5,290 research outputs found
Improving the altimetric rain record from Jason-1 & Jason-2
No full textDual-frequency rain-flagging has long been a standard part of altimetric data analysis, both for quality control of the data and for the study of rain itself, because altimeters can provide a finer spatial sampling of rain than can passive microwave instruments. However, there have been many varied implementations, using different records of the surface backscatter and different thresholds. This paper compares four different measures available for the recently-launched Jason-2. The evaluation compares these measures against clearly desired properties, finding that in most cases the adjusted backscatter and that from the ice retracker perform much better than that recommended in the users' handbook. The adjusted backscatter measure also provides a much better link to observations from Jason-1, opening up a much longer period for consistent rain investigations, and enabling greatly improved analysis of the short-scale variability of precipitation. Initial analysis shows that although the spatial and temporal gradients of backscatter increase at very low winds, the spatial gradients in rain attenuation are concentrated where rainfall is greatest, whilst the temporal changes have a simple broad latitudinal pattern
F.C. Hawthorne, Landmark papers : structure topology
No full textIn this second volume of the Mineralogical Society’s ‘Landmark’ series, Prof. Frank Hawthorne has selected a number of key papers, some of which are true milestones of mineralogy and crystallography, showing the acceleration of research and the increase in knowledge in the field of crystal-chemistry. The papers follow in chronological sequence, allowing the reader to see how crystallography and, particularly, mineralogy have evolved during the last 80 years. He has chosen the papers on the basis of three related aspects: (a) the nature of chemical bonds, and (b) their relation to bond topology, leading to (c) the prediction of bond topologies and their hierarchical organization. His commentaries on the selected papers provided a coherent narrative thread running through the volume. In the first chapter ‘Bond topology and Minerals’, Hawthorne reviews the long history of the mineralogy and crystallography, reporting the evolution of the knowledge and the experimental findings in the last 2,000 years. The author introduces the mathematical concept of topology and how to use this tool for the description of the structural configuration in crystals. In addition, he discusses the motivation of mineralogists for understanding and developing principles of bond topology. In chapter 2, two milestone papers by Linus Pauling (both published in 1929) on the structure of complex ionic crystals are reported and enriched with comments. Chapter 3 is devoted to a further milestone paper for mineralogy written by W.L. Bragg (1930), on the structure classification of the silicate minerals, the isomorphous replacement in silicates and on the application of the Pauling’s rules to this class of minerals. In Chapter 4, we jump to the 1970s with the paper of P.B. Moore (1970) on the stereoisomerism among octahedral and tetrahedral chains. Moore based his study not on a specific mineral structure, but examined the different ways in which polyhedra could link via vertices to form chains. He defines the concept of ‘‘structural hierarchy’’ as a general scheme that ties together a certain number of arrangements. An extension of the structural analysis of Moore is found in chapter 6, which is devoted to his systematic study of edge-sharing clusters, deriving the possible arrangements based solely on topological and geometrical principles according to the notions of energy minima and stability (Moore 1974). Chapter 5 deals with the paper by Brown and Shannon (1973), on the empirical bond-valence/ bond-length curves for oxides. A further refinement of the Brown and Shannon approach, into a comprehensive theory that addresses many aspects of the chemical bonding, was developed by Brown (1981), and is presented in chapter 8. Bond-valence analysis of inorganic crystal structures is an essential check on the validity of any structure determination. In his commentary on chapter 8, Hawthorne outlines the critical points introduced by Brown in the bond-valence theory, with interesting application in mineralogy, and presents the bond-valence theory as a molecular orbital theory and as an ionic theory. Chapter 7 is devoted to the paper by L.S. Dent Glasser (1979) on non-existent silicates, emphasizing that the observed arrangements in silicates represent only a small fraction of those topologically possible. Chapter 9 deals with the paper of Hawthorne (1983) on the graphical enumeration of polyhedral clusters. The author developed a ‘‘structural hierarchy’’ hypothesis which has an energetic basis and relates to paragenetic sequences. An example is Bowen’s reaction series shown as a function of the polymerization characteristic of the structure involved. A related topic is covered in chapter 10, which discusses the energetic content of bond topology with reference to the paper by Burdett et al. (1984). The last paper of this collection constituting chapter 11 is devoted to the role of OH and H2O in oxide and oxysalt minerals, based on Hawthorne (1992). The author analysed the roleplayed by (OH) , (H2O)0, (H3O)+ and (H5O2)2+ in controlling bonding topology, topological dimensionality and the role of H2O as a bond-valence transformer, which bears on the, often highly selective, uptake of interstitial cations by environmentally significant minerals. Chapter 12 is the coda, focusing on