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61.
Turbulent Transfer Between Street Canyons and the Overlying Atmospheric Boundary Layer 总被引:1,自引:1,他引:0
Pietro Salizzoni Massimo Marro Lionel Soulhac Nathalie Grosjean Richard J. Perkins 《Boundary-Layer Meteorology》2011,141(3):393-414
The turbulent exchange of momentum between a two-dimensional cavity and the overlying boundary layer has been studied experimentally,
using hot-wire anemometry and particle image velocimetry (PIV). Conditions within the boundary layer were varied by changing
the width of the canyons upstream of the test canyon, whilst maintaining the square geometry of the test canyon. The results
show that turbulent transfer is due to the coupling between the instabilities generated in the shear layer above the canyons
and the turbulent structures in the oncoming boundary layer. As a result, there is no single, unique velocity scale that correctly
characterizes all the processes involved in the turbulent exchange of momentum across the boundary layer. Similarly, there
is no single velocity scale that can characterize the different properties of the turbulent flow within the canyon, which
depends strongly on the way in which turbulence from the outer flow is entrained into the cavity and carried round by the
mean flow. The results from this study will be useful in developing simple parametrizations for momentum exchange in the urban
canopy, in situations where the street geometry consists principally of relatively long, uniform streets arranged in grid-like
patterns; they are unlikely to be applicable to sparse geometries composed of isolated three-dimensional obstacles. 相似文献
62.
Chiara Lesti Massimiliano Porreca Guido Giordano Massimo Mattei Raymond A. F. Cas Heather M. N. Wright Chris B. Folkes Josè Viramonte 《Bulletin of Volcanology》2011,73(10):1535-1565
Estimates of pyroclastic flow emplacement temperatures in the Cerro Galán ignimbrite and Toconquis Group ignimbrites were
determined using thermal remanent magnetization of lithic clasts embedded within the deposits. These ignimbrites belong to
the Cerro Galán volcanic system, one of the largest calderas in the world, in the Puna plateau, NW Argentina. Temperature
estimates for the 2.08-Ma Cerro Galán ignimbrite are retrieved from 40 sites in 14 localities (176 measured clasts), distributed
at different distances from the caldera and different stratigraphic heights. Additionally, temperature estimates were obtained
from 27 sample sites (125 measured clasts) from seven ignimbrite units forming the older Toconquis Group (5.60–4.51 Ma), mainly
outcropping along a type section at Rio Las Pitas, Vega Real Grande. The paleomagnetic data obtained by progressive thermal
demagnetization show that the clasts of the Cerro Galán ignimbrite have one single magnetic component, oriented close to the
expected geomagnetic field at the time of emplacement. Results show therefore that most of the clasts acquired a new magnetization
oriented parallel to the magnetic field at the moment of the ignimbrite deposition, suggesting that the clasts were heated
up to or above the highest blocking temperature (T
b) of the magnetic minerals (T
b = 580°C for magnetite; T
b = 600–630°C for hematite). We obtained similar emplacement temperature estimations for six out of the seven volcanic units
belonging to the Toconquis Group, with the exception of one unit (Lower Merihuaca), where we found two distinct magnetic components.
The estimation of emplacement temperatures in this latter case is constrained at 580–610°C, which are lower than the other
ignimbrites. These estimations are also in agreement with the lowest pre-eruptive magma temperatures calculated for the same
unit (i.e., 790°C; hornblende–plagioclase thermometer; Folkes et al. 2011b). We conclude that the Cerro Galán ignimbrite and Toconquis Group ignimbrites were emplaced at temperatures equal to or higher
than 620°C, except for Lower Merihuaca unit emplaced at lower temperatures. The homogeneity of high temperatures from proximal
to distal facies in the Cerro Galán ignimbrite provides constraints for the emplacement model, marked by a relatively low
eruption column, low levels of turbulence, air entrainment, surface–water interaction, and a high level of topographic confinement,
all ensuring minimal heat loss. 相似文献
63.
Francesco Di Benedetto Daniele Borrini Andrea Caneschi Gabriele Fornaciai Massimo Innocenti Alessandro Lavacchi Carlo Andrea Massa Giordano Montegrossi Werner Oberhauser Luca A. Pardi Maurizio Romanelli 《Physics and Chemistry of Minerals》2011,38(6):483-490
An extensive characterisation of the magnetic properties of synthetic powders of kuramite, with formal composition Cu3SnS4, was performed. Powders were investigated through superconducting quantum interference device (SQUID) magnetometry, electron paramagnetic resonance (EPR) spectroscopy, X-ray powder diffraction (XRPD), scanning and transmission electron microscopies (SEM and TEM) and microanalysis. SEM and TEM reveal the presence of nanodimensioned particles. XRPD clearly shows that Cu3SnS4 crystallised in a cubic sphalerite-type structural model, in spite of the stannite-type tetragonal structure described for the natural phase. This difference arises from a full random distribution of cations. Synthetic kuramite nanopowders exhibit a marked paramagnetism, originated by the presence of Cu(II), definitely assessed by EPR measurements. Moreover, the overall magnetic behaviour of the sample cannot be simply ascribed to diluted paramagnetism, and this suggests the presence of strong superexchange interactions among Cu(II) ions even at room temperature. The main consequences of these results are the definitive assessment of the chemical formula Cu(I)2Cu(II)SnS4 and of a random distribution of Cu(II), Cu(I) and Sn(IV) ions within the available tetrahedral sites. 相似文献
64.
65.
