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Multiple Time Scales (multiple + time_scale)
Selected AbstractsEstimating diurnal to annual ecosystem parameters by synthesis of a carbon flux model with eddy covariance net ecosystem exchange observationsGLOBAL CHANGE BIOLOGY, Issue 2 2005Bobby H. Braswell Abstract We performed a synthetic analysis of Harvard Forest net ecosystem exchange of CO2 (NEE) time series and a simple ecosystem carbon flux model, the simplified Photosynthesis and Evapo-Transpiration model (SIPNET). SIPNET runs at a half-daily time step, and has two vegetation carbon pools, a single aggregated soil carbon pool, and a simple soil moisture sub-model. We used a stochastic Bayesian parameter estimation technique that provided posterior distributions of the model parameters, conditioned on the observed fluxes and the model equations. In this analysis, we estimated the values of all quantities that govern model behavior, including both rate constants and initial conditions for carbon pools. The purpose of this analysis was not to calibrate the model to make predictions about future fluxes but rather to understand how much information about process controls can be derived directly from the NEE observations. A wavelet decomposition enabled us to assess model performance at multiple time scales from diurnal to decadal. The model parameters are most highly constrained by eddy flux data at daily to seasonal time scales, suggesting that this approach is not useful for calculating annual integrals. However, the ability of the model to fit both the diurnal and seasonal variability patterns in the data simultaneously, using the same parameter set, indicates the effectiveness of this parameter estimation method. Our results quantify the extent to which the eddy covariance data contain information about the ecosystem process parameters represented in the model, and suggest several next steps in model development and observations for improved synthesis of models with flux observations. [source] Hydrogeologic controls on streamflow sensitivity to climate variationHYDROLOGICAL PROCESSES, Issue 22 2008Anne Jefferson Abstract Climate models project warmer temperatures for the north-west USA, which will result in reduced snowpacks and decreased summer streamflow. This paper examines how groundwater, snowmelt, and regional climate patterns control discharge at multiple time scales, using historical records from two watersheds with contrasting geological properties and drainage efficiencies. In the groundwater-dominated watershed, aquifer storage and the associated slow summer recession are responsible for sustaining discharge even when the seasonal or annual water balance is negative, while in the runoff-dominated watershed subsurface storage is exhausted every summer. There is a significant 1 year cross-correlation between precipitation and discharge in the groundwater-dominated watershed (r = 0·52), but climatic factors override geology in controlling the inter-annual variability of streamflow. Warmer winters and earlier snowmelt over the past 60 years have shifted the hydrograph, resulting in summer recessions lasting 17 days longer, August discharges declining 15%, and autumn minimum discharges declining 11%. The slow recession of groundwater-dominated streams makes them more sensitive than runoff-dominated streams to changes in snowmelt amount and timing. Copyright © 2008 John Wiley & Sons, Ltd. [source] Spatial and temporal characteristics of droughts in the western part of BangladeshHYDROLOGICAL PROCESSES, Issue 13 2008Shamsuddin Shahid Abstract Spatial and temporal characteristics of droughts in the western part of Bangladesh have been analysed. Standardized precipitation index method is used to compute the severity of droughts from the rainfall data recorded in 12 rainfall gauge stations for the period of 1961,1999. An artificial neural network is used to estimate missing rainfall data. Geographic Information System (GIS) is used to map the spatial extent of droughts of different severities in multiple time scales. Critical analysis of rainfall is also carried to find the minimum monsoon and dry months rainfall require in different parts of the study area to avoid rainfall deficit. The study shows that the north and north-western parts of Bangladesh are most vulnerable to droughts. A significant negative relationship between multiple ENSO index and rainfall is observed in some stations. Analysis of seasonal rainfall distribution, rainfall reliability and long-term rainfall trend is also conducted to aid prediction of future droughts in the area. Copyright © 2007 John Wiley & Sons, Ltd. [source] Electromagnetic brain activity evoked by affective stimuli in schizophreniaPSYCHOPHYSIOLOGY, Issue 5 2006Brigitte Rockstroh Abstract Schizophrenia is typically associated with cognitive deficits, but symptoms also point to alterations in the processing of affective material, with potential impact on behavioral performance. This impact may unfold on multiple time scales, but initial processing of rapidly unfolding social cues may be particularly important. MEG-assessed regional brain activity associated with the capacity to process the emotional content of rapid visual stimuli (3/s) was examined in 12 individuals with schizophrenia and 12 matched controls. Patients showed less differentiation of emotional versus neutral stimuli 90,300 ms following picture onset. Together with group differences in the lateral topography of valence effects, these results are discussed as evidence of deficient automatic processing of emotionally potent stimuli in schizophrenia. [source] Multiscale variability of the evapotranspiration in eastern AmazoniaATMOSPHERIC SCIENCE LETTERS, Issue 3 2010Adriano M. L. de Sousa Abstract This article reports the characteristics of multiple time scales of signals, evapotranspiration (ET) and outgoing long-wave radiation, in the eastern Amazonia. This has to apply the technique of mathematics Morlet wavelet transform. The results show a difference in the time scales between intraseasonal, synoptic and mesoscale for events. For the non-flooded event (13 September 2004), hourly fluctuations in ET in scales from 2 to 5 h were more intense than those in the case of the flooded event (9 April 2005). This can be related to mesoscale circulations, possibly caused by local surface conditions, such as Lake Breeze. Copyright © 2010 Royal Meteorological Society [source] Relationships between morphological and sedimentological parameters in source-to-sink systems: a basis for predicting semi-quantitative characteristics in subsurface