The Venus ground-based image Active Archive: A database of amateur observations of Venus in ultraviolet and infrared light     

Geert Barentsen  Detlef Koschny 《Planetary and Space Science》2008,56(10):1444-1449

The Venus ground-based image Active Archive is an online database designed to collect ground-based images of Venus in such a way that they are optimally useful for science. The Archive was built to support ESA's Venus Amateur Observing Project, which utilizes the capabilities of advanced amateur astronomers to collect filtered images of Venus in ultraviolet, visible and near-infrared light. These images complement the observations of the Venus Express spacecraft, which cannot continuously monitor the northern hemisphere of the planet due to its elliptical orbit with apocenter above the south pole. We present the first set of observations available in the Archive and assess the useability of the data set for scientific purposes.  相似文献   

Modelling wind‐blown sediment transport around single vegetation elements     

Jakolien K. Leenders  Geert Sterk  John H. Van Boxel 《地球表面变化过程与地形》2011,36(9):1218-1229

Wind erosion is an important soil erosion and hence a soil degradation problem in the Sahelian zone of West Africa. Potentially, the characteristic dryland vegetation with scattered trees and shrubs can provide for soil erosion protection from wind erosion, but so far adequate quantification of vegetation impacts is lacking. The aim of this study was to develop a model of wind‐blown soil erosion and sediment transport around a single shrub‐type vegetation element. Starting with the selection of a suitable transport equation from four possible sediment transport equations, the effects of a single vegetation element on wind speed were parameterized. The modified wind speed was then applied to a sediment transport equation to model the change in sediment mass flux around a shrub. The model was tested with field data on wind speed and sediment transport measured around isolated shrubs in a farmer's field in the north of Burkina Faso. The simple empirical equation of Radok (Journal of Glaciology 19 : 123–129, 1977) performed best in modelling soil erosion and sediment transport, both for the entire event duration and for each minute within an event. Universal values for the empirical constants in the sediment transport equation could not be obtained because of the large variability in soil and roughness characteristics. The pattern of wind speed, soil erosion and sediment transport behind a shrub and on either side of it was modelled. The wind speed changed in the lee of the vegetation element depending on its porosity, height and downwind position. Wind speed was recovered to the upstream speed at a downwind distance of 7·5 times the height of the shrub. The variability in wind direction created a ‘rotating’ area of influence around the shrub. Compared to field measurements the model predicted an 8% larger reduction in sediment transport in the lee of the vegetation element, and a 22% larger increase beside the vegetation element. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

Evaluation of digital elevation models for delineation of hydrological response units in a Himalayan watershed     

Sameer Saran  Geert Sterk  Piet Peters  Vinay Kumar Dadhwal 《国际地球制图》2013,28(2):105-122

This study reports results from evaluation of the quality of digital elevation model (DEM) from four sources viz. topographic map (1:50,000), Shuttle Radar Topographic Mission (SRTM) (90 m), optical stereo pair from ASTER (15 m) and CARTOSAT (2.5 m) and their use in derivation of hydrological response units (HRUs) in Sitla Rao watershed (North India). The HRUs were derived using water storage capacity and slope to produce surface runoff zones. The DEMs were evaluated on elevation accuracy and representation of morphometric features. The DEM derived from optical stereo pairs (ASTER and CARTOSAT) provided higher vertical accuracies than the SRTM and topographic map-based DEM. The SRTM with a coarse resolution of 90 m provided vertical accuracy but better morphometry compared to topographic map. The HRU maps derived from the fine resolution DEM (ASTER and CARTOSAT) were more detailed but did not provide much advantage for hydrological studies at the scale of Sitla Rao watershed (5800 ha).  相似文献   

An inter-comparison of regional climate models for Europe: model performance in present-day climate   总被引：3，自引：0，他引：3  

Daniela Jacob  Lars Bärring  Ole Bøssing Christensen  Jens Hesselbjerg Christensen  Manuel de Castro  Michel Déqué  Filippo Giorgi  Stefan Hagemann  Martin Hirschi  Richard Jones  Erik Kjellström  Geert Lenderink  Burkhardt Rockel  Enrique Sánchez  Christoph Schär  Sonia I. Seneviratne  Samuel Somot  Aad van Ulden  Bart van den Hurk 《Climatic change》2007,81(1):31-52

The analysis of possible regional climate changes over Europe as simulated by 10 regional climate models within the context of PRUDENCE requires a careful investigation of possible systematic biases in the models. The purpose of this paper is to identify how the main model systematic biases vary across the different models. Two fundamental aspects of model validation are addressed here: the ability to simulate (1) the long-term (30 or 40 years) mean climate and (2) the inter-annual variability. The analysis concentrates on near-surface air temperature and precipitation over land and focuses mainly on winter and summer. In general, there is a warm bias with respect to the CRU data set in these extreme seasons and a tendency to cold biases in the transition seasons. In winter the typical spread (standard deviation) between the models is 1 K. During summer there is generally a better agreement between observed and simulated values of inter-annual variability although there is a relatively clear signal that the modeled temperature variability is larger than suggested by observations, while precipitation variability is closer to observations. The areas with warm (cold) bias in winter generally exhibit wet (dry) biases, whereas the relationship is the reverse during summer (though much less clear, coupling warm (cold) biases with dry (wet) ones). When comparing the RCMs with their driving GCM, they generally reproduce the large-scale circulation of the GCM though in some cases there are substantial differences between regional biases in surface temperature and precipitation.  相似文献   

