Sensitivity of simulated wintertime Arctic atmosphere to vertical resolution in the ARPEGE/IFS model     

Øyvind Byrkjedal  Igor Esau  Nils Gunnar Kvamstø 《Climate Dynamics》2008,30(7-8):687-701

The current state-of-the-art general circulation models, including several of those used by the IPCC, show considerable biases in the simulated present day high-latitude climate compared to observations and reanalysis data. These biases are most pronounced during the winter season. We here employ ideal vertical profiles of temperature and wind from turbulence-resolving simulations to perform a priori studies of the first-order eddy-viscosity closure scheme employed in the ARPEGE/IFS model. This reveals that the coarse vertical resolution (31 layers) of the model cannot be expected to realistically resolve the Arctic stable boundary layer. The curvature of the Arctic inversion and thus also the vertical turbulent-exchange processes cannot be reproduced by the coarse vertical mesh employed. To investigate how turbulent vertical exchange processes in the Arctic boundary layer are represented by the model parameterization, a simulation with high vertical resolution (90 layers in total) in the lower troposphere is performed. Results from the model simulations are validated against data from the ERA-40 reanalysis. The dependence of the surface air temperature on surface winds, surface energy fluxes, free atmosphere stability and boundary layer height is investigated. The coarse-resolution run reveals considerable biases in these parameters, and in their physical relations to surface air temperature. In the simulation with fine vertical resolution, these biases are clearly reduced. The physical relation between governing parameters for the vertical turbulent-exchange processes improves in comparison with ERA-40 data.  相似文献   

Numerical simulation of upwelling currents in pockmarks,and data from the Inner Oslofjord,Norway   总被引：5，自引：3，他引：2  

Øyvind Hammer  Karen E. Webb  Davy Depreiter 《Geo-Marine Letters》2009,29(4):269-275

The deflection of oceanic or tidal currents into pockmarks has been studied by both general three-dimensional computational fluid dynamics simulations and acoustic measurements in a number of pockmarks in the Inner Oslofjord, Norway. The modeling demonstrates upstream convergence of flow lines, followed by upwelling over the pockmark. This upwelling is an effect of deflected regional currents, not of expulsion of fluids or gas from the seafloor, and is sufficiently strong to prevent the settling of fine particles. The field measurements, although noisy at low vertical velocities, are consistent with the hypothesis of upwelling. The reduction in sedimentation rate inferred over the pockmarks (relative to that of the flat surrounding seabed) can explain the maintenance, or even deepening of pockmarks in the absence of fluid or gas seepage. The current pattern may also have consequences for the marine biology of pockmarks.  相似文献   

HS 2333 + 3927: a new sdB binary with a large reflection effect     

C.A. Karl  U. Heber  H. Drechsel  R. Napiwotzki  M. Altmann  R. Østensen  S. Folkes  J.E. Solheim  O. Cordes  B. Voss  D. Koester 《Astrophysics and Space Science》2004,291(3-4):283-289

We report the discovery of a binary, HS 2233 + 3927, consisting of an sdB star with a faint companion. From its lightcurve the orbital period of 14,844 s, the mass ratio, the inclination, and other system parameters are derived. The companion does not contribute to the optical light of the system except through a strong reflection effect. The semi-amplitude of the radial velocity curve K ₁= 89.6 km/s^?1 and a mass function of f(m) = 0.013 M _⊙ are determined. A preliminary spectroscopic analysis of the blue spectra using NLTE model atmospheres results in T_eff= 36 500 K, log g= 5.70, and log(n _He/n _H) =?2.15. These parameters are typical for sdB stars, the companion is probably an M dwarf.  相似文献   

Pulsation stability of helium-rich subdwarf B stars     

A. Ahmad  C.S. Jeffery  J.-E. Solheim  R. Østensen 《Astrophysics and Space Science》2004,291(3-4):435-439

We present results of the pulsation monitoring of helium-rich subdwarf B (He-sdB) stars using high-speed differential photometry observations made with the SAAO (1.0m), NOT (2.6m) and the IAC80 (0.8m) telescopes. Although we did not find a pulsator in our sample of He-sdB stars, some of the stars show hints of pulsations which need further investigation.  相似文献   

