Interaction between atmospheric flows and obstacles: Experiments in a rotating channel

We performed an experimental study using scale models in a hydrodynamic rotating channel, concerning the interactions between fluid flows and obstacles of different shapes. The study was meant to analyze the characteristics of the wakes observed on the lee side of quasi-bidimensional obstacles, in a neutral atmosphere.The obstacles were half-cylinders (with aspect ratio 0.87), placed transversally on the channel bottom and totally submerged in the fluid. We call them quasi-bidimensional since their width was a little smaller than the channel width, thus allowing the flow to partially go round their edges.The simulations were performed in the rotating hydraulic channel of ICG-CNR in Turin, and included various conditions of rotation period and flow speed. An interesting behaviour of the wakes was found on the lee side of subsynoptic-scale obstacles, modelled in conditions of Reynolds-Rossby similitude. More precisely, if a given threshold of flow velocity is exceeded, wake size is constant and is fully determined by the height of the obstacle.     

Stable isotope (O and C) and pollen trends in eastern Lake Erie,evidence for a locally-induced climatic reversal of Younger Dryas age in the Great Lakes basin

A cool period from about 11000 to 10 500 BP (11 to 10.5 ka) is recognized in pollen records from the southern Great Lakes area by the return of Picea and Abies dominance and by the persistence of herbs. The area of cooling appears centred on the Upper Great Lakes. A high-resolution record (ca. 9 mm/y) from a borehole in eastern Lake Erie reveals, in the same time interval, this pollen anomaly, isotope evidence of meltwater presence (a — 3 per mil shift in ¹⁸O and a +1.1 per mil shift in ¹³C), increased sand, and reduced detrital calcite content, all suggesting concurrent cooling of Lake Erie. The onset of cooling is mainly attributed to the effect of enhanced meltwater inflow on the relatively large upstream Main Lake Algonquin during the first eastward discharge of glacial Lake Agassiz. Termination of the cooling coincides with drainage of Lake Algonquin, and is attributed to loss of its cooling effectiveness associated with a substantial reduction in its surface area. It is hypothesized that the cold extra inflow effectively prolonged the seasonal presence of lake ice and the period of spring overturn in Lake Algonquin. The deep mixing would have greatly increased the thermal conductive capacity of this extensive lake, causing suppression of summer surface lakewater temperatures and reduction of onshore growing-degree days. Alternatively, a rapid flow of meltwater, buoyed on sediment-charged (denser) lakewater, may have kept the lake surface cold in summer. Other factors such as wind-shifted pollen deposition and possible effects from the Younger Dryas North Atlantic cooling could have contributed to the Great Lakes climatic reversal, but further studies are needed to resolve their relative significance.Contribution to Climo Locarno — Past and Present Climate Dynamics; Conference September 1990, Swiss Academy of Sciences — National Climate ProgramGeological Survey of Canada Contribution 58 890     

A time- and magnitude-predictable model for generation of shallow earthquakes in the Aegean area

Repeat times of strong shallow mainshocks have been determined by the use of instrumental and historical data for 68 seismogenic sources in the Aegean and surrounding area (34°N–43°N, 18°E–30°E). For 49 of these sources at least two interevent times (three mainshocks) are available for each source. By using the repeat times for these 49 sources the following relation has been determined: $$\log T_t = 0.36M_{\min } + 0.35M_p + a$$ whereT _t is the repeat time, measured in years,M _p the surface wave magnitude of the preceding mainshock,M _min the magnitude of the smallest earthquake considered and “a” parameter which varies from source to source. A multilinear correlation coefficient equal to 0.89 was determined for this relation. By using the same repeat times for the 49 seismogenic sources, the following relation has been determined between the magnitude,M _f , of the following mainshock andM _min andM _p . $$M_f = 0.95M_{\min } - 0.49M_p + m$$ wherem is a constant which varies from source to source. A multilinear correlation coefficient equal to 0.80 was found for this relation. The model expressed by these two relations is represented by a scheme of a time variation of stress under constant tectonic loading. In this scheme, the maximum stress values during the different seismic cycles fluctuate around a value, τ₁, in a relatively narrow stress interval, expressing the high correlation coefficient of the relation between LogT andM _p . On the contrary, the minimum stress values fluctuate around a value, τ₂, in a much broader stress interval. However, each of these minimum stress values becomes lower or higher than τ₂ if the previous one is higher or lower than τ₂, respectively, expressing the negative correlation betweenM _f andM _p .     

