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The previously derived formulations for using the relative entropy and Shannon entropy difference ( SD ) to measure information content from observations are revisited in connection with another known information measure—degrees of freedom for signal, which is defined as the statistical average of the signal part of the relative entropy. For a linear assimilation system, the statistical average of the relative entropy reduces to the SD . The formulations are extended for four-dimensional variational data assimilation (4DVar). The extended formulations reveal that the information content increases (or decreases) as the model error increase (or decrease) and/or become strongly (or weakly) correlated in 4-D space. These properties are also highlighted by illustrative examples, and the extended formulations are shown to be potential useful for designing optimum phased-array radar scan configurations to maximize the extractable information contents from radar observations by a 4DVar analysis system. 相似文献
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SEAN J. FITZSIMONS 《Sedimentology》1992,39(3):393-410
Quaternary sedimentary successions are described from the Linda Valley, a small valley in western Tasmania that was dammed by ice during Early and Middle Pleistocene glaciations. Mapping and logging of exposures suggest that an orderly sequence of deposits formed during ice incursion, occupation and withdrawal from tributary valleys. Four principal sediment assemblages record different stages of ice occupation in the valley. As the glacier advanced, a proglacial, lacustrine sediment assemblage dominated by laminated silts and muds deposited from suspension accumulated in front of the glacier. A subglacial sediment assemblage consisting of deformed lacustrine deposits and lodgement till records the overriding of lake-bottom sediments as the glacier advanced up the valley into the proglacial lake. As the glacier withdrew from the valley, a supraglacial sediment assemblage of diamict, gravel, sand and silt facies formed on melting ice in the upper part of the valley. A lacustrine regression in the supraglacial assemblage is inferred on the basis of a change from deposits mainly resulting from suspension in a subaqueous setting to relatively thin and laterally discontinuous laminated sediments, occurrence of clastic dykes, and increasing complexity of the geometry of deposits that indicate deposition in a subaerial setting. A deltaic sediment assemblage deposited during the final stage of ice withdrawal from the valley consists of steeply dipping diamict and normally graded gravel facies formed on delta foresets by subaqueous sediment gravity flows. The sediment source for the delta, which prograded toward the retreating ice margin, was the supraglacial sediment assemblage previously deposited in the upper part of the valley. A depositional model developed from the study of the Linda Valley may be applicable to other alpine glaciated areas where glaciers flowed through or terminated in medium- to high-relief topography. 相似文献
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The nature of flow, sediment transport and bed texture and topography was studied in a laboratory flume using a mixed size-density sediment under equilibrium and non-equilibrium (aggradational, degradational) conditions and compared with theoretical models. During each experiment, water depth, bed and water surface elevation, flow velocity, bed shear stress, bedload transport and bed state were continuously monitored. Equilibrium, uniform flow was established with a discharge of about 0.05 m3 s?1, a flow depth of about 0.01 m, a flow velocity of about 0.81–0.88 m s?1, a spatially averaged bed shear stress of about 1.7–2.2 Pa and a sediment transport rate of about 0.005–0.013 kg m?1 s?1 (i.e. close to the threshold of sediment transport). Such equilibrium flow conditions were established prior to and at the end of each aggradation or degradation experiment. Pebble clusters, bedload sheets and low-lying bars were ubiquitous in the experiments. Heavy minerals were relatively immobile and occurred locally in high concentrations on the bed surface as lag deposits. Aggradation was induced by (1) increasing the downstream flow depth (flume tilting) and (2) sediment overloading. Tilt-induced aggradation resulted in rapid deposition in the downstream half of the flume of a cross-stratified deposit with downstream dipping pebbles (pseudo-imbricated). and caused a slight decrease in the equilibrium mean water surface slope and total bedload transport rate. These differences between pre- and post-aggradation equilibrium flow conditions are due to a decrease in the local grain roughness of the bed. Sediment overloading produced a downstream fining and thinning wedge of sediment with upstream dipping pebbles (imbricated), whereas the equilibrium flow and sediment transport conditions remained relatively unchanged. Degradation was induced by (1) decreasing the downstream flow depth (flume tilting) and (2) cutting off the sediment feed. Tilt-induced degradation produced rapid downstream erosion and upstream deposition due to flow convergence with little change to the equilibrium flow and sediment transport conditions. The cessation of sediment feed produced degradation and armour development, a reduction in the mean water surface slope and flow velocity, an increase in flow depth, and an exponential decrease in bedload transport rate as erosion proceeded. A bedload transport model predicted total and fractional transport rates extremely well when the coarse-grained (or bedform trough) areas of the bed are used to define the sediment available to be transported. A sediment routing model, MIDAS, also reproduced the equilibrium and non-equilibrium flow conditions, total and fractional bedload transport rates and changes in bed topography and texture very well. 相似文献
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CHRISTOPHER GOLD SEAN CORMACK 《International journal of geographical information science》2013,27(2):137-148
Abstract A technique is discussed for obtaining a contour tree efficiently as a byproduct of an operational contouring system. This tree may then be used to obtain contour symbolism or interval statistics as well as for further geomorphological study. Alternatively, the tree may be obtained without the computational expense of detailed contour interpolation. The contouring system proceeds by assuming a Voronoi neighbourhood or domain about each data point and generating a dual-graph Delaunay triangulation accordingly. Since a triangulation may be traversed in a tree order, individual triangles may be processed in a guaranteed top-to-bottom sequence on the map. At the active edge of the map under construction a linked list is maintained of the contour ‘stubs’ available to be updated by the next triangle processed. Any new contour segment may extend an existing stub, open two new stubs or close (connect) two previous stubs. Extending this list of edge links backwards into the existing map permits storage of contour segments within main memory until a dump (either to plotter or disc) is required by memory overflow, contour closure, contour labelling or job completion. Maintenance of an appropriate status link permits the immediate distinction of local closure (where the newly-connected segments are themselves not connected) from global closure (where a contour loop is completed and no longer required in memory). The resulting contour map may be represented as a tree, the root node being the bounding contour of the map. The nature of the triangle-ordering procedure ensures that inner contours are closed before enclosing ones, and hence a preliminary contour tree may be generated as conventional contour generation occurs. A final scan through the resulting tree eliminates any inconsistencies. 相似文献
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The impact of modern cold glaciers on arid periglacial landscapes has received little attention compared with other glacial regimes, and there is a widely held assumption that cold glaciers are not effective geomorphological agents, despite recent studies to the contrary. This paper focuses on the processes operating at the margins of a number of glaciers in the Dry Valleys of Victoria Land, notably the Wright Lower Glacier. The glaciers are entraining primarily older drift deposits and highly weathered regolith which texturally are sandy gravels, as well as well‐sorted sands of fluvial origin. Despite basal temperatures of the order of ?16°C, frozen layers and blocks of sand and gravel are being incorporated into the base of the glaciers by folding and thrusting. The sedimentary products are ridges and aprons several metres high within which the principal lithofacies are sand, gravel, foliated glacier ice, lake ice and snow. These facies are glaciotectonized strongly. Draped over these landforms is a veneer of well‐sorted aeolian sand up to half a metre thick. Supraglacial streams flowing off the glaciers incise these landforms and the sediment is redeposited as alluvial fans, lake deltas and lake‐bottomset deposits. Overall the sediment/landform association differs markedly from those of other glacial regimes, with sand and gravel being the dominant facies, while the usual indicators of glacier working (such as facets and striations on clasts) are lacking. The preservation potential for these landforms on a thousand‐year time scale is high, as modification in this arid regime by slope processes and running water is limited. Sublimation of buried ice is so slow that ridge features are likely to remain ice‐cored almost indefinitely, modified only by wind transport and weathering. 相似文献
