Rates of aerodynamic entrainment in a developing boundary layer

Despite its significance for inception of grain transport by wind, the initial dislodgement of grains from a static surface by aerodynamic forces of drag and lift in the absence of grain collision has received little attention. This paper describes a series of wind-tunnel experiments in which the erosion of narrow strips of loose grains from the roughened surface of a flat plate exposed to a range of wind speeds was examined. The progressive downwind development of the boundary layer over the plate provided a range of airflow conditions which permitted systematic evaluation of grain entrainment rates arising from purely aerodynamic forces. Use of closely graded size fractions in flat, single grain layers resting on identical, fixed grain support eliminated the effects of surface irregularities and impacts from saltation. Results show that erosion of strips of loose grains develops with time according to an inverse exponential function in which the entrainment rate time constant relates to Shields dimensionless shear stress function. An empirical expression defining aerodynamic entrainment rate in terms of rate of strip erosion is derived and comparisons are made between present and published data. The need for additional data to resolve several questions raised by the present investigation is stressed. In addition, a simple, objective technique for accurate determination of the aerodynamic entrainment threshold of any loose, granular sediment is proposed.     

Aerodynamic entrainment threshold: effects of boundary layer flow conditions

Experimental data are presented demonstrating the influence of boundary layer flow conditions on aerodynamic entrainment of grains in the absence of intersaltation collisions. New methods are proposed for (1) the unambiguous determination of aerodynamic threshold for any grain population and (2) approximation of the probability density function (PDF) distributions of threshold shear velocity for aerodynamic entrainment. In wind tunnel experiments, the orderly spatial development of flow conditions within a developing boundary layer over the roughened surface of a flat plate constrains the aerodynamic threshold condition in terms of both mean and fluctuating values. Initial grain dislodgements and subsequent erosion from narrow strips of loose, finely fractionated ballotini were recorded photographically as wind speed was increased. Boundary layer parameters, including average threshold shear velocity (U_*t), were calculated using the momentum integral method. Direct observations show that sporadic oscillation of grains preceded dislodgement. At slightly higher velocities most grains rolled over their neighbours before entering saltation. Initial entrainment in spatially semi-organized flurries of 50 or more grains was followed by quiescent periods at airflow velocities close to threshold. These observations provide strong circumstantial evidence linking both the nature and spatial pattern of initial grain motions to sweep events during the fluid bursting process. For each grain fraction, values of U_*t were found to span an unexpectedly wide range and to decrease downwind from the leading edge of the plate as turbulence intensity increased. A probabilistic entrainment model is applied to the aerodynamic threshold condition so as to incorporate the effects of changing turbulent flow regimes over the plate. Analysis of strip erosion curves gives both an objective definition of the threshold condition and usable approximations of the PDF for U_*t required by the model and for future stochastic treatment of the threshold condition.     

Convection forced by a descending dry layer and low-level moist convergence

A narrow line of convective showers was observed over southern England on 18 July 2005 during the Convective Storm Initiation Project (CSIP). The showers formed behind a cold front (CF), beneath two apparently descending dry layers (i.e. sloping so that they descended relative to the instruments observing them). The lowermost dry layer was associated with a tropopause fold from a depression, which formed 2 d earlier from a breaking Rossby wave, located northwest of the UK. The uppermost dry layer had fragmented from the original streamer due to rotation around the depression (This rotation was also responsible for the observations of apparent descent—ascent would otherwise be seen behind a CF). The lowermost dry layer descended over the UK and overran higher  θ _w   air beneath it, resulting in potential instability. Combined with a surface convergence line (which triggered the convection but had less impact on the convective available potential energy than the potential instability), convection was forced up to 5.5 km where the uppermost dry layer capped it. The period when convection was possible was very short, thus explaining the narrowness of the shower band. Convective Storm Initiation Project observations and model data are presented to illustrate the unique processes in this case.     

The distribution of local bedrock material in some moraine forms form the inner part of northern Sweden

An inventory of bedrock material has been made from different levels in some moraine types situated in the areas around Storvindeln and Storavan. The results show that moraine forms which have a curved morphology transverse to the direction of ice flow, always have a high content of local bedrock material independent of till thickness. In many cases the local bedrock material is represented more in the upper levels of the moraine. Large transverse ridges with less curvature have a highly variable bedrock content. Drumlins may lack local bedrock material over a distance of many kilometres. The variable distribution of local bedrock material in different moraine types has been interpreted as a function of ice pressure, temperature, and movement during deglaciation.     

