Diurnal-period,wind-forced ocean variability on the inner shelf off Concepción,Chile

We characterize the response of diurnal-period ocean current variability to the sea breeze using measurements of current velocity taken off the mouth of the Itata River and wind stress collected at Hualpen Point (central Chile) in spring of 2007 and summer of 2006 and 2008. During these three periods, the winds are predominately towards the northeast, following the coastal topography, with the highest variability found in the near-diurnal and synoptic frequency bands. The sea breeze amplitude is intermittent in time and is associated with synoptic-scale variability on the order of three to 15 days, so that the diurnal-period winds (and currents) are enhanced when the alongshore wind (i.e. upwelling-favorable) is strong. The water current variability in the near-diurnal band is significant, explaining up to 40% (spring 2007) of the total current variance in the first 15 m depth.     

Estimation and propagation of volcanic source parameter uncertainty in an ash transport and dispersal model: application to the Eyjafjallajokull plume of 14–16 April 2010

Data on source conditions for the 14 April 2010 paroxysmal phase of the Eyjafjallaj?kull eruption, Iceland, have been used as inputs to a trajectory-based eruption column model, bent. This model has in turn been adapted to generate output suitable as input to the volcanic ash transport and dispersal model, puff, which was used to propagate the paroxysmal ash cloud toward and over Europe over the following days. Some of the source parameters, specifically vent radius, vent source velocity, mean grain size of ejecta, and standard deviation of ejecta grain size have been assigned probability distributions based on our lack of knowledge of exact conditions at the source. These probability distributions for the input variables have been sampled in a Monte Carlo fashion using a technique that yields what we herein call the polynomial chaos quadrature weighted estimate (PCQWE) of output parameters from the ash transport and dispersal model. The advantage of PCQWE over Monte Carlo is that since it intelligently samples the input parameter space, fewer model runs are needed to yield estimates of moments and probabilities for the output variables. At each of these sample points for the input variables, a model run is performed. Output moments and probabilities are then computed by properly summing the weighted values of the output parameters of interest. Use of a computational eruption column model coupled with known weather conditions as given by radiosonde data gathered near the vent allows us to estimate that initial mass eruption rate on 14 April 2010 may have been as high as 10⁸?kg/s and was almost certainly above 10⁷?kg/s. This estimate is consistent with the probabilistic envelope computed by PCQWE for the downwind plume. The results furthermore show that statistical moments and probabilities can be computed in a reasonable time by using 9⁴?=?6,561 PCQWE model runs as opposed to millions of model runs that might be required by standard Monte Carlo techniques. The output mean ash cloud height plus three standard deviations??encompassing c. 99.7?% of the probability mass??compares well with four-dimensional ash cloud position as retrieved from Meteosat-9 SEVIRI data for 16 April 2010 as the ash cloud drifted over north-central Europe. Finally, the ability to compute statistical moments and probabilities may allow for the better separation of science and decision-making, by making it possible for scientists to better focus on error reduction and decision makers to focus on ??drawing the line?? for risk assessment.     

Energy dependence of Ti/Fe ratio in the Galactic cosmic rays measured by the ATIC-2 experiment

Titanium is a rare, secondary nucleus among Galactic cosmic rays. Using the Silicon matrix in the ATIC experiment, Titanium has been separated. The energy dependence of the Ti to Fe flux ratio in the energy region from 5 GeV per nucleon to about 500 GeV per nucleon is presented. The article was translated by the authors.     

Evolution of X-ray cavities

A wide range of recent observations have shown that active galactic nuclei (AGN) driven cavities may provide the energy source that balances the cooling observed in the centres of 'cool-core' galaxy clusters. One tool for better understanding the physics of these cavities is their observed morphological evolution, which is dependent on such poorly understood properties as the turbulent density field and the impact of magnetic fields. Here, we combine numerical simulations that include subgrid turbulence and software that produces synthetic X-ray observations to examine the evolution of X-ray cavities in the absence of magnetic fields. Our results reveal an anisotropic size evolution of the cavities that is dramatically different from simplified, analytical predictions. These differences highlight some of the key issues that must be accurately quantified when studying AGN-driven cavities, and help to explain why the inferred pV energy in these regions appears to be correlated with their distance from the cluster centre. Interpreting that X-ray observations will require detailed modelling of effects, including mass entrainment, distortion by drag forces and projection. Current limitations do not allow a discrimination between purely hydrodynamic and magnetically dominated models for X-ray cavities.     

Stochastic models of lunar rocks and regolith. Part I. Catastrophic splitting theory

It is assumed that a rock on the lunar surface loses mass as a result of bombardment by hypervelocity meteoroids. The mass of rock and its fragments can be modeled as a nonincreasing stochastic process with independent increments. In the case of a self-similar, one-shot splitting law, Filippov's extension of Kolmogorov's results produces asymptotic mass densities (number densities) which can be of lognormal, fractional exponential (Rosin-Rammler), or inverse-power (Pareto) types. The results are extended in three directions. A new explicit formula for the number density is obtained in the case where the splitting law is a two-term polynomial. The effect of splitting laws and splitting rates which depend on randomly varying parameters, e.g., meteoroid mass and velocity, is considered. The average number density with respect to a distribution of initial rock masses and initial rock birthdays also is studied. The asymptotic average density for an inverse-power distribution of initial masses has the same shape as the unaveraged density, but a beta (Β, 1) distribution of rock birthdays strongly alters the shape of the asymptotic number density. Research supported by Office of Naval Research, under contract NONR 4010(09) awarded to Department of Statistics, The Johns Hopkins University.     

