Examination of observation impacts derived from observing system experiments (OSEs) and adjoint models

With the adjoint of a data assimilation system, the impact of any or all assimilated observations on measures of forecast skill can be estimated accurately and efficiently. The approach allows aggregation of results in terms of individual data types, channels or locations, all computed simultaneously. In this study, adjoint-based estimates of observation impact are compared with results from standard observing system experiments (OSEs) using forward and adjoint versions of the NASA GEOS-5 atmospheric data assimilation system. Despite important underlying differences in the way observation impacts are measured in the two approaches, the results show that they provide consistent estimates of the overall impact of most of the major observing systems in reducing a dry total-energy metric of 24-h forecast error over the globe and extratropics and, to a lesser extent, over the tropics. Just as importantly, however, it is argued that the two approaches provide unique, but complementary, information about the impact of observations on numerical weather forecasts. Moreover, when used together, they reveal both redundancies and dependencies between observing system impacts as observations are added or removed from the data assimilation system. Understanding these dependencies appears to pose an important challenge in making optimal use of the global observing system for numerical weather prediction.     

Post-depositional modification of Pleistocene terraces of the River Thames

The recognition of materials and structures which are unrelated to the original floodplain processes of terrace formation is essential to the proper understanding of terrace morphology and stratigraphy. Two groups of processes have been active in modifying the Lower and Middle Pleistocene terraces of the River Thames since their formation: non-fluvial deposition; and structural rearrangement by subsidence associated with solution of the underlying Chalk. Non-fluvial deposits comprise solifluction gravels which are variable in character and may incorporate a range of pre-existing deposits; and brickearths which appear in most cases to be redistributed loess mixed with non-loessic components. The distinction between fluvial and non-fluvial deposits is made using particle size, composition and fabric analysis. Structural rearrangement of terrace sediments has occurred due to subsidence into deep, narrow, steep-sided pipes. Within such pipes, disturbance of primary structures and fabrics is severe. Over a wider area. primary bedding may be inclined towards pipes, and sediments may show evidence of shearing, faulting and brecciation. Interpretative problems arising from post-depositional modification are exemplified.     

Aeolian saltation threshold: the effect of density ratio

Saltation threshold data from three wind tunnels and from hydraulic flumes are presented to show that the dimensionless threshold friction speed for small particles is a continuous function of particle-to-fluid-density ratio. In addition, the dimensionless threshold speed is a function of the grain-friction Reynolds number and an interparticle force term. The variation with density ratio seems to be due to the relative energy with which particles impact other particles to initiate saltation.     

Static breakage of granitic detritus by ice and water in comparison with breakage by flowing water

Temperature cycling between ?4·5±1·0°C and 13·0±2·0°C in the presence of added water caused significant breakage of granitic detritus. Because cycling with only adsorbed water present also caused breakage, fragmentation is attributed to both the combined action of ice and adsorbed water and to the latter acting alone. Breakage evidently resulted from the gradual tensile opening of pre-existing cracks, weakening then splitting grains. Surviving unbroken grains show evidence of a fatigue effect. Quartz split dominantly along pre-existing subplanar microfractures whereas feldspar and biotite split along crystal cleavages. Degree of breakage and product size distribution depend on the crystalline nature of the parent material, its previous history, and the nature and duration of the breakage process. With the first two factors the same, size distributions from adsorbed water breakage alone and from that due to adsorbed water plus ice differ slightly. Both contrast strongly with those of simulated fluviatile breakage. Whereas the latter preferentially produced 2–20 μm particles (probably debris of inter-particle collisions), static breakage split coarser grains wherever major weaknesses occurred, producing less selective product size distributions with greater proportions of loess-sized material (about 20--60 μ). Characteristic inflections in the size distribution curves of our experimentally produced debris are also shown by samples from the sola of some frost-affected soils. Partially healed microfractures in plutonic quartz are normally spaced at about 1–10 μ—approaching the downward asymptotic comminution limit for brittle solids (about 1 μ). Surficial physical processes are capable of reducing only a small proportion of plutonic quartz to this size before its storage in sediments.     

Field measurements of the flux and speed of wind-blown sand

A field experiment was conducted to measure the flux and speed of wind-blown sand under known conditions in a natural setting. The experiment, run at Pismo Beach, California, involved a tract 100 m long (parallel with the wind) by 20 m wide. The site was instrumented with four arrays of anemometers to obtain wind velocity profiles through the lower atmospheric boundary-layer, temperature probes to determine atmospheric stability and wind vanes to determine wind direction. From these measurements, wind friction speeds were derived for each experimental run. In order to measure sand saltation flux, a trench 3 m long by 10 m wide (transverse to the wind direction) by 0·5 m deep was placed at the downwind end of the tract and lined with 168 collector bins, forming an ‘egg-box’ pattern. The mass of particles collected in each bin was determined for four experimental runs. In order to assess various sand-trap systems used in previous experiments, 12 Leatherman traps, one Fryberger trap and one array of Ames traps were deployed to collect particles concurrently with the trench collection. Particle velocities were determined from analysis of high-speed (3000 and 5000 frames per second) motion pictures and from a particle velocimeter. Sand samples were collected from the trench bins and the various sand traps and grain size distributions were determined. Fluxes for each run were calculated using various previously published expressions, and then compared with the flux derived from the trench collection. Results show that Bagnold's (1941) model and White's (1979) equation most closely agree with values derived from the trench. Comparison of the various collector systems shows that the Leatherman and Ames traps most closely agree with the flux derived from the trench, although these systems tended to under-collect particles. Particle speeds were measured from analysis of motion pictures for saltating particles in ascending and descending parts of their trajectories. Results show that particle velocities from the velocimeter are in the range 0·5–7·0 m s^?1, compared to a wind friction velocity of 0·32–0·43 m s^?1 and a wind velocity of 2·7–3·9 m s^?1 at the height of the particle measurements. Descending particles tended to exceed the speeds of ascending particles by ～ 0·5 m s^?1.     