Fossil-pollen evidence for abrupt climate changes during the past 18 000 years in eastern North America

A quantitative measure of the rate at which fossil-pollen abundances changed over the last 18 000 years at 18 sites spread across eastern North America distinguishes local from regionally synchronous changes. Abrupt regional changes occurred at most sites in late-glacial time (at 13700, 12 300, and 10000 radiocarbon yr BP) and during the last 1000 years. The record of abrupt late-glacial vegetation changes in eastern North America correlates well with abrupt global changes in ice-sheet volume, mountain snow-lines, North Atlantic deep-water production, atmospheric CO₂, and atmospheric dust, although the palynological signal varies from site to site. Changes in vegetation during most of the Holocene, although locally significant, were not regionally synchronous. The analysis reveals non-alpine evidence for Neoglacial/Little Ice Age climate change during the last 1000 years, which was the only time during the Holocene when climate change was of sufficient magnitude to cause a synchronous vegetational response throughout the subcontinent. During the two millennia preceding this widespread synchronous change, the rate of change at all sites was low and the average rate of change was the lowest of the Holocene.Contribution to Clima Locarno Past and Present Climate Dynamics; Conference September 1990, Swiss Academy of Sciences — National Climate Program     

Time resolved simulation of lightning by lip

In this letter, we report results of time resolved spectra of lightning simulation in air by laser-induced plasma (LIP). The measured electron temperatures varied from (1.7 +/- 0.1)X10(4) K at 600 ns to (1.2 +/- 0.1)X10(4)K at 4 microseconds after the ignition of the plasma. Electron densities in the plasma varied from (7.0 +/- 1.4)X10(17)/cm3 to (9.0 +/- 1.8)X10(16)/cm3 for the above times. We also present, for the first time, a comparison of state variables for LIP and natural lightning. We find that immediately following the ignition both natural lightning and LIP reach temperatures in excess of 16,000K and relax to ambient pressure with different rates. They are expected to approach 2000 - 4000 K temperature range, which is important to chemical synthesis, in an asymptotically similar fashion.     

Fractional Crystallization and Liquid Immiscibility Processes in the Alkaline-Carbonatite Complex of Juqui (So Paulo, Brazil)

The Juqui circular intrusion, which is Cretaceous in age (130–135Ma), crops out in the Precambrian gneissic basement in Brazilover an area of 14 km2. It consists of olivine clinopyroxen-itecumulates (with minor olivine gabbros) in the northeastern sector(74 vol.%), whereas ijolites-melteigites-urtites (4%) and nephelinesyenites with minor essexites and syenodiorites (21%) outlinesubannular concentric patterns with an Mg-carbonatite core (1%), in the southwestern part of the complex. Petrographical, bulk rock, and mineral compositional trendsindicate that the origin of the complex can be largely accountedfor by shallow-level fractional crystallization of a carbonatedbasanitic parental magma. Such a magma was generated deep inthe subcontinental lithosphere by low-degree partial meltingof a garnet-phlogopite peridotite source. Mass-balance calculations in agreement with field volume estimatespermit definition of several fractionation stages of the magmaticevolution under nearly closed-system conditions, with inwarddevelopment of zonally arranged side-wall cumulates. These stagesinvolved: (1) fractionation from basanite to essexite magma(liquid fraction F = 33–5%) by crystallization of olivineclinopyroxenite plus minor olivine alkali gabbro cumulates;(2) derivation of the least differentiated mafic nepheline syenite(F = 5–5 %) from essexitic magma by subtraction of a syenodioriteassemblage; (3) exsolution of a carbonatite liquid (5%) froma CO₂-enriched mafic nepheline syenite magma, which also underwentcontinuous fractionation giving rise to ijolite-melteigite-urtitecumulates. The proportion of cumulus clinopyroxene and biotiteand intercumulus nepheline and alkali feldspar in these lastrocks, as well as the absence of alkalis in carbonatite, maybe attributed, at least in part, to loss of alkali-rich hydrousfluids released during and after the unmixing formation of thetwo conjugate liquids. The KD values determined for Mg-carbonatite/nepheline syeniteare lower (1–4–2–9) for light rare earth elements(LREE) than for REE from Eu to Yb (4–6–7–8),in contrast to recent experimental results (Hamilton et al.,1989). A possible explanation is that Juquia Mg-carbonatiterepresents an alreadydifferentiated magma, which underwent extensivefractionation of LREE-enriched calcite. In this way, the highvariability of K₀ REE patterns observed in several alkaline-carbonatitecomplexes can also be accounted for. The remarkably constant initial ⁸⁷Sr/⁸⁶Sr ratios (mostly between0–7052 and 0–7057) support the interpretation ofthe intrusion as having been generated by fractrional crystallizationand liquid immiscibility from a common parental magma. Iligherisotopic ratios (0–7060–0–7078), found mainlyin dykes and in the border facies of the intrusion, may be dueto contamination by the gecissic basement.     

