Journal: Journal of Geophysical Research

Abbreviation

J. geophys. res.

Publisher

American Geophysical Union

Journal Volumes

ISSN

0022-1406
0148-0227
0196-6928
0196-6936
2156-2202

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2003 - 2009412010 - 201918
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Publications1 - 10 of 59
  • Importance of vertical velocity variations in the cloud droplet nucleation process of marine stratus clouds
    Item type: Journal Article
    Peng, Yiran; Lohmann, Ulrike; Leaitch, Richard (2005)
    Journal of Geophysical Research
  • A map analysis of patterned-ground along a north American Arctic Transect
    Item type: Journal Article
    Raynolds, Martha K.; Walker, Donald A.; Munger, Corinne A.; et al. (2008)
    Journal of Geophysical Research
    Arctic patterned-ground features have been described individually, but never examined as parts of integrated landscape/ecosystems that vary along the Arctic climate gradient. Here we examine the complex interrelationships between patterned ground, climate, vegetation and soil along a north-south transect through all five bioclimate subzones of the North American Arctic. We mapped the vegetation, biomass, end-of-summer thaw depths, and snow cover on twenty 10 × 10-m grids. The vegetation maps illustrate the transition of vegetation types and patterns from north to south. Biomass maps showed lower biomass in the centers of patterned-ground features than in areas between features, and increasing biomass from north to south. Thaw-depth maps showed deeper thaw in the centers of features than between features, and shallow thaw on the north and south ends of the transect. Snow depth maps showed less snow on patterned-ground features subject to differential frost heave compared to areas between features which did not heave, and a north-south gradient of increasing snow depth. The maps also documented the change from small nonsorted polygons to larger nonsorted circles from north to south, and increasing pattern size with moisture. Principal components analysis revealed underlying relationships between patterned-ground landscapes and measured vegetation and environmental variables. Climate in combination with the vegetation was the most important factor affecting patterned ground on zonal sites, but soil moisture, texture and chemistry were also important.
  • Dynamics and mass balance of Taylor Glacier, Antarctica
    Item type: Journal Article
    Kavanaugh, J.L.; Cuffey, K.M.; Morse, D.L.; et al. (2009)
    Journal of Geophysical Research
    Taylor Glacier flows from the East Antarctic Ice Sheet and terminates in the McMurdo Dry Valleys, where it has left a geomorphologic record of past incursions. Here we use new data on the flow, thickness, and surface balance of Taylor Glacier to calculate ice fluxes and assess the current state of mass balance. Overall, the glacier is close to a state of zero net balance, and has largely adjusted to reduced snowfall on the Taylor Dome source region in the mid Holocene. One region of the upper ablation zone appears to be losing mass. Evidence from ice surface morphology and stable isotope profiles suggests that this mass loss is a long-term phenomenon, hence probably a lingering response to the earlier drying of Taylor Dome. Our data give a general indication of the size of ablation or accumulation changes needed for ice to advance far into Taylor Valley, as occurred 70 to 130 ka ago.
  • Snow cover sensitivity to horizontal resolution, parameterizations, and atmospheric forcing in a land surface model
    Item type: Journal Article
    Dutra, Emanuel; Kotlarski, Sven; Viterbo, Pedro; et al. (2011)
    Journal of Geophysical Research
    This paper assesses the impacts of horizontal resolution, snow physics, and atmospheric forcing in snow cover simulations by the European Centre for Medium-Range Weather Forecasts (ECWMF) land surface model Hydrology Tiled ECMWF Scheme for Surface Exchanges (HTESSEL). Off-line simulations are carried out forced by the ECMWF deterministic short-term weather forecasts (WFC) with a resolution of 25 km from March 2006 to June 2010. The horizontal-resolution impact on snow cover is addressed by performing simulations at 25, 80, and 200 km forced by WFC. The impact of atmospheric forcing on snow cover is assessed by forcing the model additionally with the ECMWF Era-Interim (ERAI) reanalysis, at 80 km resolution. Snow physics effects are analyzed by performing an extra simulation forced by WFC using a different snow scheme. The simulations are validated against four independent observational data sets: (1) snow water equivalent (SWE) over Switzerland; (2) snow cover duration in Europe (SNOWCLIM); (3) interactive multisensor snow and ice-mapping system (IMS) snow cover; and (4) Moderate Resolution Imaging Spectroradiometer (MODIS) surface albedo. ERAI forced simulations show a systematic underestimation of SWE and snow cover fraction, which is due to an underprediction of snowfall by ERAI. The snow physics experiment highlights the sensitivity of the model to the partitioning between rainfall and snowfall when rainfall interception in the snowpack is neglected. The horizontal resolution has a crucial role in characterizing the snow cover over complex terrain (e.g., orographic areas, coastal and lakes regions). However, improved snow physical parameterizations and meteorological forcing are shown to be the key elements to achieve more accurate simulations of the snowpack and of the snow-atmosphere interactions, also over flat terrain.
