An Observation Operator for Radar Refractivity Change: Comparison of Observations and Convective-Scale Simulations     

Olivier Caumont  Anouck Foray  Lucas Besson  Jacques Parent du Châtelet 《Boundary-Layer Meteorology》2013,148(2):379-397

Weather radar refractivity depends on low-level moisture, temperature, and pressure and is available at high space–time resolutions over large areas. It is of definite meteorological interest for assimilation, verification, and process-study purposes. In this study, the path-averaged refractivity change is simulated from the Arome cloud-resolving atmospheric system analyses and compared with corresponding radar observations over a 35-day period with various meteorological conditions. For that, a novel post-processing procedure is applied to radar data to improve its quality. Also, an observation operator is developed that ingests Arome analyses and simulates a 3-h path-averaged refractivity change. A sensitivity study shows that simulated path-averaged refractivity change is immune to the modelling of the beam height as long as it remains below approximately 60 m above the ground. Comparisons show overall consistency between observed and simulated path-averaged refractivity change, with discrepancies at times that suggest an improvement in analyses once radar refractivity change observations are assimilated. Finally, errors introduced when retrieving local refractivity from path-averaged refractivity are estimated and it is found for our dataset that such retrievals halve the range of usable observations.  相似文献   

Finding benchmark brown dwarfs to probe the substellar initial mass function as a function of time     

D. J. Pinfield  H. R. A. Jones  P. W. Lucas  T. R. Kendall  S. L. Folkes  A. C. Day-Jones  R. J. Chappelle  I. A. Steele 《Monthly notices of the Royal Astronomical Society》2006,368(3):1281-1295

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The strength of the lunar lithosphere     

Lucas Reindler 《Icarus》2003,162(2):233-241

A self-gravitating, elastic, spherical thick shell model is used to derive the present state of the lateral variations of density and stress differences within the lunar lithosphere. The model is allowed to deform under the load of an initial surface topography and internal density distribution, such that the resulting deformed body gives rise to the observed surface topography and gravity specified by the spherical harmonics of degree up to 70. Two main models are considered, Model A and Model B, with elastic lithospheres of thickness 300 and 210 km, respectively. Model A displays density perturbations of generally less than ±200 kg/m³ within the crustal layers, reducing rapidly to less than ±20 kg/m³ at the base of the lithosphere. The density perturbations in Model B are similar in the crust and marginally higher at the base of the lithosphere. The major stress differences in the mantle are associated with the mascon basins and are found to reach maximums of 8-10 MPa within the lower lithosphere (150-270 km) of Model A and maximums of 12-16 MPa at 150 to 180 km depth for Model B. A moderate correlation exists between the modeled stress distributions and shallow moonquake epicenters. However, the overall results of this study imply that other remnant stresses, due to processes other than density perturbations, exist and play a critical role in the large shallow moonquakes.  相似文献   

Assessment of Lateritic Gravelled Materials for Use in Road Pavements in Cameroon     

Foko Tamba  Carlos  Kengni  Lucas  Tematio  Paul  Manefouet  Bertile Ilalie  Kenfack  Jean Victor 《Geotechnical and Geological Engineering》2022,40(8):4195-4215

Geotechnical and Geological Engineering - Early surface and structural deterioration of new pavements are becoming increasingly perceptible in Cameroon. This raises serious concerns regarding the...  相似文献   

Strength and stiffness of compacted chalk putty–cement blends     

Hoch  Bruna Zakharia  Diambra  Andrea  Ibraim  Erdin  Festugato  Lucas  Consoli  Nilo Cesar 《Acta Geotechnica》2022,17(7):2955-2969

Chalk breaks easily when subjected to human action such as mechanical handling, earthworks operations or pile installation. These actions break the cemented structure of chalk, which turns into a degraded material known as putty, with lower strength and stiffness than the intact chalk. The addition of Portland cement can improve the behaviour of chalk putties. Yet, there are no studies determining the tensile strength of chalk putty–cement blends, the initial stiffness evolution during the curing time and other design parameters such as friction angle and cohesion of this material. This paper addresses this knowledge gap and provides an interpretation of new experimental results based on the dimensionless index expressed as the ratio between porosity and volumetric content of cement (η/C_iv) or its exponential modification (η/C_iv^a). This index aids the selection of the amount of cement and density for key design parameters of compacted chalk putty–cement blends required in geotechnical engineering projects such as road foundations and pavements, embankments, and also bored concrete pile foundations.

