Comparison of tropical cyclogenesis indices on seasonal to interannual timescales   总被引：1，自引：1，他引：0  

Christophe E. Menkes  Matthieu Lengaigne  Patrick Marchesiello  Nicolas C. Jourdain  Emmanuel M. Vincent  Jér?me Lefèvre  Fabrice Chauvin  Jean-Francois Royer 《Climate Dynamics》2012,38(1-2):301-321

This paper evaluates the performances of four cyclogenesis indices against observed tropical cyclone genesis on a global scale over the period 1979–2001. These indices are: the Genesis Potential Index; the Yearly Genesis Parameter; the Modified Yearly Convective Genesis Potential Index; and the Tippett et al. Index (J Clim, 2011), hereafter referred to as TCS. Choosing ERA40, NCEP2, NCEP or JRA25 reanalysis to calculate these indices can yield regional differences but overall does not change the main conclusions arising from this study. By contrast, differences between indices are large and vary depending on the regions and on the timescales considered. All indices except the TCS show an equatorward bias in mean cyclogenesis, especially in the northern hemisphere where this bias can reach 5°. Mean simulated genesis numbers for all indices exhibit large regional discrepancies, which can commonly reach up to ±50%. For the seasonal timescales on which the indices are historically fitted, performances also vary widely in terms of amplitude although in general they all reproduce the cyclogenesis seasonality adequately. At the seasonal scale, the TCS seems to be the best fitted index overall. The most striking feature at interannual scales is the inability of all indices to reproduce the observed cyclogenesis amplitude. The indices also lack the ability to reproduce the general interannual phase variability, but they do, however, acceptably reproduce the phase variability linked to El Ni?o/Southern Oscillation (ENSO)—a major driver of tropical cyclones interannual variations. In terms of cyclogenesis mechanisms that can be inferred from the analysis of the index terms, there are wide variations from one index to another at seasonal and interannual timescales and caution is advised when using these terms from one index only. They do, however, show a very good coherence at ENSO scale thus inspiring confidence in the mechanism interpretations that can be obtained by the use of any index. Finally, part of the gap between the observed and simulated cyclogenesis amplitudes may be attributable to stochastic processes, which cannot be inferred from environmental indices that only represent a potential for cyclogenesis.  相似文献   

The ability of general circulation models to simulate tropical cyclones and their precursors over the North Atlantic main development region   总被引：1，自引：1，他引：0  

Anne Sophie Daloz  Fabrice Chauvin  Kevin Walsh  Sally Lavender  Deborah Abbs  Frank Roux 《Climate Dynamics》2012,39(7-8):1559-1576

The ability of General Circulation Models (GCMs) to generate Tropical Cyclones (TCs) over the North Atlantic Main Development Region (MDR; 10–20°N, 20–80°W; Goldenberg and Shapiro in J Clim 9:1169–1187, 1996) is examined through a subset of ocean–atmosphere coupled simulations from the World Climate Research Programme (WCRP) Coupled Model Intercomparison Project phase 3 (CMIP3) multimodel data set and a high-resolution (0.5°) Sea Surface Temperature (SST)-forced simulation from the Australian Conformal-Cubic Atmospheric Model GCM. The results are compared with National Center for Environmental Prediction (NCEP-2) and European Center for Medium Range Weather Forecasts Re-Analysis (ERA-40) reanalyses over a common period from 1980 to 1998. Important biases in the representation of the TC activity are encountered over the MDR. This study emphasizes the strong link in the GCMs between African Easterly Waves (AEWs) and TC activity in this region. However, the generation of AEWs is not a sufficient condition alone for the models to produce TCs. Precipitation over the Sahel, especially rainfall over the Fouta Djallon highlands (cf. Fig.?1), is playing a role in the generation of TCs over the MDR. The influence of large-scale fields such as SST, vertical wind shear and tropospheric humidity on TC genesis is also examined. The ability of TC genesis indices, such as the Genesis Potential Index and the Convective Yearly Genesis Potential, to represent TC activity over the MDR in simulations at low to high spatial resolutions is analysed. These indices are found to be a reasonable method for comparing cyclogenesis in different models, even though other factors such as AEW activity should also be considered.  相似文献   

