High-Resolution Fibre-Optic Temperature Sensing: A New Tool to Study the Two-Dimensional Structure of Atmospheric Surface-Layer Flow   总被引：1，自引：0，他引：1  

Christoph K. Thomas  Adam M. Kennedy  John S. Selker  Ayla Moretti  Martin H. Schroth  Alexander R. Smoot  Nicholas B. Tufillaro  Matthias J. Zeeman 《Boundary-Layer Meteorology》2012,142(2):177-192

We present a novel approach based on fibre-optic distributed temperature sensing (DTS) to measure the two-dimensional thermal structure of the surface layer at high resolution (0.25 m, ≈0.5 Hz). Air temperature observations obtained from a vertically-oriented fibre-optics array of approximate dimensions 8 m × 8 m and sonic anemometer data from two levels were collected over a short grass field located in the flat bottom of a wide valley with moderate surface heterogeneity. The objectives of the study were to evaluate the potential of the DTS technique to study small-scale processes in the surface layer over a wide range of atmospheric stability, and to analyze the space–time dynamics of transient cold-air pools in the calm boundary layer. The time response and precision of the fibre-based temperatures were adequate to resolve individual sub-metre sized turbulent and non-turbulent structures, of time scales of seconds, in the convective, neutral, and stable surface layer. Meaningful sensible heat fluxes were computed using the eddy-covariance technique when combined with vertical wind observations. We present a framework that determines the optimal environmental conditions for applying the fibre-optics technique in the surface layer and identifies areas for potentially significant improvements of the DTS performance. The top of the transient cold-air pool was highly non-stationary indicating a superposition of perturbations of different time and length scales. Vertical eddy scales in the strongly stratified transient cold-air pool derived from the DTS data agreed well with the buoyancy length scale computed using the vertical velocity variance and the Brunt–Vaisala frequency, while scales for weak stratification disagreed. The high-resolution DTS technique opens a new window into spatially sampling geophysical fluid flows including turbulent energy exchange.  相似文献   

High order normal form construction near the elliptic orbit of the Sitnikov problem     

Sara Di Ruzza  Christoph Lhotka 《Celestial Mechanics and Dynamical Astronomy》2011,111(4):449-464

We consider the Sitnikov problem; from the equations of motion we derive the approximate Hamiltonian flow. Then, we introduce suitable action–angle variables in order to construct a high order normal form of the Hamiltonian. We introduce Birkhoff Cartesian coordinates near the elliptic orbit and we analyze the behavior of the remainder of the normal form. Finally, we derive a kind of local stability estimate in the vicinity of the periodic orbit for exponentially long times using the normal form up to 40th order in Cartesian coordinates.  相似文献   

Future changes in daily summer temperature variability: driving processes and role for temperature extremes     

Erich M. Fischer  Christoph Schär 《Climate Dynamics》2009,33(7-8):917-935

Anthropogenic greenhouse gas emissions are expected to lead to more frequent and intense summer temperature extremes, not only due to the mean warming itself, but also due to changes in temperature variability. To test this hypothesis, we analyse daily output of ten PRUDENCE regional climate model scenarios over Europe for the 2071–2100 period. The models project more frequent temperature extremes particularly over the Mediterranean and the transitional climate zone (TCZ, between the Mediterranean to the south and the Baltic Sea to the north). The projected warming of the uppermost percentiles of daily summer temperatures is found to be largest over France (in the region of maximum variability increase) rather than the Mediterranean (where the mean warming is largest). The underlying changes in temperature variability may arise from changes in (1) interannual temperature variability, (2) intraseasonal variability, and (3) the seasonal cycle. We present a methodology to decompose the total daily variability into these three components. Over France and depending upon the model, the total daily summer temperature variability is projected to significantly increase by 20–40% as a result of increases in all three components: interannual variability (30–95%), seasonal variability (35–105%), and intraseasonal variability (10–30%). Variability changes in northern and southern Europe are substantially smaller. Over France and parts of the TCZ, the models simulate a progressive warming within the summer season (corresponding to an increase in seasonal variability), with the projected temperature change in August exceeding that in June by 2–3 K. Thus, the most distinct warming is superimposed upon the maximum of the current seasonal cycle, leading to a higher intensity of extremes and an extension of the summer period (enabling extreme temperatures and heat waves even in September). The processes driving the variability changes are different for the three components but generally relate to enhanced land–atmosphere coupling and/or increased variability of surface net radiation, accompanied by a strong reduction of cloudiness, atmospheric circulation changes and a progressive depletion of soil moisture within the summer season. The relative contribution of these processes differs substantially between models.  相似文献   

Nowcasting of a supercell storm with VERA     

Stefan Schneider  Barbara Chimani  Hildegard Kaufmann  Benedikt Bica  Christoph Lotteraner  Simon Tschannett  Reinhold Steinacker 《Meteorology and Atmospheric Physics》2008,102(1-2):23-36

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A conservative, unconditionally stable, second-order three-point differencing scheme for the diffusion–convection equation     

Christoph Clauser  Stefan Kiesner 《Geophysical Journal International》1987,91(3):557-568

A three-point differencing scheme for the diffusion–convection equation is presented that offers all the advantages of both the central and the one-sided ('upwind') differencing scheme without suffering from their drawbacks. Specifically, the scheme is conservative, unconditionally stable, and second-order-accurate in space. It is free of oscillations and over- or undershoots, simple to code, and requires essentially no more computing time than the one-sided scheme. Although known for a relatively long time in numerical mathematics, the scheme apparently has not received sufficient attention from modellers of hydrothermal systems or contaminant transport in the geosciences. In order to fill this gap a comparison is made between this scheme and the widely used one-sided scheme for the transient diffusion–convection equation in different time discretizations. The results are discussed taking into account other approaches towards minimizing numerical diffusion.  相似文献   