systemsBASIN RESEARCH, Issue 4 2009Tor O. Sømme ABSTRACT The study of source-to-sink systems relates long-term variations in sediment flux to morphogenic evolution of erosional,depositional systems. These variations are caused by an intricate combination of autogenic and allogenic forcing mechanisms that operate on multiple time scales , from individual transport events to large-scale filling of basins. In order to achieve a better understanding of how these mechanisms influence morphological characteristics on different scales, 29 submodern source-to-sink systems have been investigated. The study is based on measurements of morphological parameters from catchments, shelves and slopes derived from a ,1 km global digital elevation model dataset, in combination with data on basin floor fans, sediment supply, water discharge and deposition rates derived from published literature. By comparing various morphological and sedimentological parameters within and between individual systems, a number of relationships governing system evolution and behaviour are identified. The results suggest that the amount of low-gradient floodplain area and river channel gradient are good indicators for catchment storage potential. Catchment area and river channel length is also related to shelf area and shelf width, respectively. Similarly to the floodplain area, these parameters are important for long-term storage of sediment on the shelf platform. Additionally, the basin floor fan area is correlative to the long-term deposition rate and the slope length. The slope length thus proves to be a useful parameter linking proximal and distal segments in source-to-sink systems. The relationships observed in this study provide insight into segment scale development of source-to-sink systems, and an understanding of these relationships in modern systems may result in improved knowledge on internal and external development of source-to-sink systems over geological time scales. They also allow for the development of a set of semi-quantitative guidelines that can be used to predict similar relationships in other systems where data from individual system segments are missing or lacking. [source] Joint Modelling of Repeated Transitions in Follow-up Data , A Case Study on Breast Cancer DataBIOMETRICAL JOURNAL, Issue 3 2005B. Genser Abstract In longitudinal studies where time to a final event is the ultimate outcome often information is available about intermediate events the individuals may experience during the observation period. Even though many extensions of the Cox proportional hazards model have been proposed to model such multivariate time-to-event data these approaches are still very rarely applied to real datasets. The aim of this paper is to illustrate the application of extended Cox models for multiple time-to-event data and to show their implementation in popular statistical software packages. We demonstrate a systematic way of jointly modelling similar or repeated transitions in follow-up data by analysing an event-history dataset consisting of 270 breast cancer patients, that were followed-up for different clinical events during treatment in metastatic disease. First, we show how this methodology can also be applied to non Markovian stochastic processes by representing these processes as "conditional" Markov processes. Secondly, we compare the application of different Cox-related approaches to the breast cancer data by varying their key model components (i.e. analysis time scale, risk set and baseline hazard function). Our study showed that extended Cox models are a powerful tool for analysing complex event history datasets since the approach can address many dynamic data features such as multiple time scales, dynamic risk sets, time-varying covariates, transition by covariate interactions, autoregressive dependence or intra-subject correlation. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Synaptic Control Of Motoneuron Excitability In Rodents: From Months To MillisecondsCLINICAL AND EXPERIMENTAL PHARMACOLOGY AND PHYSIOLOGY, Issue 1-2 2000Gd Funk SUMMARY 1. Motoneurons (MN) shape motor patterns by transforming inputs into action potential output. This transformation, excitability, is determined by an interaction between synaptic inputs and intrinsic membrane properties. Excitability is not static, but changes over multiple time scales. The purpose of the present paper is to review our recent data on synaptic factors important in the dynamic control of MN excitability over time scales ranging from weeks to milliseconds. 2. Developmental changes in modulation of MN excitability are well established. Noradrenergic potentiation of hypoglossal (XII) MN inspiratory activity in rhythmically active medullary slice preparations from rodents increases during the first two postnatal weeks. This is due to increasing ,1 - and ,-adrenoceptor excitatory mechanisms and to a decreasing inhibitory mechanism mediated by ,2 -adrenoceptors. Over a similar period, ATP potentiation of XII inspiratory activity does not change. 3. Motoneuron excitability may also change on a faster time scale, such as between different behaviours or different phases of a behaviour. Examination of this has been confounded by the fact that excitatory synaptic drives underlying behaviour can obscure smaller concurrent changes in excitability. Using the rhythmically active neonatal rat brain-stem,spinal cord preparation, we blocked excitatory inspiratory drive to phrenic MN (PMN) to reveal a reduction in PMN excitability specific to the inspiratory phase that: (i) arises from an inhibitory GABAergic input; (ii) is not mediated by recurrent pathways; and (iii) is proportional to and synchronous with the excitatory inspiratory input. We propose that the proportionality of the concurrent inhibitory and excitatory drives provides a means for phase- specific modulation of PMN gain. 4. Modulation across such diverse time scales emphasizes the active role that synaptic factors play in controlling MN excitability and shaping behaviour. [source] Analysis of multiscale methods for stochastic differential equationsCOMMUNICATIONS ON PURE & APPLIED MATHEMATICS, Issue 11 2005E Weinan We analyze a class of numerical schemes proposed [26] for stochastic differential equations with multiple time scales. Both advective and diffusive time scales are considered. Weak as well as strong convergence theorems are proven. Most of our results are optimal. They in turn allow us to provide a thorough discussion on the efficiency as well as optimal strategy for the method. © 2005 Wiley Periodicals, Inc. [source] | |||||||||||||