The linear stability of dilute particulate rings     

Henrik N. Latter  Gordon I. Ogilvie 《Icarus》2006,184(2):498-516

Irregular structure in planetary rings is often attributed to the intrinsic instabilities of a homogeneous state undergoing Keplerian shear. Previously these have been analysed with simple hydrodynamic models. We instead employ a kinetic theory, in which we solve the linearised moment equations derived in Shu and Stewart 1985 for a dilute ring. This facilitates an examination of velocity anisotropy and non-Newtonian stress, and their effects on the viscous and viscous/gravitational instabilities thought to occur in Saturn's rings. Because we adopt a dilute gas model, the applicability of our results to the actual dense rings of Saturn are significantly curtailled. Nevertheless this study is a necessary preliminary before an attack on the difficult problem of dense ring dynamics. We find the Shu and Stewart formalism admits analytic stability criteria for the viscous overstability, viscous instability, and thermal instability. These criteria are compared with those of a hydrodynamic model incorporating the effective viscosity and cooling function computed from the kinetic steady state. We find the two agree in the ‘hydrodynamic limit’ (i.e., many collisions per orbit) but disagree when collisions are less frequent, when we expect the viscous stress to be increasingly non-Newtonian and the velocity distribution increasingly anisotropic. In particular, hydrodynamics predicts viscous overstability for a larger portion of parameter space. We also numerically solve the linearised equations of the more accurate Goldreich and Tremaine 1978 kinetic model and discover its linear stability to be qualitatively the same as that of Shu and Stewart's. Thus the simple collision operator adopted in the latter would appear to be an adequate approximation for dilute rings, at least in the linear regime.  相似文献   

From modelling theory to policy: lessons from the LCS project     

Henrik Hammar  Sverker C. Jagers 《Climate Policy》2013,13(6):613-625

The Low-Carbon Society (LCS) research project (Strachan et al., 2008) is to be congratulated for providing a rich, useful source of models to inform and precipitate discussion of policy options for moving towards a low-carbon society. This commentary explores additional considerations that warrant inclusion in any policy discussion: the feasibility of political will and leadership, the limitations of technical feasibility, and clarity about what ‘the price of carbon’ means. Difficulties exist with establishing carbon prices due to differences in energy prices between countries and the distinction between baseline (existing policies) and additional parts. An alternative may be to place more emphasis on the energy price for mitigation, along with other indicators (energy efficiency, carbon intensity, renewable power diffusion) for comparison. A policy is proposed, based on the energy price combined with an emphasis on accelerating technological innovation and overcoming barriers to the adoption of energy efficiency measures.  相似文献   

Assessing future climatic changes of rainfall extremes at small spatio-temporal scales     

Ida Bülow Gregersen  Hjalte Jomo Danielsen Sørup  Henrik Madsen  Dan Rosbjerg  Peter Steen Mikkelsen  Karsten Arnbjerg-Nielsen 《Climatic change》2013,118(3-4):783-797

Climate change is expected to influence the occurrence and magnitude of rainfall extremes and hence the flood risks in cities. Major impacts of an increased pluvial flood risk are expected to occur at hourly and sub-hourly resolutions. This makes convective storms the dominant rainfall type in relation to urban flooding. The present study focuses on high-resolution regional climate model (RCM) skill in simulating sub-daily rainfall extremes. Temporal and spatial characteristics of output from three different RCM simulations with 25 km resolution are compared to point rainfall extremes estimated from observed data. The applied RCM data sets represent two different models and two different types of forcing. Temporal changes in observed extreme point rainfall are partly reproduced by the RCM RACMO when forced by ERA40 re-analysis data. Two ECHAM forced simulations show similar increases in the occurrence of rainfall extremes of over a 150-year period, but significantly different changes in the magnitudes. The physical processes behind convective rainfall extremes generate a distinctive spatial inter-site correlation structure for extreme events. All analysed RCM rainfall extremes, however, show a clear deviation from this correlation structure for sub-daily rainfalls, partly because RCM output represents areal rainfall intensities and partly due to well-known inadequacies in the convective parameterization of RCMs. The results highlight the problem urban designers are facing when using RCM output. The paper takes the first step towards a methodology by which RCM performance and other downscaling methods can be assessed in relation to the simulation of short-duration rainfall extremes.  相似文献   

AGN effect on cooling flow dynamics     

Fathallah Alouani Bibi  James Binney  Katherine Blundell  Henrik Omma 《Astrophysics and Space Science》2007,311(1-3):317-321