Long-chain monounsaturated fatty acids as biomarkers for the dispersal of organic waste from a fish enclosure     

Van Biesen G  Parrish CC 《Marine environmental research》2005,60(3):375-388

Sedimentation of food particles and fecal pellets under fish pens can lead to organic enrichment of the sediment. This study looked at the potential of lipid biomarkers as an indicator for the dispersal of organic waste from a fish enclosure. The lipid class and fatty acid composition of settling particles collected by sediment traps on a transect away from a cod enclosure was determined. The proportions of free fatty acids and of three long-chain monounsaturated fatty acids close to the fish pens were significantly higher (p<0.05) than before the fish enclosure was in operation and decreased moving away from the pens. This indicated a higher contribution of feces and/or uneaten feed to the settling particles at this location. Free fatty acids are the major lipid class in feces. Long-chain monounsaturated fatty acids are abundant in the feed and are also found in high proportions in feces because of their lower digestibility compared to other fatty acids.  相似文献   

Timing of erosion and satellite data: A multi-resolution approach to soil erosion risk mapping     

Anton Vrieling  Steven M. de Jong  Geert Sterk  Silvio C. Rodrigues 《International Journal of Applied Earth Observation and Geoinformation》2008

Erosion reduces soil productivity and causes negative downstream impacts. Erosion processes occur on areas with erodible soils and sloping terrain when high-intensity rainfall coincides with limited vegetation cover. Timing of erosion events has implications on the selection of satellite imagery, used to describe spatial patterns of protective vegetation cover. This study proposes a method for erosion risk mapping with multi-temporal and multi-resolution satellite data. The specific objectives of the study are: (1) to determine when during the year erosion risk is highest using coarse-resolution data, and (2) to assess the optimal timing of available medium-resolution images to spatially represent vegetation cover during the high erosion risk period. Analyses were performed for a 100-km² pasture area in the Brazilian Cerrados. The first objective was studied by qualitatively comparing three-hourly TRMM rainfall estimates with MODIS NDVI time series for one full year (August 2002–August 2003). November and December were identified as the months with highest erosion risk. The second objective was examined with a time series of six available ASTER images acquired in the same year. Persistent cloud cover limited image acquisition during high erosion risk periods. For each ASTER image the NDVI was calculated and classified into five equally sized classes. Low NDVI was related to high erosion risk and vice versa. A DEM was used to set approximately flat zones to very low erosion risk. The six resulting risk maps were compared with erosion features, visually interpreted from a fine-resolution QuickBird image. Results from the October ASTER image gave highest accuracy (84%), showing that erosion risk mapping in the Brazilian Cerrados can best be performed with images acquired shortly before the first erosion events. The presented approach that uses coarse-resolution temporal data for determining erosion periods and medium-resolution data for effective erosion risk mapping is fast and straightforward. It shows good potential for successful application in other areas with high spatial and temporal variability of vegetation cover.  相似文献   

The first multi-model ensemble of regional climate simulations at kilometer-scale resolution part 2: historical and future simulations of precipitation     

Pichelli  Emanuela  Coppola  Erika  Sobolowski  Stefan  Ban  Nikolina  Giorgi  Filippo  Stocchi  Paolo  Alias  Antoinette  Belu&#;i&#;  Danijel  Berthou  Segolene  Caillaud  Cecile  Cardoso  Rita M.  Chan  Steven  Christensen  Ole B&#;ssing  Dobler  Andreas  de Vries  Hylke  Goergen  Klaus  Kendon  Elizabeth J.  Keuler  Klaus  Lenderink  Geert  Lorenz  Torge  Mishra  Aditya N.  Panitz  Hans-Juergen  Sch&#;r  Christoph  Soares  Pedro M. M.  Truhetz  Heimo  Vergara-Temprado  Jesus 《Climate Dynamics》2021,56(11):3581-3602

This paper presents the first multi-model ensemble of 10-year, “convection-permitting” kilometer-scale regional climate model (RCM) scenario simulations downscaled from selected CMIP5 GCM projections for historical and end of century time slices. The technique is to first downscale the CMIP5 GCM projections to an intermediate 12–15 km resolution grid using RCMs, and then use these fields to downscale further to the kilometer scale. The aim of the paper is to provide an overview of the representation of the precipitation characteristics and their projected changes over the greater Alpine domain within a Coordinated Regional Climate Downscaling Experiment Flagship Pilot Study and the European Climate Prediction system project, tasked with investigating convective processes at the kilometer scale. An ensemble of 12 simulations performed by different research groups around Europe is analyzed. The simulations are evaluated through comparison with high resolution observations while the complementary ensemble of 12 km resolution driving models is used as a benchmark to evaluate the added value of the convection-permitting ensemble. The results show that the kilometer-scale ensemble is able to improve the representation of fine scale details of mean daily, wet-day/hour frequency, wet-day/hour intensity and heavy precipitation on a seasonal scale, reducing uncertainty over some regions. It also improves the representation of the summer diurnal cycle, showing more realistic onset and peak of convection. The kilometer-scale ensemble refines and enhances the projected patterns of change from the coarser resolution simulations and even modifies the sign of the precipitation intensity change and heavy precipitation over some regions. The convection permitting simulations also show larger changes for all indices over the diurnal cycle, also suggesting a change in the duration of convection over some regions. A larger positive change of frequency of heavy to severe precipitation is found. The results are encouraging towards the use of convection-permitting model ensembles to produce robust assessments of the local impacts of future climate change.