Anil Maheshwari A.N. Sial Massimo Coltorti V.K. Chittora Manoel J.M. Cruz 《Gondwana Research》2001,4(1)
The bimodal Malani suite, West of Barmer, Rajasthan is characterized by discontinuous, ring shaped outcrops of Siwana peralkaline granite with minor outcrops of basalt. The peralkaline, within- plate and A-type nature of granite are evident by its chemical characteristics. The granite is characterized by high Na2O+K2O, Fe/Mg, Zr, Nb, Y, Zn; low Al2O3, CaO and Sr and is significantly low in absolute abundance of trace and REE elements compared to type area Siwana granite. The granite is correlated to the “Pan-African” event and its petrogenesis and tectonic significance are discussed. 相似文献
66.
Bioremediation of petroleum hydrocarbons in anoxic marine sediments: Consequences on the speciation of heavy metals 总被引:3,自引:0,他引:3
Antonio Dell’Anno Francesca Beolchini Massimo Gabellini Laura Rocchetti Antonio Pusceddu Roberto Danovaro 《Marine pollution bulletin》2009,58(12):1808-1814
We investigated the effects of biostimulation and bioagumentation strategies applied to harbor sediments displaying reducing conditions and high concentrations of petroleum hydrocarbons and heavy metals. We compared the microbial efficiency of hydrocarbon removal from sediments maintained for 60 days in anoxic conditions and inoculated with acetate, sulfate-reducing bacterial strains and acetate and sulfate-reducing bacteria. All treatments determined a significant increase in the microbial growth and significant decreases of hydrocarbon contents and of redox potential values. The addition of sulfate-reducing bacterial strains to the sediment was the most efficient treatment for the hydrocarbon removal. In all experiments, significant changes of the heavy metals’ phase repartition were observed. The results reported here suggest that the biodegradation of petroleum hydrocarbons in anoxic marine sediments may be enhanced by stimulating microbial anaerobic metabolism, but care should be applied to monitor the potential changes in the mobility and bioavailability of heavy metals induced by bio-treatments. 相似文献
67.
Previous studies have shown how biogenic silica particles undergo conversion to aluminosilicate phases in large tropical deltaic systems, thus affecting the world ocean budget of major seawater cations. This study tackles the important question of the silica budget in the coastal zone of the Mississippi River Delta, providing evidence for the role of biogenic silica diagenesis in this subtropical system from direct examination of individual diatom particles, sediment leachates and pore-water composition. The estimated reactive silica stored in the study area (5990 km2) is based on operational leaches that account for altered biogenic silica particles and other authigenic aluminosilicate phases in addition to fresh biogenic silica. Early diagenesis of silica in the delta front occurs mainly where more siliceous material is deposited. An inner-shelf area, where hypoxic conditions are found, significantly contributes to the formation of authigenic products of Si alteration. Data suggest that the limiting factor of silica alteration processes is the availability of detrital phases such as Al and Fe. The estimated total reactive silica accumulation in the study area is 1.45×1010 mol Si year−1, representing ∼2.2% of the long-term bulk sediment accumulation. On the basis of a conservative appraisal, the authigenic mineral components account for ∼40% of the long-term reactive silica storage. This study shows that non-tropical deltaic systems are significantly more important sinks of silica than previously thought and that, where conditions are favourable, a consistent portion of reactive silica not leaving the shelf is stored within the delta in the form of authigenic components. 相似文献
68.
Guido Giordano Massimiliano Porreca Pietro Musacchio Massimo Mattei 《Bulletin of Volcanology》2008,70(10):1221-1236
The edifice of Stromboli volcano gravitationally collapsed several times during its volcanic history (>100 ka–present). The
largest Holocene event occurred during the final stage of the Neostromboli activity (∼13–5 ka), and was accompanied by the
emplacement of phreatomagmatic and lahar deposits, known as the Secche di Lazzaro succession. A stratigraphic and paleomagnetic
study of the Secche di Lazzaro deposits allows the interpretation of the emplacement and the eruptive processes. We identify
three main units within the succession that correspond to changing eruption conditions. The lower unit (UA) consists of accretionary
lapilli-rich, thinly bedded, parallel- to cross-stratified ash deposits, interpreted to indicate the early stages of the eruption
and emplacement of dilute pyroclastic density currents. Upward, the second unit (UB) of the deposit is more massive and the
beds thicker, indicating an increase in the sedimentation rate from pyroclastic density currents. The upper unit (UC) caps
the succession with thick, immediately post-eruptive lahars, which reworked ash deposited on the volcano’s slope. Flow directions
obtained by Anisotropy of Magnetic Susceptibility (AMS) analysis of the basal bed of UA at the type locality suggest a provenance
of pyroclastic currents from the sea. This is interpreted to be related to the initial base-surges associated with water–magma
interaction that occurred immediately after the lateral collapse, which wrapped around the shoulder of the sector collapse
scar. Upward in the stratigraphy (upper beds of UA and UB) paleoflow directions change and show a provenance from the summit
vent, probably related to the multiple collapses of a vertical, pulsatory eruptive column. 相似文献
69.
卫星遥感结合地面观测估算非均匀地表区域能量通量 总被引:49,自引:1,他引:48
卫星遥感在研究非均匀陆面上地-气间能量和水循环过程时有其独到的作用。文中介绍了一种利用LANDSATTM资料进行非均匀陆面上区域能量平衡研究的参数化方案。并以夏季和深秋两个景的TM资料为个例,结合“黑河实验”(HEIFE)期间的地面观测资料分析了实验区非均匀陆面上地表特征参数及能量平衡各分量的区域分布及季节差异,同时将所得的结果与地面观测的“真值”作了比较,所得的这些结果是基本可信的。这种方法仍然处于发展的初始阶段,对此也进行了讨论。 相似文献
70.