New VLBI2010 scheduling strategies and implications on the terrestrial reference frames   总被引：1，自引：1，他引：0  

Jing Sun  Johannes Böhm  Tobias Nilsson  Hana Krásná  Sigrid Böhm  Harald Schuh 《Journal of Geodesy》2014,88(5):449-461

In connection with the work for the next generation VLBI2010 Global Observing System (VGOS) of the International VLBI Service for Geodesy and Astrometry, a new scheduling package (Vie_Sched) has been developed at the Vienna University of Technology as a part of the Vienna VLBI Software. In addition to the classical station-based approach it is equipped with a new scheduling strategy based on the radio sources to be observed. We introduce different configurations of source-based scheduling options and investigate the implications on present and future VLBI2010 geodetic schedules. By comparison to existing VLBI schedules of the continuous campaign CONT11, we find that the source-based approach with two sources has a performance similar to the station-based approach in terms of number of observations, sky coverage, and geodetic parameters. For an artificial 16 station VLBI2010 network, the source-based approach with four sources provides an improved distribution of source observations on the celestial sphere. Monte Carlo simulations yield slightly better repeatabilities of station coordinates with the source-based approach with two sources or four sources than the classical strategy. The new VLBI scheduling software with its alternative scheduling strategy offers a promising option with respect to applications of the VGOS.  相似文献   

Observational evidences of Walker circulation change over the last 30 years contrasting with GCM results     

B. J. Sohn  Sang-Wook Yeh  Johannes Schmetz  Hwan-Jin Song 《Climate Dynamics》2013,40(7-8):1721-1732

In order to examine the changes in Walker circulation over the recent decades, we analyzed the sea surface temperature (SST), deep convective activities, upper tropospheric moistening, sea level pressure (SLP), and effective wind in the boundary layer over the 30-year period of 1979–2008. The analysis showed that the eastern tropical Pacific has undergone cooling while the western Pacific has undergone warming over the past three decades, causing an increase in the east–west SST gradient. It is indicated that the tropical atmosphere should have responded to these SST changes; increased deep convective activities and associated upper tropospheric moistening over the western Pacific ascending region, increased SLP over the eastern Pacific descending region in contrast to decreased SLP over the western Pacific ascending region, and enhanced easterly wind in the boundary layer in response to the SLP change. These variations, recognized from different data sets, occur in tandem with each other, strongly supporting the intensified Walker circulation over the tropical Pacific Ocean. Since the SST trend was attributed to more frequent occurrences of central Pacific-type El Niño in recent decades, it is suggested that the decadal variation of El Niño caused the intensified Walker circulation over the past 30 years. An analysis of current climate models shows that model results deviate greatly from the observed intensified Walker circulation. The uncertainties in the current climate models may be due to the natural variability dominating the forced signal over the tropical Pacific during the last three decades in the twentieth century climate scenario runs by CMIP3 CGCMs.  相似文献   

Assessment of different metrics for physical climate feedbacks     

Daniel Klocke  Johannes Quaas  Bjorn Stevens 《Climate Dynamics》2013,41(5-6):1173-1185

We quantify the feedbacks from the physical climate system on the radiative forcing for idealized climate simulations using four different methods. The results differ between the methods and differences are largest for the cloud feedback. The spatial and temporal variability of each feedback is used to estimate the averaging scale necessary to satisfy the feedback concept of one constant global mean value. We find that the year-to-year variability, combined with the methodological differences, in estimates of the feedback strength from a single model is comparable to the model-to-model spread in feedback strength of the CMIP3 ensemble. The strongest spatial and temporal variability is in the short-wave component of the cloud feedback. In our simulations, where many sources of natural variability are neglected, long-term averages are necessary to get reliable feedback estimates. Considering the large natural variability and relatively small forcing present in the real world, as compared to the forcing imposed by doubling CO₂ concentrations in the simulations, implies that using observations to constrain feedbacks is a challenging task and requires reliable long-term measurements.  相似文献   

Climate-model induced differences in the 21st century global and regional glacier contributions to sea-level rise   总被引：4，自引：0，他引：4  

