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External Forcing (external + forcing)
Selected AbstractsGeological constraints on Antarctic palaeo-ice-stream retreatEARTH SURFACE PROCESSES AND LANDFORMS, Issue 4 2008Colm Ó Cofaigh Abstract Submarine landforms preserved in bathymetric troughs on the Antarctic continental shelf show that the style of ice stream retreat across the shelf following the last glacial maximum varied between different troughs. Three styles of retreat are inferred from the geological evidence: rapid, episodic and slow. Rapid retreat by ice stream floatation and calving is recorded by the preservation of a landform assemblage of unmodified streamlined subglacial bedforms including mega-scale glacial lineations (MSGLs) that record streaming flow along these troughs. These elongate bedforms are not overprinted by recessional glacial landforms formed transverse to ice flow such as moraines or grounding-zone wedges, and overlying deglacial sediments are thin. A second type of landform assemblage consists of MSGLs overprinted or interrupted by transverse grounding-zone wedges. This assemblage implies episodic retreat between successive grounding-zone positions. The third type of landform assemblage is that of numerous, closely spaced, recessional moraines and intermittent grounding-zone wedges that overlie and interrupt MSGLs. This assemblage records the slow retreat of grounded ice across the shelf. Variation in the style of ice stream retreat between the different bathymetric troughs indicates that Antarctic palaeo-ice-streams did not respond uniformly to external forcing at the end of the last glacial cycle. Rather, their diachronous retreat reflects the dominance of local controls in the form of bathymetry and drainage basin size. More broadly, these data show that retreat of marine-based ice sheets in areas of reverse bed slope is not necessarily catastrophic, and they provide important constraints for numerical models that attempt to predict the dynamics of large polar ice sheets. Copyright © 2008 John Wiley & Sons, Ltd. [source] The Kalman filter for the pedologist's tool kitEUROPEAN JOURNAL OF SOIL SCIENCE, Issue 6 2006R. Webster Summary The Kalman filter is a tool designed primarily to estimate the values of the ,state' of a dynamic system in time. There are two main equations. These are the state equation, which describes the behaviour of the state over time, and the measurement equation, which describes at what times and in what manner the state is observed. For the discrete Kalman filter, discussed in this paper, the state equation is a stochastic difference equation that incorporates a random component for noise in the system and that may include external forcing. The measurement equation is defined such that it can handle indirect measurements, gaps in the sequence of measurements and measurement errors. The Kalman filter operates recursively to predict forwards one step at a time the state of the system from the previously predicted state and the next measurement. Its predictions are optimal in the sense that they have minimum variance among all unbiased predictors, and in this respect the filter behaves like kriging. The equations can also be applied in reverse order to estimate the state variable at all time points from a complete series of measurements, including past, present and future measurements. This process is known as smoothing. This paper describes the ,predictor,corrector' algorithm for the Kalman filter and smoother with all the equations in full, and it illustrates the method with examples on the dynamics of groundwater level in the soil. The height of the water table at any one time depends partly on the height at previous times and partly on the precipitation excess. Measurements of the height of water table and their errors are incorporated into the measurement equation to improve prediction. Results show how diminishing the measurement error increases the accuracy of the predictions, and estimates achieved with the Kalman smoother are even more accurate. Le filtre de Kalman comme outil pour le pédologue Résumé Le filtre de Kalman est un outil conçu essentiellement pour estimer les valeurs de l'état d'un système dynamique dans le temps. Il comprend deux équations principales. Celles-ci sont l'équation d'état, qui décrit l'évolution de l'état pendant le temps, et l'équation de mesure qui decrit à quel instants et de quelle façon on observe l'état. Pour le filtre discret de Kalman, décrit dans cet article, l'équation d'état est une équation stochastique différentielle qui comprend une composante aléatoire pour le bruit dans le système et qui peut inclure une force extérieure. On définit l'équation de mesure de façon à ce qu'elle puisse traiter des mesures indirectes, des vides dans des séquences de mesures et des erreurs de mesure. Le filtre de Kalman fonctionne récursivement