  • Characterization of the shallow structures of active fault zones using 3-D ground-penetrating radar data
    Item type: Journal Article
    McClymont, Alastair F.; Green, Alan G.; Villamor, Pilar; et al. (2008)
    Journal of Geophysical Research
    Where they can be correlated with geological exposures and trenches, 3-D ground-penetrating radar (GPR) data can contribute critical subsurface information to paleoseismic investigations. Because active faults are typically characterized by complicated near-surface structures that vary with the styles of faulting and the types of rock that are ruptured, GPR data can be difficult to interpret. We have acquired 3-D GPR data sets across three active fault zones within New Zealand that have different deformation styles: the strike-slip Wellington fault zone, reverse faults of the Ostler fault zone, and normal faults of the Maleme fault zone. To improve our interpretation of the processed GPR volumes, we employed two suites of geometric attributes. The first suite was computed using a coherence-based algorithm. It provided estimates of the coherency, azimuth, and dip of reflections. The second suite quantified the volumetric textures of reflections, which allowed different reflection facies to be defined objectively. We have demonstrated how some attributes were more successful at visualizing certain structural or depositional characteristics than others. For example, the coherency attribute was an excellent tool for highlighting normal faults within volcanic deposits of the Maleme fault zone, whereas the texture-based attributes were most useful for discriminating between the gravel and metasediment units juxtaposed by the Wellington fault zone. Our GPR data sets and associated attribute volumes showed details of near-surface fault geometry that were not obvious from surface mapping, and they revealed evidence of off-fault deformation, gravitational collapse, and topple structures.
  • Spectral signatures of characteristic spatial scales and nonfractal structure in landscapes
    Item type: Journal Article
    Perron, J. Taylor; Kirchner, James W.; Dietrich, William E. (2008)
    Journal of Geophysical Research
    Landscapes are sometimes argued to be scale-invariant or random surfaces, yet qualitative observations suggest that they contain characteristic spatial scales. We quantitatively investigate the existence of characteristic landscape scales by analyzing two-dimensional Fourier power spectra derived from high-resolution topographic maps of two landscapes in California. In both cases, we find that spectral power declines sharply above a frequency that corresponds roughly to hillslope length, implying that the landscape is relatively smooth at finer scales. The spectra also show that both landscapes contain quasiperiodic ridge-and-valley structures, and we derive a robust measure of the ridge-valley wavelength. By comparing the spectra with the statistical properties of spectra derived from randomly generated topography, we show that such uniform valley spacing is unlikely to occur in a random surface. We describe several potential applications of spectral analysis in geomorphology beyond the identification of characteristic spatial scales, including a filtering technique that can be used to measure topographic attributes, such as local relief, at specific scales or in specific orientations.
  • Aftershock asymmetry on a bimaterial interface
    Item type: Journal Article
    Rubin, Allan M.; Ampuero, Jean-Paul (2007)
    Journal of Geophysical Research
    To better understand the asymmetric distribution of microearthquake aftershocks along the central San Andreas fault, we study dynamic models of slip-weakening ruptures on an interface separating differing elastic half-spaces. Subshear ruptures grow as slightly asymmetric bilateral cracks, with larger propagation velocities, slip velocities, and normal stress changes at the rupture front moving in the direction of slip of the medium with the lower shear wave speed (the southeast front, in the context of the San Andreas). When the SE front encounters a stress barrier, the tensile stress perturbation behind the rupture front continues forward and for a wide range of barrier strengths nucleates a dying slip pulse. This slip pulse smooths the stress field and reduces the static stress change beyond the SE front. Furthermore, because the tensile stress that carried the slip pulse into the barrier is a purely dynamic phenomenon, the SE rupture front can be left far below the failure threshold, while the NW front remains quite close to failure. Both mechanisms could contribute to the observed aftershock asymmetry. Formation of a robust slip pulse requires a peak tensile stress perturbation that approaches the nominal strength drop of the slip-weakening law. To achieve this while minimizing off-fault damage requires either substantial velocity contrasts or small reductions in friction. The simulations also show a pronounced asymmetry in the timescales over which barriers to the SE and NW experience increasing stresses, a result that has implications for the asymmetric distribution of subevents in compound earthquakes.