the prediction of bond topology and of the stoichiometry of stable compounds in a given chemical system. I think that the re-publication of these landmark papers, accompanied by the commentaries of Prof. Hawthorne, will be useful not only for undergraduate or PhD students, but for all structural mineralogists. This collection provides valuable insights into the evolution of structural mineralogy and its wider application to the petrology. As several of the milestone papers collected in this book are published in German journals (Zeitschrift fu ̈r Kristallographie, Neues Jahrbuch fu ̈r Mineralogie Monatshefte), I did a little inquiry and I found that these journals are often not readily available in departmental libraries, and so this is another good reason to have this book in your own library. In conclusion, I warmly recommend this volume to all mineralogists and to Earth sciences libraries.
G. DIEGO GATT
Mozambique Channel Eddies in GCMs: A question of resolution and slippage
No full textHydrographic observations in the 21st century have shown that the flow within the MozambiqueChannel is best described by a series of large poleward-propagating anticyclonic eddies, rather than, aspreviously thought, a continuous intense western boundary current. The portrayal of this region in various runs of the NEMO 75-level model is found to vary between those two descriptions depending upon the resolution used and the implementation of the model's lateral boundary conditions. In a comparison of 1/4 ? resolution runs, the change of these conditions from free-slip to no-slip leads to the mean southward flow moving further offshore, with greater variability in the zonal and meridional velocities as the flow organises itself into eddies, and a reduction in total transport. If a realization of a model is unable to get these aspects of the physical flow correct, then this will significantly reduce its ability to show a realistic biological signal or long-term response to climate change. Further south, beyond Durban, the application of no-slip conditions similarly causes the mean Agulhas Current to lie further offshore, making it much more able to simulate Natal Pulses.<br/
Altimeter accuracy requirements for detecting changes in sea level rise
No full textAll nations with a maritime border are concerned about changes in sea level, with an increase in sea level leading to flooding of coastal areas, damage to property, salinification of fresh water aquifers and destruction of valuable agricultural land. Around the change of the millenium global sea level rise was estimated to be 2.7 mm/yr, but various climate models have suggested the rate of rise will increase markedly necessitating governments to take action more swiftly. This report looks briefly at the accuracy requirements and time series duration needed to be able to detect reliably a significant change in the rate of sea level rise. One constraint on detecting an increased trend is the natural interannual variability of the climate system, which implies that a minimum duration of around 10-20 years is required in order to detect a trend with confidence to within 1 mm/yr. Added to this will be the effect of efficiencies in the observing system. This is investigated through a series of simulations, mimicking the sampling of a long-time series by altimeters with random bias and drift terms. Whilst not directly addressing issues relating to the choice of orbit configuration, this preliminary work prototypes a methodology for assessing the design of a long-term altimetry observing system. Naturally the maintenance of multiple altimeter systems plus a representative global network of tide gauges provides the best basis for such monitoring. However, considering only a single system, one notes that the required dataset duration can be between 10 and 60 years depending upon the quality of the altimeter missions. Due to the difficulty of tying separate missions to a common datum, a single short interruption to precise monitoring may add more than a decade to the time required to detect an increased rate of sea level rise
High pressure behaviour of AIP04-5 in penetrating/ non penetrating pressure medium
No full textAluminophosphate are objects of a growing research interest due to their potential technologieal and industriaI applications [e.g 1,2]. Their large channels serve as ideaI host for organie compounds and small polymers. Among those, AIP0-5 is a synthetic zeolite characterized by an open-framework of (P,AI)O4 tetrahedra. The tetrahedra are connected to form six-and twelve-membered rings, in such a way that a large channel (0~7.3À), parallel to the [001] direction, occurs. Klap et al. [3] underlines that every crystal of AIP0-5 is built up by three different microdomains, in which the positions of the framework oxygen atoms are slightly different; the main effect of the structural disorder is the very large anisotropie displacement parameters of the framework oxygens. We performed two in situ single-crystal synchrotron X-ray diffraction experiments using both penetrating (methanol:ethanol:H20 mix, m:e:w) and non-penetrating (silicon oil) pressure media [4]. The structure refinements showed that: 1) for compression in m:e:w mix, H20 molecules are absorbed at low-P regime, forming a H20-network by H-bonding interaction; 2) the elastic parameters of the super-hydrated AIP04 5 are different if compared to the one compressed in silicon oil; 3) the structural deformation mechanisms of super-hydrated and regular AIP04 -5 are different; 4) evidence of a incommensurately modulated structure occur (according to [3]), and there is an evolution of the non-Bragg reflections with pressure.
