Observations in a sediment-laden flow by use of laser-doppler velocimetry

The laser-Doppler velocimetry technique was adapted for use in sediment-laden flows. The developed instrumentation was used to make one-dimensional, instantaneous measurements of both fluid and sediment grain velocities throughout the water column in such a flow. The velocimetry results were obtained in a steady, uniform flow over a natural sediment bed in the high-transport, flat bed regime. Laser-Doppler velocimetry is particularly attractive for use in sediment-laden flows as no calibration is required and no probe is introduced into the flow field. Measurements of the fluid velocity and the occurrence and velocity of individual sediment grains are possible with the instrumentation developed in this study. The major difficulties encountered are the possible conditional sampling, hence possible biasing, of the fluid velocity data and the failure of the instrumentation to record or resolve individual sediment grains at higher sediment transport rates. The instrumentation employed in this study is still in the developmental stages and suggestions for its improvement are given. Despite the difficulties encountered, the data obtained in this study give some insights into the mechanics of suspension and entrainment of sediment during transport by water. The longitudinal turbulence intensity does not seem to be significantly affected by the presence of suspended sediment; the turbulence intensities observed in the sediment-laden flow of this study do not differ greatly from the values reported by previous investigators for clear fluid flows. The mean and standard deviation of the sediment grain velocity were observed to be less than those for the fluid velocity in the lower portion of the flow, but respectively greater near the water surface. The data demonstrate the shortcoming of the continuum approach to the mechanics of the suspension on sediment. The length (or time) scales of the fluid turbulence are smaller than the length (or time) scale of a set of sediment grains required to define suspended sediment concentration. Near the water surface, where the velocimeter acts as a grain counter, the probability density functions of the sediment grain inter-arrival times, the time between the detection of successive sediment grains, were observed to be negative exponentials. The transport of individual sediment grains might be modeled as a Poisson process. This work is the foundation of an ongoing experimental program of direct measurements of the fine-scale, time-fluctuating characteristics of sediment-laden flows. This study developed and implemented instrumentation capable of making such measurements and established a conceptual framework for the subsequent interpretation of the data obtained. Two-dimensional measurements, with improved instrumentation, will give additional insights into the mechanics of sediment transport

Deriving Daytime Variables From the AmeriFlux Standard Eddy Covariance Data Set

A gap-filled, quality assessed eddy covariance dataset has recently become available for the AmeriFluxnetwork. This dataset uses standard processing and produces commonly used science variables. This shared dataset enables robust comparisons across different analyses. Of course, there are many remaining questions. One of those is how to define 'during the day' which is an important concept for many analyses. Some studies have used local time ?for example 9am to 5pm; others have used thresholds on photosynthetic active radiation (PAR). A related question is how to derive quantities such as the Bowen ratio. Most studies compute the ratio of the averages of the latent heat (LE) and sensible heat (H). In this study, we use different methods of defining 'during the day' for GPP, LE, and H. We evaluate the differences between methods in two ways. First, we look at a number of statistics of GPP. Second, we look at differences in the derived Bowen ratio. Our goal is not science per se, but rather informatics in support of the science

Fault Tolerance and Scaling in e-Science Cloud Applications: Observations from the Continuing Development of MODISAzure

It can be natural to believe that many of the traditional issues of scale have been eliminated or at least greatly reduced via cloud computing. That is, if one can create a seemingly wellfunctioning cloud application that operates correctly on small or moderate-sized problems, then the very nature of cloud programming abstractions means that the same application will run as well on potentially significantly larger problems. In this paper, we present our experiences taking MODISAzure, our satellite data processing system built on the Windows Azure cloud computing platform, from the proof-of-concept stage to a point of being able to run on significantly larger problem sizes (e.g., from national-scale data sizes to global-scale data sizes). To our knowledge, this is the longest-running eScience application on the nascent Windows Azure platform. We found that while many infrastructure-level issues were thankfully masked from us by the cloud infrastructure, it was valuable to design additional redundancy and fault-tolerance capabilities such as transparent idempotent task retry and logging to support debugging of user code encountering unanticipated data issues. Further, we found that using a commercial cloud means anticipating inconsistent performance and black-box behavior of virtualized compute instances, as well as leveraging changing platform capabilities over time. We believe that the experiences presented in this paper can help future eScience cloud application developers on Windows Azure and other commercial cloud providers