  • Review of the formulation of present-generation stratospheric, chemistry-climate models and associated external forcings
    Item type: Journal Article
    Morgenstern, O.; Giorgetta, M.A.; Shibata, K.; et al. (2010)
    Journal of Geophysical Research
    The goal of the Chemistry-Climate Model Validation (CCMVal) activity is to improve understanding of chemistry-climate models (CCMs) through process-oriented evaluation and to provide reliable projections of stratospheric ozone and its impact on climate. An appreciation of the details of model formulations is essential for understanding how models respond to the changing external forcings of greenhouse gases and ozone-depleting substances, and hence for understanding the ozone and climate forecasts produced by the models participating in this activity. Here we introduce and review the models used for the second round (CCMVal-2) of this intercomparison, regarding the implementation of chemical, transport, radiative, and dynamical processes in these models. In particular, we review the advantages and problems associated with approaches used to model processes of relevance to stratospheric dynamics and chemistry. Furthermore, we state the definitions of the reference simulations performed, and describe the forcing data used in these simulations. We identify some developments in chemistry-climate modeling that make models more physically based or more comprehensive, including the introduction of an interactive ocean, online photolysis, troposphere-stratosphere chemistry, and non-orographic gravity-wave deposition as linked to tropospheric convection. The relatively new developments indicate that stratospheric CCM modeling is becoming more consistent with our physically based understanding of the atmosphere.
  • Changes in ozone over Europe
    Item type: Journal Article
    Logan, J.A.; Staehelin, J.; Megretskaia, I.A.; et al. (2012)
    Journal of Geophysical Research
    We use ozone observations from sondes, regular aircraft, and alpine surface sites in a self-consistent analysis to determine robust changes in the time evolution of ozone over Europe. The data are most coherent since 1998, with similar interannual variability and trends. Ozone has decreased slowly since 1998, with an annual mean trend of −0.15 ppb yr⁻¹ at ∼3 km and the largest decrease in summer. There are some substantial differences between the sondes and other data, particularly in the early 1990s. The alpine and aircraft data show that ozone increased from late 1994 until 1998, but the sonde data do not. Time series of differences in ozone between pairs of locations reveal inconsistencies in various data sets. Differences as small as few ppb for 2–3 years lead to different trends for 1995–2008, when all data sets overlap. Sonde data from Hohenpeissenberg and in situ data from nearby Zugspitze show ozone increased by ∼1 ppb yr⁻¹ during 1978–1989. We construct a mean alpine time series using data for Jungfraujoch, Zugspitze, and Sonnblick. Using Zugspitze data for 1978–1989, and the mean time series since 1990, we find that the ozone increased by 6.5–10 ppb in 1978–1989 and 2.5–4.5 ppb in the 1990s and decreased by 4 ppb in the 2000s in summer with no significant changes in other seasons. It is hard to reconcile all these changes with trends in emissions of ozone precursors, and in ozone in the lowermost stratosphere. We recommend data sets that are suitable for evaluation of model hindcasts.
  • Variability of heterotrophic metabolism in small stream corridors of an early successional watershed
    Item type: Journal Article
    Gerull, Linda; Frossard, Aline; Gessner, Mark O.; et al. (2011)
    Journal of Geophysical Research
    Metabolic activity in stream corridors is regulated by a complex combination of factors that are difficult to disentangle in mature ecosystems. Chicken Creek in Germany, an experimentally created watershed in an early successional stage, offers the opportunity to assess the spatiotemporal variation in metabolic activity in a simplified system. We measured microbial respiration in soils and sediments along the hydrologic flow path from upland terrestrial to ephemeral to perennial sites of three stream corridors. Dry soils and sediments were rewetted before respiration measurements to mimic periods of activity during and after rainfall. Respiration rates and organic matter contents of soil and sediment were generally low. The presence of algae and accretion of vascular plant fragments in the perennial stream reaches increased respiration rates, pointing to the importance of particulate organic matter. Contrary to expectation, respiration rates of rewetted soil and sediment from dry stream channels were similar to rates measured with sediments collected in the perennial channel sections. This suggests that permanent water availability was not a main factor determining metabolic potential in the early successional Chicken Creek watershed. Carbon turnover in perennial channels was fourfold to eightfold higher than in ephemeral channels and terrestrial sites, as water was permanently available. However, this magnitude was insufficient for perennial channels to compensate for the large surface area of terrestrial soils: extrapolated to a year and the whole watershed, stream channels contributed only 5% to total carbon turnover, 95% being due to soils during and after rainfall events.
Publications1 - 10 of 59