35 research outputs found
Planary Symmetric Static Worlds with Massless Scalar Sources
Motivated by the recent wave of investigations on plane domain wall
spacetimes with nontrivial topologies, the present paper deals with (probably)
the most simple source field configuration which can generate a spatially
planary symmetric static spacetime, namely a minimally coupled massless scalar
field that depends only upon a spacelike coordinate, . It is shown that the
corresponding exact solutions are
algebraically special, type , and represent globally
pathologic spacetimes with a - group of motion acting on orbits. In spite of the model simplicity, these
- generated worlds possess naked timelike singularities (reached within
a finite universal time by normal non-spacelike geodesics), are completely free
of Cauchy surfaces and contain into the - leveled sections points which can
not be jointed by - trajectories images of oblique non-spacelike
geodesics. Finally, we comment on the possibility of deriving from two other physically interesting ^^ ^^ -
generated'' spacetimes, by appropiate jonction conditions in the -
plane.Comment: 14 pages, LaTeX format, figures not include
Evolution of the Bianchi I, the Bianchi III and the Kantowski-Sachs Universe: Isotropization and Inflation
We study the Einstein-Klein-Gordon equations for a convex positive potential
in a Bianchi I, a Bianchi III and a Kantowski-Sachs universe. After analysing
the inherent properties of the system of differential equations, the study of
the asymptotic behaviors of the solutions and their stability is done for an
exponential potential. The results are compared with those of Burd and Barrow.
In contrast with their results, we show that for the BI case isotropy can be
reached without inflation and we find new critical points which lead to new
exact solutions. On the other hand we recover the result of Burd and Barrow
that if inflation occurs then isotropy is always reached. The numerical
integration is also done and all the asymptotical behaviors are confirmed.Comment: 22 pages, 12 figures, Self-consistent Latex2e File. To be published
in Phys. Rev.
SIRTA, a ground-based atmospheric observatory for cloud and aerosol research
Ground-based remote sensing observatories have a crucial role to play in providing data to improve our understanding of atmospheric processes, to test the performance of atmospheric models, and to develop new methods for future space-borne observations. Institut Pierre Simon Laplace, a French research institute in environmental sciences, created the Site Instrumental de Recherche par Télédétection Atmosphérique (SIRTA), an atmospheric observatory with these goals in mind. Today SIRTA, located 20km south of Paris, operates a suite a state-of-the-art active and passive remote sensing instruments dedicated to routine monitoring of cloud and aerosol properties, and key atmospheric parameters. Detailed description of the state of the atmospheric column is progressively archived and made accessible to the scientific community. This paper describes the SIRTA infrastructure and database, and provides an overview of the scientific research associated with the observatory. Researchers using SIRTA data conduct research on atmospheric processes involving complex interactions between clouds, aerosols and radiative and dynamic processes in the atmospheric column. Atmospheric modellers working with SIRTA observations develop new methods to test their models and innovative analyses to improve parametric representations of sub-grid processes that must be accounted for in the model. SIRTA provides the means to develop data interpretation tools for future active remote sensing missions in space (e.g. CloudSat and CALIPSO). SIRTA observation and research activities take place in networks of atmospheric observatories that allow scientists to access consistent data sets from diverse regions on the globe
BLOOM: A 176B-Parameter Open-Access Multilingual Language Model
Large language models (LLMs) have been shown to be able to perform new tasks
based on a few demonstrations or natural language instructions. While these
capabilities have led to widespread adoption, most LLMs are developed by
resource-rich organizations and are frequently kept from the public. As a step
towards democratizing this powerful technology, we present BLOOM, a
176B-parameter open-access language model designed and built thanks to a
collaboration of hundreds of researchers. BLOOM is a decoder-only Transformer
language model that was trained on the ROOTS corpus, a dataset comprising
hundreds of sources in 46 natural and 13 programming languages (59 in total).
We find that BLOOM achieves competitive performance on a wide variety of
benchmarks, with stronger results after undergoing multitask prompted
finetuning. To facilitate future research and applications using LLMs, we
publicly release our models and code under the Responsible AI License
Orogenic squalline observed with Doppler polarimetric radars during the MAP experiment.