The author acknowledges the ltalian Ministry of Education, MIUR-Project: "Futuro in Ricerca 2012 -ImPACT-RBFR12CLQD".
[lJ Tang Z.K. et al. Applied Physies Letters 1998; 73, 2287-2289.
[2] Yang W.S. et al. Microporous and mesoporous materials 20i6; 219,87-92. [3J Klap G.J. et al. Mieroporous and mesoporous materials 2000; 38,403-412. [4J Gatta, G.D. Mieroporous and Mesoporous Material 2010; 128, 78-84
Ultra-fast escape of a deformable jet-propelled body
No full textIn this work a cephalopod-like deformable body that fills an internal cavity with fluid and expels it to propel an escape manoeuvre, while undergoing a drastic external shape change through shrinking, is shown to employ viscous as well as mainly inviscid hydrodynamic mechanisms to power an impressively fast start. First, we show that recovery of added-mass energy enables a shrinking rocket in a dense inviscid flow to achieve greater escape speed than an identical rocket in a vacuum. Next, we extend the shrinking body results of Weymouth & Triantafyllou (J. Fluid Mech., vol. 702, 2012, pp. 470–487) to three-dimensional bodies and show that three hydrodynamic mechanisms must be combined to achieve rapid escape performance in a viscous fluid: added-mass energy recovery; flow separation elimination; and an optimized energy storage and recovery. In particular, we show that the mechanism of separation elimination achieved through rapid body shrinking, coordinated with the mechanism of recovering the initially imparted added-mass energy, is critical to achieving a high escape speed. Hence a flexible, collapsing body can be vastly superior to a rigid-shell jet-propelled body
Cs-zeolites under extreme conditions: comparative thermoelastic behaviour of Cs-ABW,Cs-CAS and Cs-ANA
No full text1. Introduction
Cs-bearing zeolites are extremely rare in nature. Pollucite, the Cs-counterpart of analcime, is the only natural Cs-rich zeolite. In the last decades, synthetic Cs-aluminosilicates have been prepared in search for suitable crystalline phases potentially usable as solid hosts for 137Cs γ-radiation source to be used in sterilization applications, or for fixation and deposition of radioactive isotopes of Cs. The thermo-elastic behaviour, the phase-stability and the main P-induced deformation mechanisms of two synthetic Cs-bearing zeolites, Cs-ABW (CsAlSiO4) and Cs-CAS (CsAlSi5O12), and one natural zeolite, pollucite Cs-ANA [(Cs,Na)AlSi2O6 x nH2O] have been recently investigated. Here we provide a comparative analysis of the response at high-pressure and high-temperature of the three mentioned Cs-rich zeolites.
