Means and Covariance Functions for Geostatistical Compositional Data: an Axiomatic Approach     

Denis Allard  Thierry Marchant 《Mathematical Geosciences》2018,50(3):299-315

This work focuses on the characterization of the central tendency of a sample of compositional data. It provides new results about theoretical properties of means and covariance functions for compositional data, with an axiomatic perspective. Original results that shed new light on geostatistical modeling of compositional data are presented. As a first result, it is shown that the weighted arithmetic mean is the only central tendency characteristic satisfying a small set of axioms, namely continuity, reflexivity, and marginal stability. Moreover, this set of axioms also implies that the weights must be identical for all parts of the composition. This result has deep consequences for spatial multivariate covariance modeling of compositional data. In a geostatistical setting, it is shown as a second result that the proportional model of covariance functions (i.e., the product of a covariance matrix and a single correlation function) is the only model that provides identical kriging weights for all components of the compositional data. As a consequence of these two results, the proportional model of covariance function is the only covariance model compatible with reflexivity and marginal stability.  相似文献   

A Comparative Study of Radiation Fog and Quasi-Fog Formation Processes During the ParisFog Field Experiment 2007     

Martial Haeffelin  Jean-Charles Dupont  Neda Boyouk  Darrel Baumgardner  Laurent Gomes  Greg Roberts  Thierry Elias 《Pure and Applied Geophysics》2013,170(12):2283-2303

Fog is an atmospheric phenomenon that has important environmental consequences related to visibility, air quality and climate change on local and regional scales. The formation of radiation fog results from a complex balance between surface radiative cooling, turbulent mixing in the surface layer, aerosol growth by deliquescence and activation of fog droplets. During the ParisFog field experiment, out of 16 events forecasted for radiation fog, activated fog materialized in seven events, while in five other events the visibility dropped to 1–2 km but haze particle size remained below the critical size of activation. To better understand the conditions that lead to or do not lead to sustained fog droplet activation, we performed a comparative study of dynamic, thermal, radiative and microphysical processes occurring between sunset and fog (or quasi-fog) onset. We selected two radiation fog events and two quasi-radiation fog events that occurred under similar large-scale conditions for this comparative study. We identified that aerosol growth by deliquescence and droplet activation actually occurred in both quasi-fog events, but only during <1 h. Based on ParisFog measurements, we found that the main factors limiting sustained activation of droplets at fog onset in the Paris metropolitan area are (1) lack of mixing in the surface layer (typically wind speed <0.5 ms^?1), (2) relative humidity exceeding 90 % throughout the residual layer, (3) low cooling rate in the surface layer (typically less than ?1 °C per hour on average) due to weak radiative cooling (0 to ?30 Wm^?2) and near zero sensible heat fluxes, and (4) a combination of the three factors listed above during the critical phase of droplet activation preventing the transfer of cooling from the surface to the liquid layer. In addition, we found some evidence of contrasted aerosol growth by deliquescence under high relative humidity conditions in the four events, possibly associated with the chemical nature of the aerosols, which could be another factor impacting droplet activation.  相似文献   

Threshold of erosion of submarine bedrock landscapes by tidal currents     

Neil C. Mitchell  John M. Huthnance  Thierry Schmitt  Brian Todd 《地球表面变化过程与地形》2013,38(6):627-639

Since sea level stabilized 7000 yr bp , shelf seas experiencing semi‐diurnal tides will have been affected by streaming four times per day. If tidal erosion of bedrock were even only marginally efficient, the ~10 million streamings since then should have left geomorphological imprints. We examine high‐resolution multibeam sonar data from three areas with extreme tidal currents. The Minas Passage (Bay of Fundy) experiencing 8‐knot surface tidal currents was surveyed in 2007 with a multibeam sonar. In an area near to transverse dunes, which are evidence for bedload transport, the data show local overhanging surfaces near to the sediment‐rock contact, potentially created by abrasion by saltating particles. However, they are uncommon. In the Straits of Messina, where surface currents reach 10 knots, surveying revealed ridges lying oblique to the flow that are not obviously broken into separate outcrops by erosion. In the Bristol Channel, UK, sonar data collected where currents reach 3·4 knots at 1·5 m above the bed reveal outcrops of limestone with superimposed sand dunes, but only minor rounding of blocks. Holocene tidal currents have apparently been generally ineffective at eroding bedrock. We examine this issue further by compiling extreme tidal streams around the UK and from them estimate shear stresses, representing a macro‐tidal environment where peak surface currents reach 9·7 knots. Those data are compared with shear stresses in mountainous rivers where long‐term rates of erosion are comparable with tectonic uplift rates and are thus geomorphologically significant. Whereas river stresses reach 10²–10³ Pa, the largest tidal stresses are generally 10¹ and only rarely approach 10² Pa, too small for quarrying to operate generally. However, the vertical faces in the Minas Passage may represent the onset of abrasion. Given this limited evidence for abrasion, we explore conditions in the geological past for tides that may have locally eroded bedrock. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

