AGU journal highlights

2010-09-10

(Press-News.org) The following highlights summarize research papers that have been recently published in Journal of Geophysical Research-Atmospheres (JGR-D) and Geophysical Research Letters (GRL).

In this release:

Landmass shape affects extent of Arctic sea ice Estimating how much rain forests intercept How compliant fault zones respond to nearby earthquakes Measuring the rate of mountain building in New Zealand Interplanetary magnetic field causes changes in the polar cap ionosphere Oxygen and hydrogen follow different escape paths from Venus's atmosphere

Anyone may read the scientific abstract for any already-published paper by clicking on the link provided at the end of each Highlight. You can also read the abstract by going to http://www.agu.org/pubs/search_options.shtml and inserting into the search engine the full doi (digital object identifier), e.g. 10.1029/2010GL043741. The doi is found at the end of each Highlight below.

Journalists and public information officers (PIOs) at educational or scientific institutions, who are registered with AGU, also may download papers cited in this release by clicking on the links below. Instructions for members of the news media, PIOs, and the public for downloading or ordering the full text of any research paper summarized below are available at http://www.agu.org/news/press/papers.shtml.

1. Landmass shape affects extent of Arctic sea ice

Arctic sea ice has retreated significantly in recent years, reaching a record low in September 2007. It is known that the seasonal cycle in Arctic sea ice extent is not symmetric—seasonal ice retreat proceeds gradually during early summer and then accelerates toward end of summer, while in winter, ice growth is rapid at first and then slows later in the season. Scientists have observed that ice cover has retreated far more rapidly in September than during other times of the year.

Some scientists have suggested that this seasonal asymmetry is due to factors such as temperature changes. However, Eisenman finds that the seasonal differences in rate of ice growth or retreat are caused by the geometry of the landmasses surrounding the Arctic Ocean. Because the Arctic Ocean is mostly surrounded by land, coastlines block the southward extent of sea ice growth during the winter, but coastlines have little effect on the extent of ice during the summer.

The author suggests that to better interpret changes in Arctic sea ice, instead of considering sea ice areal extent, scientists should track the line marking the latitude of the Arctic sea ice edge, averaged zonally over locations where the edge is free to move. He finds that this line moves northward or southward at a steady pace over the course of the year, with no seasonal asymmetry. In recent years, this line has been migrating northward at a rate of about 8 kilometers (5 miles) per year, consistent with overall ice loss. The study explains some aspects of the seasonal Arctic sea ice cycle and could help scientists better interpret sea ice evolution in the future.

Title: Geographic muting of changes in the Arctic sea ice cover

Author: Ian Eisenman: Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA and Department of Atmospheric Sciences, University of Washington, Seattle, Washington, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2010GL043741, 2010 http://dx.doi.org/10.1029/2010GL043741

2. Estimating how much rain forests intercept

Although most rainfall over continents reaches the ground, a significant portion is caught by the forest canopy and evaporates directly. To better understand feedbacks between evaporation and climate, and to estimate the effects of climate change and deforestation on water resources, scientists need to know how much rainfall is intercepted by the canopy.

Miralles et al. apply satellite rainfall data to an existing model to create global maps of estimated forest rainfall interception. They find that interception loss accounts for 13 percent of rainfall over broadleaf evergreen forests, 19 percent in broadleaf deciduous forests, and 22 percent in needle-leaf forests. The researchers compare their model estimates with field observations and find they are in good agreement.

Title: Global canopy interception from satellite observations

Authors: Diego G. Miralles: Department of Hydrology and Geo-environmental Sciences, VU University Amsterdam, Amsterdam, Netherlands;

John H. Gash: Department of Hydrology and Geo-environmental Sciences, VU University Amsterdam, Amsterdam, Netherlands and Centre for Ecology and Hydrology, Natural Environment Research Council, Wallingford, UK;

Thomas R. H. Holmes: Department of Hydrology and Geo-environmental Sciences, VU University Amsterdam, Amsterdam, Netherlands and Hydrology and Remote Sensing Laboratory, USDA Agricultural Research Service, Beltsville, Maryland, USA;

Richard A. M. de Jeu and A. J. Dolman: Department of Hydrology and Geo-environmental Sciences, VU University Amsterdam, Amsterdam, Netherlands.

Source: Journal of Geophysical Research-Atmospheres (JGR-D) paper 10.1029/2009JD013530, 2010 http://dx.doi.org/10.1029/2009JD013530

3. How compliant fault zones respond to nearby earthquakes

Compliant fault zones are weaker than surrounding rocks and are more responsive to small stress changes caused by nearby earthquakes. Duan models how compliant fault zones around existing faults respond to nearby earthquakes. He finds that some portions of the fault zone undergo inelastic deformation, while other areas deform elastically.