2. Experimental Methods and Results
2.1. Cs-ABW
The elastic behavior of the synthetic zeolite CsAlSiO4 (a~9.446, b~5.439, and c~ 8.927 Å, space group Pc21n)[1] is under investigation by in-situ powder synchrotron X-ray diffraction with a diamond anvil cell under hydrostatic conditions. Data are currently available up to ~7 GPa. The material preserves its crystallinity and no phase transition appears to occur within the P-range investigated. Fitting the P-V data with a third-order Birch-Murnaghan equation-of-state (BM-EoS), we obtained: V0 = 457.9(4) Å3, KT0 = 42(1) GPa and K’ = 3.9(3). The “axial moduli” were calculated with a third-order “linearized” BM-EoS, substituting the cube of the individual lattice parameter (a3, b3, c3) for the volume. The refined axial-EoS parameters are: KT0a = 271(9) GPa (b_a = 0.00123(4) GPa-1), K’a = 4 (fixed) for the a-axis; KT0b = 181(3) GPa (b_b = 0.00184(3) GPa-1), K’b = 4 (fixed) for the b-axis; KT0c =14.5(5) GPa (b_c = 0.0230(8) GPa-1), K’c = 2.6(1) for the c-axis (KT0a : KT0b : KT0c = 19 : 12 : 1).
Previous high-temperature experiments showed that Cs-ABW transforms irreversibly to Cs-ANA framework-type zeolite at 1423 K [2].
2.2. Cs-CAS
The elastic and structural behavior of the synthetic zeolite CsAlSi5O12 (a~16.753, b~13.797 and c~5.023 Å, space group Ama2) were investigated up to ~8.5 GPa by in-situ single-crystal X-ray diffraction with a diamond anvil cell under hydrostatic conditions [3]. No phase-transition occurs within the P-range investigated. Fitting the P-V data with a third-order BM-EoS gives: V0 = 1155(4) Å3, KT0 = 20(1) GPa and K’ = 6.5(7). The “axial moduli”, calculated with a third-order “linearized” BM-EoS, are: KT0a = 14(2) GPa (b_a = 0.024(3) GPa-1), K’a = 6.2(8) for the a-axis; KT0b = 21(3) GPa (b_b = 0.016(2) GPa-1), K’b = 10(2) for the b-axis; KT0c = 33(3) GPa (b_c = 0.010(1) GPa-1), K’c = 3.2(8) for the c-axis (KT0a : KT0b : KT0c = 1 : 1.50 : 2.36). The HP-crystal structure evolution was studied on the basis of several structural refinements at different pressures. The main deformation mechanisms at high-pressure are governed by tetrahedral tilting, the tetrahedra behaving as rigid-units. A change in the compressional mechanisms was observed at P ≤ 2 GPa. The P-induced structural rearrangement from 0.0001 up to 8.5 GPa is completely reversible.
Further experiments have been devoted to the high-temperature behavior of CsAlSi5O12. In-situ high-temperature single-crystal and powder X-ray diffraction experiments were performed to describe its anisotropic thermal expansion [4]. The evolution of the unit-cell constants show a significant decrease in expansion above 773 K. At 773 K, a displacive phase transition from the acentric low-T space group Ama2 to the high-T centrosymmetric Amam was found. Thermal expansion below the phase-transition is governed by rigid-body tetrahedra rotations, accompanied by stretching of T–O–T angles. Temperature-dependent unpolarized Raman spectra between room temperature and 1270 K confirm the nature of the phase-transition (i.e. disordered static–disordered dynamic type) and the crystallinity of CsAlSi5O12 at least up to 1270 K.
2.3. Cs-ANA
The elastic behavior and the phase-stability of a natural pollucite, (Cs,Na)AlSi2O6 x nH2O, were investigated at hydrostatic pressure by in-situ single-crystal X-ray diffraction with a diamond anvil cell [5]. Pollucite experiences a P-induced phase transition at P=0.66 +/- 0.12 GPa from cubic (Ia d) to triclinic symmetry (P ). The phase transition is completely reversible and without any appreciable hysteresis effect. No further phase transition has been observed up to ~9 GPa. Fitting the P-V data of the low-P cubic polymorph with a BM-EoS, we obtained: V0=2558.3(4)Å3, KT0= 41(2)GPa and K’T = 4 (fixed). For the high-P triclinic polymorph, a third-order BM-EoS fit gives: V0=2577.5(40)Å3, KT0=25.1(9)GPa and K’T = 6.5(4). The axial bulk moduli of the high-pressure triclinic polymorph were calculated with a third-order “linearized” BM-EoS. The EoS parameters are: KT0(a)= 25.5(17)GPa (b_a = 0.0131(11) GPa-1) and K’T(a)= 6.8(6) for the a-axis; KT0(b)= 23.2(15)GPa (b_b = 0.0145(10) GPa-1) and K’T(b)= 7.7(7) for the b-axis; KT0(c) = 25.2(10)GPa (b_c = 0.0132(6) GPa-1) and K’T(c) = 6.8(4) for the c-axis, resulting in a modest elastic anisotropy (KT0(a):KT0(b):KT0(c) = 1.10:1:1.09).