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SEAN J. FITZSIMONS ERIC A. COLHOUN GUUS VAN DE GEER ROBERT S. HILL 《Boreas: An International Journal of Quaternary Research》1990,19(1):1-15
Interpretation of weathered glacial and interglacial deposits in the lower King Valley suggests that the Early Pleistocene Thureau Formation is conformably overlain by the Regency Formation. Pollen analysis of the Regency Formation provides evidence of a new climatic stage, the Regency Interglacial. The interglacial deposit consists of humified peat overlain by drifted wood and leaves. The pollen analysis shows a transition from montane scrub rainforest to lowland temperate rainforest dominated by Lagarostrobos franklinii, Nothofagus cunninghamii and Phyllocladus aspleniifolius. Trace quantities of the species Quintinia psi-latispora and Gothanipollis perplexus , now both extinct in Tasmania, were also recorded. On the basis of intense chemical weathering and correlation with sediments that have a reversed magnetization the weathered glacial deposits of the Thureau Formation are thought to be older than 730,000 B.P. The eroded interglacial deposit rests on the weathered deposits and is buried by outwash gravels of the David Formation, which was deposited during an ice advance of the Middle Pleistocene Henty Glaciation (c. 150,000 B.P.). Comparison of the Regency site with a site 2 km to the south at Baxter Rivulet shows that the unconformity between the interglacial deposit and the overlying outwash gravel represents the erosion of the evidence for the Middle Pleistocene Moore Glaciation. 相似文献
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Niels BORMANN David DUNCAN Stephen ENGLISH Sean HEALY Katrin LONITZ Keyi CHEN Heather LAWRENCE Qifeng LU 《大气科学进展》2021,38(8):1285-1298
This paper reviews the data quality and impact of observations from the FY-3 satellite series used operationally in the ECMWF system. This includes data from the passive microwave radiometers MWHS-1, MWHS-2 and MWRI, as well as observations from the radio occultation receiver GNOS. Evaluations against background equivalents show that the quality of the observations is broadly comparable to that of similar instruments on other polar-orbiting satellites, even though biases for the passive microwave observations can be somewhat larger and more complex for some channels. An observing system experiment shows that the FY-3 instruments jointly contribute significantly to the forecast skill in the ECMWF system. Positive impact of up to 2% is seen for most variables out to the day-2 forecasts over hemispheric scales, with significant benefits for total column water vapor or for temperature and wind in the stratosphere out to day 4. 相似文献
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There has been a huge surge in interest in the preservation of Madagascar's forests in the past two decades, but despite the investment of hundreds of millions of dollars, the goal remains elusive. Recent legislation has given the government the authority to enter into contractual arrangements with communities for the management of the country's public forests, so it has become crucial to grapple with the significant challenges involved. This paper explores the enormity of the challenge of forest governance in Madagascar in an era of decentralization. By examining several forests in one part of the country, it reveals a dizzying range of forest types and forms of use and governance within a fairly small portion of the country. These examples make it apparent that the history of forest management in Madagascar constitutes a broad-ranging experiment with forest governance. Simply monitoring the dynamics of the forest canopy is a significant technical challenge. However, this pales in comparison to the difficulties inherent in explaining those dynamics and assessing the sustainability and equity of different management regimes. Of the forests considered in the study, those where the Malagasy state has partnered with international conservation and development organizations seem to stand out, both in terms of stabilized, or even growing, forest cover, as well as a balance of interests among users. 相似文献
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Interest in the role cooperative networks can play in small business development is increasing. Evidence from a survey of three network promoting small business associations based in three different cities is examined and a number of different ways the agencies could deliver their services are suggested. Two networking initiatives by the New Zealand Trade Development Board are then considered. It is concluded that individual networking initiatives need to be developed and evaluated as part of a broad policy push to facilitate the development of an environment more conducive to business cooperation. 相似文献