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lmd_Risi2012_abstracts.html

2012 .

(9 publications)

J.-L. Lacour, C. Risi, L. Clarisse, S. Bony, D. Hurtmans, C. Clerbaux, and P.-F. Coheur. Mid-tropospheric δD observations from IASI/MetOp at high spatial and temporal resolution. Atmospheric Chemistry & Physics, 12:10817-10832, November 2012. [ bib | DOI | ADS link ]

In this paper we retrieve atmospheric HDO, H2O concentrations and their ratio δD from IASI radiances spectra. Our method relies on an existing radiative transfer model (Atmosphit) and an optimal estimation inversion scheme, but goes further than our previous work by explicitly considering correlations between the two species. A global HDO and H2O a priori profile together with a covariance matrix were built from daily LMDz-iso model simulations of HDO and H2O profiles over the whole globe and a whole year. The retrieval parameters are described and characterized in terms of errors. We show that IASI is mostly sensitive to δD in the middle troposphere and allows retrieving δD for an integrated 3-6 km column with an error of 38 on an individual measurement basis. We examine the performance of the retrieval to capture the temporal (seasonal and short-term) and spatial variations of δD for one year of measurement at two dedicated sites (Darwin and Izaña) and a latitudinal band from -60deg to 60deg for a 15 day period in January. We report a generally good agreement between IASI and the model and indicate the capabilities of IASI to reproduce the large scale variations of δD (seasonal cycle and latitudinal gradient) with good accuracy. In particular, we show that there is no systematic significant bias in the retrieved δD values in comparison with the model, and that the retrieved variability is similar to the one in the model even though there are certain local differences. Moreover, the noticeable differences between IASI and the model are briefly examined and suggest modeling issues instead of retrieval effects. Finally, the results further reveal the unprecedented capabilities of IASI to capture short-term variations in δD, highlighting the added value of the sounder for monitoring hydrological processes.

R. D. Field, C. Risi, G. A. Schmidt, J. Worden, A. Voulgarakis, A. N. LeGrande, A. H. Sobel, and R. J. Healy. A Tropospheric Emission Spectrometer HDO/H2O retrieval simulator for climate models. Atmospheric Chemistry & Physics, 12:10485-10504, November 2012. [ bib | DOI | ADS link ]

Retrievals of the isotopic composition of water vapor from the Aura Tropospheric Emission Spectrometer (TES) have unique value in constraining moist processes in climate models. Accurate comparison between simulated and retrieved values requires that model profiles that would be poorly retrieved are excluded, and that an instrument operator be applied to the remaining profiles. Typically, this is done by sampling model output at satellite measurement points and using the quality flags and averaging kernels from individual retrievals at specific places and times. This approach is not reliable when the model meteorological conditions influencing retrieval sensitivity are different from those observed by the instrument at short time scales, which will be the case for free-running climate simulations. In this study, we describe an alternative, ”categorical” approach to applying the instrument operator, implemented within the NASA GISS ModelE general circulation model. Retrieval quality and averaging kernel structure are predicted empirically from model conditions, rather than obtained from collocated satellite observations. This approach can be used for arbitrary model configurations, and requires no agreement between satellite-retrieved and model meteorology at short time scales. To test this approach, nudged simulations were conducted using both the retrieval-based and categorical operators. Cloud cover, surface temperature and free-tropospheric moisture content were the most important predictors of retrieval quality and averaging kernel structure. There was good agreement between the δD fields after applying the retrieval-based and more detailed categorical operators, with increases of up to 30 over the ocean and decreases of up to 40 over land relative to the raw model fields. The categorical operator performed better over the ocean than over land, and requires further refinement for use outside of the tropics. After applying the TES operator, ModelE had δD biases of -8 over ocean and -34 over land compared to TES δD, which were less than the biases using raw model δD fields.

