Author
Camille Risi
Other affiliations: École Normale Supérieure, University of Colorado Boulder, Pierre-and-Marie-Curie University ...read more
Bio: Camille Risi is an academic researcher from Centre national de la recherche scientifique. The author has contributed to research in topics: Precipitation & Water vapor. The author has an hindex of 44, co-authored 105 publications receiving 6042 citations. Previous affiliations of Camille Risi include École Normale Supérieure & University of Colorado Boulder.
Topics: Precipitation, Water vapor, Ice core, δ18O, Troposphere
Papers
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TL;DR: In this article, the authors established a database of precipitation δ18O and used different models to evaluate the climatic controls of precipitation over the Tibetan Plateau (TP), revealing three distinct domains associated with the influence of the westerlies (northern TP), Indian monsoon (southern TP), and transition in between.
Abstract: The stable oxygen isotope ratio (δ18O) in precipitation is an integrated tracer of atmospheric processes worldwide. Since the 1990s, an intensive effort has been dedicated to studying precipitation isotopic composition at more than 20 stations in the Tibetan Plateau (TP) located at the convergence of air masses between the westerlies and Indian monsoon. In this paper, we establish a database of precipitation δ18O and use different models to evaluate the climatic controls of precipitation δ18O over the TP. The spatial and temporal patterns of precipitation δ18O and their relationships with temperature and precipitation reveal three distinct domains, respectively associated with the influence of the westerlies (northern TP), Indian monsoon (southern TP), and transition in between. Precipitation δ18O in the monsoon domain experiences an abrupt decrease in May and most depletion in August, attributable to the shifting moisture origin between Bay of Bengal (BOB) and southern Indian Ocean. High-resolution atmospheric models capture the spatial and temporal patterns of precipitation δ18O and their relationships with moisture transport from the westerlies and Indian monsoon. Only in the westerlies domain are atmospheric models able to represent the relationships between climate and precipitation δ18O. More significant temperature effect exists when either the westerlies or Indian monsoon is the sole dominant atmospheric process. The observed and simulated altitude-δ18O relationships strongly depend on the season and the domain (Indian monsoon or westerlies). Our results have crucial implications for the interpretation of paleoclimate records and for the application of atmospheric simulations to quantifying paleoclimate and paleo-elevation changes.
604 citations
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TL;DR: The probability that hurricane winds will affect any given point in space by combining an estimate of the probability that a hurricane will pass within some given radius of the point in question with an estimates of the spatial probability density of storm winds.
Abstract: Hurricanes are lethal and costly phenomena, and it is therefore of great importance to assess the long-term risk they pose to society. Among the greatest threats are those associated with high winds and related phenomena, such as storm surges. Here we assess the probability that hurricane winds will affect any given point in space by combining an estimate of the probability that a hurricane will pass within some given radius of the point in question with an estimate of the spatial probability density of storm winds. To assess the probability that storms will pass close enough to a point of interest to affect it, we apply two largely independent techniques for generating large numbers of synthetic hurricane tracks. The first treats each track as a Markov chain, using statistics derived from observed hurricane-track data. The second technique begins by generating a large class of synthetic, time-varying wind fields at 850 and 250 hPa whose variance, covariance, and monthly means match NCEP–NCAR reanalysis d...
476 citations
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TL;DR: In this article, stable water isotopes (H218O and HDO) have been introduced in a single column model including the Emanuel convection parameterization, and the physical processes underlying the amount effect and propose a methodology to quantify their relative contributions.
