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51.
Johanna K. Spiegel Nina Buchmann Olga L. Mayol-Bracero Luis A. Cuadra-Rodriguez Carlos J. Valle Díaz Kimberly A. Prather Stephan Mertes Werner Eugster 《Pure and Applied Geophysics》2014,171(9):2443-2459
We investigated cloud properties of warm clouds in a tropical montane cloud forest at Pico del Este (1,051 m a.s.l.) in the northeastern part of Puerto Rico to address the question of whether cloud properties in the Caribbean could potentially be affected by African dust transported across the Atlantic Ocean. We analyzed data collected during 12 days in July 2011. Cloud droplet size spectra were measured using the FM-100 fog droplet spectrometer that measured droplet size distributions in the range from 2 to 49 µm, primarily during fog events. The droplet size spectra revealed a bimodal structure, with the first peak (D < 6 µm) being more pronounced in terms of droplet number concentrations, whereas the second peak (10 µm < D < 20 µm) was found to be the one relevant for total liquid water content (LWC) of the cloud. We identified three major clusters of characteristic droplet size spectra by means of hierarchical clustering. All clusters differed significantly from each other in droplet number concentration ( \(N_{\rm tot}\) ), effective diameter (ED), and median volume diameter (MVD). For the cluster comprising the largest droplets and the lowest droplet number concentrations, we found evidence of inhomogeneous mixing in the cloud. Contrastingly, the other two clusters revealed microphysical behavior, which could be expected under homogeneous mixing conditions. For those conditions, an increase in cloud condensation nuclei—e.g., from processed African dust transported to the site—is supposed to lead to an increased droplet concentration. In fact, one of these two clusters showed a clear shift of cloud droplet size spectra towards smaller droplet diameters. Since this cluster occurred during periods with strong evidence for the presence of long-range transported African dust, we hypothesize a link between the observed dust episodes and cloud characteristics in the Caribbean at our site, which is similar to the anthropogenic aerosol indirect effect. 相似文献
52.
Kimberly Genareau Greg A. Valentine Gordon Moore Richard L. Hervig 《Bulletin of Volcanology》2010,72(5):593-607
Explosive activity at Lathrop Wells volcano, Nevada, U.S.A. originated with weak Strombolian (WS) eruptions along a short
fissure, and transitioned to violent Strombolian (VS) activity from a central vent, with lava effusion during both stages.
The cause for this transition is unknown; it does not reflect a compositional change, as evidenced by the consistent bulk
geochemistry of all the eruptive products. However, comparison of agglutinate samples from the early, WS events with samples
of scoria from the later, VS events reveal differences in the abundance and morphology of groundmass phases and variable textures
in the rims of olivine phenocrysts. Scanning electron microscope (SEM) examination of thin sections from the WS samples show
euhedral magnetite microlites in the groundmass glass and olivine phenocrysts show symplectite lamellae in their rims. Secondary
ion mass spectrometry (SIMS) depth profiles of these symplectites indicate they are diffusion-controlled. The calculated DFe-Mg allows an estimation of the oxygen fugacity (fO2) and indicates an increased fO2 during eruption of the WS products. Conversely, the VS samples show virtually no magnetite microlites in the groundmass glass,
a lack of symplectites in the olivines, and a lower calculated fO2. These microtextural features suggest that the Lathrop Wells trachybasalt experienced increased oxidation during WS activity.
As magma ascended through the original fissure, exsolved bubbles were concentrated in the wider part(s) (the protoconduit)
and this bubble flux drove convective circulation that oxidized the magma through exposure to atmosphere and recirculation.
This oxidation resulted in groundmass crystallization of magnetite within the melt and formation of symplectites within the
olivine phenocrysts. Bubble-driven convection mixed magma vertically within the protoconduit, keeping it fluid and driving
Strombolian bursts, while microlite crystallization in narrower parts of the fissure helped to focus flow. Development of
a central conduit increased the magma ascent velocity (due to a greater product volume in the later eruptive stages) and caused
the shift in eruption intensity. Consequently, variations in microtextures of the Lathrop Wells products reveal how a combination
of fluid dynamic and crystallization processes in the ascending magma resulted in different styles of activity while the products
maintained a consistent bulk composition. 相似文献
53.