S. C. Sherwood and C. Risi. The HDO/H2O relationship in tropospheric water vapor in an idealized last-saturation model. Journal of Geophysical Research (Atmospheres), 117:19205, October 2012. [ bib | DOI | ADS link ]

Previous model studies have shown that the isotopic composition of tropospheric water vapor is sensitive to atmospheric water transport processes, but compositional information is difficult to interpret due to the complexity of the models. Here an attempt is made to clarify the sensitivity by computing the relationship between tropospheric HDO (via δD) and H2O (via specific humidity q) in an idealized model atmosphere based on a ”last-saturation” framework that includes convection coupled to a steady large-scale circulation with prescribed horizontal mixing. Multiple physical representations of convection and mixing allow key structural as well as parametric uncertainties to be explored. This model has previously been shown to reproduce the essential aspects of the humidity distribution. Variations ofδD or qindividually are dominated by local dynamics, but their relationship is preserved advectively, thus revealing conditions in regions of convection. The model qualitatively agrees with satellite observations, and reproduces some parametric sensitivities seen in previous GCM experiments. Sensitivity to model assumptions is greatest in the upper troposphere, apparently because in-situ evaporation and condensation processes in convective regions are more dominant in the budget there. In general, vapor recycling analogous to that in continental interiors emerges as the crucial element in explaining whyδD exceeds that predicted by a simple Rayleigh process; such recycling involves coexistent condensation sinks and convective moisture sources, induced respectively by (for example) waves and small-scale convective mixing. The relative humidity distribution is much less sensitive to such recycling.

J.-E. Lee, C. Risi, I. Fung, J. Worden, R. A. Scheepmaker, B. Lintner, and C. Frankenberg. Asian monsoon hydrometeorology from TES and SCIAMACHY water vapor isotope measurements and LMDZ simulations: Implications for speleothem climate record interpretation. Journal of Geophysical Research (Atmospheres), 117:15112, August 2012. [ bib | DOI | ADS link ]

Observations show that heavy oxygen isotope composition in precipitation (δ18Op) 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 δ18Op is low over SE China as a result of local and upstream condensation and that δ18Op is high over NW China because of evaporative enrichment of 18O as raindrops fall through dry air. We show that δ18Op 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 δ18Op over large areas of southern and central China, consistent with coherent speleothem δ18Op variations over different parts of China. Previous studies have documented high correlations between speleothem δ18Op and millennial timescale climate forcings, and we suggest that the high correlation between insolation and speleothem δ18Op in southern China reflects the variations of hydrologic processes over the Indian monsoon region on millennial and orbital timescales. The δ18Op in the drier part (north of 30degN) 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.

G. Tremoy, F. Vimeux, S. Mayaki, I. Souley, O. Cattani, C. Risi, G. Favreau, and M. Oi. A 1-year long δ18O record of water vapor in Niamey (Niger) reveals insightful atmospheric processes at different timescales. Geophysical Research Letters, 39:8805, April 2012. [ bib | DOI | ADS link ]

We present a 1-year long representative δ18O record of water vapor (δ18Ov) in Niamey (Niger) using the Wavelength Scanned-Cavity Ring Down Spectroscopy (WS-CRDS). We explore how local and regional atmospheric processes influence δ18Ov variability from seasonal to diurnal scale. At seasonal scale, δ18Ov exhibits a W-shape, associated with the increase of regional convective activity during the monsoon and the intensification of large scale subsidence North of Niamey during the dry season. During the monsoon season, δ18Ov records a broad range of intra-seasonal modes in the 25-40-day and 15-25-day bands that could be related to the well-known modes of the West African Monsoon (WAM). Strong δ18Ov modulations are also seen at the synoptic scale (5-9 days) during winter, driven by tropical-extra-tropical teleconnections through the propagation of a baroclinic wave train-like structure and intrusion of air originating from higher altitude and latitude. δ18Ov also reveals a significant diurnal cycle, which reflects mixing process between the boundary layer and the free atmosphere during the dry season, and records the propagation of density currents associated with meso-scale convective systems during the monsoon season.

