Atmospheric circulation anomalies during two persistent north american droughts: 1932�C1939 and 1948�C1957     

Benjamin I. Cook  Richard Seager  Ron L. Miller 《Climate Dynamics》2011,36(11-12):2339-2355

We use an early twentieth century (1908?C1958) atmospheric reanalysis, based on assimilation of surface and sea level pressure observations, to contrast atmospheric circulation during two periods of persistent drought in North America: 1932?C1939 (the ??Dust Bowl??) and 1948?C1957. Primary forcing for both droughts is believed to come from anomalous sea surface temperatures (SSTs): a warm Atlantic and a cool eastern tropical Pacific. For boreal winter (October?CMarch) in the 1950s, a stationary wave pattern originating from the tropical Pacific is present, with positive centers over the north Pacific and north Atlantic ocean basins and a negative center positioned over northwest North America and the tropical/subtropical Pacific. This wave train is largely absent for the 1930s drought; boreal winter height anomalies are organized much more zonally, with positive heights extending across northern North America. For boreal summer (April?CSeptember) during the 1930s, a strong upper level ridge is centered over the Great Plains; this feature is absent during the 1950s and appears to be linked to a weakening of the Great Plains low-level jet (GPLLJ). Subsidence anomalies are co-located over the centers of each drought: in the central Great Plains for the 1930s and in a band extending from the southwest to the southeastern United States for the 1950s. The location and intensity of this subsidence during the 1948?C1957 drought is a typical response to a cold eastern tropical Pacific, but for 1932?C1939 deviates in terms of the expected intensity, location, and spatial extent. Overall, circulation anomalies during the 1950s drought appear consistent with the expected response to the observed SST forcing. This is not the case for the 1930s, implying some other causal factor may be needed to explain the Dust Bowl drought anomalies. In addition to SST forcing, the 1930s were also characterized by massive alterations to the land surface, including regional-scale devegetation from crop failures and intensive wind erosion and dust storms. Incorporation of these land surface factors into a general circulation model greatly improves the simulation of precipitation and subsidence anomalies during this drought, relative to simulations with SST forcing alone. Even with additional forcing from the land surface, however, the model still has difficulty reproducing some of the other circulation anomalies, including weakening of the GPLLJ and strengthening of the upper level ridge during AMJJAS. This may be due to either weaknesses in the model or uncertainties in the boundary condition estimates. Still, analysis of the circulation anomalies supports the conclusion of an earlier paper (Cook et?al. in Proc Natl Acad Sci 106:4997, 2009), demonstrating that land degradation factors are consistent with the anomalous nature of the Dust Bowl drought.  相似文献   

Geomagnetic field intensity: How high can it get? How fast can it change? Constraints from Iron Age copper slag     

Ron Shaar  Erez Ben-Yosef  Hagai Ron  Lisa Tauxe  Amotz Agnon  Ronit Kessel 《Earth and Planetary Science Letters》2011,301(1-2):297-306

The intensity of the geomagnetic field varies over different time scales. Yet, constraints on the maximum intensity of the field as well as for its maximum rate of change are inadequate due to poor temporal resolution and large uncertainties in the geomagnetic record. The purpose of this study is to place firm limits on these fundamental properties by constructing a high-resolution archaeointensity record of the Levant from the 11th century to the early 9th century BCE, a period over which the geomagnetic field reached its maximum intensity in Eurasia over the past 50,000 years. We investigate a ¹⁴C-dated sequence of ten layers of slag material, which accumulated within an ancient industrial waste mound of an Iron Age copper-smelting site in southern Israel. Depositional stratigraphy constrains relative ages of samples analyzed for paleointensity, and ¹⁴C dates from different horizons of the mound constrain the age of the whole sequence. The analysis yielded 35 paleointenisty data points with accuracy better than 94% and precision better than 6%, covering a period of less than 350 years, most probably 200 years. We construct a new high-resolution quasi-continuous archaeointensity curve of the Levant that displays two dramatic spikes in geomagnetic intensity, each corresponding to virtual axial dipole moment (VADM) in excess of 200 ZAm². The geomagnetic spikes rise and fall over a period of less than 30 years and are associated with VADM fluctuations of at least 70 ZAm². Thus, the Levantine archaeomagnetic record places new constraints on maximum geomagnetic intensity as well as for its rate of change. Yet, it is not clear whether the geomagnetic spikes are local non-dipolar features or a geomagnetic dipolar phenomenon.  相似文献   