Abstract: [1] In the tropics, the proportion of heavier water isotopes in precipitation is anticorrelated with the precipitation amount. The physical processes underlying this so-called amount effect are still poorly understood and quantified. In the present study, stable water isotopes (H218O and HDO) have been introduced in a single column model including the Emanuel convection parameterization. We investigate the physical processes underlying the amount effect and propose a methodology to quantify their relative contributions. We focus on convective processes, since the idealized framework of the single column models does not allow us to consider the effects of large-scale horizontal advections of air masses of different isotopic signatures. We show that two kinds of processes predominantly explain the amount effect: first, the reevaporation of the falling rain and the diffusive exchanges with the surrounding vapor; and second, the recycling of the subcloud layer vapor feeding the convective system by convective fluxes. This highlights the importance of a detailed representation of rain evaporation processes to simulate accurately the isotopic composition of precipitation in the tropics. The variability of the isotopic composition on different timescales (from days to months) is also studied using a unidimensional simulation of the Tropical Ocean–Global Atmosphere–Coupled Ocean-Atmosphere Response Experiment (TOGA-COARE) campaign. The amount effect is best observable at intraseasonal or longer timescales. The period of time over which convective activity significantly affects the isotopic composition of precipitation is related to the residence time of water within atmospheric reservoirs.
466 citations
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TL;DR: In this article, the LMDZ-iso general circulation model was used to simulate water-stable isotopes from a midlatitude station and evaluated at different time scales (synoptic to interannual).
Abstract: We present simulations of water-stable isotopes from the LMDZ general circulation model (the LMDZ-iso GCM) and evaluate them at different time scales (synoptic to interannual). LMDZ-iso reproduces reasonably well the spatial and seasonal variations of both delta O-18 and deuterium excess. When nudged with reanalyses, LMDZ-iso is able to capture the synoptic variability of isotopes in winter at a midlatitude station, and the interannual variability in mid and high latitudes is strongly improved. The degree of equilibration between the vapor and the precipitation is strongly sensitive to kinetic effects during rain reevaporation, calling for more synchronous vapor and precipitation measurements. We then evaluate the simulations of two past climates: Last Glacial Maximum (21 ka) and Mid-Holocene (6 ka). A particularity of LMDZ-iso compared to other isotopic GCMs is that it simulates a lower d excess during the LGM over most high-latitude regions, consistent with observations. Finally, we use LMDZ-iso to explore the relationship between precipitation and delta O-18 in the tropics, and we discuss its paleoclimatic implications. We show that the imprint of uniform temperature changes on tropical delta O-18 is weak. Large regional changes in delta O-18 can, however, be associated with dynamical changes of precipitation. Using LMDZ as a test bed for reconstructing past precipitation changes through local delta O-18 records, we show that past tropical precipitation changes can be well reconstructed qualitatively but not quantitatively. Over continents, nonlocal effects make the local reconstruction even less accurate.
315 citations
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Cooperative Institute for Research in Environmental Sciences1, Centre national de la recherche scientifique2, California Institute of Technology3, Karlsruhe Institute of Technology4, Spanish National Research Council5, University of Toronto6, University of York7, Agencia Estatal de Meteorología8, National Institute of Water and Atmospheric Research9, University of Bremen10, University of Wollongong11, University of Liège12, Harvard University13, University of the Ryukyus14, Stockholm University15
TL;DR: In this paper, a large number of isotopic data sets (four satellite, sixteen ground-based remote-sensing, five surface in situ and three aircraft data sets) are analyzed 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).
Abstract: 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. Copyright © 2012 by the American Geophysical Union.
292 citations
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TL;DR: In this article, the authors present a document, redatto, voted and pubblicato by the Ipcc -Comitato intergovernativo sui cambiamenti climatici - illustra la sintesi delle ricerche svolte su questo tema rilevante.
Abstract: Cause, conseguenze e strategie di mitigazione Proponiamo il primo di una serie di articoli in cui affronteremo l’attuale problema dei mutamenti climatici. Presentiamo il documento redatto, votato e pubblicato dall’Ipcc - Comitato intergovernativo sui cambiamenti climatici - che illustra la sintesi delle ricerche svolte su questo tema rilevante.