We analyzed the feedback of AGN jets on cooling flow clusters using three-dimensional AMR hydrodynamic simulations. We studied the interaction of the jet with the intracluster medium and creation of low X-ray emission cavities (Bubbles) in cluster plasma. The distribution of energy input by the jet into the system was quantified in its different forms, i.e. internal, kinetic and potential. We find that the energy associated with the bubbles, (pV+γ pV/(γ−1)), accounts for less than 10 percent of the jet energy.  相似文献   

Observations of martian gullies and constraints on potential formation mechanisms: II. The northern hemisphere     

Jennifer L. Heldmann  Ella Carlsson  Henrik Johansson  Owen B. Toon 《Icarus》2007,188(2):324-344

The formation process(es) responsible for creating the observed geologically recent gully features on Mars has remained the subject of intense debate since their discovery. We present new data and analysis of northern hemisphere gullies from Mars Global Surveyor data which is used to test the various proposed mechanisms of gully formation. We located 137 Mars Orbiter Camera (MOC) images in the northern hemisphere that contain clear evidence of gully landforms and analyzed these images in combination with Mars Orbiter Laser Altimeter (MOLA) and Thermal Emission Spectrometer (TES) data to provide quantitative measurements of numerous gully characteristics. Parameters we measured include apparent source depth and distribution, vertical and horizontal dimensions, slopes, orientations, and present-day characteristics that affect local ground temperatures. Northern hemisphere gullies are clustered in Arcadia Planitia, Tempe Terra, Acidalia Planitia, and Utopia Planitia. These gullies form in craters (84%), knobby terrain (4%), valleys (3%), other/unknown terrains (9%) and are found on all slope orientations although the majority of gullies are equator-facing. Most gullies (63%) are associated with competent rock strata, 26% are not associated with strata, and 11% are ambiguous. Assuming thermal conductivities derived from TES measurements as well as modeled surface temperatures, we find that 95% of the gully alcove bases with adequate data coverage lie at depths where subsurface temperatures are greater than 273 K and 5% of the alcove bases lie within the solid water regime. The average alcove length is 470 m and the average channel length is 690 m. Based on a comparison of measured gully features with predictions from the various models of gully formation, we find that models involving carbon dioxide, melting ground ice in the upper few meters of the soil, dry landslide, and surface snowmelt are the least likely to describe the formation of the martian gullies. Although some discrepancies still exist between prediction and observation, the shallow and deep aquifer models remain as the most plausible theories. Interior processes involving subsurface fluid sources are generally favored over exogenic processes such as wind and snowfall for explaining the origin of the martian gullies. These findings gleaned from the northern hemisphere data are in general agreement with analyses of gullies in the southern hemisphere [Heldmann, J.L., Mellon, M.T., 2004. Icarus 168, 285-304].  相似文献   

Applications of multivariate statistical analysis in remote sensing of agriculture     

Kjeld Rasmussen  cand.silv. Henrik Hagen Olesen 《Geografisk tidskrift / udgivet af Bestyrelsen for Det Kongelige danske geografiske selskab》2013,113(1):100-107

Rasmussen, Kjeld & Olesen, Henrik Hagen: Applications of multivariate statistical analysis in remote sensing of agriculture. Geografisk Tidsskrift 88:100–107. Copenhagen 1988.

Applications of satellite remote sensing to agriculture involve two main objectives, the identification and mapping of crops, including estimation of acreages, and monitoring of plant growth or production factors, aiming at estimation/prediction of yields.

Deterministic models of the interaction of electromagnetic radiation and plant canopies are used to relate the measured reflected or emitted radiation to crop type and agronomically relevant parameters. The great natural variation of reflectance properties of crops does, however, call for use of a statistical approach. The high dimensionality of the data-sets involved, very often more than ten, requires the use of multivariate techniques.

This paper will deal with the use of multivariate statistical techniques for both crop identification and crop monitoring based on high-resolution satellite remote sensing data, such as those produced by Landsat MSS and -TM and SPOT. Emphasis will be placed upon use of statistical methods in classification and on removal of redundancy in multi-dimensional data-sets. The relative merits of deterministic and statistical methods will be discussed as will the possibilities of incorporating spatial information into statistical methods.  相似文献   

Estimation of the surface energy balance in the Sahelian zone of Western Africa     

Henrik Søgaard 《Geografisk tidskrift / udgivet af Bestyrelsen for Det Kongelige danske geografiske selskab》2013,113(1):108-115

Søgaard, Henrik: Estimation of the surface energy balance in the Sahelian zone of Western Africa. Geografisk Tidsskrift 88: 108- 115. Copenhagen 1988.