Micro-relief development in polygonal patterned ground in the Dry Valleys of Antarctica     

Bernard Hallet  Ron Sletten  Kevin Whilden 《Quaternary Research》2011,75(2):347-355

Polygonal patterned ground in polar regions of both Earth and Mars has received considerable attention. In comparison with the size, shape, and arrangement of the polygons, the diverse micro-relief and topography (termed here simply “relief”) of polygonal patterned ground have been understudied. And yet, the relief reflects important conditions and processes occurring directly below the ground surface, and it can be observed readily in the field and through remote sensing. Herein, we describe the relief characteristic of the simplest and relatively young form of patterned ground in the Dry Valleys of Antarctic. We also develop a numerical model to examine the generation of relief due to subsurface material being shouldered aside contraction cracks by incremental sand wedges growth, and to down-slope creep of loose granular material on the surface. We model the longterm subsurface deformation of ice-cemented permafrost as a non-linear viscous material. Our modeling is guided and validated using decades of field measurements of surface displacements of the permafrost and relief. This work has implications for assessing the activity of surfaces on Earth and Mars, and much larger scale potential manifestations of incremental wedging in icy material, namely the distinct paired ridges on Europa.  相似文献   

Arild Holt‐Jensen,Geography. History and Concepts: A Student's Guide (fourth edition)     

Ron Johnston 《Singapore journal of tropical geography》2011,32(2):267-268

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Ten eclipsing binaries in the Small Magellanic Cloud: fundamental parameters and Cloud distance     

Tim J. Harries  Ron W. Hilditch  Ian D. Howarth 《Monthly notices of the Royal Astronomical Society》2003,339(1):157-172

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Annual General Meeting of the Royal Geographical Society (with the Institute of British Geographers) 2 June 2003     

Sir Ron Cooke 《The Geographical journal》2003,169(3):278-281

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LAGEOS II perigee rate and eccentricity vector excitations residuals and the Yarkovsky-Schach effect     

David M. Lucchesi  Ignazio Ciufolini  José I. Andrés  Roberto Peron  Ron Noomen 《Planetary and Space Science》2004,52(8):699-710

We have analysed LAGEOS II perigee rate and eccentricity vector excitation residuals over a period of about 7.8 years, adjusting and computing the satellite orbit with the full set of dynamical models included in the GEODYN II software code. The long-term behaviour of these orbital residuals appears to be characterised by several distinct frequencies which are a clear signature of the Yarkovsky-Schach perturbing effect. This non-gravitational perturbation is not included in the GEODYN II models for the orbit determination and analysis. Through an independent numerical analysis, and using the new LOSSAM model to represent the spin-axis behaviour of the satellite, we propagated the Yarkovsky-Schach effect on LAGEOS II perigee rate and compared the results obtained with the orbital residuals. We have thus been able to satisfactorily fit the amplitude of the Yarkovsky-Schach effect to the observed residuals. Our approach here has proven very successful with very positive results. We have been able to obtain a fractional reduction of about 40% of the post-fit rms with respect to the pre-fit value. When analysing the eccentricity vector residuals, we have been able to obtain a better result in the case of the real component, with a fractional reduction of the post-fit rms of about 49% of the initial value. The analysis of the effect's imaginary component in the eccentricity vector rate is more complicated and deserves additional scrutiny. In this case we need a deeper study which includes the analysis of other unmodelled and mismodelled effects acting on the imaginary component. The study performed in this paper will be of significant relevance not only for the geophysical applications involving LAGEOS II orbit analysis, but also for a refined re-analysis of the general relativistic precession produced by the Earth angular momentum, i.e., the Lense-Thirring effect.  相似文献   