The Petrology and Geochemistry of Cone-sheets from the Cuillin Igneous Complex, Isle of Skye: Evidence for Combined Assimilation and Fractional Crystallization during Lithospheric Extension

The origin and evolution of the magma(s) involved in the formationof the olivine tholeiite cone-sheets which invade the 60-MaCuillin Igneous Complex, Isle of Skye, have been investigatedusing mineral chemistry data together with whole-rock major-and trace-element and isotope (Sr and Nd) geochemistry data.The most primitive compositions identified are almost identicalto those of the basalts being erupted at the present day alongthe spreading centre in Iceland. Rare examples of lavas fromthe slightly older lava pile of west-central and north Skye,together with a large number of dykes from the axial zone ofthe Skye Main Swarm, are of similar composition and are referredto as being of the Preshal More Basalt type (spelling is accordingto current Ordnance Survey of Great Britain maps). The intra-suitecompositional variation exhibited by the cone-sheets can beexplained in terms of relatively low-pressure fractionationof the three-phase assemblage olivine+clinopyroxene+plagioclasein their cotectic proportions of 10: 35: 55. Modelling of major-and compatible trace-element data indicates that the most evolvedcomposition may be derived by 60% crystallization of the leastevolved composition. Incompatible trace-element modelling impliesgreater degrees of crystal-liquid fractionation: Y and Zr indicate64 and 68% crystallization, respectively, whereas the rare earthelements (Eu, Yb, Gd, and Ce) indicate between 71 and 75% crystallization.This decoupled behaviour of compatible and incompatible elementsis attributed to the cone-sheet magma(s) evolving in a subjacentmagma chamber, before final emplacement in the overlying crust.Sr- and Nd-isotope data indicate that the cone-sheet magma(s)assimilated upper-crustal lithologies during fractional crystallization,most likely involving amphibolite facies gneiss of the LewisianGneiss Complex. This style of simultaneous assimilation andfractional crystallization (AFC) process in the upper crustfor the Skye cone-sheet magma(s) is in direct contrast to theprocesses identified for the magmas which produced the bulkof the lava field of west-central and north Skye, where assimilationoccurred after fractionation. The cone-sheet magmas were generatedby relatively large degrees of partial melting of a depletedmantle source associated with significant lithospheric stretching.The cause of this depletion, together with the temporal relationshipsbetween the cone-sheet magmas and the dominant transitionalmagmas of the Skye lava field and sill complex, are also discussed. ^* Reprint request to B. R. Bell     

Modelling Diverse Processes in the Petrogenesis of a Composite Batholith: the Central Bohemian Pluton, Central European Hercynides

The multiple intrusions making up the Central Bohemian Plutonin the Central European Hercynides have petrographic and geochemicalfeatures consistent with the presence of four main granitoidsuites. Major-element, trace-element and Sr–Nd isotopiccompositions are used to model their petrogenesis. Partial meltingof metabasic rocks or of a CHUR-like mantle source are interpretedto have produced melts parental to the most primitive calc-alkalineSázava suite. Interaction of basic with more acidic magmasfollowed by extensive amphibole–plagioclase-dominatedfractionation accounts for the production of trondhjemites.Alternatively, the trondhjemites correspond to small-degreemelts of a metabasic source. AFC (assimilation–fractionalcrystallization) modelling with a paragneiss as a contaminantand increasing D_Nd values simulates the characteristics of theBlatná suite. Closed-system fractionation of stronglyenriched mantle-derived magmas or their interaction with leucograniticmelts is deduced for the petrogenesis of the shoshonitic     

Calcretes,fluviolacustrine sediments and subsidence patterns in Permo‐Triassic salt‐walled minibasins of the south Urals,Russia

The south Uralian foreland basin forms part of the giant, yet sparsely documented, PreCaspian salt tectonic province. The basin can potentially add much to the understanding of fluviolacustrine sedimentation within salt‐walled minibasins, where the literature has been highly reliant on only a few examples (such as the Paradox Basin of Utah). This paper describes the Late Permian terrestrial fill of the Kul’chumovo salt minibasin near Orenburg in the south Urals in which sediments were deposited in a range of channel, overbank and lacustrine environments. Palaeomagnetic stratigraphy shows that, during the Late Permian, the basin had a relatively slow and uniform subsidence pattern with widespread pedogenesis and calcrete development. Angular unconformities or halokinetic sequence boundaries cannot be recognized within the relatively fine‐grained fill, and stratigraphic and spatial variations in facies are therefore critical to understanding the subsidence history of the salt minibasin. Coarse‐grained channel belts show evidence for lateral relocation within the minibasin while the development of a thick stack of calcrete hardpans indicates that opposing parts of the minibasin became largely inactive for prolonged periods (possibly in the order of one million years). The regular vertical stacking of calcrete hardpans within floodplain mudstones provides further evidence that halokinetic minibasin growth is inherently episodic and cyclical.     

Development of a Journal: Some brief comments relating to the Part Index for the Australian Geographer, 1928-98

This commentary concentrates on the physical production of the Australian Geographer and developments of a related nature. As I cannot pretend to have read every paper published over the last 71 years, comment on content is very limited. I also wish to avoid overlap with already published retrospectives included in Volume 19, Number 1, and Volume 27, Number 1, of this journal. A full issue-by-issue index is included in Volume 30, Number 1, of the Australian Geographer (pp. 103-26): all major papers, but not some minor pieces, are listed. Some of the latter, when part of a series, are listed in Tables 1 to 4 in this commentary. A full alphabetical author index for the 71 years of the journal will be included in the next issue (July 1999) as Appendix B.