A previous experiment based on in-situ high-temperature powder X-ray diffraction up to 1470 K (at room P) showed that synthetic cubic pollucite (CsAlSi2O6) preserves its crystallinity within the T-range investigated, without any evidence of phase transition between 290 and 1470 K [6].
3. Discussion
The experiments aimed to describe the phase-stability fields, the thermo-elastic behavior and the P/T-induced structure evolutions of Cs-ABW, Cs-CAS and Cs-ANA show that these three compounds are crystalline at least up to 8-9 GPa. This results is surprising if we consider their microporous nature.
Pollucite only shows a P-induced phase-transition, at a modest pressure (~0.7 GPa). The high-pressure polymorph of Cs-ANA shows an almost isotropic elastic behaviour (i.e. KT0(a):KT0(b):KT0(c) = 1.10 : 1 : 1.09). More anisotropic is the elastic response of Cs-CAS (i.e. KT0(a):KT0(b):KT0(c) = 1 : 1.50 : 2.36). In contrast, Cs-ABW appears to be one of the most anisotropic crystalline materials, with KT0(a) : KT0(b) : KT0(c) = 19 : 12 : 1. The elastic anisotropy in Cs-CAS and Cs-ABW reflects the configuration of the channels, and follow a general principle concerning the HP-behavior of microporous materials [7]: the open framework structures tend to accommodate the effect of pressure, by cooperative rotation of the tetrahedra, usually increasing the ellipticity of the channel systems and maintaining the original elliptical configuration, without any “inversion” in ellipticity. An in-situ high-pressure single-crystal diffraction experiment on Cs-ABW is in progress, aimed to describe the main deformation mechanisms responsible for the so drastic anisotropic behavior of this zeolites.
The thermal stability of the three mentioned microporous materials is also surprising. Despite its microporous structure, pollucite is currently considered as a “ceramic” material [6], with potential technological applications due to its modest thermal expansion. Previous experiments suggest that, among the crystalline phases of the Cs2O-Al2O3-SiO2 system, Cs-ANA shows the highest T-stability [2].
Even the chemical stability of the Cs-bearing zeolites is someway surprising. Pollucite, for example, is able to retain Cs when immersed into a fluid phase, even under hydrothermal conditions, better than several other Cs-bearing materials [5]. This behavior is ascribable to the topological configuration of the Cs-polyhedron and its bonding environment, to the small dimension of the sub-nanopores where the Cs-sites lie and to the high flexibility of the ANA framework type. The Cs-retention of CsAlSi5O12 was recently examined by treating the powdered compound in boiling 1M NaCl solution [8]. After more than one month of exposure at extreme exchange conditions, only small amounts of Cs occupying the eight-membered ring channels are extracted. Results show that Cs is strongly bonded to the tetrahedral framework.
The key to explain the thermal and chemical stability of the mentioned zeolites is in: 1) the significantly modest, or absent, amount of H2O as extra-framework molecules, 2) the (long) ionic radius of Cs+, which allows high coordination-numbers with the framework anions, 3) the modest free-diameters of the channel systems, which hinder the cation migration. Only natural Cs-ANA contains a low amount of H2O (<2wt%), whereas Cs-ABW and Cs-CAS are anhydrous materials.
On the basis of their high thermo-elastic and chemical stability, the three aforementioned Cs-bearing zeolites, especially Cs-ANA, may be considered as functional materials usable for fixation and deposition of radioactive isotopes of Cs, or as solid hosts for 137Cs γ-radiation source to be used in sterilization applications. However, further experiments are needed to investigated potential self-radiation damage of the crystal structures due to 137Cs.
