The Ashima/MIT Mars GCM and argon in the martian atmosphere

We investigate the ability of modern general circulation models (GCMs) to simulate transport in the martian atmosphere using measurements of argon as a proxy for the transport processes. Argon provides the simplest measure of transport as it is a noble gas with no sinks or sources on seasonal timescales. Variations in argon result solely from ‘freeze distillation’, as the atmosphere condenses at the winter poles, and from atmospheric transport. Comparison of all previously published models when rescaled to a common definition of the argon enhancement factor (EF) suggest that models generally do a poor job in predicting the peak enhancement in southern winter over the winter pole – the time when the capability of the model transport approaches are most severely tested. Despite observed peak EF values of ～6, previously published model predictions peaked at EF values of only 2–3. We introduce a new GCM that provides a better treatment of mass conservation within the dynamical core, includes more sophisticated tracer transport approaches, and utilizes a cube–sphere grid structure thus avoiding the grid-point convergence problem at the pole that exists for most current Mars GCMs. We describe this model – the Ashima Research/Massachusetts Institute of Technology Mars General Circulation Model (Ashima/MIT Mars GCM) and use it to demonstrate the significant sensitivity of peak EF to the choices of transport approach for both tracers and heat. We obtain a peak EF of 4.75 which, while over 50% higher than any prior model, remains well short of the observed value. We show that the polar EF value in winter is primarily determined by the competition between two processes: (1) mean meridional import of lower-latitude air not enriched in argon and (2) the leakage of enriched argon out of the polar column by eddies in the lowest atmospheric levels. We suggest possibilities for improving GCM representation of the CO₂ cycle and the general circulation that may further improve the simulation of the argon cycle. We conclude that current GCMs may be insufficient for detailed simulation of transport-sensitive problems like the water cycle and potentially also the dust cycle.     

Mineralogy and petrogenesis of lunar magnesian granulitic meteorite Northwest Africa 5744

Lunar meteorite Northwest Africa (NWA) 5744 is a granulitic breccia with an anorthositic troctolite composition that may represent a distinct crustal lithology not previously described. This meteorite is the namesake and first‐discovered stone of its pairing group. Bulk rock major element abundances show the greatest affinity to Mg‐suite rocks, yet trace element abundances are more consistent with those of ferroan anorthosites. The relatively low abundances of incompatible trace elements (including K, P, Th, U, and rare earth elements) in NWA 5744 could indicate derivation from a highlands crustal lithology or mixture of lithologies that are distinct from the Procellarum KREEP terrane on the lunar nearside. Impact‐related thermal and shock metamorphism of NWA 5744 was intense enough to recrystallize mafic minerals in the matrix, but not intense enough to chemically equilibrate the constituent minerals. Thus, we infer that NWA 5744 was likely metamorphosed near the lunar surface, either as a lithic component within an impact melt sheet or from impact‐induced shock.     

A geochemical approach to constraining the formation of glassy fallout debris from nuclear tests

Glassy nuclear fallout debris from near-surface nuclear tests is fundamentally reprocessed earth material. A geochemical approach to analysis of glassy fallout is uniquely suited to determine the means of reprocessing and shed light on the mechanisms of fallout formation. An improved understanding of fallout formation is of interest both for its potential to guide post-detonation nuclear forensic investigations and in the context of possible affinities between glassy debris and other glasses generated by high-energy natural events, such as meteorite impacts and lightning strikes. This study presents a large major-element compositional dataset for glasses within aerodynamic fallout from the Trinity nuclear test (“trinitite”) and a geochemically based analysis of the glass compositional trends. Silica-rich and alkali-rich trinitite glasses show compositions and textures consistent with formation through melting of individual mineral grains—quartz and alkali feldspar, respectively—from the test-site sediment. The volumetrically dominant glass phase—called the CaMgFe glass—shows extreme major-element compositional variability. Compositional trends in the CaMgFe glass are most consistent with formation through volatility-controlled condensation from compositionally heterogeneous plasma. Radioactivity occurs only in CaMgFe glass, indicating that co-condensation of evaporated bulk ground material and trace device material was the main mechanism of radioisotope incorporation into trinitite. CaMgFe trinitite glasses overlap compositionally with basalts, rhyolites, fulgurites, tektites, and microtektites but display greater compositional diversity than all of these naturally formed glasses. Indeed, the most refractory CaMgFe glasses compositionally resemble early solar system condensates—specifically, CAIs.     