pour prédire en avance une démarche à temps l'état du système de la démarche prédite antérieure plus l'observation prochaine. Ses prédictions sont optimales dans le sens qu'elles minimisent la variance parmi toutes les prédictions non-biasées, et à cet égard le filtre se comporte comme le krigeage. On peut appliquer, aussi, les équations dans l'ordre inverse pour estimer la variable d'état à toutes pointes à toutes les instants d'une série complète d'observations, y compris les observations du passé, du présent et du futur. Ce processus est connu comme ,smoothing'. Cet article décrit l'algorithme ,predictor,corrector' du filtre de Kalman et le ,smoother' avec toutes les équations entières. Il illustre cette méthode avec des exemples de la dynamique du niveau de la nappe phréatique dans le sol. Le niveau de la nappe à un instant particulier dépend en partie du niveau aux instants précédents et en partie de l'excès de la précipitation. L'équation d'état fournit la relation générale entre les deux variables et les prédictions. On incorpore les mesures du niveau de la nappe et leurs erreurs pour améliorer les prédictions. Les résultats mettent en évidence que lorsqu'on diminue l'erreur de mesure la précision des prédictions augmente, et aussi que les estimations avec le ,smoother' de Kalman sont encore plus précises. [source] Optimal modal reduction of vibrating substructuresINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 3 2003Paul E. Barbone Abstract A structure which consists of a main part and a number of attached substructures is considered. A ,model reduction' scheme is developed and applied to each of the discrete substructures. Linear undamped transient vibrational motion of the structure is assumed, with general external forcing and initial conditions. The goal is to replace each discrete substructure by another substructure with a much smaller number of degrees of freedom, while minimizing the effect this reduction has on the dynamic behaviour of the main structure. The approach taken here involves Ritz reduction and the Dirichlet-to-Neumann map as analysis tools. The resulting scheme is based on a special form of modal reduction, and is shown to be optimal in a certain sense, for long simulation times. The performance of the scheme is demonstrated via numerical examples, and is compared to that of standard modal reduction. Copyright © 2003 John Wiley & Sons, Ltd. [source] An Arctic and antarctic perspective on recent climate changeINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 3 2007John Turner Abstract We contrast recent climatic and environmental changes and their causes in the Arctic and the Antarctic. There are continuing increases in surface temperatures, losses of sea ice and tundra, and warming of permafrost over broad areas of the Arctic, while most of the major increase in Antarctic temperatures is on the Antarctic Peninsula associated with sea ice loss in the Bellingshausen,Amundsen Seas sector. While both natural atmospheric and oceanic variability, and changes in external forcing including increased greenhouse gas concentrations, must be considered in the quest for understanding such changes, the interactions and feedbacks between system components are particularly strong at high latitudes. For the 1950s to date in the Arctic and for 1957 to date in the Antarctic, positive trends in large-scale atmospheric circulation represented by the Arctic oscillation (AO) and Antarctic oscillations (AAO) and the Pacific North American (PNA) pattern contribute to the long-term temperature trends. However, continuing Arctic trends during the last decade of near neutral AO will require alternate explanations. The trend in the AAO since 1950 is larger than expected from natural variability and may be associated with the decrease in stratospheric ozone over Antarctic. The persistence shown in many Arctic and Antarctic Peninsula components of climate and their influence through possible feedback supports continuation of current trends over the next decade. One can expect large spatial and temporal differences, however, from the relative contributions of intrinsic variability, external forcing, and internal feedback/amplifications. It is particularly important to resolve regional feedback processes in future projections based on modeling scenarios. Copyright © 2006 Royal Meteorological Society. [source] Signals of anthropogenic influence on European warming as seen in the trend patterns of daily temperature varianceINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 1 2005A. M. G. Klein Tank Abstract Signals of anthropogenic warming over Europe are searched for in the spatial trend patterns for the variance and skewness (expressed by the 10th and 90th percentiles) of the distribution of daily mean temperature. Comparisons are made between these patterns in the station records of the European Climate