Inelastic deformation occurs if the preexisting stress level in the rock is close to the rock strength. If elastic deformation takes place, retrograde motion (in the opposite direction of a long-term slip) occurs across the fault zone. If inelastic deformation takes place, sympathetic motion of neighboring faults occurs across the fault zone. The results suggest that observations of inelastic deformation could be used to constrain the stress state in the crust. The study could also help researchers better understand how fault zones respond to earthquakes.

Title: Inelastic response of compliant fault zones to nearby earthquakes

Author: Benchun Duan: Center for Tectonophysics, Department of Geology and Geophysics, Texas A&M University, College Station, Texas, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2010GL044150, 2010 http://dx.doi.org/10.1029/2010GL044150

4. Measuring the rate of mountain building in New Zealand

For the past 20 million years, the Southern Alps Mountains on the South Island of New Zealand have been growing as continental plates converge. Beavan et al. use 10 years of Global Positioning System data to measure the present-day rates of vertical movement across the Southern Alps. They find that the highest uplift rates, about 5 millimeters per year, occur near the crest of the mountains, about 20󈞊 km (about 12󈝿 miles) southeast of the Alpine Fault, while the mountains are growing at a slower rate at lower elevations.

Title: Distribution of present‐day vertical deformation across the Southern Alps, New Zealand, from 10 years of GPS data

Authors: J. Beavan: GNS Science, Lower Hutt, New Zealand;

P. Denys, M. Denham: School of Surveying, Otago University, Dunedin, New Zealand;

B. Hager, T. Herring: Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA;

P. Molnar: Department of Geological Sciences and Cooperative Institute for Research in Environmental Science, University of Colorado at Boulder, Boulder, Colorado, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2010GL044165, 2010 http://dx.doi.org/10.1029/2010GL044165

5. Interplanetary magnetic field causes changes in the polar cap ionosphere

Electron density variations in the ionosphere are important for understanding space weather, as they affect the Global Positioning System and radio communications. Bahcivan et al. use some of the first measurements from the U.S. National Science Foundation's Resolute incoherent scatter radar in Resolute Bay, Canada (near the geomagnetic pole), to investigate the highly structured polar cap ionosphere.

They compare the ionospheric measurements with measurements of the interplanetary magnetic field (IMF). The IMF is the magnetic field of the Sun, carried away from the Sun toward Earth by the solar wind. The researchers find that times when patches of enhanced ionospheric electron density appear correspond to times when the IMF is oriented southward. They also observe some internal structures within the patches of enhanced electron density. The study contributes to scientists' understanding of how solar wind variations affect the ionosphere and could help lead to improved forecasts of ionospheric conditions.

Title: Initial ionospheric observations made by the new Resolute incoherent scatter radar and comparison to solar wind IMF

Authors: Hasan Bahcivan, Roland Tsunoda, Michael Nicolls, and Craig Heinselman: Center for Geospace Studies, SRI International, Menlo Park, California, USA.

Source: Geophysical Research Letters paper 10.1029/2010GL043632, 2010 http://dx.doi.org/10.1029/2010GL043632

6. Oxygen and hydrogen follow different escape paths from Venus's atmosphere

In the upper atmosphere of Venus, ions are exposed to magnetic and electric fields created by the solar wind. These electric and magnetic fields enable oxygen and hydrogen ions, the components of water, to escape from the atmosphere. Previous studies have mainly focused on oxygen's escape from Venus. Jarvinen et al. use a simulation to study both oxygen and hydrogen ion escape from Venus. They find that the two types of ions have very different escape patterns and leave the atmosphere at different rates.

Hydrogen ions simply drift away from Venus in the direction determined by the electric and magnetic fields. Oxygen ions, which are much heavier, are subject to other effects. The study could help scientists interpret measured ion escape rates and improve understanding of the evolution of planetary atmospheres.

Title: Widely different characteristics of oxygen and hydrogen ion escape from Venus

Authors: R. Jarvinen, E. Kallio, S. Dyadechkin, P. Janhunen: Finnish Meteorological Institute, Helsinki, Finland;

I. Sillanpää: Southwest Research Institute, San Antonio, Texas, USA.

Source: Geophysical Research Letters paper 10.1029/2010GL044062, 2010 http://dx.doi.org/10.1029/2010GL044062

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AGU journal highlights

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