(17-21 juin 2002
FRAMEA: an experimental campaign for severe storm survey and flood warning, associating an X-band weather radar and a discharge hydrological model.
International audienc
Summer Mistral at the Exit of the Rhone Valley
The paper examines the three-dimensional structure and dynamics of the mistral at the RhĂŽne valley exit on 28 June 2001. The mistral refers to a severe wind that develops along the RhĂŽne valley in southern France. This summer mistral event was documented in the framework of the ESCOMPTE field experiment. The dynamical processes driving the circulation of the mistral in the RhĂŽne valley and particularly wake formation and planetary boundary layer (PBL) inhomogeneity at the scale of RhĂŽne valley delta are investigated. Several important data sources are used (airborne Doppler lidar, radiosondes and surface stations) as well as non-hydrostatic mesoscale simulations. This paper analyses experimentally, numerically and theoretically the mechanism of wake formation. It shows that the flow impinging on the Alpine range and the Massif Central becomes supercritical all along the ridge line, including the RhĂŽne valley and continues to accelerate in the lee regions until a hydraulic jump occurs. It leads to the formation of wakes behind and close to the mountain peaks. Compared to the Massif Central wake, the origin of the western Alps wake is rather complicated. In this study, the observations and simulations suggest a combined wall separation/gravity wave breaking mechanism to explain the western Alps wake. Indeed, it is shown that in addition to the flow descending the western Alps slopes and experiencing a strong hydraulic jump, the point where the mistral flow separates from the eastern flank of the RhĂŽne valley located at about 44°N is associated with a 'flank-shock' which is an oblique hydraulic jump (i.e.the downstream Froude number is supercritical). Wake formation in the lee of the Alps and the Massif Central causes large inhomogeneity of the PBL with differences between land and sea. In the Massif Central and western Alps wakes, the continental PBL is deeper (1.8 km) than in the mistral flow (1 km), which is consistent with a subcritical regime associated with enhanced turbulent mixing. The supercritical air flow, descending the Massif Central and Alps slopes and transitioning to subcritical flow, increases the near-surface air temperature due to the föhn effect. Over the Mediterranean, the surface heat fluxes are slightly negative (betweenâ50 and 0 W m â2) and the main source of PBL turbulence is mechanical (wind shear). The PBL depth within the mistral flow does not vary over land (1 km), whereas the absence of convection but also of strong winds prevent PBL development over the sea in the wakes of the Massif Central and the Alps (PBL depth of about 0.5 km)
Gestion des CRues par Intégration des SystÚmes transfrontaliers de prévision et de prévention des bassins versants Alpins
[Departement_IRSTEA]Eaux [TR1_IRSTEA]ARCEAULe projet « FRAMEA » (2004-2007) a vu naĂźtre deux radars compacts de nouvelle technologie en bande X. CĂŽtĂ© français, ce radar mĂ©tĂ©o, prĂ©nommĂ© « Hydrix » et installĂ© sur le Mont Vial Ă 1400 m dâaltitude, permet depuis juin 2007 une couverture optimale du territoire des Alpes Maritimes, qui nâĂ©tait jusque lĂ pas ou peu couvert par les radars existants. AprĂšs cette premiĂšre expĂ©rience concluante, le travail de coopĂ©ration a Ă©tĂ© poursuivi dĂšs 2008 Ă travers le projet « Cristal » pour dĂ©velopper des applications opĂ©rationnelles pour ces deux territoires. Les expĂ©rimentations menĂ©es au cours de ce projet par les Ă©quipes scientifiques ont permis de dĂ©montrer la puissance de lâoutil radar et lâefficacitĂ© des outils de prĂ©vision dans la gestion des crises hydromĂ©tĂ©orologiques sur le territoire des Alpes-Maritimes, que ce soit Ă lâĂ©chelle de petits bassins urbains que de bassins versants plus importants comme la Siagne, les Paillons et la Roya Ă cheval sur les deux pays