Assessment dataset for the 1976,99 period, the patterns associated with natural variability in the observations (which were empirically derived from the observations in the 1946,75 period), and the patterns of future warming and natural variability as simulated by the National Center for Atmospheric Research Community Climate System Model in the Challenge ensemble experiment. The results indicate that, on the basis of the patterns for the variance, a distinction can be made between temperature change due to natural variability and temperature change due to changes in external forcing. The observed variance trend patterns for the spring (March,May) and summer (June,August) warming 1976,99 are clearly different from the patterns for the change in variance associated with a warming due to natural variability in the observations. This led us to conclude that a change in an external forcing has to be invoked to explain the observed spring and summer warming. From the evaluation of the greenhouse and natural variability patterns in the climate model simulations, we infer that the observed spring and summer variance trend patterns contain imprints consistent with anthropogenic warming. The analysis of the variance trend patterns for the winter (December,February) season is inconclusive about identifying causes of the observed warming for that season. Unlike the other three seasons, the autumn (September,November) is for Europe a period of cooling in recent decades. The observed variance trend pattern for this season closely resembles the estimated pattern for the change in variance associated with a cooling due to natural variability, indicating that the observed autumn cooling can be ascribed to random weather variations in the period under consideration. Copyright © 2005 Royal Meteorological Society [source] The influence of natural conditions on the spatial variation of climate in Lapland, northern FinlandINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 9 2003Andrea Vajda Abstract At high latitudes in Lapland, near the climatological timberline, forestry and other environmental research require detailed information about the spatial variation of climate. In this study, the influence of local geographical factors on the climate in northern Finland (Lapland), as well as the applicability of the kriging interpolation method in the case of detailed spatial resolution, were examined. The spatial analysis of mean, maximum, minimum temperatures, length of the frost-free season, degree-days and daily range was made using a 1 km × 1 km resolution. The time period used was 1971,2000. We studied whether taking account of external forcing, such as lake coverage and altitude, would improve the accuracy of spatial interpolation of climatological parameters. The geographical factors of coordinates, elevation, lakes and sea influence on the regional features of the climate were examined. According to the results of this study, only geographical position and local relief have a significant influence on regional climate in Lapland. The effect of lakes and sea seems to be secondary. Copyright © 2003 Royal Meteorological Society [source] Local forcing of a nonlinear surface reaction: CO oxidation on Pt(100)AICHE JOURNAL, Issue 1 2009Daniel Bilbao Abstract A novel spatiotemporal perturbation method for nonlinear surface reactions is reported, thus allowing the creation of new spatially localized structures. Forcing was achieved by dosing reactant gases through a capillary positioned near the catalyst surface, providing control over the local surface coverage and reaction rate. The emergence of localized concentration patterns and oscillations in an otherwise stable system is attributed to a local modification of the catalytic properties of the surface due to external forcing. Based on the spatial orientation, the temporal and thermal stability of the modified surface, as well as the affinity of CO toward the perturbed surface, subsurface O is proposed as a possible source of the observed localized patterning and surface memory effect. © 2008 American Institute of Chemical Engineers AIChE J, 2009 [source] Tephra-linked peat humification records from Irish ombrotrophic bogs question nature of solar forcing at 850 cal. yr BC,JOURNAL OF QUATERNARY SCIENCE, Issue 1 2006G. Plunkett Abstract This paper investigates evidence for palaeoclimatic changes during the period ca. 1500,500,cal.,yr BC through peat humification studies on seven Irish ombrotrophic bogs. The sites are well-correlated by the identification of three mid-first millennium BC tephras, which enable the humification records at specific points in time to be directly compared. Phases of temporarily increased wetness are suggested at ca. 1300,1250,cal.,yr BC, ca. 1150,1050 cal.,yr BC, ca. 940,cal.,yr BC and ca. 740,cal.,yr BC. The last of these is confirmed to be synchronous at five