Uranium series disequilibria in ocean island basalts (OIB) provide evidence for the presence of garnet in their source region. It has been suggested that enriched OIB signatures derive from mantle lithologies other than peridotite, such as eclogite or pyroxenite, and, in particular, that silica-poor garnet pyroxenite is the source lithology for alkali basalts. To test the ability of such a source to produce the U–Th disequilibria observed in alkali OIB, we determined experimentally clinopyroxene-melt and garnet-melt partition coefficients for a suite of trace elements, including U and Th, at 2.5 GPa and 1420–1450 °C. The starting composition for the experiments was a 21% partial melt of a silica-poor garnet pyroxenite. Experimentally determined clinopyroxene-melt partition coefficients range from 0.0083 ± 0.0006 to 0.020 ± 0.002 for Th and from 0.0094 ± 0.0006 to 0.024 ±0.002 for U, and garnet-melt partition coefficients are 0.0032 ± 0.0004 for Th and 0.013 ± 0.002 for U. Comparison of our experimental results with partition coefficients from previous experimental studies shows that the relative compatibilities of U and Th in both garnet and clinopyroxene are different for different mineral compositions, leading to varying degrees of U/Th fractionation with changing lithology. For a given melting rate and extent of partial melting, mafic lithologies tend to produce larger 230Th excesses than peridotite. However, this effect is minimized by the greater overall extents of melting experienced by eclogites and pyroxenites relative to peridotite. Results from chromatographic, batch, and fractional melting calculations with binary mixing between partial melts of pyroxenite and peridotite, carried out using our new partitioning data for the pyroxenite component and taking into account variable productivities and different solidus depths for the two lithologies, suggest that OIB are not the product of progressive melting of a source containing a fixed quantity of garnet pyroxenite. Melting a peridotite with enriched signatures, and mixing those melts with melts of a depleted, “normal” peridotite, is an alternative explanation for the trends seen in Hawaiian, Azores and Samoan lavas. 相似文献
54.
Turbulent wall pressure data acquired during tests of a full-scale experimental towed array over a range of tow speeds in straight tows and turns is presented. The experimental towed array contained a linear array of sensors mounted at the fluid–solid interface to measure the spectra of the wall pressure fluctuations due to the cylindrical turbulent boundary layer. The physics are dominated by the growth of a thick, high Reynolds number turbulent boundary layer at arc length Reynolds numbers as high as 9×108. The measured wavenumber-frequency spectra, autospectra, cross-spectral decay and convection velocities are presented. A well-defined convective ridge exists in the wavenumber-frequency spectra obtained during straight tows and turns. Turns give rise to a complicated fluid–structure interaction problem, but do not lead to the separation of the turbulent boundary layer. As the array moves through a turn, flow-induced vibrations of the array are shown to dominate the spectra at low frequencies, with more rapid decay in the measured coherence occurring at higher frequencies. The use of tow speed as a velocity scale is shown to collapse autospectra and convection velocities. 相似文献
55.
M.C. Towner M.R. Patel J.C. Zarnecki M.R. Sims A.-M. Harri C.F. Wilson R.C. Quinn M.H. Hecht 《Planetary and Space Science》2004,52(13):1141-1156
A suite of instruments on the Beagle 2 Mars lander was designed and built in order to investigate the environmental conditions at the landing site. The sensor suite was capable of measuring air temperature at two heights, surface level pressure, wind speed and direction, saltated particle momentum, UV flux (diffuse and direct at five wavelengths), the total accumulated radiation dose and investigating the nature of the oxidising environment. The scientific goals of the instruments are discussed within the context of current understanding of the environmental conditions on Mars, and the instruments themselves are described in detail. Beagle 2 landed on Mars in late 2003, as part of the ESA Mars Express mission. The expected lifetime of the lander on the surface was 180 sols, with a landing site in Isidis Planitia, but has not responded to attempts to contact it, and has now been declared lost. The Environmental Sensor Suite (ESS) was intended to monitor and characterise the current local meteorological parameters, investigating specific areas of scientific interest raised from previous missions, most notably dust transport and transient phenomena, and additionally to add context to the conditions that any possible martian micro-organisms would have to face. The design of the instrument suite was strongly influenced by mass limitations, with eight sensor subsystems having a total mass of approximately 100 g. Although Beagle 2 has been now declared lost, the scientific goals of an Environmental Sensors Suite still remain a valid target for any future astrobiology orientated missions. 相似文献
56.
57.