The NOT pulsating sdB search programme     

J.-E. Solheim  R. Østensen  R. Silvotti  U. Heber 《Astrophysics and Space Science》2004,291(3-4):419-426

A search program for pulsating sdBs was conducted with the Nordic Optical Telescope in the years 1999–2001. Possible candidates were selected from the HS, HE and PG surveys. In total 10 pulsators were found, some quite bright. In addition one more was found from a small SDSS sample observed in Oct. 2002.  相似文献   

Physics of solar-like oscillations     

JØrgen Christensen-Dalsgaard 《Solar physics》2004,220(2):137-168

The physics of solar and stellar oscillations determines their observable properties: frequencies, amplitudes, lifetimes, line asymmetries and phase relations. In the solar case these quantities have been measured, often with high precision, and much has been learned about the properties of the solar interior, and the properties of the oscillations. With recent advances in observational techniques, such seismic investigations are now being extended to solar-like oscillations in distant stars. I provide a brief overview of the basic properties of stellar oscillations, and of the information about stellar properties that may be inferred from them, concentrating mostly on the low-degree modes for which information may be expected for distant stars. In addition, I consider the current state of investigations of solar-like oscillations in other stars, and the prospects for an improved understanding of the physics of such oscillations.  相似文献   

The heat goes on—changes in indices of hot extremes in Poland     

Graczyk  Dariusz  Pińskwar  Iwona  Kundzewicz  Zbigniew W.  Hov  Øystein  Førland  Eirik J.  Szwed  Małgorzata  Choryński  Adam 《Theoretical and Applied Climatology》2017,129(1-2):459-471

Theoretical and Applied Climatology - On the basis of temperature observations at 60 meteorological stations in Poland, changes in the indices associated with the presence of extremely high air...  相似文献   

Discovery of A New Class of Pulsating Stars: Gravity-mode Pulsators among Subdwarf B Stars     

Elizabeth M. Green  Keith Callerame  Ivo R. Seitenzahl  Brooke A. White  Elaina A. Hyde  Melissa Giovanni  Mike Reed  Gilles Fontaine  Roy Østensen 《Astrophysics and Space Science》2003,284(1):65-68

During the course of an ongoing CCD monitoring program to investigate low-level light variations in subdwarf B (sdB) stars, weserendipitously discovered a new class of multimode pulsators withperiods of the order of an hour. These periods are a factor of tenlonger than those of previously known multimode sdB pulsators (EC14026 stars), implying the new pulsations are due to gravity modes rather than pressure modes. The iron opacity instability that drives the short period EC 14026 stars is effective in hot sdB's. Thelong period pulsators are found only among cooler sdB stars, wherethey are surprisingly common. The mechanism responsible for excitingthe deeper g-modes in cool sdB's is currently unknown, but thetemperature and gravity range in which these stars occur must be animportant clue. We present the first observational results for thisnew class of pulsating sdB stars, and discuss some possible implications.  相似文献   

The water level in the bosphorus strait and its dependence on atmospheric forcing     

SØren Andersen  Flemming Jakobsen  Bedri Alpar  Deniz Bilimleri 《Ocean Dynamics》1997,49(4):467-476

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Existence of the frequentistic estimate for power-law regression with a location parameter, with applications for making discharge rating curves     

Trond Reitan  Asgeir Petersen-Øverleir 《Stochastic Environmental Research and Risk Assessment (SERRA)》2006,20(6):445-453

Practical application of the power-law regression model with an unknown location parameter can be plagued by non-finite least squares parameter estimates. This presents a serious problem in hydrology, since stream flow data is mainly obtained using an estimated stage–discharge power-law rating curve. This study provides a set of sufficient requirements for the data to ensure the existence of finite least squares parameter estimates for a power-law regression with an unknown location parameter. It is shown that in practice, these requirements act as necessary for having a finite least squares solution, in most cases. Furthermore, it is proved that there is a finite probability for the model to produce data having non-finite least squares parameter estimates. The implications of this result are discussed in the context of asymptotic predictions, inference and experimental design. A Bayesian approach to the actual regression problem is recommended.  相似文献