Assessing spatial and attribute errors in large national datasets for population distribution models: a case study of Philadelphia county schools

Geospatial technologies and digital data have developed and disseminated rapidly in conjunction with increasing computing efficiency and Internet availability. The ability to store and transmit large datasets has encouraged the development of national infrastructure datasets in geospatial formats. National datasets are used by numerous agencies for analysis and modeling purposes because these datasets are standardized and considered to be of acceptable accuracy for national scale applications. At Oak Ridge National Laboratory a population model has been developed that incorporates national schools data as one of the model inputs. This paper evaluates spatial and attribute inaccuracies present within two national school datasets, Tele Atlas North America and National Center of Education Statistics (NCES). Schools are an important component of the population model, because they are spatially dense clusters of vulnerable populations. It is therefore essential to validate the quality of school input data. Schools were also chosen since a validated schools dataset was produced in geospatial format for Philadelphia County; thereby enabling a comparison between a local dataset and the national datasets. Analyses found the national datasets are not standardized and incomplete, containing 76 to 90 percent of existing schools. The temporal accuracy of updating annual enrollment values resulted in 89 percent inaccuracy for 2003. Spatial rectification was required for 87 percent of NCES points, of which 58 percent of the errors were attributed to the geocoding process. Lastly, it was found that by combining the two national datasets, the resultant dataset provided a more useful and accurate solution.     

Harvest patterns and effort dynamics of indigenous and non-indigenous commercial sectors of the eastern Torres Strait reef line fishery

The reef line fishery (RLF) in eastern Torres Strait (ETS) is unique in that it has both a commercial indigenous sector and a commercial non-indigenous sector. Recently, concerns have been expressed by all stakeholders about the long-term sustainability of the fishery. These concerns have been exacerbated by the lack of detailed catch and effort information from both sectors, which has precluded any formal assessment of the fishery. In this paper, we characterise the harvest patterns and effort dynamics of the indigenous and non-indigenous commercial sectors of the ETS RLF using a range of data sources including commercial logbooks, community freezer records, voluntary logbooks and observer surveys. We demonstrate that bycatch is a significant component of the catch for both sectors and identify substantial differences in harvest patterns and effort dynamics between the sectors. Differences between sectors were observed in species composition and spatial and temporal patterns in catch, effort and catch per unit effort. These results highlight the inherent variation in catch and effort dynamics between the two commercial sectors of the ETS RLF and provide valuable information for the development of future assessments and appropriate management strategies for the fishery. The more reliable estimates of harvest patterns and effort dynamics for both sectors obtained from observer surveys will also assist in resolving issues relating to allocation of reef fish resources in Torres Strait.     

A transport‐distance approach to scaling erosion rates: 1. Background and model development

The process basis of existing soil‐erosion models is shown to be ill‐founded. The existing literature builds directly or indirectly on Bennett's (1974) paper, which provided a blueprint for integrated catchment‐scale erosion modelling. Whereas Bennett recognized the inherent assumptions of the approach suggested, subsequent readings of the paper have led to a less critical approach. Most notably, the assumption that sediment movement could be approximated by a continuity equation that related to transport in suspension has produced a series of submodels that assume that all movement occurs in suspension. For commonly occurring conditions on hillslopes, this case is demonstrably untrue both on theoretical grounds and from empirical observations. Elsewhere in the catchment system, it is only partially true, and the extent to which the assumption is reasonable varies both spatially and temporally. A second ground‐breaking paper – that of Foster and Meyer (1972) – was responsible for subsequent uncritical application of a first‐order approximation to deposition based on steady‐state analysis and again a weak empirical basis. We describe in this paper an alternative model (Mahleran – Model for Assessing Hillslope‐Landscape Erosion, Runoff And Nutrients) based upon particle‐travel distance that overcomes existing limitations by incorporating parameterizations of the different detachment and transport mechanisms that occur in water erosion in hillslopes and small catchments. In the second paper in the series, we consider the sensitivity and general behaviour of Mahleran , and test it in relation to data from a large rainfall‐simulation experiment. The third paper of the sequence evaluates the model using data from plots of different sizes in monitored rainfall events. From this evaluation, we consider the scaling characteristics of the current form of Mahleran and suggest that integrated modelling, laboratory and field approaches are required in order to advance the state of the art in soil‐erosion modelling. Copyright © 2008 John Wiley & Sons, Ltd.     