Experimental studies of mineralogical assemblages of metasedimentary rocks at Earth's mantle transition zone conditions

Metasedimentary rocks, a major component of the continental crust, are abundant within ultra‐high pressure (UHP) metamorphic terranes related to continental collisions. The presence of diamond, coesite, and relics of decompressed minerals in these rocks suggests that they were subducted to a depth of more than 150–250 km. Reconnaissance experiments at 9–12 GPa and 1000–1300 °C on compositions corresponding to felsic rocks from diamond‐bearing UHP terranes of Germany and Kazakhstan show that at higher pressures they consist of majoritic garnet, Al‐Na‐rich clinopyroxene, stishovite, solid solution of KAlSi₃O₈‐NaAlSi₃O₈ hollandite, topaz‐OH, and TiO₂ with α‐PbO₂ structure. Comparison of our data with experiments conducted by others at similar P–T conditions shows differences, which are due to variations in bulk chemistry and the type of starting material (gel, oxides, minerals). These differences may affect correct establishment of the ‘point of no return’ of subducted continental lithologies. This paper discusses the implication of the experimental data with regard to naturally existing UHP metamorphic rocks and their significance for our understanding of the deep subduction of continental material.     

GEOGRAPHERS AND THE TENNESSEE VALLEY AUTHORITY*

ABSTRACT. The Tennessee Valley Authority (TVA) was the largest, most comprehensive, and most controversial regional development and planning project in U.S. history. Geographers were involved from its inception and made impressive contributions. Aside from the unit area method of data gathering and mapping, little is known about their contributions, some of which were truly ahead of their time. Although their work and recommendations were often discarded and unheeded because of political turbulence, the geographers rarely complained or entered into the political arena. Their work in the TVA has generally gone unheralded and even unappreciated within the geography profession. The primary purpose of this article is to document their contributions.     

Facies model for sedimentation in the glaciolacustrine environment

Diamictons which have characteristics of both basal tills and lacustrine sedimenis have been called by various authors waterlaid tills, lacuatrotills, subaqueous tills, aqualills, underwater tills, and other terms with the word till replaced by moraine. It is proposed that a more restrictive definition of these terms be applied based on criteria indicative of the environment of deposition.
The term lacustrotill is proposed for the till-like sediments deposited in the lacustrine environment by flow mechanisms. Their clasts are often striated and may exhibit preferred orientation unrelated to glacier movement. Deformation structures suggestive of slumping or flowage during deposition are usually present.
The term waterlaid till is proposed for sediments deposited beneath a floating glacier, where the water depth does not allow appreciable size separation during settling: glacial drift dumped in standing water at the snout of a glacier grounded on a lake bottom: or till deposited during re advances (perhaps annually) into a lake basin and subsequently slightly reworked by lacustrine processes.
These sediments contain glacially abraded clasts which may even show a weak preferred orientation related to glacial movement; deformation and flow structures will not normally be present: and they may he massive, or exhibit crude stratification.     

Mössbauer spectroscopy studies of the valence state of iron in chromite from the Luobusa massif of Tibet: implications for a highly reduced deep mantle

Mössbauer spectroscopy was applied to study the valence state of iron in chromite from massive, nodular and disseminated podiform chromitite ores of the Luobasa ophiolite massif of Tibet. The results show that Fe³⁺/ΣFe = 0.42 in chromite from massive ore, and Fe³⁺/ΣFe = 0.22 in chromite from nodular and disseminated ores. The massive ore records traces of ultra high pressure mineralogical assemblages, such as diamond inclusions in OsIr alloys, exsolution lamellae of coesite and diopside in chromite, inclusions of metal‐nitrides, native iron and others, which suggests a strongly reducing environment. In contrast, chromite from nodular and disseminated ore contains abundant low‐pressure OH‐bearing mineral inclusions whose formation requires a more oxidizing environment. The high value of Fe³⁺/ΣFe in the ‘reduced’ massive ore is explained by crystallographic stabilization of Fe³⁺ in a high‐pressure polymorph of chromite deep in the upper mantle despite low ambient fO₂ conditions. The presence of high‐pressure phases within the massive chromitite ore requires that the latter, together with its host peridotite, was transported in the solid state from a highly reduced deep mantle environment to shallow depths beneath an ocean spreading centre. It is suggested that in the low‐pressure environment of the spreading centre, the deep‐seated, reduced, massive chromitites partially reacted with their host peridotite in the presence of hydrous melt, yielding the nodular and disseminated chromitite ores. The preponderance of evidence suggests that the latter interaction involved boninitic melts in a supra‐subduction zone environment as proposed previously.