Hydrodynamical Simulations of Molecular Dynamics in Supersonic Turbulent Flow

Here we present results from simulations of turbulence in star forming environments obtained by coupling three-dimensional hydrodynamical models with appropriate chemical processes. We investigate regimes of decaying high-speed molecular turbulence. Here we analyse PDFs of density for the volume, mass, molecular mass and the energy distribution over the range of scales. We compare our results to those previously obtained for isothermal turbulence and suggest possible explanations.     

Relative effects of local and landscape factors on wetland algal biomass over a salinity gradient

Wetlands in south-eastern Australia and other arid regions of the world are experiencing increases in salinity due to dryland salinization and climate change. We investigated changes in wetland ecological function, measured as phytoplankton and benthic algal Chl a, over a large salinity gradient (0.047–226 mS cm⁻¹) and in relation to several local water chemistry variables that may be important predictors of algal biomass. We investigated the relative importance of landscape variables that may affect input pollution and hydrology of wetlands at four spatial scales (100, 500, 1,000 and 5000 m). We explored the strength and form of the relationships between algal biomass and local and landscape predictors with emphasis on the effects of local and landscape salinity. We found local variables were more important than landscape variables in influencing algal biomass. We also found salinity of wetlands was not a good predictor of phytoplankton biomass but it did predict benthic algal biomass.     

Wavelet-multivariate relevance vector machine hybrid model for forecasting daily evapotranspiration

Evapotranspiration (ET) is one of the main components of the hydrological cycle. It is a complex process driven mainly by weather parameters, and as such, is characterized by high non-linearity and non-stationarity. This paper introduces a methodology combining wavelet multiresolution analysis with a machine learning algorithm, the multivariate relevance vector machine (MVRVM), in order to predict 16 days of future daily reference evapotranspiration (ETo). This methodology lays the ground for forecasting the spatial distribution of ET using Landsat satellite imagery, hence the choice of 16 days, which corresponds with the Landsat overpass cycle. An accurate prediction of daily ETo is needed to improve the management of irrigation schedules as well as the operations of water supply facilities like canals and reservoirs. In this paper, various wavelet decompositions were performed and combined with MVRVM to develop hybrid models to predict ETo over a 16-days period. These models were compared to a MVRVM model, and models accuracy and robustness were evaluated. The addition of 10 days of forecasted air temperature as additional inputs to the forecasting models was also investigated. The results of the wavelet-MVRVM hybrid modeling methodology showed that a reliable forecast of ETo up to 16 days ahead is possible.     

High-resolution calculations of asteroid impacts into the Venusian atmosphere.

We present results from a number of 2D high-resolution hydrodynamical simulations of asteroids striking the atmosphere of Venus. These cover a wide range of impact parameters (velocity, size, and incidence angle), but the focus is on 2-3 km diameter asteroids, as these are responsible for most of the impact craters on Venus. Asteroids in this size range are disintegrated, ablated, and significantly decelerated by the atmosphere, yet they retain enough impetus to make large craters when they meet the surface. We find that smaller impactors (diameter <1-2 km) are better described by a "pancaking" model in which the impactor is compressed and distorted, while for larger impactors (>2-3 km) fragmentation by mechanical ablation is preferred. The pancaking model has been modified to take into account effects of hydrodynamical instabilities. The general observation that most larger impactors disintegrate by shedding fragments generated from hydrodynamic instabilities spurs us to develop a simple heuristic model of the mechanical ablation of fragments based on the growth rates of Rayleigh-Taylor instabilities. Although in principle the model has many free parameters, most of these have little effect provided that they are chosen reasonably. In practice the range of model behavior can be described with one free parameter. The resulting model reproduces the mass and momentum fluxes rather well, doing so with reasonable values of all physical parameters.     

Hurst scaling with crossover of a drought indicator: a case study in Belem and Manaus,Brazil

Natural Hazards - Some Amazonia regions are vulnerable to natural disasters. Hurst analysis of hydrological events can provide more information than classical statistical methods. The objectives of...     

High-Resolution Simulations of the Impacts of Asteroids into the Venusian Atmosphere II: 3D Models

We compare high-resolution 2D and 3D numerical hydrocode simulations of asteroids striking the atmosphere of Venus. Our focus is on aerobraking and its effect on the size of impact craters. We consider impacts both by spheres and by the real asteroid 4769 Castalia, a severely nonspherical body in a Venus-crossing orbit. We compute mass and momentum fluxes as functions of altitude as global measures of the asteroid's progress. We find that, on average, the 2D and 3D simulations are in broad agreement over how quickly an asteroid slows down, but that the scatter about the average is much larger for the 2D models than for the 3D models. The 2D models appear to be rather strongly susceptible to the “butterfly effect,” in which tiny changes in initial conditions (e.g., 0.05% change in the impact velocity) produce quite different chaotic evolutions. By contrast, the global properties of the 3D models appear more reproducible despite seemingly large differences in initial conditions. We argue that this difference between 2D and 3D models has its root in the greater geometrical constraints present in any 2D model, and in particular in the global conservation of enstrophy in 2D that forces energy to pool in large-scale structures. It is the interaction of these artificial large-scale structures that causes slightly different 2D models to diverge so greatly. These constraints do not apply in 3D and large scale structures are not observed to form. A one-parameter modified pancake model reproduces the expected crater diameters of the 3D Castalias reasonably well.     

Comparative study of the kinetics of the thermal decomposition of synthetic and natural siderite samples

The mechanism of the thermal decomposition of two siderites (a pure synthetic and a natural Mg-containing sample) has been determined from comparison of the results obtained from linear heating rate (TG) and constant rate thermal analysis (CRTA) experiments in high vacuum. The thermal decomposition of the synthetic siderite takes place approximately 200 K below the decomposition temperature of the natural sample. The mechanism and the product of the thermal decomposition are different for the siderite samples. In fact, an A₂ kinetic model describes the thermal decomposition of the synthetic siderite, whereas the thermal decomposition of the natural sample obeys an F₁ kinetic law. Decomposition products of the synthetic siderite are iron and magnetite, those of the natural siderite are wüstite and minor magnetite. Received: 22 July 1999 / Accepted: 12 February 2000     

The DRASTIC-Sg model: an extension to the DRASTIC approach for mapping groundwater vulnerability in aquifers subject to differential land subsidence,with application to Mexico City

Mexico City relies on groundwater for most of its domestic supply. Over the years, intensive pumping has led to significant drawdowns and, subsequently, to severe land subsidence. Tensile cracks have also developed or reactivated as a result. All such processes cause damage to urban infrastructure, increasing the risk of spills and favoring contaminant propagation into the aquifer. The effects of ground deformation are frequently ignored in groundwater vulnerability studies, but can be relevant in subsiding cities. This report presents an extension to the DRASTIC methodology, named DRASTIC-Sg, which focuses on evaluating groundwater vulnerability in urban aquifers affected by differential subsidence. A subsidence parameter is developed to represent the ground deformation gradient (Sg), and then used to depict areas where damage risk to urban infrastructure is higher due to fracture propagation. Space-geodetic SqueeSAR data and global positioning system (GPS) validation were used to evaluate subsidence rates and gradients, integrating hydrogeological and geomechanical variables into a GIS environment. Results show that classic DRASTIC approaches may underestimate groundwater vulnerability in settings such as the one at hand. Hence, it is concluded that the Sg parameter is a welcome contribution to develop reliable vulnerability assessments in subsiding basins.