sites, suggesting external forcing on a regional scale. The timing of this wet-shift is constrained by two closely dated tephras and is demonstrated to be distinct from the widely reported changes to cooler/wetter conditions associated with a solar minimum at 850,760,cal.,yr BC, at which time the Irish sites appear instead to experience drier conditions. The results suggest the possibility of either non-uniform responses to solar forcing in northwest Europe at this time, or the existence of unrelated climate events in the early first millennium BC. The findings caution against the correlation of loosely dated palaeoclimate data if the effects of forcing mechanisms are to be understood. Copyright © 2006 John Wiley & Sons, Ltd. [source] Forced Alveolar Flows and Mixing in the LungPROCEEDINGS IN APPLIED MATHEMATICS & MECHANICS, Issue 1 2009David Borer The air flows deep inside the lung are not only important in gas exchange processes but they also determine the efficiency of particle deposition and retention. The study aims at quantifying the relative influence of different flow components in the transport of small particles in alveolar geometries such as convective breathing patterns, wall movement, gravitational settling and Brownian motion. In addition, the possibility and efficiency of external forcing is studied, relying on the mechanism of internal acoustic streaming. A viscous oscillating boundary layer flow is converted into a steady, viscosity-independent bulk motion which is very efficient at low Reynolds numbers. The streaming can be controlled by external parameters (excitation amplitude, frequency, beam shape) and may thus be of diagnostic and therapeutic relevance. Numerical simulations are performed to analyze the flow patterns in 3D model geometries and to measure deposition rates. (© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Influence of Ohmic diffusion on the excitation and dynamics of MRIASTRONOMISCHE NACHRICHTEN, Issue 1 2010M.J. Korpi Abstract In this paper we make an effort to understand the interaction of turbulence generated by the magnetorotational instability (MRI) with turbulence from other sources, such as supernova explosions (SNe) in galactic disks. First we perform a linear stability analysis (LSA) of non-ideal MRI to derive the limiting value of Ohmic diffusion that is needed to inhibit the growth of the instability for different types of rotation laws. With the help of a simple analytical expression derived under first-order smoothing approximation (FOSA), an estimate of the limiting turbulence level and hence the turbulent diffusion needed to damp the MRI is derived. Secondly, we perform numerical simulations in local cubes of isothermal nonstratified gas with external forcing of varying strength to see whether the linear result holds for more complex systems. Purely hydrodynamic calculations with forcing, rotation and shear are made for reference purposes, and as expected, non-zero Reynolds stresses are found. In the magnetohydrodynamic calculations, therefore, the total stresses generated are a sum of the forcing and MRI contributions. To separate these contributions, we perform reference runs with MRI-stable shear profiles (angular velocity increasing outwards), which suggest that the MRI-generated stresses indeed become strongly suppressed as function of the forcing. The Maxwell to Reynolds stress ratio is observed to decrease by an order of magnitude as the turbulence level due to external forcing exceeds the predicted limiting value, which we interpret as a sign of MRI suppression. Finally, we apply these results to estimate the limiting radius inside of which the SN activity can suppress the MRI, arriving at a value of 14 kpc (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Geophysical evidence for Holocene lake-level change in southern California (Dry Lake)BOREAS, Issue 1 2010BROXTON W. BIRD Bird, B. W., Kirby, M. E., Howat, I. M. & Tulaczyk, S. 2009: Geophysical evidence for Holocene lake-level change in southern California (Dry Lake). Boreas, 10.1111/j.1502-3885.2009.00114.x. ISSN 0300-9483. Ground penetrating radar (GPR) data are used in combination with previously published sediment cores to develop a Holocene history of basin sedimentation in a small, alpine lake in southern California (Dry Lake). The GPR data identify three depositional sequences spanning the past 9000 calendar years before present (cal. yr BP). Sequence I represents the first phase of an early Holocene highstand. A regression between <8320 and >8120 cal. yr BP separates Sequence I from Sequence II, perhaps associated with the 8200 cal. yr BP cold event. Sequence II represents the second phase of the early-to-mid Holocene highstand. Sequence IIIa represents a permanent shift to predominantly low lake stands beginning ,5550 cal. yr BP. This mid-Holocene shift was accompanied by a dramatic decrease in sedimentation