Streamflow generation in a headwater basin on the precambrian shield     

Christoph Wels  Colin H. Taylor  R. Jack Cornett  Bruce D. Lazerte 《水文研究》1991,5(2):185-199

Current conceptual runoff models hypothesize that stormflow generation on the Canadian Shield is a combination of subsurface stormflow and saturation overland flow. This concept was tested during spring runoff in a small (3.3 ha) headwater basin using: (1) isotopic and chemical hydrograph separation and (2) field mapping and direct tracing of saturated areas. Isotopic and chemical hydrograph separation indicated three runoff components: (1) pre-melt subsurface flow; (2) subsurface flow of new (event) water; and (3) direct precipitation on to saturated areas (DPS). During early thaw-freeze cycles, their relative contributions to total flow remained constant (65 per cent, 30 per cent, and 5 per cent respectively). It is hypothesized that lateral flow along the bedrock/mineral soil interface, possibly through macropores, supplied large volumes of subsurface flow (of both old and new water) rapidly to the stream channel. Much higher contributions of DPS were observed during an intensive rain-on-snow event (15 per cent of total flow). Mapping and direct tracing of saturated areas using lithium bromide, suggested that saturated area size was positively correlated to stream discharge but its response lagged behind that of discharge. These observations suggest that the runoff mechanisms, and hence the sources of stream flow, will vary depending on storm characteristics.  相似文献   

Basement control on oblique thrust sheet evolution: seismic imaging of the active deformation front of the Central Andes in Bolivia     

Ralph Hinsch  Charlotte M. Krawczyk  Christoph Gaedicke  Raul Giraudo  Daniel Demuro 《Tectonophysics》2002,355(1-4)

In the area of the Bolivian Orocline, we examine the deformation pattern associated with the active development of a new thrust sheet. A dense grid of reprocessed 2-D seismic lines from hydrocarbon exploration industry is interpreted and a 3-D simplified structural and kinematic model is deduced. In the Boomerang Hills, onlapping Paleozoic and foredeep sediments are detached from the underlying S-dipping basement. They are thrust northeastwards by less than 2 km. Two zones can be differentiated along the Andean deformation front: (1) a W–E to NW–SE striking frontal segment of predominantly orthogonal shortening, comprising a thrust and anticline system; (2) a WSW–ENE striking lateral zone of oblique shortening within a complex system of thin-skinned strike–slip faults and minor folds. The deformation front always follows a pronounced edge in the topography of the top basement surface close to the boundary of the Paleozoic basin. The observed deformation pattern indicates intensified strain partitioning caused by the interaction of contraction direction and basement topography, which provides a near oblique ramp for the onlapping wedge of sediments. The SW–NE thrusting direction is divided into orthogonal and tangential components. These are accommodated by convergent and strike–slip structures, respectively, which sole into a common detachment horizon. The structural evolution of the new thrust sheet in the Bolivian Orocline is primarily controlled by the paleorelief of the Brazilian Shield because: (1) the shape of the basement affects the taper of the thrust wedge and localizes the deformation front and (2) small asperities in/close to the top of the basement promote fault localization. The coincidence of a relatively high basement position and a structural high of the Eastern Cordillera leads to the conclusion that the shape of the Brazilian Shield also controls the structural evolution of the pronounced eastern border of the Bolivian Orocline.  相似文献   

Shallow water wave modelling with nonlinear dissipation     

Christoph Schneggenburger  Heinz Günther  Wolfgang Rosenthal 《Ocean Dynamics》1997,49(2-3):431-444

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Tracer experiment with sulfur hexafluoride to quantify the vertical transport in a meromictic pit lake     

Christoph von Rohden  Johann Ilmberger 《Aquatic Sciences - Research Across Boundaries》2001,63(4):417-431

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On the magnitude and apparent range of variation of the von Karman constant in the atmospheric surface layer   总被引：1，自引：1，他引：0  

Paul Frenzen  Christoph A. Vogel 《Boundary-Layer Meteorology》1995,72(4):371-392

The half-century history of the experimental evaluation of the von Karman constant in the atmospheric surface layer is reviewed, an evidence indicating that this well-known scaling factor,k, is actually a weak variable that decreases with increasing Reynolds number is discussed. A combined average ofk=0.390 with a standard error of ±0.010 is found from two field studies, a result which indicates that there is only one chance in 40 that the true value of the scaling factor in the population from which the experimental sample was drawn could have been as large as the laboratory value of 0.40. Based on experimental and theoretical results given by others, it is suggested thatk varies from a maximum of 0.41 in light winds over open water and smooth land surfaces, to a minimum near 0.37 in stronger winds over forests and cities. This range is shown to imply that a working value ofk=0.39±1% is appropriate for flows over surfaces characterized by 0.7<z ₀<8.7 cm, a roughness subrange which corresponds to a wide variety of terrain types from sparsely vegetated level plains to mixed croplands with occasional hedges and trees.sponsored in part by the Global Change Post Doctoral Program, Office of Health and Environmental Research, U. S. Department of Energy.Work sponsored by the National Science Foundation under Grant No. ATM-9019682.  相似文献