In studies of desertification in the Sahelian zone of Western Africa an improved knowledge of the water and surface energy balance is recognized to be of major importance. Based on micro-climatological measurements collected during an ongoing field study in the northern part of Burkina Faso, a number of methods for deriving surface energy balance are examined. It is found that for the actual case, with profile measurements restricted to two levels above the surface, exact values of the sensible heat flux can accurately be obtained by applying non-dimensional gradients based on the Monin-Obukhov turbulence theory. Utilization of the results for deriving actual evapotranspiration from standard observations is demonstrated, and the paper finally discusses applicability of the results in satellite remote sensing.  相似文献   

Biomarker responses in Atlantic cod (Gadus morhua) exposed to produced water from a North Sea oil field: laboratory and field assessments     

Sundt RC  Ruus A  Jonsson H  Skarphéðinsdóttir H  Meier S  Grung M  Beyer J  Pampanin DM 《Marine pollution bulletin》2012,64(1):144-152

Biological markers of produced water (PW) exposure were studied in Atlantic cod (Gadus morhua) in both laboratory and field experiments, using authentic PW from a North Sea oil field. In the laboratory study, the PW exposure yielded significantly elevated levels of metabolites of polycyclic aromatic hydrocarbons (PAHs) and alkylphenols (APs) in bile even at the lowest exposure dose (0.125% PW). Other biomarkers (hepatic CYP1A induction and DNA adduct formation) responded at 0.25% and 0.5% PW concentrations. In the field study, bile metabolite markers and hepatic CYP1A were clearly increased in fish caged close to the PW outfall. Induction of plasma vitellogenin was not found in laboratory or field exposures, suggesting that the levels of oestrogen agonists (such as APs) might not have been sufficient to elicit induction, under the present conditions. The applicability of the biomarkers for use in water column biomonitoring programs is discussed.  相似文献   

Stochastic rainfall-runoff forecasting: parameter estimation,multi-step prediction,and evaluation of overflow risk   总被引：2，自引：2，他引：0  

Roland Löwe  Peter Steen Mikkelsen  Henrik Madsen 《Stochastic Environmental Research and Risk Assessment (SERRA)》2014,28(3):505-516

Probabilistic runoff forecasts generated by stochastic greybox models can be notably useful for the improvement of the decision-making process in real-time control setups for urban drainage systems because the prediction risk relationships in these systems are often highly nonlinear. To date, research has primarily focused on one-step-ahead flow predictions for identifying, estimating, and evaluating greybox models. For control purposes, however, stochastic predictions are required for longer forecast horizons and for the prediction of runoff volumes, rather than flows. This article therefore analyzes the quality of multistep ahead forecasts of runoff volume and considers new estimation methods based on scoring rules for k-step-ahead predictions. The study shows that the score-based methods are, in principle, suitable for the estimation of model parameters and can therefore help the identification of models for cases with noisy in-sewer observations. For the prediction of the overflow risk, no improvement was demonstrated through the application of stochastic forecasts instead of point predictions, although this result is thought to be caused by the notably simplified setup used in this analysis. In conclusion, further research must focus on the development of model structures that allow the proper separation of dry and wet weather uncertainties and simulate runoff uncertainties depending on the rainfall input.  相似文献   

Inertial waves near corotation in three-dimensional hydrodynamical discs     

Henrik N. Latter  Steven A. Balbus 《Monthly notices of the Royal Astronomical Society》2009,399(2):1058-1073

This paper concerns the interaction between non-axisymmetric inertial waves and their corotation resonances in a hydrodynamical disc. Inertial waves are of interest because they can localize in resonant cavities circumscribed by Lindblad radii and, as a consequence, can exhibit discrete oscillation frequencies that may be observed. It is often hypothesized that these trapped eigenmodes are affiliated with the poorly understood quasi-periodic oscillation phenomenon. We demonstrate that a large class of non-axisymmetric three-dimensional (3D) inertial waves cannot manifest as trapped normal modes. This class includes any inertial wave whose resonant cavity contains a corotation singularity. Instead, these 'singular' modes constitute a continuous spectrum and, as an ensemble, are convected with the flow, giving rise to shearing waves. Finally, we present a simple demonstration of how the corotation singularity stabilizes 3D perturbations in a slender torus.  相似文献