Rianne H. Giesen  Johannes Oerlemans 《Climate Dynamics》2013,41(11-12):3283-3300

The large uncertainty in future global glacier volume projections partly results from a substantial range in future climate conditions projected by global climate models. This study addresses the effect of global and regional differences in climate input data on the projected twenty-first century glacier contribution to sea-level rise. Glacier volume changes are calculated with a surface mass balance model combined with volume-area scaling, applied to 89 glaciers in different climatic regions. The mass balance model is based on a simplified energy balance approach, with separated contributions by net solar radiation and the combined other fluxes. Future mass balance is calculated from anomalies in air temperature, precipitation and atmospheric transmissivity, taken from eight global climate models forced with the A1B emission scenario. Regional and global sea-level contributions are obtained by scaling the volume changes at the modelled glaciers to all glaciers larger than 0.1 km² outside the Greenland and Antarctic ice sheets. This results in a global value of 0.102 ± 0.028 m (multi-model mean and standard deviation) relative sea-level equivalent for the period 2012–2099, corresponding to 18 ± 5 % of the estimated total volume of glaciers. Glaciers in the Antarctic, Alaska, Central Asia and Greenland together account for 65 ± 4 % of the total multi-model mean projected sea-level rise. The projected sea-level contribution is 35 ± 17 % larger when only anomalies in air temperature are taken into account, demonstrating an important compensating effect by increased precipitation and possibly reduced atmospheric transmissivity. The variability in projected precipitation and atmospheric transmissivity changes is especially large in the Arctic regions, making the sea-level contribution for these regions particularly sensitive to the climate model used. Including additional uncertainties in the modelling procedure and the input data, the total uncertainty estimate for the future projections becomes ±0.063 m.  相似文献   

Drivers of change in Brazil’s carbon dioxide emissions     

Manfred Lenzen  Roberto Schaeffer  Jonas Karstensen  Glen P. Peters 《Climatic change》2013,121(4):815-824

Brazil’s economic development has been underpinned by a diverse and – in a global comparison – unusual set of energy carriers, notably hydroelectricity and ethanol from sugar cane. Its energy mix makes Brazil one of the least energy-related carbon-intensive economies worldwide. Given that the country is fast becoming one of the world’s economic powerhouses, decision-makers need to understand the drivers underlying past and current carbon dioxide emissions trends. We therefore investigate a) which key long-term drivers have led to Brazil’s unique emissions profile, and b) the implications of these drivers for Brazil’s national policies. We show that Brazil’s emissions are growing mainly due to increasing individual standards of living, exports and population size, and that this growth is so far unchallenged by technological and structural improvements toward lower emissions intensities and more efficient production structures. As these trends are likely to continue amidst growing international pressure on key economies to reduce their carbon emissions, a decoupling of drivers from emissions is needed to simultaneously meet development and environmental goals.  相似文献   

How can the ISC location procedures be improved?   总被引：2，自引：0，他引：2  

Johannes Schweitzer 《Physics of the Earth and Planetary Interiors》2006,158(1):19-26

For many decades the International Seismological Centre (ISC) has used a well defined procedure to locate seismic events using first P-onsets and the Jeffreys-Bullen tables ([Jeffreys, H., Bullen, K.E., 1940. Seismological Tables. British Association for the Advancement of Science, Gray Milne Trust, London, 50 pp]) as the travel-time reference. However, during the last two decades, more accurate spherical Earth models have been published and enhanced computer capabilities make it easier to implement more sophisticated data inversion schemes. Several features that may improve the location procedure at the ISC were systematically tested using the location program HYPOSAT. The investigated features were the influence of

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the usage of the spherical Earth models JB, PREM, IASP91, SP6, and AK135;

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the usage of later onsets;

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travel-time corrections for local crustal structure based on CRUST 5.1;

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different weighting of the residuals;

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reducing the amount of defining data at a late stage of the inversion process.

Application of different combinations of these factors led to a reduction of the location errors for the 156 test events, of which the epicenter is known with an accuracy of less than 5 km. However, no clear rule of common factors to achieve this result could be defined. Most promising is the application of AK135 as model for travel-time calculations, applying crust specific station corrections and corrections for the reflection points of surface reflections, a combined usage of surface and core reflections, and removing data which have large residuals or do not much contribute to the solution for the last iterations.  相似文献   

Lake Baikal: Some topical aspects of current research     

Roger J. Flower  David Ryves  Richard W. Battarbee  Johannes Mueller  Michael Sturm 《Journal of Paleolimnology》1999,22(2):223-224

