Estimation of dispersion in estuaries by a self-consistent method

Longitudinal and transverse dispersion in estuaries may be determined in principle by measuring the change in time of the concentration field of a water soluble dye. In practice, measurements may take the form of several sampling runs crossing the dye patch. The coefficients of dispersion are usually estimated from the second moments of the concentration field; the use of second moments, in particular, introduces errors because measurements at long times or distance are unduly emphasized.A self-consistent method, free of the disadvantages inherent in the use of higher moments, is described in which the distribution of concentration in space is related to the zeroth spatial moment of concentration. In this formulation, based upon sampling runs across the dye patch, the concentration field in the estuary may be directly and simply related to the dispersion coefficients in both the intercept and gradient of a linear graph.The methods are illustrated by the characterization of a series of experiments in the Inner Estuary of Milford Haven.     

Seasonal variations in prey selection by estuarine black-headed gulls (Larus ridibundus)

The number of black-headed gulls (Larus ridibundus) in the Clyde Estuary is large. In summer the average density has reached 1350 gulls km^?2 and in winter 180 gulls km^?2. This paper compares prey selection and feeding efficiency in gulls during summer and winter on tidal flats, and considers how seasonal differences may be adaptations to cope with seasonal changes in prey availability.Gross and net rates of energy intake were highest in summer because gulls captured more of the polychaete N. diversicolor than the amphipod C. volutator. In winter, gulls selected for C. volutator and therefore an energetically less profitable diet. Throughout the year gulls selected more C. volutator relative to N. diversicolor than expected on energetic grounds and so apparently did not maximize potential net rate of energy intake.Gulls used three techniques to capture prey and made most intensive use of the ‘crouch’ technique. Crouching gulls attained a much higher net rate of energy intake than ‘upright’ or ‘paddling’ gulls.A log-linear model showed that (a) season, water depth and gull density determined feeding technique and (b) feeding technique and season independently determined foraging success and prey selection. Thus gull density and water depth acted on prey selection through imposed variations in feeding technique.Reasons for gulls selecting energetically unprofitable C. volutator and for the use of several distinct feeding techniques are discussed.     

Robertson-Walker type model with conformally invariant scalar field with trace-free energy momentum tensor

A non-static exact solution of the Einstein equations corresponding to a conformally invariant scalar field with trace-free energy momentum tensor is obtained for the Robertson-Walker type metric. Some physical properties of the solution are discussed.     

Rigidly rotating modes of the solar magnetic field

Discrete rigidly rotating components (modes) of the large-scale solar magnetic field have been investigated. We have used a specially calculated basic set of functions to resolve the observed magnetic field into discrete components. This adaptive set of functions, as well as the expansion coefficients, have been found by processing a series of digitized synoptic maps of the background magnetic field over a 20-year period. As a result, dependences have been obtained which describe the spatial structure and the temporal evolution of the 27-day and 28-day rigidly rotating modes of the Sun's magnetic field.The spatial structure of the modes has been compared with simulations based on the known flux-transport equation. In the simulations, the rigidly rotating modes were regarded as stationary states of the magnetic field whose rigid rotation and stability were maintained by a balance between the emergence of magnetic flux from stationary sources located at low latitudes and the horizontal transport of flux by turbulent diffusion and poleward directed meridional flow. Under these assumptions, the structure of the modes is determined solely by the horizontal velocity field of the plasma, except for the low-latitude zone where sources of magnetic flux concentrate. We have found a detailed agreement between the simulations and the results of the data analysis, provided that the amplitude of the meridional flow velocity and the diffusion constant are equal to 9.5 m s^–1 and 600 km² s^–1, respectively.The analysis of the expansion coefficients has shown that the rigidly rotating modes undergo rapid step-like variations which occur quasi-periodically with a period of about two years. These variations are caused by separate surges of magnetic flux in the photosphere, so that each new surge gives rise to a rapid replacement of old large-scale magnetic structures by newly arisen ones.     

Chaos and elliptical galaxies

Recent results on chaos in triaxial galaxy models are reviewed. Central mass concentrations like those observed in early-type galaxies - either stellar cusps, or massive black holes — render most of the box orbits in a triaxial potential stochastic. Typical Liapunov times are 3–5 crossing times, and ensembles of stochastic orbits undergo mixing on timescales that are roughly an order of magnitude longer. The replacement of the regular orbits by stochastic orbits reduces the freedom to construct self-consistent equilibria, and strong triaxiality can be ruled out for galaxies with sufficiently high central mass concentrations.     

Soil moisture: A critical focus for global change studies

The scientific and human dimensions of global change have many overlapping themes which offer a focus on processes occurring at the continental surface. Soil moisture is of critical importance to the physical processes governing energy and water exchanges at the land/air boundary. Soil moisture controls the extent to which plants can exploit sunlight in photosynthesis and the effectiveness with which agriculture, forestry and freshwater resources can be developed. The importance of the soil moisture to many, diverse communities has resulted in a very large collection of numerical models all of which simulate soil moisture. This paper outlines why and how a series of soil moisture simulation intercomparisons were conducted in a one-year exercise jointly sponsored by the International Geosphere Biosphere Programme and the World Climate Research Programme.     

Brief description of bare essentials of surface transfer and results from simulations with the HAPEX-MOBILHY data

The Bare Essentials of Surface Transfer (BEST) land surface scheme is briefly described and the key physical parameterisations discussed. Results are then presented to illustrate how the model performs, with forcing data for HAPEX-MOBILHY, compared to a series of other schemes in the simulation of evaporation and sensible heat. The implications of the models performance, and some indications of the future development of the scheme are provided. The basic version of BEST was found to overestimate evaporation for the HAPEX-MOBILHY data, simulating 816 mm yr⁻¹ compared to a range of 550 to 816 mm yr⁻¹ for all models. A modification to the β parameterisation reduced the evaporation to 759 mm yr⁻¹ which, although an improvement, is still probably too high.     

The time-scale for core collapse in spherical star clusters

The collapse time for a cluster of equal-mass stars is usually stated to be either 330 central relaxation times (t_rc) or 12-19 half-mass relaxation times (t_rh). But the first of these times applies only to the late stage of core collapse, and the second only to low-concentration clusters. To clarify how the time depends on the density profile, the Fokker-Planck equation is solved for the evolution of a variety of isotropic cluster models, including King models, models with power-law density cusps of ρ ∼ r^−γ, and models with nuclei. The collapse times for King models vary considerably with the cluster concentration when expressed in units of t_rc or t_rh, but vary much less when expressed in units of t_rc divided by a dimensionless measure of the temperature gradient in the core. Models with cusps have larger temperature gradients and evolve faster than King models, but not all of them collapse: those with 0 < γ < 2 expand because they start with a temperature inversion. Models with nuclei collapse or expand as the nuclei would in isolation if their central relaxation times are short; otherwise their evolution is more complicated. Suggestions are made for how the results can be applied to globular clusters, galaxies, and clusters of dark objects in the centers of galaxies.Scott D. Tremaine