Transient and Transition Factors in Modeling Permafrost Thaw and Groundwater Flow

Permafrost covers approximately 24% of the Northern Hemisphere, and much of it is degrading, which causes infrastructure failures and ecosystem transitions. Understanding groundwater and heat flow processes in permafrost environments is challenging due to spatially and temporarily varying hydraulic connections between water above and below the near-surface discontinuous frozen zone. To characterize the transitional period of permafrost degradation, a three-dimensional model of a permafrost plateau that includes the supra-permafrost zone and surrounding wetlands was developed. The model is based on the Scotty Creek basin in the Northwest Territories, Canada. FEFLOW groundwater flow and heat transport modeling software is used in conjunction with the piFreeze plug-in, to account for phase changes between ice and water. The Simultaneous Heat and Water (SHAW) flow model is used to calculate ground temperatures and surface water balance, which are then used as FEFLOW boundary conditions. As simulating actual permafrost evolution would require hundreds of years of climate variations over an evolving landscape, whose geomorphic features are unknown, methodologies for developing permafrost initial conditions for transient simulations were investigated. It was found that a model initialized with a transient spin-up methodology, that includes an unfrozen layer between the permafrost table and ground surface, yields better results than with steady-state permafrost initial conditions. This study also demonstrates the critical role that variations in land surface and permafrost table microtopography, along with talik development, play in permafrost degradation. Modeling permafrost dynamics will allow for the testing of remedial measures to stabilize permafrost in high value infrastructure environments.     

Multistatic estimation of high-frequency radar surface currents in the region of Toulon

Ocean Dynamics - The high-frequency radar coastal network in Toulon operates in multistatic mode for the monitoring of the ocean circulation in the Northwestern Mediterranean Sea. With 2...     

Effects of antecedent moisture and macroporosity on infiltration and water flow in frozen soil

Infiltration into frozen soil plays an important role in soil freeze–thaw and snowmelt-driven hydrological processes. To better understand the complex thermal energy and water transport mechanisms involved, the influence of antecedent moisture content and macroporosity on infiltration into frozen soil was investigated. Ponded infiltration experiments on frozen macroporous and non-macroporous soil columns revealed that dry macroporous soil produced infiltration rates reaching 10³ to 10⁴ mm day⁻¹, two to three orders of magnitude larger than dry non-macroporous soil. Results suggest that rapid infiltration and drainage were a result of preferential flow through initially air-filled macropores. Using recorded flow rates and measured macropore characteristics, calculations indicated that a combination of both saturated flow and unsaturated film flow likely occurred within macropores. Under wet conditions, regardless of the presence of macropores, infiltration was restricted by the slow thawing rate of pore ice, producing infiltration rates of 2.8 to 5.0 mm day⁻¹. Reduced preferential flow under wet conditions was attributed to a combination of soil swelling, due to smectite-rich clay (that reduced macropore volume), and pore ice blockage within macropores. In comparison, dry soil column experiments demonstrated that macropores provided conduits for water and thermal energy to bypass the frozen matrix during infiltration, reducing thaw rates compared with non-macroporous soils. Overall, results showed the dominant control of antecedent moisture content on the initiation, timing, and magnitude of infiltration and flow in frozen macroporous soils, as well as the important role of macropore connectivity. The study provides an important data set that can aid the development of hydrological models that consider the interacting effects of soil freeze–thaw and preferential flow on snowmelt partitioning in cold regions.