Galactic cosmic ray bombardment provides a permanent background ionosphere in planetary atmospheres. A transport technique is used to compute the cosmic ray ionization rate profile in a model of the Venusian atmosphere at altitudes between 55 and 100 km. These ionization rates are then applied to a model of ion chemistry to predict equilibrium electron and ion density profiles. Ionization rates for typical solar flare proton events are available from earlier calculations and have been included. 相似文献
58.
Zhao-Cheng Zeng Liping Lei Kimberly Strong Dylan B. A. Jones Lijie Guo Min Liu 《International Journal of Digital Earth》2017,10(4):426-456
ABSTRACTThis study presents an approach for generating a global land mapping dataset of the satellite measurements of CO2 total column (XCO2) using spatio-temporal geostatistics, which makes full use of the joint spatial and temporal dependencies between observations. The mapping approach considers the latitude-zonal seasonal cycles and spatio-temporal correlation structure of XCO2, and obtains global land maps of XCO2, with a spatial grid resolution of 1° latitude by 1° longitude and temporal resolution of 3 days. We evaluate the accuracy and uncertainty of the mapping dataset in the following three ways: (1) in cross-validation, the mapping approach results in a high correlation coefficient of 0.94 between the predictions and observations, (2) in comparison with ground truth provided by the Total Carbon Column Observing Network (TCCON), the predicted XCO2 time series and those from TCCON sites are in good agreement, with an overall bias of 0.01?ppm and a standard deviation of the difference of 1.22?ppm and (3) in comparison with model simulations, the spatio-temporal variability of XCO2 between the mapping dataset and simulations from the CT2013 and GEOS-Chem are generally consistent. The generated mapping XCO2 data in this study provides a new global geospatial dataset in global understanding of greenhouse gases dynamics and global warming. 相似文献
59.
J. Revel Sims 《Urban geography》2016,37(1):26-56
Little is understood about displacement in urban contexts. While some of the difficulties are methodological, the more serious problem is conceptual. Outside of the rent gap hypothesis or the philosophy of property rights, there has been little theoretical inquiry into the causal dynamics of displacement. In this article, I present a study of evictions in Los Angeles that addresses these conceptual and empirical shortcomings. A spatial analysis of more than 70,000 georeferenced evictions between 1994 and 1999 documents the existence of four distinct geographies of displacement, each produced by separate types of causal circumstances. Gentrification explains only one of the four displacement geographies, while the other three are nongentrifying or pre-gentrifying contexts and more appropriately described through growth machine models, global city theory, and financial restructuring. The extent of displacement in pre- and nongentrifying areas reinforces Mark Davidson’s emphasis on Lefebvre’s production of space as a crucial framework for understanding displacement processes. 相似文献
60.
Mark K. Reagan Simon Turner Kenneth W.W. Sims 《Geochimica et cosmochimica acta》2008,72(17):4367-4378
Phonolite pumice found floating offshore of Tristan da Cunha following intense seismic activity southeast of the island July 29-30, 2004 was analyzed for 238U- and 232Th-series nuclides to determine initial 230Th, 226Ra, 210Pb, 210Po, 228Ra, and 228Th activities. The initial (210Po/210Pb) value of 0.15 for the phonolite shows that, like most subaerial lavas, this subaqueous tephra degassed most of its 210Po upon eruption. The (230Th/232Th) and (238U/232Th) values for the phonolite are similar to those of the trachyandesites erupted in 1961 from Tristan da Cunha. However, the relative activities of 210Pb, 226Ra, and 230Th in the phonolite contrast with those of the trachyandesites, in that 210Pb and 230Th are both strongly enriched with respect to 226Ra. In addition, the phonolite had a small deficit in 228Ra with respect to 232Th. The Ra deficits likely resulted from partitioning into feldspars and hornblende in a time frame that extended over several decades to a century. These disequilibria can be explained by crystal fractionation at a decreasing rate through time at an average of 3-5 × 10−3 year−1. The calculated crystallization rate is about an order of magnitude faster than has been calculated for most other phonolites and trachytes, and about half that calculated for crystallization of the Makaopuhi lava lake. These data imply that the 2004 magma was not the differentiated cap of a much larger body that remained at depth. Instead, it was likely the residue of a relatively small body of more mafic magma that was injected into the crust southeast Tristan and underwent extensive and rapid crystal fractionation before it erupted. 相似文献