A note on algebraic potentials and Morales–Ramis theory     

Thierry Combot 《Celestial Mechanics and Dynamical Astronomy》2013,115(4):397-404

We present various properties of algebraic potentials, and then prove that some Morales–Ramis theorems readily apply for such potentials even if they are not in general meromorphic potentials. This allows in particular to precise some non-integrability proofs in Celestial Mechanics, where the mutual distances between the bodies appear in the potential, and thus making this analysis unavoidable.  相似文献   

Photoabsorption in Ganymede’s atmosphere     

Gaël Cessateur  Jean Lilensten  Mathieu Barthélémy  Thierry Dudok de Wit  Cyril Simon Wedlund  Guillaume Gronoff  Hélène Ménager  Matthieu Kretzschmar 《Icarus》2012,218(1):308-319

In the framework of future space missions to Ganymede, a pre-study of this satellite is a necessary step to constrain instrument performances according to the mission objectives. This work aims at characterizing the impact of the solar UV flux on Ganymede’s atmosphere and especially at deriving some key physical parameters that are measurable by an orbiter. Another objective is to test several models for reconstructing the solar flux in the Extreme-UV (EUV) in order to give recommendations for future space missions.Using a Beer–Lambert approach, we compute the primary production of excited and ionized states due to photoabsorption, neglecting the secondary production that is due to photoelectron impacts as well as to precipitated suprathermal electrons. Ions sputtered from the surface are also neglected. Computations are performed at the equator and close to the pole, in the same conditions as during the Galileo flyby. From the excitations, we compute the radiative relaxation leading to the atmospheric emissions. We also propose a simple chemical model to retrieve the stationary electron density. There are two main results: (i) the modelled electron density and the one measured by Galileo are in good agreement. The main atmospheric visible emission is the atomic oxygen red line at 630 nm, both in equatorial and in polar conditions, in spite of the different atmospheric compositions. This emission is measurable from space, especially for limb viewing conditions. The OH emission (continuum between 260 and 410 nm) is also probably measurable from space. (ii) The input EUV solar flux may be directly measured or reconstructed from only two passbands solar observing diodes with no degradation of the modelled response of the Ganymede’s atmosphere. With respect to these results, there are two main conclusions: (i) future missions to Ganymede should include the measurement of the red line as well as the measurement of OH emissions in order to constrain the atmospheric model. (ii) None of the common solar proxies satisfactorily describes the level of variability of the solar EUV irradiance. For future atmospheric planetary space missions, it would be more appropriate to derive the EUV flux from a small radiometer rather than from a full-fledged spectrometer.  相似文献   

Non-integrability of the equal mass n-body problem with non-zero angular momentum     

Thierry Combot 《Celestial Mechanics and Dynamical Astronomy》2012,114(4):319-340

We prove an integrability criterion and a partial integrability criterion for homogeneous potentials of degree ?1 which are invariant by rotation. We then apply it to the proof of the meromorphic non-integrability of the n-body problem with Newtonian interaction in the plane on a surface of equation (H, C) = (H ₀, C ₀) with (H ₀, C ₀) ?? (0, 0) where C is the total angular momentum and H the Hamiltonian, in the case where the n masses are equal. Several other cases in the 3-body problem are also proved to be non integrable in the same way, and some examples displaying partial integrability are provided.  相似文献   

A model for conductive faults with non-matching grids     

Xavier Tunc  Isabelle Faille  Thierry Gallou?t  Marie Christine Cacas  Pascal Havé 《Computational Geosciences》2012,16(2):277-296

In this paper, we are interested in modeling single-phase flow in a porous medium with known faults seen as interfaces. We mainly focus on how to handle non-matching grids problems arising from rock displacement along the fault. We describe a model that can be extended to multi-phase flow where faults are treated as interfaces. The model is validated in an academic framework and is then extended to 3D non K-orthogonal grids, and a realistic case is presented.  相似文献   

Sensitivity of coastal polynyas and high-salinity shelf water production in the Ross Sea,Antarctica, to the atmospheric forcing   总被引：1，自引：1，他引：0  

Pierre Mathiot  Nicolas C. Jourdain  Bernard Barnier  Hubert Gallée  Jean Marc Molines  Julien Le Sommer  Thierry Penduff 《Ocean Dynamics》2012,62(5):701-723

