Observations of the Early Evening Boundary-Layer Transition Using a Small Unmanned Aerial System     

Timothy Bonin  Phillip Chilson  Brett Zielke  Evgeni Fedorovich 《Boundary-Layer Meteorology》2013,146(1):119-132

The evolution of the lower portion of the planetary boundary layer is investigated using the Small Multifunction Research and Teaching Sonde (SMARTSonde), an unmanned aerial vehicle developed at the University of Oklahoma. The study focuses on the lowest 200 m of the atmosphere, where the most noticeable thermodynamic changes occur during the day. Between October 2010 and February 2011, a series of flights was conducted during the evening hours on several days to examine the vertical structure of the lower boundary layer. Data from a nearby Oklahoma Mesonet tower was used to supplement the vertical profiles of temperature, humidity, and pressure, which were collected approximately every 30?min, starting 2 h before sunset and continuing until dusk. From the profiles, sensible and latent heat fluxes were estimated. These fluxes were used to diagnose the portion of the boundary layer that was most affected by the early evening transition. During the transition period, a shallow cool and moist layer near the ground was formed, and as the evening progressed the cooling affected an increasingly shallower layer just above the surface.  相似文献   

Mine Overburden Dump Failure: A Case Study     

Brett Poulsen  Manoj Khanal  A. Manohar Rao  Deepak Adhikary  Rao Balusu 《Geotechnical and Geological Engineering》2014,32(2):297-309

In any open cast mine, the management of stripped spoil during mining is crucial to the mine’s successful operation. The improper management of the overburden (OB) dump can result in stability issues which may affect safety and production of the mine. Various literatures have reported the failure of open pit dumps and the consequences in loss of life, production and impact on neighbouring amenities. Recently, the failure of an out-of-pit OB dump at an Indian Colliery was reported. The failed OB dump displaced spoil approximately 70 m from the original location of the dump toe and impacted on neighbouring amenities. This paper back analyse material properties and investigates the probable mechanism of this OB failure. Well established tools including limiting equilibrium and continuum numerical methods have been used to understand and identify the failure kinematics of this dump. It has been found that the residual friction angle of the material comprising the dump structural unit dominates stability. Mobilisation of residual strength can occur by operational induced strains and/or the presence of water. The shear strength of the foundation was fully mobilised and provided a slip surface for the overlying dump material. The resultant failure is manifest as a bilinear wedge movement of two ridged blocks defined by linear rupture planes.  相似文献   

Olivine or impact melt: Nature of the “Orange” material on Vesta from Dawn     

Lucille Le Corre  Vishnu Reddy  Nico Schmedemann  Kris J. Becker  David P. O’Brien  Naoyuki Yamashita  Patrick N. Peplowski  Thomas H. Prettyman  Jian-Yang Li  Edward A. Cloutis  Brett W. Denevi  Thomas Kneissl  Eric Palmer  Robert W. Gaskell  Andreas Nathues  Michael J. Gaffey  David W. Mittlefehldt  William B. Garry  Holger Sierks  Christopher T. Russell  Carol A. Raymond  Maria C. De Sanctis  Eleonora Ammanito 《Icarus》2013

NASA’s Dawn mission observed a great variety of colored terrains on asteroid (4) Vesta during its survey with the Framing Camera (FC). Here we present a detailed study of the orange material on Vesta, which was first observed in color ratio images obtained by the FC and presents a red spectral slope. The orange material deposits can be classified into three types: (a) diffuse ejecta deposited by recent medium-size impact craters (such as Oppia), (b) lobate patches with well-defined edges (nicknamed “pumpkin patches”), and (c) ejecta rays from fresh-looking impact craters. The location of the orange diffuse ejecta from Oppia corresponds to the olivine spot nicknamed “Leslie feature” first identified by Gaffey (Gaffey, M.J. [1997]. Icarus 127, 130–157) from ground-based spectral observations. The distribution of the orange material in the FC mosaic is concentrated on the equatorial region and almost exclusively outside the Rheasilvia basin. Our in-depth analysis of the composition of this material uses complementary observations from FC, the visible and infrared spectrometer (VIR), and the Gamma Ray and Neutron Detector (GRaND). Several possible options for the composition of the orange material are investigated including, cumulate eucrite layer exposed during impact, metal delivered by impactor, olivine–orthopyroxene mixture and impact melt. Based on our analysis, the orange material on Vesta is unlikely to be metal or olivine (originally proposed by Gaffey (Gaffey, M.J. [1997]. Icarus 127, 130–157)). Analysis of the elemental composition of Oppia ejecta blanket with GRaND suggests that its orange material has ∼25% cumulate eucrite component in a howarditic mixture, whereas two other craters with orange material in their ejecta, Octavia and Arruntia, show no sign of cumulate eucrites. Morphology and topography of the orange material in Oppia and Octavia ejecta and orange patches suggests an impact melt origin. A majority of the orange patches appear to be related to the formation of the Rheasilvia basin. Combining the interpretations from the topography, geomorphology, color and spectral parameters, and elemental abundances, the most probable analog for the orange material on Vesta is impact melt.  相似文献   