Surface chemistry and reactivity of plant phytoliths in aqueous solutions     

Fabrice Fraysse  Oleg S. Pokrovsky  Jacques Schott  Jean-Dominique Meunier 《Chemical Geology》2009,258(3-4):197-206

In order to better understand the reactivity of plant phytoliths in soil solutions, we determined the solubility, surface properties (electrophoretic mobilities and surface charge) and dissolution kinetics of phytoliths extracted from fresh biomass of representative plant species (larch tree and elm, horsetail, fern, and four grasses) containing significant amount of biogenic silica. The solubility product of larch, horsetail, elm and fern phytoliths is close to that of amorphous silica and soil bamboo phytoliths. Electrophoretic measurements yield isoelectric point pH_IEP = 0.9, 1.1, 2.0 and 2.2 for four grasses, elm, larch and horsetail phytoliths respectively, which is very close to that of quartz or amorphous silica. Surface acid–base titrations allowed generation of a 2-pK surface complexation model (SCM) for larch, elm and horsetail phytoliths. Phytoliths dissolution rates, measured in mixed-flow reactors at far from equilibrium conditions at 1 ≤ pH ≤ 8, were found to be very similar among the species, and close to those of soil bamboo phytoliths. Mechanistic treatment of all plant phytoliths dissolution rates provided three-parameters equation sufficient to describe phytoliths reactivity in aqueous solutions:$R(\text{mol}/{\text{cm}}^2/\text{s})=6?{10}^{?16}?a_{\text{H+}}+5.0?{10}^{?18}+3.5?{10}^{?13}?a_{\text{OH}?}^{0.33}$Alternatively, the dissolution rate dependence on pH can be modeled within the concept of surface coordination theory assuming the rate proportional to concentration of > SiOH₂⁺, > SiOH⁰ and > SiO^? species. In the range of Al concentration from 20 to 5000 ppm in the phytoliths, we have not observed any correlation between their Al content and solubility, surface acid–base properties and dissolution kinetics.It follows from the results of this study that phytoliths dissolution rates exhibit a minimum at pH ～ 3. Mass-normalized dissolution rates are similar among all four types of plant species studied and these rates are an order of magnitude higher than those of typical soil clay minerals. The minimal half life time of larch and horsetail phytoliths in the interstitial soil solution ranges from 10–12 years at pH = 2–3 to < 1 year at pH above 6, comparable with mean residence time of phytoliths in soil from natural observations.  相似文献   

Reply to comments on “Intrusive rocks and tectono-metamorphic evolution of the Mako Paleoproterozoic belt (Eastern Senegal, West Africa)”     

Mamadou Gueye  Papa Malick Ngom  Mouhamadane Dine  Yaouba Thiam  Siegfried Siegesmund  Klaus Wemmer  Sabine Pawlig 《Journal of African Earth Sciences》2009,55(5):269-270

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Rate of hydrogen-iron redox exchange in silicate melts and glasses     

Fabrice Gaillard  Steven Mackwell 《Geochimica et cosmochimica acta》2003,67(13):2427-2441