Thermal expansion and spontaneous strain associated with the normal-incommensurate phase transition in melilites

The linear thermal expansions of åkermanite (Ca₂MgSi₂O₇) and hardystonite (Ca₂ZnSi₂O₇) have been measured across the normal-incommensurate phase transition for both materials. Least-squares fitting of the high temperature (normal phase) data yields expressions linear in T for the coefficients of instantaneous linear thermal expansion, $$\alpha _1 = \frac{1}{l}\frac{{dl}}{{dT}}$$ for åkermanite: $$\begin{gathered} \alpha _{[100]} = 6.901(2) \times 10^{ - 6} + 1.834(2) \times 10^{ - 8} T \hfill \\ \alpha _{[100]} = - 2.856(1) \times 10^{ - 6} + 11.280(1) \times 10^{ - 8} T \hfill \\ \end{gathered} $$ for hardystonite: $$\begin{gathered} \alpha _{[100]} = 15.562(5) \times 10^{ - 6} - 1.478(3) \times 10^{ - 8} T \hfill \\ \alpha _{[100]} = - 11.115(5) \times 10^{ - 6} + 11.326(3) \times 10^{ - 8} T \hfill \\ \end{gathered} $$ Although there is considerable strain for temperatures within 10° C of the phase transition, suggestive of a high-order phase transition, there appears to be a finite ΔV of transition, and the phase transition is classed as “weakly first order”.     

Computation of surface energies in an electrostatic point charge model: I. Theory

The attachment energies, the slice energies and the specific surface energies can be calculated in an electrostatic point charge model using the formula derived by Madelung for the potential introduced by an infinite row of equally spaced point charges. Power series are given for the Hankel function iH ₍₀₎ ⁽¹⁾ (iy) and Ψ(x)=d ln x!/dx. The logarithmic expression in the Madelung formula converges rapidly when applying a power series, which combines equally charged cations and anions. Besides the specific surface energy (γ ^hkl), the slice energy (E _s ^hkl ) and the attachment energy (E _a ^hkl ) can be considered as special categories of surface energies as they depend on surface configurations as well. The specific surface energy γ is the energy per unit area of surface needed to split the crystal parallel to a face (hkl). The attachment energy (E _a) is the energy released per mole, when a new slice of thickness d _hkl crystallizes on an already existing crystal face (hkl). The growth rate of the crystal face (hkl) is a function of its attachment energy. The slice energy (E _s) is the energy released per mole, when a new slice d _hkl is formed from the vapour neglecting the influence of edge energies. The lattice energy (E _c) which is the energy released per mole of a crystal crystallizing from the vapour, is given by the following relation: E _c=E _a+E _s.     

Structure of the αFexAl1:xOOH solid solution

The three-dimensional local organisation around Fe atoms in a natural diaspore (Al_0.9955Fe_0.0045OOH) has been investigated by angular measurements of X-ray absorption spectra. It is demonstrated that in a single crystal of diaspore, the absorption cross-section exhibits the special case of trichroism where three independent measurements are needed to determine the absorption cross-section for any direction of polarization. Extended X-ray absorption fine structure (EXAFS) spectra were thus recorded at the following orientations of the polarization vector: , , and . The incoming white beam was monochromatized using two Si(331) crystals, which deliver at the Fe K-edge a completely linearly polarized X-ray beam. The reliability of our measurements was checked by comparing the isotropic EXAFS spectrum calculated from the three orthogonal measurements to the one of the diaspore powder recorded at the magic angle. It is shown that Fe³⁺ ions are not randomly distributed within the diaspore framework. Furthermore, only part of the Fe³⁺ ions substitutes Al, the others being located in the channels of the structure. The 3D local structure of the Fe domains has been modeled assuming hematite-like clusters of three Fe octahedra topotactically grafted to aluminous chains. These Fe clusters are thought to represent ancient multinuclear Fe surface complexes having formed at the time of the diaspore growth, and being sealed in its bulk structure. The whole crystal of diaspore is then thought to have kept the memory of the heterogeneous nucleation mechanism of these hematite nuclei. In addition to the well-known examples of atom segregations, ion vacancies, and intergrowths of discrete phases, this new type of crystal defect represents another evidence of non-equilibrium crystallization process under the thermodynamic and kinetic conditions that prevail at the earth's surface.     

Computation of surface energies in an electrostatic point charge model: II. Application to zircon (ZrSiO4)

The attachment energies, the slice energies and the specific surface energies can be calculated in an electrostatic point charge model using the formula derived by Madelung for the potential introduced by an infinite row of equally spaced point charges. Power series are given for the Hankel function iH ₍₀₎ ⁽¹⁾ (iy) and (x)=d ln x!/dx. The logarithmic expression in the Madelung formula converges rapidly when applying a power series, which combines equally charged cations and anions. Besides the specific surface energy ( ^hkl), the slice energy (E _s ^hkl ) and the attachment energy (E _a ^hkl ) can be considered as special categories of surface energies as they depend on surface configurations as well. The specific surface energy is the energy per unit area of surface needed to split the crystal parallel to a face (hkl). The attachment energy (E _a) is the energy released per mole, when a new slice of thickness d _hkl crystallizes on an already existing crystal face (hkl). The growth rate of the crystal face (hkl) is a function of its attachment energy. The slice energy (E _s) is the energy released per mole, when a new slice d _hkl is formed from the vapour neglecting the influence of edge energies. The lattice energy (E _c) which is the energy released per mole of a crystal crystallizing from the vapour, is given by the following relation: E _c=E _a+E _s.