| 441. |
- Treat, C., et al.
(författare)
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Timescale dependence of environmental and plant-mediated controls on CH4 flux in a temperate fen.
- 2007
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 112:G01014
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Tidskriftsartikel (refereegranskat)abstract
- This study examined daily, seasonal, and interannual variations in CH4 emissions at a temperate peatland over a 5-year period. We measured net ecosystem CO2 exchange (NEE), CH4 flux, water table depth, peat temperature, and meteorological parameters weekly from the summers (1 May to 31 August) of 2000 through 2004 at Sallie's Fen in southeastern New Hampshire, United States. Significant interannual differences, driven by high variability of large individual CH4 fluxes (ranging from 8.7 to 3833.1 mg CH4 m−2 d−1) occurring in the late summer, corresponded with a decline in water table level and an increase in air and peat temperature. Monthly timescale yielded the strongest correlations between CH4 fluxes and peat and air temperature (r2 = 0.78 and 0.74, respectively) and water table depth (WTD) (r2 = 0.53). Compared to daily and seasonal timescales, the monthly timescale was the best timescale to predict CH4 fluxes using a stepwise multiple regression (r2 = 0.81). Species composition affected relationships between CH4 fluxes and measures of plant productivity, with sedge collars showing the strongest relationships between CH4 flux, water table, and temperature. Air temperature was the only variable that was strongly correlated with CH4 flux at all timescales, while WTD had either a positive or negative correlation depending on timescale and vegetation type. The timescale dependence of controls on CH4 fluxes has important implications for modeling.
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| 442. |
- Tregnaghi, M., et al.
(författare)
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Stochastic determination of entrainment risk in uniformly sized sediment beds at low transport stages : 2. Experiments
- 2012
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 117:4, s. F04005-
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Tidskriftsartikel (refereegranskat)abstract
- Fluvial sediment transport is caused by a complex interaction of interdependent grain and fluid processes many of which are stochastic in nature and cannot be adequately represented by deterministic equations. Random variable analysis has been used previously but limited data are available to describe the variability of grain resistance combined with particle arrangements, and thus validate such analysis. In this study low to medium bed load transport tests were carried out in a flume where sediment movement was monitored using a three-camera 3D PIV system. Simultaneous grain motion and flow velocity measurements were made on a plane located slightly above and parallel to the sediment bed. Detailed statistical velocity information was acquired to model the velocity distribution at the bed level. This was combined with the joint probabilistic distribution of particle exposures and grain resistance to motion, which were obtained from discrete particle modeling (DPM) simulations. DPM simulations were used to provide a stochastic mathematical description of the risk that a stationary particle is entrained by the flow. Predictions from the stochastic model equations replicated the observed pulsation in sediment transport. This demonstrates that it is possible to simulate sediment entrainment and transport at a high resolution by adequately modeling all the sub-processes. A number of flow patterns were identified that caused large fluctuations of the entrainment rate. These all exhibit high velocity flow structures, but they selectively cause the dislodgement of individual particles located at different positions. This selective behavior follows from the variability of the interaction between the near-bed flow and the particles having different exposure.
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| 443. |
- Tregnaghi, M., et al.
(författare)
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Stochastic determination of entrainment risk in uniformly sized sediment beds at low transport stages : 1. Theory
- 2012
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 117:4, s. F04004-
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Tidskriftsartikel (refereegranskat)abstract
- The entrainment of sediments in rivers exhibits an intermittent behavior. Incipient motion should therefore be described as a random process requiring a stochastic predictive approach. The effect of near-bed turbulence on grain entrainment and the variation in bed particle stability due to local surface heterogeneity are included into a probabilistic framework. The intuitive evidence that hydrodynamic forces acting on the sediment bed and the resistance to motion of the bed particles are two mutually dependent aspects of a unique process is modeled by introducing a conditional independence hypothesis. Based on this concept, new insights into the stochastic aspects of incipient motion are obtained. For low ratios of the boundary shear stress to the critical shear stress, by including the mutual dependence of different processes the new theoretical development predicts up to 50% larger probabilities of grain removal from the bed surface compared to Grass' original formulation. This follows from the entrainment risk being not only dependent on the distributions of fluid forces and grain resistance, but also on the correlation that these distributions exhibit in relation to the geometrical configuration of the sediment bed. This complex interaction is neglected in existing probabilistic models of sediment transport. It is then demonstrated that such additional contribution explains better the influence of both flow turbulence and particle arrangement on key features of the overall grain entrainment process.
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| 444. |
- Tunved, Peter, et al.