  相似文献   

Seismic risk assessment for the mega-city of Istanbul: Ductility, strength and maximum interstory drift demands     

Pelin Gundes Bakir  Guido De Roeck  Geert Degrande  Kevin K.F. Wong 《Soil Dynamics and Earthquake Engineering》2007,27(12):1101-1117

This study investigates the seismic demands due to the catastrophic 1999 M_w=7.4 Kocaeli and the M_w=7.1 Duzce earthquakes. The inelastic response spectra for the Kocaeli and Duzce earthquakes are investigated for systems with known strength and ductility. An analytical fiber element model is developed for a typical reinforced concrete building in Turkey. The interstory drifts are calculated from nonlinear dynamic analysis using 26 recorded strong-motion data from the 1999 Kocaeli and the Duzce earthquakes. In the dynamic analysis, the structural members are modeled by employing distributed plasticity fiber elements and both geometrical as well as material nonlinearities are taken into account. This study shows that the ductility and interstory drift demands due to the Kocaeli and the Duzce earthquakes were very severe (well above the code prescribed values) even for moderately inelastic structures. It is apparent from the results that the forward directivity effect is the most influential factor on the interstory drift demand. Both the distance to the fault rupture and the site conditions affect seismic demands, but the site conditions and the local topography are more influential than the distance from the fault rupture. This study shows that substantial damage should be expected in a future earthquake at all districts of Istanbul, but especially at Avcilar, Cekmece, Fatih, Bakirkoy and the Zeytinburnu districts. It is also shown that base isolation may substantially improve the performance of a structure in the inelastic domain and base isolated structures may be designed for lower minimum lateral strengths and higher strength reduction factors.  相似文献   

Research Note: Sparsity‐Aware Multiple Microseismic Event Localization Blind to the Source Time‐Function          下载免费PDF全文

Zijian Tang  Xander Campman  Alexander Droujinine  Geert Leus 《Geophysical Prospecting》2015,63(1):70-77

We consider the problem of simultaneously estimating three parameters of multiple microseimic events, i.e., the hypocenter, moment tensor, and origin time. This problem is of great interest because its solution could provide a better understanding of reservoir behavior and can help to optimize the hydraulic fracturing process. The existing approaches employing spatial source sparsity have advantages over traditional full‐wave inversion‐based schemes; however, their validity and accuracy depend on the knowledge of the source time‐function, which is lacking in practical applications. This becomes even more challenging when multiple microseimic sources appear simultaneously. To cope with this shortcoming, we propose to approach the problem from a frequency‐domain perspective and develop a novel sparsity‐aware framework that is blind to the source time‐function. Through our simulation results with synthetic data, we illustrate that our proposed approach can handle multiple microseismic sources and can estimate their hypocenters with an acceptable accuracy. The results also show that our approach can estimate the normalized amplitude of the moment tensors as a by‐product, which can provide worthwhile information about the nature of the sources.  相似文献   

SST and circulation trend biases cause an underestimation of European precipitation trends   总被引：2，自引：0，他引：2  

Ronald van Haren  Geert Jan van Oldenborgh  Geert Lenderink  Matthew Collins  Wilco Hazeleger 《Climate Dynamics》2013,40(1-2):1-20

Clear precipitation trends have been observed in Europe over the past century. In winter, precipitation has increased in north-western Europe. In summer, there has been an increase along many coasts in the same area. Over the second half of the past century precipitation also decreased in southern Europe in winter. An investigation of precipitation trends in two multi-model ensembles including both global and regional climate models shows that these models fail to reproduce the observed trends. In many regions the model spread does not cover the trend in the observations. In contrast, regional climate model (RCM) experiments with observed boundary conditions reproduce the observed precipitation trends much better. The observed trends are largely compatible with the range of uncertainties spanned by the ensemble, indicating that the boundary conditions of RCMs are responsible for large parts of the trend biases. We find that the main factor in setting the trend in winter is atmospheric circulation, for summer sea surface temperature (SST) is important in setting precipitation trends along the North Sea and Atlantic coasts. The causes of the large trends in atmospheric circulation and summer SST are not known. For SST there may be a connection with the well-known ocean circulation biases in low-resolution ocean models. A quantitative understanding of the causes of these trends is needed so that climate model based projections of future climate can be corrected for these precipitation trend biases.  相似文献