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An optical spectroscopic HR diagram for low-mass stars and brown dwarfs in Orion     

F. C. Riddick  P. F. Roche  P. W. Lucas 《Monthly notices of the Royal Astronomical Society》2007,381(3):1077-1092

The masses and temperatures of young low-mass stars and brown dwarfs in star-forming regions are not yet well established because of uncertainties in the age of individual objects and the spectral type–temperature scale appropriate for objects with ages of only a few Myr. Using multi-object optical spectroscopy, 45 low-mass stars and brown dwarfs in the Trapezium Cluster in Orion have been classified and 44 of these confirmed as bona fide cluster members. The spectral types obtained have been converted to effective temperatures using a temperature scale intermediate between those of dwarfs and giants, which is suitable for young pre-main-sequence objects. The objects have been placed on a Hertzsprung–Russell (HR) diagram overlaid with theoretical isochrones. The low-mass stars and the higher mass substellar objects are found to be clustered around the 1 Myr isochrone, while many of the lower mass substellar objects are located well above this isochrone. An average age of 1 Myr is found for the majority of the objects. Assuming coevality of the sources and an average age of 1 Myr, the masses of the objects have been estimated and range from  0.018 to 0.44 M_⊙  . The spectra also allow an investigation of the surface gravity of the objects by measurement of the sodium doublet equivalent width. With one possible exception, all objects have low gravities, in line with young ages, and the Na indices for the Trapezium objects lie systematically below those of young stars and brown dwarfs in Chamaeleon, suggesting that the 820 nm Na index may provide a sensitive means of estimating ages in young clusters.  相似文献   

The UKIRT Infrared Deep Sky Survey (UKIDSS)     

A. Lawrence  S. J. Warren  O. Almaini  A. C. Edge  N. C. Hambly  R. F. Jameson  P. Lucas  M. Casali  A. Adamson  S. Dye  J. P. Emerson  S. Foucaud  P. Hewett  P. Hirst  S. T. Hodgkin  M. J. Irwin  N. Lodieu  R. G. McMahon  C. Simpson  I. Smail  D. Mortlock  M. Folger 《Monthly notices of the Royal Astronomical Society》2007,379(4):1599-1617

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Optical spectroscopic classification and membership of young M dwarfs in star-forming regions     

F. C. Riddick  P. F. Roche  P. W. Lucas 《Monthly notices of the Royal Astronomical Society》2007,381(3):1067-1076

The spectral type is a key parameter in calibrating the temperature which is required to estimate the mass of young stars and brown dwarfs. We describe an approach developed to classify low-mass stars and brown dwarfs in the Trapezium Cluster using red optical spectra, which can be applied to other star-forming regions. The classification uses two methods for greater accuracy: the use of narrow-band spectral indices which rely on the variation of the strength of molecular lines with spectral type and a comparison with other previously classified young, low-mass objects in the Chamaeleon I star-forming region. We have investigated and compared many different molecular indices and have identified a small number of indices which work well for classifying M-type objects in nebular regions. The indices are calibrated for young, pre-main-sequence objects whose spectra are affected by their lower surface gravities compared with those on the main sequence. Spectral types obtained are essentially independent of both reddening and nebular emission lines.
Confirmation of candidate young stars and brown dwarfs as bona fide cluster members may be accomplished with moderate resolution spectra in the optical region by an analysis of the strength of the gravity-sensitive Na doublet. It has been established that this feature is much weaker in these very young objects than in field dwarfs. A sodium spectral index is used to estimate the surface gravity and to demonstrate quantitatively the difference between young (1–2 Myr) objects, and dwarf and giant field stars.  相似文献   

Sheared Flow as a Stabilizing Mechanism in Astrophysical Jets     

Lucas F. Wanex  Erik Tendeland 《Astrophysics and Space Science》2007,307(1-3):83-86

It has been hypothesized that the sustained narrowness observed in the asymptotic cylindrical region of bipolar outflows from Young Stellar Objects (YSO) indicates that these jets are magnetically collimated. The j _z × B _ϕ force observed in z-pinch plasmas is a possible explanation for these observations. However, z-pinch plasmas are subject to current driven instabilities (CDI). The interest in using z-pinches for controlled nuclear fusion has lead to an extensive theory of the stability of magnetically confined plasmas. Analytical, numerical, and experimental evidence from this field suggest that sheared flow in magnetized plasmas can reduce the growth rates of the sausage and kink instabilities. Here we propose the hypothesis that sheared helical flow can exert a similar stabilizing influence on CDI in YSO jets.  相似文献   

VI Carboniferous     

H.G.S. Cribb  K.G. Lucas  W.G.H. Maxwell  G.W. Tweedale  D.A. White  D.H. Wyatt 《Australian Journal of Earth Sciences》2013,60(1-2):165-181

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