A comparison of forest gap models: Model structure and behaviour

Forest gap models share a common structure for simulating tree population dynamics, and many models contain the same or quite similar ecological factors. However, a wide variety of formulations are being used to implement this general structure. The comparison of models incorporating different formulations is important for model validation, for assessing the reliability of model projections obtained under scenarios of climatic change, and for the development of models with a wide range of applicability. This paper reviews qualitative and quantitative comparisons of the structure and behaviour of forest gap models.As examples of qualitative model comparisons, the different formulations used for the heightdiameter relationship, for the maximum growth equation, and for the effects of temperature and drought on tree growth are reviewed. The variety of formulations currently in use has the potential to influence simulation results considerably, but we conclude that little is known on the sensitivity of the models in this respect.The quantitative model comparisons performed so far allow us to draw the following conclusions: (1) Gap models are quite sensitive to the formulation of climate-dependent processes under current climate, and this sensitivity is even more pronounced under a changed climate. (2) Adaptations of forest gap models to specific regions have required detailed sub-models of species life history, thus complicating model comparison. (3) Some of the complex models developed for region-specific applications can be simplified without hampering the realism with which they simulate species composition. (4) Attempts to apply the models without modification beyond the area for which they were developed have produced controversial results.It is concluded that the sensitivity of forest gap models to the exact process formulations should be examined carefully, and that more systematic comparisons of model behaviour at a range of test sites would be desirable. Such studies could improve our understanding of forest dynamics considerably, and they would help to focus future research activities with gap models.     

Mapping the spatial and temporal distributions of woody debris in streams of the Greater Yellowstone Ecosystem, USA

The objectives of this study were: (1) to document spatial and temporal distributions of large woody debris (LWD) at watershed scales and investigate some of the controlling processes; and (2) to judge the potential for mapping LWD accumulations with airborne multispectral imagery. Field surveys were conducted on the Snake River, Soda Butte Creek, and Cache Creek in the Greater Yellowstone Ecosystem, USA. The amount of woody debris per kilometer is highest in 2nd order streams, widely variable in 3rd and 4th order streams, and relatively low in the 6th order system. Floods led to increases in woody debris in 2nd order streams. Floods redistributed the wood in 3rd and 4th order streams, removing it from the channel and stranding it on bars, but appeared to generate little change in the total amount of wood throughout the channel system. The movement of woody debris suggests a system that is the reverse of most sediment transport systems in mountains. In 1st and 2nd order tributaries, the wood is too large to be moved and the system is transport-limited, with floods introducing new material through undercutting, but not removing wood through downstream transport. In the intermediate 3rd and 4th order channels, the system displays characteristics of dynamic equilibrium, where the channel is able remove the debris at approximately the same rate that it is introduced. The spatial distribution and quantity of wood in 3rd and 4th order reaches varies widely, however, as wood is alternatively stranded on gravel bars or moved downstream during periods of bar mobilization. In the 6th order and larger channels, the system becomes supply-limited, where almost all material in the main stream can be transported out of the central channel by normal stream flows and deposition occurs primarily on banks or in eddy pool environments. Attempts to map woody debris with 1-m resolution digital four-band imagery were generally unsuccessful, primarily because the imagery could not distinguish the narrow logs within a pixel from the surrounding sand and gravel background and due to problems in precisely coregistering imagery and field maps.     

First-order drainage basin morphology—definition and distribution

First-order drainage basin morphology consists of two complementary regions: a headwater region, the valley head; and a stream region, the channelway (Figure 1). Each subbasin's morphology is represented by a set of principal components factors that include the properties of area, length, slope, relief, elongation, and plan curvature. The channelway region is a highly-integrated morphological unit that is dominated by a size-shape factor, indicative of an organized flow system and the presence of a permanent channel. The valley head region shows little integration of its morphological factor set, and this is reflected by its lack of a permanent channel. The valley head-channelway definition is utilized to classify first-order basins into three morphological groups or types. Basin type is related to basin location within the larger drainage network, and this relation helps to explain variations in subbasin morphology. The channelway's morphologic properties are influenced by the location of the first-order basin's bifurcation or junction within the higher-ordered network; and valley head morphology is related to the location of the basin's divide position within the drainage net.     

New terrestrial heat flow measurements on the Nazca Plate

Sixty-seven new heat flow measurements on the Nazca Plate are reported, and the thermal regimes of three specific areas on the plate are examined. The Nazca Ridge is an aseismic ridge which may have been generated as an “island trail” from the Easter Island “hot spot” and/or may be a fossil transform fault. The Nazca Ridge has lower heat flow than the surrounding sea floor implying that the ridge might have low “effective” thermal conductivity causing heat to preferentially flow or refract to surrounding ocean crust which has higher conductivity, or, the low heat flow values may be caused by hydrothermal circulation on the ridge. The Carnegie Plateau is an elevated region south of the Carnegie Ridge on the northeastern Nazca Plate with high heat flow and shallow topography consistent with an age of less than 20 m.y. B.P. The central Nazca Plate is an area of highly variable heat flow which is possibly related to thin sediment and to rough regional topography.