rate as well as a contraction of the basin's area of sedimentation. By ,1860 cal. yr BP (Sequence IIIb), the lake was restricted to the modern, central basin. Taken together, the GPR and core data indicate a wet early Holocene followed by a long-term Holocene drying trend. The similarity in ages of the early Holocene highstand across the greater southern California region suggests a common external forcing , perhaps modulation of early Holocene storm activity by insolation. However, regional lake level records are less congruous following the initial early Holocene highstand, which may indicate a change in the spatial domain of climate forcing(s) throughout the Holocene in western North America. [source] Impact of CO2 concentration changes on the biosphere-atmosphere system of West AfricaGLOBAL CHANGE BIOLOGY, Issue 12 2002GUILING WANG Abstract Vegetation dynamics plays a critical role in causing the decadal variability of precipitation over the Sahel region of West Africa. However, the potential impact of changes in CO2 concentration on vegetation dynamics and precipitation variability of this region has not been addressed by previous studies. In this paper, we explore the role of CO2 concentration in the regional climate system of West Africa using a zonally symmetric, synchronously coupled biosphere-atmosphere model. We first document the response of precipitation and vegetation to incremental changes of CO2 concentration; the impact of CO2 concentration on the variability of the regional biosphere-atmosphere system is then addressed using the second half of the twentieth century as an example. An increase of CO2 concentration causes the regional biosphere-atmosphere system to become wetter and greener, with the radiative effect of CO2 and improved plant-water relation dominant in the Sahelian grassland region and the direct enhancement of leaf carbon assimilation dominant in the tree-covered region to the south. Driven by the observed sea surface temperature (SST) of the tropical Atlantic Ocean during the period 1950,97 and with CO2 concentration prescribed at a pre-industrial level 300ppmv, the model simulates a persistent Sahel drought during the period of 1960s,1990s. The simulated drought takes place in the form of a transition of the coupled biosphere-atmosphere system from a wet/green regime in the 1950s to a dry/barren regime after the 1960s. This climate transition is triggered by SST forcing and materialized through vegetation-climate interactions. The same SST forcing does not produce such a persistent drought when a constant modern CO2 concentration of 350ppmv is specified, indicating that the biosphere-atmosphere system at higher CO2 level is more resilient to drought-inducing external forcings. This finding suggests that the regional climate in Sahel, which tends to alternate between dry and wet spells, may experience longer (or more frequent) wet episodes and shorter (or less frequent) dry episodes in the future than in the past. Our study has significant implications regarding the impact of climate change on regional socio-economic development. [source] Causes of variability in the summertime Antarctic boundary-layer climateINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 13 2007Dirk van As Abstract A high-resolution one-dimensional atmospheric model is used to assess the contribution of various surface characteristics and external forcings on the structure and dynamics of the atmospheric boundary layer (ABL) over the Antarctic Plateau in summer. The reference run simulates the boundary layer over a mildly sloping surface (1.5 m km,1) for a clear sky near the end of the Antarctic summer (31 January-3 February). The ABL depth is approximately 100 m. At night, a low-level jet forms due to the combined effect of katabatic forcing and an inertial oscillation. During the day a convective mixed layer is present. As expected, the ABL is very sensitive to surface slope; a larger slope forces higher wind speeds and a deeper boundary layer. Over a horizontal surface, a nocturnal jet is also found as a result of the inertial oscillation. A modest change in surface albedo alters the mixed-layer temperature and the height and strength of the nocturnal jet considerably. Rotating the large-scale wind relative to the slope direction also has a large impact on ABL depth and structure. The deepest boundary layer and largest wind speed over a northward down-sloping surface are found for an easterly (cross slope) large-scale wind, as is typical for Antarctica. A very shallow ABL with low wind speed is found for the opposite large-scale wind direction. ABL sensitivity to surface roughness was found to be small. For all experiments, the ABL sensitivity is enhanced due to the positive feedback between the cooling of the ABL and katabatic wind speed. Copyright © 2007 Royal Meteorological Society [source] | |||||||||||||