Fluxnet Synthesis Dataset Collaboration Infrastructure

The Fluxnet synthesis dataset originally compiled for the La Thuile workshop contained approximately 600 site years. Since the workshop, several additional site years have been added and the dataset now contains over 920 site years from over 240 sites. A data refresh update is expected to increase those numbers in the next few months. The ancillary data describing the sites continues to evolve as well. There are on the order of 120 site contacts and 60proposals have been approved to use thedata. These proposals involve around 120 researchers. The size and complexity of the dataset and collaboration has led to a new approach to providing access to the data and collaboration support and the support team attended the workshop and worked closely with the attendees and the Fluxnet project office to define the requirements for the support infrastructure. As a result of this effort, a new website (http://www.fluxdata.org) has been created to provide access to the Fluxnet synthesis dataset. This new web site is based on a scientific data server which enables browsing of the data on-line, data download, and version tracking. We leverage database and data analysis tools such as OLAP data cubes and web reports to enable browser and Excel pivot table access to the data

Integration of MODIS land and atmosphere products with a coupled-process model to estimate gross primary productivity and evapotranspiration from 1 km to global scales

We propose the Breathing Earth System Simulator (BESS), an upscaling approach to quantify global gross primary productivity and evapotranspiration using MODIS with a spatial resolution of 1-5 km and a temporal resolution of 8 days. This effort is novel because it is the first system that harmonizes and utilizes MODIS Atmosphere and Land products on the same projection and spatial resolution over the global land. This enabled us to use the MODIS Atmosphere products to calculate atmospheric radiative transfer for visual and near infrared radiation wave bands. Then we coupled atmospheric and canopy radiative transfer processes, with models that computed leaf photosynthesis, stomatal conductance and transpiration on the sunlit and shaded portions of the vegetation and soil. At the annual time step, the mass and energy fluxes derived from BESS showed strong linear relations with measurements of solar irradiance (r(2) = 0.95, relative bias: 8%), gross primary productivity (r(2) = 0.86, relative bias: 5%) and evapotranspiration (r(2) = 0.86, relative bias: 15%) in data from 33 flux towers that cover seven plant functional types across arctic to tropical climatic zones. A sensitivity analysis revealed that the gross primary productivity and evapotranspiration computed in BESS were most sensitive to leaf area index and solar irradiance, respectively. We quantified the mean global terrestrial estimates of gross primary productivity and evapotranpiration between 2001 and 2003 as 118 +/- 26 PgC yr(-1) and 500 +/- 104 mm yr(-1) (equivalent to 63,000 +/- 13,100 km(3) yr(-1)), respectively. BESS-derived gross primary productivity and evapotranspiration estimates were consistent with the estimates from independent machine-learning, data-driven products, but the process-oriented structure has the advantage of diagnosing sensitivity of mechanisms. The process-based BESS is able to offer gridded biophysical variables everywhere from local to the total global land scales with an 8-day interval over multiple years.Keywords: Energy fluxes, Water vapor exchange, Foliage clumping index, Carbon dioxide, Stomal conductance, Net ecosystem exchange, Sitchensis bong carr, Radiative transfer, Oak hickory forest, Leaf area inde

The FLUXNET2015 dataset and the ONEFlux processing pipeline for eddy covariance data

The FLUXNET2015 dataset provides ecosystem-scale data on CO2, water, and energy exchange between the biosphere and the atmosphere, and other meteorological and biological measurements, from 212 sites around the globe (over 1500 site-years, up to and including year 2014). These sites, independently managed and operated, voluntarily contributed their data to create global datasets. Data were quality controlled and processed using uniform methods, to improve consistency and intercomparability across sites. The dataset is already being used in a number of applications, including ecophysiology studies, remote sensing studies, and development of ecosystem and Earth system models. FLUXNET2015 includes derived-data products, such as gap-filled time series, ecosystem respiration and photosynthetic uptake estimates, estimation of uncertainties, and metadata about the measurements, presented for the first time in this paper. In addition, 206 of these sites are for the first time distributed under a Creative Commons (CC-BY 4.0) license. This paper details this enhanced dataset and the processing methods, now made available as open-source codes, making the dataset more accessible, transparent, and reproducible.Peer reviewe