| 461. |
- Wallin, Marcus, et al.
(author)
-
Temporal and spatial variability of dissolved inorganic carbon in a boreal stream network: Concentrations and downstream fluxes
- 2010
-
In: Journal of Geophysical Research: Biogeosciences. - 2169-8953 .- 2169-8961. ; 115, s. G02014-
-
Journal article (peer-reviewed)abstract
- Carbon dioxide (CO(2)) and dissolved inorganic carbon (DIC) concentrations and export were analyzed throughout a 67 km(2) boreal stream network in northern Sweden. 700 DIC and CO(2) samples from 14 subcatchments were collected in 2006 and 2007. All sites were consistently supersaturated in CO(2) with respect to the atmosphere. Temporal variability of DIC and CO(2) concentration was best correlated with discharge, with concentrations generally diluting at high discharge. However, the variability in CO(2) concentration was also dependent on the specific pH range of the stream, as variability was greatest in acidic headwater streams and lowest in larger circumneutral streams. In the larger ones the increase in the CO(2) proportion of DIC at increased discharge counteracts the dilution of CO(2). The shift toward proportionally more CO(2) of the DIC at higher discharge is caused by decline in pH. Spatial patterns showed that DIC and CO(2) concentrations were best correlated with peatland coverage of the subcatchment. The highest concentrations were found in headwater streams draining peatlands. The downstream export of DIC from the catchment outlet constitutes 19% of the total downstream export of carbon (DIC + DOC), or 0.7 (+/-0.09) g C m(-2) yr(-1). This study demonstrates the importance of including fluvial fluxes of inorganic carbon in landscape carbon budgets via runoff, and also highlights the need to account for stream evasion of CO(2) to the atmosphere in such estimates since it can be larger than the downstream DIC export.
|
|
| 462. |
- Walter, Jacob I., et al.
(author)
-
Transient slip events from near-field seismic and geodetic data on a glacier fault, Whillans Ice Plain, West Antarctica
- 2011
-
In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 116:F1, s. F01021-
-
Journal article (peer-reviewed)abstract
- Bidaily, tidally modulated stick-slip speed-ups of the Whillans Ice Plain (WIP) provide insight into glacier dynamics and failure at a naturally repeating fault asperity. We installed a network of continuously operating GPS receivers in 2007 and deployed on-ice broadband seismometers during the austral summer of 2008 on Whillans Ice Stream (WIS), West Antarctica, and recorded 26 glacier speed-up events. Previous work during the 2004 field season suggested that these speed-ups initiate as failure of an asperity on or near "ice raft A" that triggers rupture across the entire WIP. Our results for 2008 locate the slip initiation farther to the south of this feature, closer to the grounding line and the southernmost extent of the Ross Ice Shelf. The initiation may be controlled by a discontinuity in basal boundary conditions at the suture between two ice streams. A strong correlation between the amplitude of seismic waves generated at the rupture front and the total slip achieved over the duration of the slip event (similar to 30 min) suggests slip-predictable behavior, i.e., the ability to forecast the eventual slip based on the first minute of seismic radiation. Successive slip events propagate with different rupture speeds (100-300 m/s) that strongly correlate (R-2 = 0.73) with the interevent duration. In addition, the amount of slip achieved during each event appears to be correlated with the rupture speed. We use these observations to constrain basal shear stress to be 4 kPa by calculating conditions for basal freezing. Our observations yield information regarding mechanics and dynamics of ice streambeds at the scale of tens to hundreds of kilometers. Subglacial processes are notoriously difficult to constrain on these large scales, which are relevant to the understanding of regional and continental ice motion.
|
|
| 463. |
- Wang, L., et al.