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Subglacial water-escape structures and till structures,sléttjökull,Iceland     

Jaap J. M Van Der Meer  Kurt H. Kjaer  Johannes Krüger 《第四纪科学杂志》1999,14(3):191-205

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Upward continuation of Dome-C airborne gravity and comparison with GOCE gradients at orbit altitude in east Antarctica     

Hasan Yildiz  Rene Forsberg  Carl Christian Tscherning  Daniel Steinhage  Graeme Eagles  Johannes Bouman 《Studia Geophysica et Geodaetica》2017,61(1):53-68

An airborne gravity campaign was carried out at the Dome-C survey area in East Antarctica between the 17th and 22nd of January 2013, in order to provide data for an experiment to validate GOCE satellite gravity gradients. After typical filtering for airborne gravity data, the cross-over error statistics for the few crossing points are 11.3 mGal root mean square (rms) error, corresponding to an rms line error of 8.0 mGal. This number is relatively large due to the rough flight conditions, short lines and field handling procedures used. Comparison of the airborne gravity data with GOCE RL4 spherical harmonic models confirmed the quality of the airborne data and that they contain more high-frequency signal than the global models. First, the airborne gravity data were upward continued to GOCE altitude to predict gravity gradients in the local North-East-Up reference frame. In this step, the least squares collocation using the ITGGRACE2010S field to degree and order 90 as reference field, which is subtracted from both the airborne gravity and GOCE gravity gradients, was applied. Then, the predicted gradients were rotated to the gradiometer reference frame using level 1 attitude quaternion data. The validation with the airborne gravity data was limited to the accurate gradient anomalies (T_XX, T_YY, T_ZZ and T_XZ) where the long-wavelength information of the GOCE gradients has been replaced with GOCO03s signal to avoid contamination with GOCE gradient errors at these wavelengths. The comparison shows standard deviations between the predicted and GOCE gradient anomalies T_XX, T_YY, T_ZZ and T_XZ of 9.9, 11.5, 11.6 and 10.4 mE, respectively. A more precise airborne gravity survey of the southern polar gap which is not observed by GOCE would thus provide gradient predictions at a better accuracy, complementing the GOCE coverage in this region.  相似文献   

Sediment budgeting of short-term backfilling processes: The erosional collapse of a Carolingian canal construction     

Johannes Schmidt  Lukas Werther  Johannes Rabiger-Völlmer  Franz Herzig  Birgit Schneider  Ulrike Werban  Peter Dietrich  Stefanie Berg  Sven Linzen  Peter Ettel  Christoph Zielhofer 《地球表面变化过程与地形》2020,45(14):3449-3462

Sediment budgeting concepts serve as quantification tools to decipher the erosion and accumulation processes within a catchment and help to understand these relocation processes through time. While sediment budgets are widely used in geomorphological catchment-based studies, such quantification approaches are rarely applied in geoarchaeological studies. The case of Charlemagne's summit canal (also known as Fossa Carolina) and its erosional collapse provides an example for which we can use this geomorphological concept and understand the abandonment of the Carolingian construction site. The Fossa Carolina is one of the largest hydro-engineering projects in Medieval Europe. It is situated in Southern Franconia (48.9876°N, 10.9267°E; Bavaria, southern Germany) between the Altmühl and Swabian Rezat rivers. It should have bridged the Central European watershed and connected the Rhine–Main and Danube river systems. According to our dendrochronological analyses and historical sources, the excavation and construction of the Carolingian canal took place in AD 792 and 793. Contemporary written sources describe an intense backfill of excavated sediment in autumn AD 793. This short-term erosion event has been proposed as the principal reason for the collapse and abandonment of the hydro-engineering project. We use subsurface data (drillings, archaeological excavations, and direct-push sensing) and geospatial data (a LiDAR digital terrain model (DTM), a pre-modern DTM, and a 3D model of the Fossa Carolina] for the identification and sediment budgeting of the backfills. Dendrochronological findings and radiocarbon ages of macro remains within the backfills give clear evidence for the erosional collapse of the canal project during or directly after the construction period. Moreover, our quantification approach allows the detection of the major sedimentary collapse zone. The exceedance of the manpower tipping point may have caused the abandonment of the entire construction site. The spatial distribution of the dendrochronological results indicates a north–south direction of the early medieval construction progress. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd  相似文献