Coastal polynyas around Antarctica are the place of intense air–sea exchanges which eventually lead to the formation of high-salinity shelf waters (HSSW) over continental shelves. Here, the influence of atmospheric forcing on coastal polynyas in the Ross Sea is studied by contrasting the response of a regional ocean/sea-ice circulation model to two different atmospheric forcing sets. A first forcing (DFS3) is based on ERA40 atmospheric surface variables and satellite products. A second forcing (MAR) is produced on the basis of ERA40 with a dynamical downscaling procedure. As compared to DFS3, MAR forcing is shown to improve substantially the representation of small-scale patterns of coastal winds with stronger katabatic winds along the coast. The response of the ocean/sea-ice model to the two forcing sets shows that the MAR forcing improves substantially the geographical distribution of polynyas in the Ross Sea. With the MAR forcing, the polynya season is also shown to last longer with a greater ice-production rate. As a consequence, a greater flow of dense water out of the polynyas is found with the MAR forcing and the properties of HSSW are notably improved as compared to the DFS3 forcing. The factors contributing to the activity of Terra Nova Bay and Ross Ice Shelf polynyas in the model are studied in detail. The general picture that emerges from our simulations is that the properties of HSSW are mostly set by brine rejection when the polynya season resume. We found that coastal polynyas in the Ross Sea export about 0.4 Sv of HSSW which then flows along three separate channels over the Ross Shelf. A 6-month time lag is observed between the peak of activity of polynyas and the maximum transport across the sills in the channels with a maximum transport of about 1 Sv in February. This lag corresponds to the time it takes to the newly formed HSSW to spread from the polynya to the sills (at a speed of nearly 2 cm s⁻¹).  相似文献   

The origin and fate of mode water in the southern Pacific Ocean     

Audrey Hasson  Ariane Koch-Larrouy  Rosemary Morrow  Mélanie Juza  Thierry Penduff 《Ocean Dynamics》2012,62(3):335-354

Understanding the origin and fate of mode and intermediate waters (MW) in the subtropical Pacific Ocean is critical for climate, as MW store and export a large volume of CO₂, heat, nutrients and salinity to lower latitudes at depths isolated from the atmosphere. A realistic 4D simulation has been used to track and quantify the MW routes and their property characteristics at the last region of subduction. It also allows us to quantify the water transformation after subduction. The simulation has been compared to available observations using a collocation method that interpolated model data onto observations in time and space. The comprehensive comparisons gave us confidence in the model’s capacity to reproduce MW characteristics. A quantitative Lagrangian analysis was performed on the model output to depict the origin, the fate and the route of MW circulating in the southern Pacific Ocean, selected in the density range of 26.8–27.4 kg m⁻³. We found 18 Sv of MW were transported northward in patches through the 42° S section, mostly between 200 and 800 m depth. Of this transport, 8 Sv enters the Pacific Ocean in the upper layer south of Tasmania and subducts in the Pacific. The remainder is not ventilated in the Pacific sector: 4 Sv is advected from the Indian Ocean south of Tasmania at intermediate depth and finally 6 Sv is part of an intermediate depth recirculation within the Pacific Ocean. Particles take up to 30 years to travel northward through our domain before crossing the 42° S section. Southward transport branches also exist: 3 Sv flows southward following the eastern New Zealand coast and then exits through Drake Passage. An additional 4 Sv passes southward in the Tasman Sea, following the eastern Tasmanian coast and enters the Indian Ocean south of Tasmania, as part of the Tasman Leakage. Four different formation sites have been identified, where the MW are last ventilated. These formation sites have different water masses with specific salinity ranges. A study on the evolution of the physical characteristics of each of these water masses has been performed. All MW characteristics become more homogeneous at 42° S than they were when they left the mixed layer. This study confirms the homogenisation of mode waters at intermediate depth in the Pacific Ocean as previously revealed in the Indian Ocean using the same methodology. Transformations are shown to be mostly isopycnal in the Tasman Sea and diapycnal farther east.  相似文献   

Stochastic tracking of mesoscale convective systems: evaluation in the West African Sahel     

Alexandros Makris  Clémentine Prieur  Théo Vischel  Guillaume Quantin  Thierry Lebel  Rémy Roca 《Stochastic Environmental Research and Risk Assessment (SERRA)》2016,30(2):681-691

In this work we apply a recently proposed Bayesian multiple target tracking model to mesoscale convective systems tracking. This stochastic model follows the multiple hypothesis tracking paradigm and can handle a varying number of targets while detecting the target birth, death, split, and merge events. The model is tested experimentally with real MCS targets detected from meteosat IR data over the Sahelian region. The performance of the stochastic tracking is evaluated by comparing it qualitatively and quantitatively with well established deterministic methods.  相似文献