(author)
-
Improving snow process modeling with satellite-based estimation of near-surface-air-temperature lapse rate
- 2016
-
In: Journal of Geophysical Research - Atmospheres. - 0148-0227 .- 2156-2202. ; 121:20
-
Journal article (peer-reviewed)abstract
- In distributed hydrological modeling, surface air temperature (Tair) is of great importance in simulating cold region processes, while the near-surface-air-temperature lapse rate (NLR) is crucial to prepare Tair (when interpolating Tair from site observations to model grids). In this study, a distributed biosphere hydrological model with improved snow physics (WEB-DHM-S) was rigorously evaluated in a typical cold, large river basin (e.g., the upper Yellow River basin), given a mean monthly NLRs. Based on the validated model, we have examined the influence of the NLR on the simulated snow processes and streamflows. We found that the NLR has a large effect on the simulated streamflows, with a maximum difference of greater than 24% among the various scenarios for NLRs considered. To supplement the insufficient number of monitoring sites for near-surface-air-temperature at developing/undeveloped mountain regions, the nighttime Moderate Resolution Imaging Spectroradiometer land surface temperature is used as an alternative to derive the approximate NLR at a finer spatial scale (e.g., at different elevation bands, different land covers, different aspects, and different snow conditions). Using satellite-based estimation of NLR, the modeling of snow processes has been greatly refined. Results show that both the determination of rainfall/snowfall and the snowpack process were significantly improved, contributing to a reduced summer evapotranspiration and thus an improved streamflow simulation. ©2016. American Geophysical Union. All Rights Reserved.
|
|
| 464. |
- Wang, Rongsheng, et al.
(author)
-
Asymmetry in the current sheet and secondary magnetic flux ropes during guide field magnetic reconnection
- 2012
-
In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 117, s. A07223-
-
Journal article (peer-reviewed)abstract
- A magnetic reconnection event with a moderate guide field encountered by Cluster in the near-Earth tail on 28 August 2002 is reported. The guide field points dawnward during this event. The quadrupolar structure of the Hall magnetic field within the ion diffusion region is distorted toward the northern hemisphere in the earthward part while toward the southern hemisphere tailward part of X-line. Observations of current density and electron pitch angle distribution indicate that the distorted quadrupolar structure is formed due to a deformed Hall electron current system. Cluster crossed the ion diffusion region from south to north earthward of the X-line. An electron density cavity is confirmed in the northern separatrix layer while a thin current layer (TCL) is measured in the southern separatrix layer. The TCL is formed due to electrons injected into the X-line along the magnetic field. These observations are different from simulation results where the cavity is produced associated with inflow electrons along the southern separatrix while the strong current sheet appears with the outflow electron beam along the northern separatrix. The energy of the inflowing electron in the separatrix layer could extend up to 10 keV. Energetic electron fluxes up to 50 keV have a clear peak in the TCL. The length of the separatrix layer is estimated to be at least 65 c/omega(pi). These observations suggest that electrons could be pre-accelerated before they are ejected into the X-line region along the separatrix. Multiple secondary flux ropes moving earthward are observed within the diffusion region. These secondary flux ropes are all identified earthward of the observed TCL. These observations further suggest there are numerous small scale structures within the ion diffusion region.
|
|
| 465. |
- Westerberg, Lars-Göran, et al.
(author)
-
3D MHD reconnection model coupled with Cluster multi-spacecraft data : 3D reconnection model
- 2008
-
In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 113
-
Journal article (peer-reviewed)abstract
- We present a model where a three-dimensional viscous/resistive analytical model of the large scale plasma flow in the outflow region about a reconnection site at the dayside magnetopause, is coupled with multi-spacecraft measurements from the Cluster satellite armada. A total of ten magnetopause crossings have been analyzed during the period 10:29-11:05 UT January 26, 2001, where nine are shown to be suitable to be coupled with the analytical model. Five of these nine reconnection events results in a successful coupling. By fitting the analytical expressions of the plasma velocity and magnetic field behaviour during the transition of the magnetopause, with data from the Cluster spacecraft we obtain estimates of the location of the X-line together with estimates on the anomalous transport coeffcients of kinematic viscosity (v) and magnetic diffusivity (ηd). We also obtain the development of the magnetopause transition layer away from the X-line. It is shown that the distance from the spacecraft to the X-line varies between 0.01 and 1 Earth radii, while v + ηd have a value between 4 . 104 km2/ s and 4 . 106 km2/ s. Furthermore it is shown that the transition layer thickness follows the analytical pattern where it grows proportional to the square root of the distance from the reconnection site. The estimated thickness obtained from each spacecraft is shown to follow the analytical structure well.
|
|
| 466. |
- Westlake, J. H., et al.