4,187 citations
01 Dec 2013
TL;DR: This paper found that the most intensive glacier shrinkage is in the Himalayan region, whereas glacial retreat in the Pamir Plateau region is less apparent, due to changes in atmospheric circulations and precipitation patterns.
Abstract: Glacial melting in the Tibetan Plateau affects the water resources of millions of people. This study finds that—partly owing to changes in atmospheric circulations and precipitation patterns—the most intensive glacier shrinkage is in the Himalayan region, whereas glacial retreat in the Pamir Plateau region is less apparent.
1,599 citations
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TL;DR: In this paper, a new technique for inferring tropical cyclone climatology from the output of global models, extend it to predict genesis climatologies, and apply it to current and future climate states simulated by a suite of global climate models developed in support of the most recent Intergovernmental Panel on Climate Change report.
Abstract: Changes in tropical cyclone activity are among the more potentially consequential results of global climate change, and it is therefore of considerable interest to understand how anthropogenic climate change may affect such storms. Global climate models are currently used to estimate future climate change, but the current generation of models lacks the horizontal resolution necessary to resolve the intense inner core of tropical cyclones. Here we review a new technique for inferring tropical cyclone climatology from the output of global models, extend it to predict genesis climatologies (rather than relying on historical climatology), and apply it to current and future climate states simulated by a suite of global models developed in support of the most recent Intergovernmental Panel on Climate Change report. This new technique attacks the horizontal resolution problem by using a specialized, coupled ocean-atmosphere hurricane model phrased in angular momentum coordinates, which provide a high resolution ...
800 citations
01 Dec 2010
TL;DR: In this paper, spatial variations in sea surface temperature (SST) and rainfall changes over the tropics are investigated based on ensemble simulations for the first half of the twenty-first century under the greenhouse gas emission scenario A1B with coupled ocean-atmosphere general circulation models of the Geophysical Fluid Dynamics Laboratory (GFDL) and National Center for Atmospheric Research (NCAR).
Abstract: Spatial variations in sea surface temperature (SST) and rainfall changes over the tropics are investigated based on ensemble simulations for the first half of the twenty-first century under the greenhouse gas (GHG) emission scenario A1B with coupled ocean–atmosphere general circulation models of the Geophysical Fluid Dynamics Laboratory (GFDL) and National Center for Atmospheric Research (NCAR). Despite a GHG increase that is nearly uniform in space, pronounced patterns emerge in both SST and precipitation. Regional differences in SST warming can be as large as the tropical-mean warming. Specifically, the tropical Pacific warming features a conspicuous maximum along the equator and a minimum in the southeast subtropics. The former is associated with westerly wind anomalies whereas the latter is linked to intensified southeast trade winds, suggestive of wind–evaporation–SST feedback. There is a tendency for a greater warming in the northern subtropics than in the southern subtropics in accordance ...
782 citations
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TL;DR: Increases in tropical cyclone activity are most prominent in the western North Pacific, but are evident in other regions except for the southwestern Pacific, consistent with increases in a genesis potential index based on monthly mean global model output.
Abstract: A recently developed technique for simulating large [O(104)] numbers of tropical cyclones in climate states described by global gridded data is applied to simulations of historical and future climate states simulated by six Coupled Model Intercomparison Project 5 (CMIP5) global climate models Tropical cyclones downscaled from the climate of the period 1950–2005 are compared with those of the 21st century in simulations that stipulate that the radiative forcing from greenhouse gases increases by over preindustrial values In contrast to storms that appear explicitly in most global models, the frequency of downscaled tropical cyclones increases during the 21st century in most locations The intensity of such storms, as measured by their maximum wind speeds, also increases, in agreement with previous results Increases in tropical cyclone activity are most prominent in the western North Pacific, but are evident in other regions except for the southwestern Pacific The increased frequency of events is consistent with increases in a genesis potential index based on monthly mean global model output These results are compared and contrasted with other inferences concerning the effect of global warming on tropical cyclones
661 citations