Future evolution of bound superclusters in an accelerating Universe

The evolution of marginally bound supercluster-like objects in an accelerating Λ cold dark matter (ΛCDM) Universe is followed, by means of cosmological simulations, from the present time to an expansion factor   a = 100  . The objects are identified on the basis of the binding density criterion introduced by Dünner et al. Superclusters are identified with the ones whose mass   M > 10¹⁵  h ⁻¹ M_⊙  , the most massive one with   M ∼ 8 × 10¹⁵  h ⁻¹ M_⊙  , comparable to the Shapley supercluster. The spatial distribution of the superclusters remains essentially the same after the present epoch, reflecting the halting growth of the cosmic web as Λ gets to dominate the expansion of the Universe. The same trend can be seen in the stagnation of the development of the mass function of virialized haloes and bound objects. The situation is considerably different when looking at the internal evolution, quantified in terms of their shape, compactness and density profile, and substructure in terms of their multiplicity function. We find a continuing evolution from a wide range of triaxial shapes at   a = 1  to almost perfect spherical shapes at   a = 100  . We also find a systematic trend towards a higher concentration. Meanwhile, we see their substructure gradually disappearing, as the surrounding subclumps fall in and merge to form one coherent, virialized system.     

Effects of wet meadow riparian vegetation on streambank erosion. 1. Remote sensing measurements of streambank migration and erodibility

We quantified how rates of stream channel migration in a montane meadow vary as a function of the riparian vegetation community. The South Fork of the Kern River at Monache Meadow, located in California's southern Sierra Nevada range, supports two distinct types of vegetation: a dry meadow community dominated by sagebrush and non‐native grasses (xeric scrub and meadow), and a wet meadow community dominated by rushes and sedges (hydric graminoids). We measured rates of lateral stream migration for dry versus wet meadow reaches from aerial photographs spanning a 40‐year period (1955–1995). While stream migration rates averaged only 0·24 ± 0·02 m a^?1 in the wet meadow, the dry meadow channel migrated an average of 1·4 ± 0·3 m a^?1. We used a linear model of meander migration to calculate coefficients that characterize bank migration potential, or bank erodibility, independent of channel curvature. These calculations demonstrate that, at Monache Meadow, banks without wet meadow vegetation are roughly ten times more susceptible to erosion than banks with wet meadow vegetation. Where stream bank heights consistently exceed 1 m, low water availability creates riparian habitats dominated by dry meadow vegetation. Thus, channel incision may reduce bank stability not only by increasing bank height, but also by converting banks from wet meadow to dry meadow vegetation. Copyright © 2002 John Wiley & Sons, Ltd.     

Pinatubo eruption winter climate effects: model versus observations

Large volcanic eruptions, in addition to the well-known effect of producing global cooling for a year or two, have been observed to produce shorterterm responses in the climate system involving non-linear dynamical processes. In this study, we use the ECHAM2 general circulation model forced with stratospheric aerosols to test some of these ideas. Run in a perpetual-January mode, with tropical stratospheric heating from the volcanic aerosols typical of the 1982 El Chichón eruption or the 1991 Pinatubo eruption, we find a dynamical response with an increased polar night jet in the Northern Hemisphere (NH) and stronger zonal winds which extend down into the troposphere. The Azores High shifts northward with increased tropospheric westerlies at 60°N and increased easterlies at 30°N. Surface temperatures are higher both in northern Eurasia and North America, in agreement with observations for the NH winters of 1982–83 and 1991–92 as well as the winters following the other 10 largest volcanic eruptions since 1883.This paper was presented at the Second International Conference on Modelling of Global Climate Variability, held in Hamburg 7–11 September 1992 under the auspices of the Max Planck Institute for Meteorology. Guest Editor for these papers is L. Dümenil     

The influence of induced polarization on long-offset transient electromagnetic data

The diffusion of electromagnetic fields is dependent not only on conductivity, but also on magnetic permeability, dielectric permittivity and polarizability, i.e. dispersive conductivity. The long‐offset transient electromagnetic (LOTEM) method is mainly used to determine the spatial distribution of conductivity in the subsurface. However, earlier work on loop‐loop TEM suggests that transient EM methods can also be affected by induced polarization (IP). Numerous 1D forward calculations were carried out to study the IP effect on LOTEM data, using the Cole‐Cole relaxation model to simulate the polarizability of the ground. Besides the polarizability of each layer, the IP effect depends on the LOTEM field set‐up and the spatial distribution of conductivity in the ground. In particular, near‐surface layers with high chargeabilities can significantly distort the late time transients of the electric field components in the vicinity of the transmitter. The influence of polarizable layers on the magnetic field components can be neglected under normal circumstances. In 1997 and 1999, LOTEM measurements were carried out at Mt. Vesuvius in Italy to explore the geological structure of the volcano. Sensitivity studies on the effect of polarizable layers suggest that high chargeabilities in connection with conductive layers at greater depths would result in a detectable distortion of the electric field transients. Although the simultaneous IP measurements revealed high chargeabilities in a near‐surface layer, no evidence of IP effects could be found in the measured LOTEM data. We conclude that the observed chargeabilities are local and that 3D effects are probably present in the data. Another aspect is the measurement of the system response, which is usually measured by placing a receiver very close to the transmitter. Therefore, large distortions can be expected if near‐surface polarizable layers exist. This was verified in practice by field measurements in an area with high chargeabilities in Longerich, Cologne.     

Modulation of the inflow into the Caribbean Sea by North Brazil Current rings

A two-year long record from a triangular mooring array between the Lesser Antilles islands Tobago, Barbados, and St. Lucia is used to investigate the inflow into the Caribbean Sea, the amount of South Atlantic Water (SAW) carried with the inflow, and the role of North Brazil Current (NBC) rings in the observed variability. The data set consists of time series from temperature/conductivity recorders and current meters in the moorings, bottom-mounted inverted echo sounders at the Tobago and St. Lucia mooring positions, and supplementary shipboard measurements. The acoustic travel time measurements of the inverted echo sounders and the conductivity/temperature time series are used for continuous estimation of dynamic height profiles and geostrophic currents between the surface and 1000 dbar as well as the amount of SAW found at the mooring positions.The observations show a domination of intraseasonal variability between 0 and 15 Sv, superimposed on the long-term fluctuations. With time scales of one to three months, these represent the signature of the NBC rings. The baroclinic transport time series shows nine periods of increased variability, indicative of the rings interacting with the Lesser Antilles island arc; with the exception of one, these periods were associated with corresponding sea surface height anomalies. No marked seasonality was observed in the transport variability or the ring frequency.The arrival of individual rings leads to a weakening of the inflow into the Caribbean. Nevertheless, the rings carry large amounts of SAW into the area, and the immediate increase of the transport towards the end of a ring event suggests a subsequent flow of this SAW-rich water into the Caribbean. At St. Lucia, rings sometimes cause a short-term decrease of SAW content, indicative of an influx of northern hemispheric water and a blocking situation. The average transport of SAW into the Caribbean south of St. Lucia during the observations amounted to 5.5 Sv, with no significant seasonal cycle, but a small positive trend in SAW fraction as well as in transport of about 15% and 1 Sv, respectively; a corresponding trend in the baroclinic volume transport was not observed.     

Particle energization in galactic centre ejections

For ejections in the galactic centre (the galactic centre lobe, the galactic centre jet, and the low-energy jet), which were discovered recently, the efficiency of two low plasma- electron re-acceleration processes (Alfvén resonance heating and dissipation by magnetic non-equilibrium) considered recently by one of us (T.M.K.) are discussed and order of magnitude calculations for the dissipated power are given.