LMDZ5Aiso. During our monitoring period, the signals in the 2 m air temperature T , humidity mixing ratio q and both water vapor isotopes δD and δ18O are dominated by the presence of diurnal cycles. Both AGCMs simulate similar diurnal cycles with a mean amplitude 30 to 70 % lower than observed, possibly due to an incorrect simulation of the surface energy balance and the boundary layer dynamics. In parallel, snow surface samples were collected each hour over 35 h, with a sampling depth of 2–5 mm. A diurnal cycle in the isotopic composition of the snow surface is observed in phase with the water vapor, reaching a peak-to-peak amplitude of 3 ‰ for δD over 24 h (compared to 36 ‰ for δD in the water vapor). A simple box model treated as a closed system has been developed to study the exchange of water molecules between an air and a snow reservoir. In the vapor, the box model simulations show too much isotopic depletion compared to the observations. Mix- ing with other sources (advection, free troposphere) has to be included in order to fit the observations. At the snow surface, the simulated isotopic values are close to the observations with a snow reservoir of ∼ 5 mm depth (range of the snow sample depth). Our analysis suggests that fractionation oc- curs during sublimation and that vapor–snow exchanges can no longer be considered insignificant for the isotopic compo- sition of near-surface snow in polar regions

Process-evaluation of tropospheric humidity simulated by general circulation models using water vapor isotopic observations: 2. Using isotopic diagnostics to understand the mid and upper tropospheric moist bias in the tropics and subtropics

Evaluating the representation of processes controlling tropical and subtropical tropospheric relative humidity (RH) in atmospheric general circulation models (GCMs) is crucial to assess the credibility of predicted climate changes. GCMs have long exhibited a moist bias in the tropical and subtropical mid and upper troposphere, which could be due to the mis-representation of cloud processes or of the large-scale circulation, or to excessive diffusion during water vapor transport. The goal of this study is to use observations of the water vapor isotopic ratio to understand the cause of this bias. We compare the three-dimensional distribution of the water vapor isotopic ratio measured from space and ground to that simulated by several versions of the isotopic GCM LMDZ. We show that the combined evaluation of RH and of the water vapor isotopic composition makes it possible to discriminate the most likely cause of RH biases. Models characterized either by an excessive vertical diffusion, an excessive convective detrainment or an underestimated in situ cloud condensation will all produce a moist bias in the free troposphere. However, only an excessive vertical diffusion can lead to a reversed seasonality of the free tropospheric isotopic composition in the subtropics compared to observations. Comparing seven isotopic GCMs suggests that the moist bias found in many GCMs in the mid and upper troposphere most frequently results from an excessive diffusion during vertical water vapor transport. This study demonstrates the added value of water vapor isotopic measurements for interpreting shortcomings in the simulation of RH by climate models