(författare)
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Aerosol characteristics of air masses in Northern Europe – influences of location, transport, sinks and sources
- 2005
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Ingår i: Journal of Geophysical Research. - : American Physical Union. - 0148-0227 .- 2156-2202. ; 110:D7, s. D07201-
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Tidskriftsartikel (refereegranskat)abstract
- Synoptic-scale air masses at different stations were classified following a definition based on Berliner Wetterkarte. This air mass classification has been related to 1 year of aerosol number size distributions measurements performed at four different stations extending from Aspvreten in Sweden (58.8 degrees N) to Pallas in northern Finland (68 degrees N). The air mass classification describes both class of air mass, based on the origin of the air mass, and character of air in terms of marine, mixed, and continental air masses. The aerosol size distribution properties were evaluated in relation to the air masses. Emphasis was put on the differences between marine, mixed, and continental character air masses. It is shown that continental air masses exceed marine and mixed character air masses both in number and mass concentration. Different classes of air masses were further associated with different aerosol size distribution properties. It is also shown that although serving as a somewhat good qualifier for the aerosol at individual stations, the air mass classification cannot be used to estimate the aerosol burden over large geographical areas. Instead, a sharp gradient was shown to exist between different stations, although aerosol properties were observed in equal air masses according to the definition by Berliner Wetterkarte. This gradient manifests as a south-northerly decrease in aerosol total number and volume, indicating that the aerosol properties including the aerosol size distribution are less conservative than the thermodynamic properties (e.g., pseudo-potential temperature and humidity profiles) that characterize the different air masses. Further, using a pseudo-Lagrangian approach, the aerosol turnover time was estimated for different sized aerosols in air moving from south to north (i.e., depletion of aerosols in air arriving from the continent). Turnover time of Aitken particles was found to be in the range of 1-2 days, while accumulation mode turnover time was estimated to be in the order of 2-3 days
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| 445. |
- Urban, Joachim, 1964, et al.
(författare)
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Odin/SMR Limb Observations of Stratospheric Trace Gases: Level 2 Processing of ClO, N2O, O3, and HNO3
- 2005
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 110:D14, s. 1-20
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Tidskriftsartikel (refereegranskat)abstract
- The Sub-Millimetre Radiometer (SMR) on board the Odin satellite, launched on 20 February 2001, observes key species with respect to stratospheric chemistry and dynamics such as O-3, ClO, N2O, and HNO3 using two bands centered at 501.8 and 544.6 GHz. We present the adopted methodology for level 2 processing and the achieved in-orbit measurement capabilities of the SMR radiometer for these species in terms of altitude range, altitude resolution, and measurement precision. The characteristics of the relevant level 2 data versions, namely version 1.2 of the operational processor as well as versions 222 and 223 of the reference code, are discussed and differences are evaluated. An analysis of systematic retrieval errors, resulting from spectroscopic and instrumental uncertainties, is also presented.
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| 446. |
- Urban, Joachim, 1964, et al.
(författare)
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Odin/SMR limb observations of stratospheric trace gases: Validation of N2O
- 2005
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 110:9, s. D09301-20
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Tidskriftsartikel (refereegranskat)abstract
- The Sub-Millimetre Radiometer (Odin/SMR) on board the Odin satellite, launched on 20 February 2001, performs regular measurements of the global distribution of stratospheric nitrous oxide (N2O) using spectral observations of the J = 20→ 19 rotational transition centered at 502.296 GHz. We present a quality assessment for the retrieved N2O profiles (level 2 product) by comparison with independent balloonborne and aircraftborne validation measurements as well as by cross-comparing with preliminary results from other satellite instruments. An agreement with the airborne validation experiments within 28 ppbv in terms of the root mean square (RMS) deviation is found for all SMR data versions (v222, v223, and v1.2) under investigation. More precisely, the agreement is within 19 ppbv for N2O volume mixing ratios (VMR) lower than 200 ppbv and within 10% for mixing ratios larger than 150 ppbv. Given the uncertainties due to atmospheric variability inherent to such comparisons, these values should be interpreted as upper limits for the systematic error of the Odin/SMR N2O measurements. Odin/SMR N2O mixing ratios are systematically slightly higher than nonvalidated data obtained from the Improved Limb Atmospheric Spectrometer-II (ILAS-II) on board the Advanced Earth Observing Satellite-II (ADEOS-II). Root mean square deviations are generally within 23 ppbv (or 20% for VMR-N2O > 100 ppbv) for versions 222 and 223. The comparison with data obtained from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on the Envisat satellite yields a good agreement within 9-17 ppbv (or 10% for VMR-N2O > 100 ppbv) for the same data versions. Odin/SMR version 1.2 data show somewhat larger RMS deviations and a higher positive bias.
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| 447. |
- Usoskin, Ilya G., et al.