A kinetic model for the rate of iron-hydrogen redox exchange in silicate glasses and melts has been derived from time-series experiments performed on natural rhyolitic obsidians. Cylinders of the starting glasses were exposed to reducing mixtures composed of H₂-Ar-CO₂-CO in 1-atm furnaces and H₂-Ar in a cold seal pressure vessel. Overall, runs covered the temperature range 300 to 1000°C. The progression of a front of ferric iron reduction within the quenched melt was observed optically through a change of color. For all run conditions, the advancement of the front (ξ) was proportional to the square root of time, revealing the reaction as a diffusion-limited process. Iso-fO₂ runs performed in CO₂-CO, H₂-Ar, and H₂-CO₂ gases have shown that fH₂ rather than fO₂ is the dominant parameter controlling the reaction rate. The fH₂ dependence of the rate constant was characterized in the range 0.02 to 70 bar. The growth of the reduced layer, which is accompanied by an increase in reaction-derived OH-group content, was fitted considering that the reaction rate is controlled by the migration of a free mobile species (H₂) immobilized in the form of OH subsequent to reaction with ferric iron. The reaction rate is thus a function of both solubility and diffusivity of H₂ weighted by the concentration of its sink (ferric iron). We extracted a single law for both solubility and diffusivity of H₂ in amorphous silicates that applies over a range of temperatures below and above the glass transition temperature. Melt/glass structure (degree of polymerization) does not seem to significantly affect both solubility and diffusivity of H₂. We therefore provide a model that allows the prediction of oxidation-reduction rates in the presence of hydrogen for a wide range of compositions of amorphous glasses and melts. Comparisons with previous work elucidating rate of redox exchange in dry systems allow us to anticipate the fH₂-T domains where different redox mechanisms may apply. We conclude that equilibration of redox potential in nature should be dominated by H₂ transfer at a rate controlled by both H₂ solubility and diffusion. Numerical applications of the model illustrate redox exchanges in natural magmas and in glasses exposed to weathering under near surface conditions. We show that crustal events such as magmas mixing should not modify the iron redox state of magmas. In the case of nuclear-waste-bearing glasses, the fH₂ conditions in the host terrain are clearly a parameter that must be taken into account to predict glass durability.  相似文献   

Experimental determination of activities of FeO and Fe₂O₃ components in hydrous silicic melts under oxidizing conditions     

Fabrice Gaillard  Michel Pichavant  Bruno Scaillet 《Geochimica et cosmochimica acta》2003,67(22):4389-4409

The critical role of iron on crystal-silicate liquid relationships and melt differentiation is mainly controlled by the redox conditions prevailing in magmas, but the presently available database merely constrains the thermodynamic properties of iron-bearing components in strongly reduced and anhydrous molten silicate where iron is in the ferrous form. This paper provides new standard states for pure ferrous (FeO^liq) and ferric (Fe₂O₃^liq) molten iron oxides and extends the experimental database towards oxidizing and water-bearing domains. Iron-iridium, iron-platinum alloys, magnetite or hematite were equilibrated with synthetic silicic liquids at high temperature and high pressure under controlled oxygen fugacity (fO₂) to determine activity-composition relationships for FeO^liq and Fe₂O₃^liq. Between 1000 and 1300°C, the fO₂ ranges from that in air to 3-log units below that of the nickel-nickel oxide buffer (NNO). Experiments were performed on both anhydrous and hydrous melts containing up to 6-wt.% water. Incorporation of water under reducing conditions increases the activity coefficient of FeO^liq but has an opposite effect on Fe₂O₃^liq. As calcium is added to system, the effect of water becomes weaker and is inverted for Fe₂O₃^liq. Under oxidizing conditions, water has a negligible effect on both activities of FeO^liq and Fe₂O₃^liq. In contrast, changes in redox conditions dominate the activity coefficients of both FeO^liq and Fe₂O₃^liq, which increase significantly with increasing fO₂. The present results combined with the previous work provide a specific database on the energetics of iron in silicate melts that cover most of the condition prevailing in natural magmas.  相似文献   

A numerical and statistical investigation of a seiche oscillation of the Ligurian Sea     

Lorenzo Papa 《Ocean Dynamics》1981,34(1):15-25

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A numerical model of an elliptic vortex in the Ligurian Sea     

Dr. L. Papa 《Meteorology and Atmospheric Physics》1980,29(1-2):157-165

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Palaeozoic (Silurian and Devonian) radiolarians and conodonts from chert olistoliths of the Volissos Turbidites, Chios island, Greece     

Christiano Larghi  Fabrice Cordey  Carlo Corradini  Maurizio Gaetani  Alda Nicora 《Eclogae Geologicae Helvetiae》2005,98(1):123-131

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Temperature dependence of shear wave attenuation in partially molten gabbronorite at seismic frequencies     

Fabrice R. Fontaine  Benoit Ildefonse  Nickolai S. Bagdassarov 《Geophysical Journal International》2005,163(3):1025-1038

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