Satellite searches at Pluto and Mars     

Philip D. Nicholson  Brett J. Gladman 《Icarus》2006,181(1):218-222

We report the results of searches for outer satellites of Pluto and Mars, carried out with the Hale 5-m telescope in 1999 and 2001, respectively. No new satellites were found down to limiting magnitudes of mR=25.0 at Pluto and mV∼22 at Mars, corresponding to diameters of 35 and 1 km, respectively, for an assumed geometric albedo of 0.07. A faint trans-neptunian object, 1999 LB₃₇, was discovered in the Pluto fields; given the depth of our survey, discovery of one such object in the background Kuiper belt is in reasonable agreement with expectations.  相似文献   

A characterization of volatile organic compounds and secondary organic aerosol at a mountain site in the Southeastern United States     

Michael Link  Yong Zhou  Brett Taubman  James Sherman  Hadi Morrow  Ian Krintz  Luke Robertson  Ryan Cook  Justine Stocks  Matthew West  Barkley C. Sive 《Journal of Atmospheric Chemistry》2015,72(2):81-104

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River network travel time is correlated with dissolved organic matter composition in rivers of the contiguous United States     

Jacob D. Hosen  George H. Allen  Giuseppe Amatulli  Sara Breitmeyer  Matthew J. Cohen  Byron C. Crump  YueHan Lu  Jérôme P. Payet  Brett A. Poulin  Aron Stubbins  Byungman Yoon  Peter A. Raymond 《水文研究》2021,35(5):e14124

Most terrestrial allochthonous organic matter enters river networks through headwater streams during high flow events. In headwaters, allochthonous inputs are substantial and variable, but become less important in streams and rivers with larger watersheds. As allochthonous dissolved organic matter (DOM) moves downstream, the proportion of less aromatic organic matter with autochthonous characteristics increases. How environmental factors converge to control this transformation of DOM at a continental scale is less certain. We hypothesized that the amount of time water has spent travelling through surface waters of inland systems (streams, rivers, lakes, and reservoirs) is correlated to DOM composition. To test this hypothesis, we used established river network scaling relationships to predict relative river network flow-weighted travel time (FWTT) of water for 60 stream and river sites across the contiguous United States (3090 discrete samples over 10 water years). We estimated lentic contribution to travel times with upstream in-network lake and reservoir volume. DOM composition was quantified using ultraviolet and visible absorption and fluorescence spectroscopy. A combination of FWTT and lake and reservoir volume was the best overall predictor of DOM composition among models that also incorporated discharge, specific discharge, watershed area, and upstream channel length. DOM spectral slope ratio (R² = 0.77) and Freshness Index (R² = 0.78) increased and specific ultraviolet absorbance at 254 nm (R² = 0.68) and Humification Index (R² = 0.44) decreased across sites as a function of FWTT and upstream lake volume. This indicates autochthonous-like DOM becomes continually more dominant in waters with greater FWTT. We assert that river FWTT can be used as a metric of the continuum of DOM composition from headwaters to rivers. The nature of the changes to DOM composition detected suggest this continuum is driven by a combination of photo-oxidation, biological processes, hydrologically varying terrestrial subsidies, and aged groundwater inputs.  相似文献   

Mechanisms for avulsion on alluvial fans: Insights from high-frequency topographic data     

Anya Leenman  Brett Eaton 《地球表面变化过程与地形》2021,46(6):1111-1127

Avulsion is a key process in building alluvial fans, but it is also a formidable natural hazard. Based on laboratory experiments monitored with novel high-frequency photogrammetry, we present a new model for avulsion on widely graded gravel fans. Previous experimental studies of alluvial fans have suggested that avulsion occurs in a periodic autogenic cycle, that is thought to be mediated by the gradient of the fan and fan-channel. However, those studies measured gradients at low spatial or temporal resolutions, which capture temporally or spatially averaged topographic evolution. Here, we present high-resolution (1 mm), high-frequency (1-minute) topographic data and orthophotos from an alluvial fan experiment. Avulsions in the experiment were rapid and, in contrast to some previous experimental studies, avulsion occurrence was aperiodic. Moreover, we found little evidence of the back-filling observed at coarser temporal and spatial resolutions. Our observations suggest that avulsion is disproportionately affected by sediment accumulation in the channel, particularly around larger, less mobile grains. Such in-channel deposition can cause channel shifting that interrupts the autogenic avulsion cycle, so that avulsions are aperiodic and their timing is more difficult to predict.  相似文献   