(författare)
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Short-term production and synoptic influences on atmospheric Be-7 concentrations
- 2009
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 114, s. D06108-
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Tidskriftsartikel (refereegranskat)abstract
- Variations of the cosmogenic radionuclide Be-7 in the global atmosphere are driven by cooperation of processes of its production, air transports, and removal. We use a combination of the Goddard Institute for Space Studies ModelE and the OuluCRAC: Be-7 production model to simulate the variations in the Be-7 concentration in the atmosphere for the period from 1 January to 28 February 2005. This period features significant synoptic variability at multiple monitoring stations around the globe and spans an extreme solar energetic particle (SEP) event that occurred on 20 January. Using nudging from observed horizontal winds, the model correctly reproduces the overall level of the measured Be-7 concentration near ground and a great deal of the synoptic variability at timescales of 4 days and longer. This verifies the combined model of production and transport of the Be-7 radionuclide in the atmosphere. The impact of an extreme SEP event of January 2005 is seen dramatically in polar stratospheric Be-7 concentration but is small near the surface (about 2%) and indistinguishable given the amount of intrinsic variability and the uncertainties of the surface observations.
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| 448. |
- Vaivads, A, et al.
(författare)
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Energy transport during O+ energization by ELF waves observed by the Freja satellite
- 1999
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 104:A2, s. 2563-2572
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Tidskriftsartikel (refereegranskat)abstract
- We present a study of strong ELF waves correlated with Of ion heating. We investigate the kinetic energy flux of oxygen ions and electrons parallel to the ambient magnetic field and the Poynting flux along the magnetic field of waves in different frequency regions. We show that the Poynting flux of lower hybrid waves and waves with frequencies below the oxygen gyrofrequency is downward but much less than the upgoing oxygen ion energy flux. The Poynting flux of ELF waves with frequencies at about half the local proton gyrofrequency is also downward, and it is comparable to, or larger, than the upward oxygen energy flux. Our results are consistent with a scenario where ELF waves with frequencies around half the proton gyrofrequency are generated above the satellite and transfer energy to oxygen ions in a region below the satellite.
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| 449. |
- van Camp, Michel, et al.
(författare)
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Repeated absolute gravity measurements for monitoring slow intraplate vertical deformation in Western Europe
- 2011
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 116:B8, s. B08402-
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Tidskriftsartikel (refereegranskat)abstract
- In continental plate interiors, ground surface movements are at the limit of the noise level and close to or below the accuracy of current geodetic techniques. Absolute gravity measurements are valuable to quantify slow vertical movements, as this instrument is drift free and, unlike GPS, independent of the terrestrial reference frame. Repeated absolute gravity (AG) measurements have been performed in Oostende (Belgian coastline) and at 8 stations along a southwest-northeast profile across the Belgian Ardennes and the Roer Valley Graben (Germany), in order to estimate the tectonic deformations in the area. The AG measurements, repeated once or twice a year, can resolve elusive gravity changes with a precision better than 3.7 nm/s²/yr (95% confidence interval) after 11 years, even in difficult conditions. After 8-15 years (depending on the station), we find that the gravity rates of change lie in the [-3.1, 8.1] nm/s²/yr interval and result from a combination of anthropogenic, climatic, tectonic, and Glacial Isostatic Adjustment (GIA) effects. After correcting for the GIA, the inferred gravity rates and consequently, the vertical land movements, reduce to zero within the uncertainty level at all stations except Jülich (due to man-induced subsidence) and Sohier (possibly, an artifact due to the shortness of the time series at that station).
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| 450. |
- Vapirev, A. E., et al.
(författare)
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Formation of a transient front structure near reconnection point in 3-D PIC simulations
- 2013
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Ingår i: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202 .- 2169-9380. ; 118:4, s. 1435-1449
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Tidskriftsartikel (refereegranskat)abstract
- Massively parallel numerical simulations of magnetic reconnection are presented in this study. Electromagnetic full-particle implicit code iPIC3D is used to study the dynamics and 3-D evolution of reconnection outflows. Such features as Hall magnetic field, inflow and outflow, and diffusion region formation are very similar to 2-D particle-in-cell (PIC) simulations. In addition, it is well known that instabilities develop in the current flow direction or oblique directions. These modes could provide for anomalous resistivity and diffusive drag and can serve as additional proxies for magnetic reconnection. In our work, the unstable evolution of reconnection transient front structures is studied. Reconnection configuration in the absence of guide field is considered, and it is initialized with a localized perturbation aligned in the cross-tail direction. Our study suggests that the instabilities lead to the development of finger-like density structures on ion-electron hybrid scales. These structures are characterized by a rapid increase of the magnetic field, normal to the current sheet (Bz). A small decrease in the magnetic field component parallel to the reconnection X line and the component perpendicular to the current sheet is observed in the region ahead of the front. The instabilities form due to fact that the density gradient inside the front region is opposite to the direction of the acceleration Lorentz force. Such density structures may possibly further develop into larger-scale earthward flux transfer events during magnetotail reconnection. In addition, oscillations mainly in the magnetic and electric fields and the electron density are observed shortly before the arrival of the main front structure which is consistent with recent THEMIS observations. Key PointsThree dimensional particle-in-cell simulation of reconnection in the magnetotailEvolution of dipolarization front at reconnection and associated plasma flowDevelopment of instabilities in the plasma population
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