Terra Comes of Age     

Ron Oxburgh 《地学学报》1993,5(3):212-212

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Regionalizing mean annual flow and daily flow variability for basin‐scale sediment and nutrient modelling     

Scott N. Wilkinson  William J. Young  Ron C. DeRose 《水文研究》2006,20(13):2769-2786

River discharges vary strongly through time and space, and quantifying this variability is fundamental to understanding and modelling river processes. The river basin is increasingly being used as the unit for natural resource planning and management; to facilitate this, basin‐scale models of material supply and transport are being developed. For many basin‐scale planning activities, detailed rainfall‐runoff modelling is neither necessary nor tractable, and models that capture spatial patterns of material supply and transport averaged over decades are sufficient. Nevertheless, the data to describe the spatial variability of river discharge across large basins for use in such models are often limited, and hence models to predict river discharge at the basin scale are required. We describe models for predicting mean annual flow and a non‐dimensional measure of daily flow variability for every river reach within a drainage network. The models use sparse river gauging data, modelled grid surfaces of mean annual rainfall and mean annual potential evapotranspiration, and a network accumulation algorithm. We demonstrate the parameterization and application of the models using data for the Murrumbidgee basin, in southeast Australia, and describe the use of these predictions in modelling sediment transport through the river network. The regionalizations described contain less uncertainty, and are more sensitive to observed spatial variations in runoff, than regionalizations based on catchment area and rainfall alone. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

Mineralogical and spectroscopic investigation of the Tagish Lake carbonaceous chondrite by X‐ray diffraction and infrared reflectance spectroscopy     

Matthew R. M. IZAWA  Roberta L. FLEMMING  Penelope L. KING  Ron C. PETERSON  Phil J. A. McCAUSLAND 《Meteoritics & planetary science》2010,45(4):675-698

Abstract– We have carried out a sample‐correlated spectroscopic and mineralogical investigation of samples from seven different collection sites of the Tagish Lake C2 chondrite. Rietveld refinement of high‐resolution powder X‐ray diffraction (XRD) data was used to determine quantitative major mineral abundances. Thermal infrared (400–4500 cm⁻¹, 2.2–25.0 μm) spectra of the same samples were obtained using diffuse (biconical) reflectance infrared Fourier transform spectroscopy (DRIFTS). Our results are in good agreement with previous studies of the mineralogy of the Tagish Lake meteorite; we find however that Tagish Lake is more varied in major mineralogy than has previously been reported. In particular, we observed two new distinct lithologies, an inclusion‐poor magnetite‐ and sulfide‐rich lithology, and a carbonate‐rich, siderite‐dominated lithology in addition to the previously documented carbonate‐rich and carbonate‐poor lithologies. Grain density for each Tagish Lake sample was calculated from the measured mineral modal abundances and known mineral densities. For powders from three originally intact inclusion‐rich samples, the calculated grain density is 2.77 ± 0.05 g cm⁻³, in excellent agreement with those reported in the literature for other intact inclusion‐rich Tagish Lake samples. Tagish Lake disaggregated samples have a significantly higher calculated grain density due to their lower saponite‐serpentine content, likely a result of mineral separation in the meltwater holes from which they were collected; the disaggregated samples may not therefore adequately represent bulk samples of the Tagish Lake meteorite. The predominance of very fine‐grained material in the Tagish Lake samples investigated in this study is expected to produce infrared spectra representative of asteroidal regolith. Gypsum and talc have been found by XRD in powders from the inclusion‐rich, intact Tagish Lake samples in this study, and may have been present in the parent body; if present, these hydrous sulfates would complicate the interpretation of possible hydrated mineral features in asteroid infrared spectra.  相似文献