The orbital distribution of Near-Earth Objects inside Earth’s orbit     

Sarah Greenstreet  Henry Ngo  Brett Gladman 《Icarus》2012,217(1):355-366

Canada’s Near-Earth Object Surveillance Satellite (NEOSSat), set to launch in early 2012, will search for and track Near-Earth Objects (NEOs), tuning its search to best detect objects with a < 1.0 AU. In order to construct an optimal pointing strategy for NEOSSat, we needed more detailed information in the a < 1.0 AU region than the best current model (Bottke, W.F., Morbidelli, A., Jedicke, R., Petit, J.M., Levison, H.F., Michel, P., Metcalfe, T.S. [2002]. Icarus 156, 399–433) provides. We present here the NEOSSat-1.0 NEO orbital distribution model with larger statistics that permit finer resolution and less uncertainty, especially in the a < 1.0 AU region. We find that Amors = 30.1 ± 0.8%, Apollos = 63.3 ± 0.4%, Atens = 5.0 ± 0.3%, Atiras (0.718 < Q < 0.983 AU) = 1.38 ± 0.04%, and Vatiras (0.307 < Q < 0.718 AU) = 0.22 ± 0.03% of the steady-state NEO population. Vatiras are a previously undiscussed NEO population clearly defined in our integrations, whose orbits lie completely interior to that of Venus. Our integrations also uncovered the unexpected production of retrograde orbits from main-belt asteroid sources; this retrograde NEA population makes up ?0.1% of the steady-state NEO population. The relative NEO impact rate onto Mercury, Venus, and Earth, as well as the normalized distribution of impact speeds, was calculated from the NEOSSat-1.0 orbital model under the assumption of a steady-state. The new model predicts a slightly higher Mercury impact flux.  相似文献   

Lipid biomarker analysis of cyanobacteria-dominated microbial mats in meltwater ponds on the McMurdo Ice Shelf,Antarctica     

Anne D. Jungblut  Michelle A. Allen  Brendan P. Burns  Brett A. Neilan 《Organic Geochemistry》2009,40(2):258-269

The lipid biomarker composition of microbial mat communities from three meltwater ponds from the McMurdo Ice Shelf, Antarctica, was investigated for the first time. Hydrocarbons, ether-linked components, fatty acids (FAs), wax esters, hopanols and sterols from Fresh, Orange and Salt Ponds were analysed. The dominance of cyanobacteria and the presence of bacterial sulfate reducers were confirmed using signature FAs in all three mats. Wax ester analysis suggested the presence of Chloroflexus spp. The dominance of short chain hydrocarbons, wax esters and FAs indicated that microorganisms are the major source of organic matter in these meltwater ponds. A variety of sterols were present in different relative abundances. The greatest diversity of sterols was in Salt Pond, followed by Fresh Pond, which was attributed to differences in the present eukaryotic diversity. Lipid profiles of the three communities were similar despite the presence of a salinity gradient. Analysis of lipid biomarkers allowed the creation of profiles for these unique Antarctic cryo-ecosystems. This will assist in the comparison of present and past microbial communities and in the monitoring of Antarctic biodiversity in response to global climate change and other environmental perturbations.  相似文献   

Mercurian impact ejecta: Meteorites and mantle     

Brett GLADMAN  Jaime COFFEY 《Meteoritics & planetary science》2009,44(2):285-291

Abstract— We have examined the fate of impact ejecta liberated from the surface of Mercury due to impacts by comets or asteroids, in order to study 1) meteorite transfer to Earth, and 2) reaccumulation of an expelled mantle in giant‐impact scenarios seeking to explain Mercury's large core. In the context of meteorite transfer during the last 30 Myr, we note that Mercury's impact ejecta leave the planet's surface much faster (on average) than other planets in the solar system because it is the only planet where impact speeds routinely range from 5 to 20 times the planet's escape speed; this causes impact ejecta to leave its surface moving many times faster than needed to escape its gravitational pull. Thus, a large fraction of Mercurian ejecta may reach heliocentric orbit with speeds sufficiently high for Earth‐crossing orbits to exist immediately after impact, resulting in larger fractions of the ejecta reaching Earth as meteorites. We calculate the delivery rate to Earth on a time scale of 30 Myr (typical of stony meteorites from the asteroid belt) and show that several percent of the high‐speed ejecta reach Earth (a factor of 2–3 less than typical launches from Mars); this is one to two orders of magnitude more efficient than previous estimates. Similar quantities of material reach Venus. These calculations also yield measurements of the re‐accretion time scale of material ejected from Mercury in a putative giant impact (assuming gravity is dominant). For Mercurian ejecta escaping the gravitational reach of the planet with excess speeds equal to Mercury's escape speed, about one third of ejecta reaccretes in as little as 2 Myr. Thus collisional stripping of a silicate proto‐Mercurian mantle can only work effectively if the liberated mantle material remains in small enough particles that radiation forces can drag them into the Sun on time scale of a few million years, or Mercury would simply re‐accrete the material.  相似文献