(author)
-
Titan's ionospheric composition and structure : Photochemical modeling of Cassini INMS data
- 2012
-
In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 117, s. E01003-
-
Journal article (peer-reviewed)abstract
- Titan's upper atmosphere produces an ionosphere at high altitudes from photoionization and electron impact that exhibits complex chemical processes in which hydrocarbons and nitrogen-containing molecules are produced through ion-molecule reactions. The structure and composition of Titan's ionosphere has been extensively investigated by the Ion and Neutral Mass Spectrometer (INMS) onboard the Cassini spacecraft. We present a detailed study using linear correlation analysis, 1-D photochemical modeling, and empirical modeling of Titan's dayside ionosphere constrained by Cassini measurements. The 1-D photochemical model is found to reproduce the primary photoionization products of N(2) and CH(4). The major ions, CH(5)(+), C(2)H(5)(+), and HCNH(+) are studied extensively to determine the primary processes controlling their production and loss. To further investigate the chemistry of Titan's ionosphere we present an empirical model of the ion densities that calculates the ion densities using the production and loss rates derived from the INMS data. We find that the chemistry included in our model sufficiently reproduces the hydrocarbon species as observed by the INMS. However, we find that the chemistry from previous models appears insufficient to accurately reproduce the nitrogen-containing organic compound abundances observed by the INMS. The major ion, HCNH(+), is found to be overproduced in both the empirical and 1-D photochemical models. We analyze the processes producing and consuming HCNH(+) in order to determine the cause of this discrepancy. We find that a significant chemical loss process is needed. We suggest that the loss process must be with one of the major components, namely C(2)H(2), C(2)H(4), or H(2).
|
|
| 467. |
- Whiter, D. K., et al.
(author)
-
Using multispectral optical observations to identify the acceleration mechanism responsible for flickering aurora
- 2010
-
In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 115, s. A12315-
-
Journal article (peer-reviewed)abstract
- We present an analysis of flickering (2-10 Hz) auroras observed with a state-of-the-art multispectral imaging system, Auroral Structure and Kinetics, located in Tromso, Norway. Short (1-2 s) periods of flickering aurora have been identified in which the frequency of the brightness oscillations decreases or increases smoothly over time. To the authors' knowledge this is the first detailed analysis of such "chirps" in flickering aurora or field-aligned bursts. We have found that the electron precipitation energy is strongly anticorrelated with the flickering frequency during all identified chirps. This result is consistent with the theory that flickering aurora is caused by the resonance interaction between electromagnetic ion cyclotron waves and precipitating electrons and that the wave parallel phase velocity is the primary factor determining the electron acceleration produced by this mechanism. Other current theories known to the authors cannot completely explain our observations.
|
|
| 468. |
- Wik, Martin, et al.
(author)
-
Bubbles trapped in arctic lake ice : Potential implications for methane emissions
- 2011
-
In: Journal of Geophysical Research. - Washington, DC, USA : American Geophysical Union. - 0148-0227 .- 2156-2202. ; 116, s. G03044-
-
Journal article (peer-reviewed)abstract
- The amount of methane (CH(4)) emitted from northern lakes to the atmosphere is uncertain but is expected to increase as a result of arctic warming. A majority of CH4 is thought to be released through ebullition (bubbling), a pathway with extreme spatial variability that limits the accuracy of measurements. We assessed ebullition during early and late winter by quantifying bubbles trapped in the ice cover of two lakes in a landscape with degrading permafrost in arctic Sweden using random transect sampling and a digital image processing technique. Bubbles covered up to similar to 8% of the lake area and were largely dominated by point source emissions with spatial variabilities of up to 1056%. Bubble occurrence differed significantly between early and late season ice, between the two lakes and among different zones within each lake (p < 0.001). Using a common method, we calculated winter fluxes of up to 129 +/- 486 mg CH(4) m(-2) d(-1). These calculations are, on average, two times higher than estimates from North Siberian and Alaskan lakes and four times higher than emissions measured from the same lakes during summer. Therefore, the calculations are likely overestimates and point to the likelihood that estimating CH(4) fluxes from ice bubble distributions may be more difficult than believed. This study also shows that bubbles quantified using few transects will most likely be unsuitable in making large-scale flux estimates. At least 19 transects covering similar to 1% of the lake area were required to examine ebullition with high precision in our studied lakes.
|
|
| 469. |
- Wild, J. A., et al.