C. Risi, D. Noone, J. Worden, C. Frankenberg, G. Stiller, M. Kiefer, B. Funke, K. Walker, P. Bernath, M. Schneider, S. Bony, J. Lee, D. Brown, and C. Sturm. 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. Journal of Geophysical Research (Atmospheres), 117:5304, March 2012. [ bib | DOI | ADS link ]

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.

C. Risi, D. Noone, J. Worden, C. Frankenberg, G. Stiller, M. Kiefer, B. Funke, K. Walker, P. Bernath, M. Schneider, D. Wunch, V. Sherlock, N. Deutscher, D. Griffith, P. O. Wennberg, K. Strong, D. Smale, E. Mahieu, S. Barthlott, F. Hase, O. GarcíA, J. Notholt, T. Warneke, G. Toon, D. Sayres, S. Bony, J. Lee, D. Brown, R. Uemura, and C. Sturm. Process-evaluation of tropospheric humidity simulated by general circulation models using water vapor isotopologues: 1. Comparison between models and observations. Journal of Geophysical Research (Atmospheres), 117:5303, March 2012. [ bib | DOI | ADS link ]

The goal of this study is to determine how H2O 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/H2O 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.

M. Berkelhammer, C. Risi, N. Kurita, and D. C. Noone. The moisture source sequence for the Madden-Julian Oscillation as derived from satellite retrievals of HDO and H2O. Journal of Geophysical Research (Atmospheres), 117:3106, February 2012. [ bib | DOI | ADS link ]

A number of competing theories to explain the initiation mechanism, longevity and propagation characteristics of the Madden-Julian Oscillation (MJO) have been developed from observational analysis of the tropical climate and minimal dynamical models. Using the isotopic composition of atmospheric moisture from paired satellite retrievals of H2O and HDO from the boundary layer and mid troposphere, we identify the different sources of moisture that feed MJO convection during its life cycle. These fluxes are then associated with specific dynamical processes. The HDO/H2O isotope ratio data show that during the early phase of the MJO, the mid-troposphere is dominated by moisture evaporated from the ocean surface that was transported vertically undergoing minimal distillation. The contribution from the evaporative source diminishes during early convective activity but reappears during the peak of MJO activity along with an isotopically depleted flux, which is hypothesized to originate from easterly convergence. The contribution of different moisture sources as shown from the HDO/H2O data is consistent with model results where the sustaining of deep convection requires a feedback between convergence, precipitation strength and evaporation. In the wake of an MJO event, the weak vertical isotopic gradient, depletion in boundary layer δD and the uniquely moist and depleted vapor in the mid troposphere all point toward a prominent presence of moisture originated from rainfall re-evaporation, which confirms the prediction that the transition from convective to stratiform rains is important to the moisture budget of the MJO.

C. Shi, V. Daux, Q.-B. Zhang, C. Risi, S.-G. Hou, M. Stievenard, M. Pierre, Z. Li, and V. Masson-Delmotte. Reconstruction of southeast Tibetan Plateau summer climate using tree ring δ18O: moisture variability over the past two centuries. Climate of the Past, 8:205-213, February 2012. [ bib | DOI | ADS link ]

A tree-ring δ18O chronology of Linzhi spruce, spanning from AD 1781 to 2005, was developed in Bomi, Southeast Tibetan Plateau (TP). During the period with instrumental data (AD 1961-2005), this record is strongly correlated with regional CRU (Climate Research Unit) summer cloud data, which is supported by a precipitation δ18O simulation conducted with the isotope-enabled atmospheric general circulation model LMDZiso. A reconstruction of a regional summer cloud index, based upon the empirical relationship between cloud and diurnal temperature range, was therefore achieved. This index reflects regional moisture variability in the past 225 yr. The climate appears drier and more stable in the 20th century than previously. The drying trend in late 19th century of our reconstruction is consistent with a decrease in the TP glacier accumulation recorded in ice cores. An exceptional dry decade is documented in the 1810s, possibly related to the impact of repeated volcanic eruptions on monsoon flow.

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