References
[1] GATTA G.D., ROTIROTI N., ZANAZZI P.F., RIEDER M., DRABEK M., WEISS Z., KLASKA R., Am. Mineral. 93 (2008), 988-995.
[2] DIMITRIJEVIC R., DONDUR V., PETRANOVIC N., J. Solid State Chem. 95 (1991), 335–345.
[3] GATTA G.D., ROTIROTI N., FISCH M., KADIYSKI M., ARMBRUSTER T., Phys. Chem. Miner. 35 (2008), 521-533.
[4] FISCH M., ARMBRUSTER T., KOLESOV B., J. Solid State Chem. 181 (2008), 423-431.
[5] GATTA G.D., ROTIROTI N., BOFFA BALLARAN T., SANCHEZ-VALLE C., PAVESE A., Am. Mineral., 94, (2009), 1137-1143.
[6] KOBAYASHI H., YANASE I., MITAMURA T., J. Am. Ceramic Soc. 80 (1997), 2161–2164.
[7] GATTA G.D., LEE Y., Phys. Chem. Minerals 32 (2006), 726 – 732.
[8] FISCH M., ARMBRUSTER T., LIBOWITZKY E., Topics in Chemistry and Materials Science, vol 4 (2010) 61-70. Advanced Micro- and Mesoporous Materials – 08, Eds. K. Hadjiivanov, V. Valtchev, S. Mintova, G. Vayssilov, Sofia, Heron Press Ltd, ISSN 1314-0795
Multi-sensor satellite monitoring of ocean climate
No full textWave height can be measured as accurately from satellite altimeters as from surface buoys and data accumulated since 1985 have been used to show both that the NE Atlantic became rougher and that fluctuations in different oceans may be linked. Planetary waves are an important mechanism for carrying information across ocean basins and may modify currents such as the Gulf Stream and Kuroshio. Detection of one type (Rossby waves) from satellite measurements of sea level, sea temperature and ocean colour will be presented; the inferred characteristics have already led to revisions of standard theories with implications for the way the oceans affect climate. Although unforeseen when the sensors were designed altimeters can also be used to identify rain events. A new precipitation climatology, derived from measured backscatter intensity, will be described and compared with previous work. It has the advantage of using a single sensor-type for tropical to polar latitudes
Inibitori di PCSK9 e dislipidemie: le evidenze cliniche
No full textElevated plasma LDL cholesterol (LDL-C) levels are associated with cardiovascular diseases and statin therapy was proven to decrease LDL-C and reduce cardiovascular death. However, in patients at high cardiovascular risk, achievement of optimal LDL-C levels is challenging, and therefore additional strategies for further loweing LDL-C levels are under development. Recently, silencing of apolipoprotein B gene and MTP inhibition have been approved for the treatment of patients with familial hypercholesterolemia, and there is great interest in the inhibition of proprotein convertase subtilisin/kexin 9 (PCSK9). PCSK9 promotes the degradation of the LDL receptor. The inhibition of PCSK9 favors LDL catabolism and reduces plasma LDLC levels. Monoclonal antibodies against PCSK9 represent so far the most advanced approach in clinical development, with alirocumab, evolocumab and bococizumab under advanced clinical development. Recent data from the first phase III studies show LDL-C reduction in monotherapy and on top of statins. Long-term studies on cardiovascular endpoints are ongoing and the results will be crucial to translate the benefit of this promising approach into clinical practice
A Letter from Sir Charles G.D. Roberts (A Personal Memoir)
No full textSkala recounts her first and subsequent meetings with Charles G.D. Roberts whom she met when he was in his late seventies, and she in her teens. Roberts was a man of paradox, an author of emotionally-distanced verse yet a man of strong sentiment. He was a chameleon, a person who was everything to everybody; he played the role of gentle poet, mentor, and editor; he was a husband, father, and friend. He wrote about unique characters, animal or human, and he was quite naturally an elitist, though his "elite" could have been chosen from all ranks of society, and, indeed, from all societies. Also discussed are some of the critical views on Roberts' work, praiseworthy and not, as well as Roberts' own critical assessment of Skala's early verse
- …