(author)
-
Midnight sector observations of auroral omega bands
- 2011
-
In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 116, s. A00I30-
-
Journal article (peer-reviewed)abstract
- We present observations of auroral omega bands on 28 September 2009. Although generally associated with the substorm recovery phase and typically observed in the morning sector, the features presented here occurred just after expansion phase onset and were observed in the midnight sector, dawnward of the onset region. An all-sky imager located in northeastern Iceland revealed that the omega bands were similar to 150 x 200 km in size and propagated eastward at similar to 0.4 km s(-1) while a colocated ground magnetometer recorded the simultaneous occurrence of Ps6 pulsations. Although somewhat smaller and slower moving than the majority of previously reported omega bands, the observed structures are clear examples of this phenomenon, albeit in an atypical location and unusually early in the substorm cycle. The THEMIS C probe provided detailed measurements of the upstream interplanetary environment, while the Cluster satellites were located in the tail plasma sheet conjugate to the ground-based all-sky imager. The Cluster satellites observed bursts of 0.1-3 keV electrons moving parallel to the magnetic field toward the Northern Hemisphere auroral ionosphere; these bursts were associated with increased levels of field-aligned Poynting flux. The in situ measurements are consistent with electron acceleration via shear Alfven waves in the plasma sheet similar to 8 R-E tailward of the Earth. Although a one-to-one association between auroral and magnetospheric features was not found, our observations suggest that Alfven waves in the plasma sheet are responsible for field-aligned currents that cause Ps6 pulsations and auroral brightening in the ionosphere. Our findings agree with the conclusions of earlier studies that auroral omega bands have a source mechanism in the midtail plasma sheet.
|
|
| 470. |
- Wynn, Jonathan, et al.
(author)
-
Processes of multibathyal aragonite undersaturation in the Arctic Ocean
- 2016
-
In: Journal of Geophysical Research - Oceans. - 0148-0227 .- 2156-2202. ; 121:11, s. 8248-8267
-
Journal article (peer-reviewed)abstract
- During 3 years of study (2010–2012), the western Arctic Ocean was found to have unique aragonite saturation profiles with up to three distinct aragonite undersaturation zones. This complexity is produced as inflow of Atlantic-derived and Pacific-derived water masses mix with Arctic-derived waters, which are further modified by physiochemical and biological processes. The shallowest aragonite undersaturation zone, from the surface to ∼30 m depth is characterized by relatively low alkalinity and other dissolved ions. Besides local influence of biological processes on aragonite undersaturation of shallow coastal waters, the nature of this zone is consistent with dilution by sea-ice melt and invasion of anthropogenic CO2 from the atmosphere. A second undersaturated zone at ∼90–220 m depth (salinity ∼31.8–35.4) occurs within the Arctic Halocline and is characterized by elevated pCO2 and nutrients. The nature of this horizon is consistent with remineralization of organic matter on shallow continental shelves bordering the Canada Basin and the input of the nutrients and CO2 entrained by currents from the Pacific Inlet. Finally, the deepest aragonite undersaturation zone is at greater than 2000 m depth and is controlled by similar processes as deep aragonite saturation horizons in the Atlantic and Pacific Oceans. The comparatively shallow depth of this deepest aragonite saturation horizon in the Arctic is maintained by relatively low temperatures, and stable chemical composition. Understanding the mechanisms controlling the distribution of these aragonite undersaturation zones, and the time scales over which they operate will be crucial to refine predictive models.
|
|
