AGU journal highlights -- April 12, 2010

The following highlights summarize research papers that have recently been published in Geophysical Research Letters (GRL).

In this release:

1. Wet spells getting longer in Europe
2. Changes in Martian gullies indicate liquid water
3. Thick water ice observed in lunar craters
4. Increasing subtropical humidity of warming Earth explained
5. Model gives insights into sprites
6. Greenland ice loss spreading northwest

Anyone may read the scientific abstract for any of these papers 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. 10.1029/2010GL042468. 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

As the world's climate changes, precipitation patterns are changing as well. Previous studies have found that in Europe, the amount of precipitation has been increasing, and a new study shows an increase in the duration of wet periods, potentially affecting the frequency of catastrophic floods. Using daily rain gauge data from nearly 700 rain gauges in Europe covering the period 1950-2008, Zolina et al. study changes in the duration of wet spells, defined as consecutive days with significant precipitation (more than 1 millimeter/day: 0.04 inches/day). The authors find that although the total number of wet days has not changed significantly over the past 60 years, the duration of wet periods has been increasing and short rain events have regrouped into longer rainy spells. In addition, heavy precipitation events are now more commonly part of prolonged periods of rain and have become more intense. Because flooding depends not only on the amount of rain but also on the duration of rainfall, the changing duration of rainy periods could alter the frequency and intensity of floods, which could affect large populations in Europe.

Title: Changing structure of European precipitation: Longer wet periods leading to more abundant rainfalls

Authors: Olga Zolina: Meteorologisches Institut, Universitaet Bonn, Bonn, Germany, and P. P. Shirshov Institute of Oceanology, Moscow, Russia;

Clemens Simmer, Stefan Kollet: Meteorologisches Institut, Universitaet Bonn, Bonn, Germany;

Sergey K. Gulev: P. P. Shirshov Institute of Oceanology, Moscow, Russia;

Source: Geophysical Research Letters (GRL) paper 10.1029/2010GL042468, 2010
http://dx.doi.org/10.1029/2010GL042468

2. Changes in Martian gullies indicate liquid water

Present-day gully activity on Mars provides new evidence for transient liquid water. Reiss et al. study images of the Russell crater dune field on Mars taken by the High Resolution Imaging Science Experiment from November 2006 to May 2009. The authors observe that the length of a 2-meter (6.56 feet) wide gully channel had increased by about 50 m (164 ft) in one year and about 120 m (394 ft) within the second year. On the basis of the morphology of the channel and other factors, the researchers believe that these changes in the length of the gully are best explained by erosional processes triggered by the melting of small amounts of water ice.

Title: Evidence for present day gully activity on the Russell crater dune field, Mars

Authors: D. Reiss, G. Erkeling, K. E. Bauch, H. Hiesinger: Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Münster, Germany;

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL042192, 2010
http://dx.doi.org/10.1029/2009GL042192

3. Thick water ice observed in lunar craters

Water ice exists in large quantities in many small craters near the Moon's north pole, according to a new study. Spudis et al. present initial results from the miniature synthetic aperture radar (Mini-SAR) experiment on board the Indian Chandrayaan 1 spacecraft, which mapped most of the area near the north pole of the Moon between February and April 2009. The Mini-SAR instrument collected data on the polarization of radio waves reflected off the lunar surface. These data give the researchers insight into lunar surface features. For instance, high values of the circular polarization ratio (CPR) indicate either surface roughness or ice. The observations suggest that water ice exists in many of the small craters in permanent darkness near the north pole of the Moon. According to the scientists' analysis, the CPR values recorded indicate that this ice must be at least 2-3 meters (6.5-9.8 feet) thick. The researchers note that the ice is not distributed evenly over the lunar surface, suggesting that it may have been deposited through an episodic deposition process such as comet or asteroid impact. Because these lunar craters near the north pole are in permanent shadow, ice would remain stable there indefinitely. The new study supports and extends other recent discoveries of water on the Moon.

Title: Initial results for the north pole of the Moon from Mini-SAR, Chandrayaan-1 mission

Authors: P. D. Spudis: Lunar and Planetary Institute, Houston, Texas, USA, and others (for the complete list of authors, please see: http://dx.doi.org/10.1029/2009GL042259)

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL042259, 2010
http://dx.doi.org/10.1029/2009GL042259

4. Increasing subtropical humidity of warming Earth explained

Global circulation model (GCM) experiments carried out for the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) predict that as the Earth undergoes global warming, humidity will increase in subtropical regions. Hurley and Galewsky use an IPCC AR4 GCM simulation to study the source of this increased atmospheric water vapor under global warming. The authors find that the increased subtropical atmospheric water vapor can be attributed primarily to the warmer temperatures encountered by air parcels at the time the parcels were last saturated with water vapor. Furthermore, the researchers note that increased evaporation does not explain the increased subtropical humidity and that circulation changes have only a minimal impact on humidity in their simulations.

Title: A last-saturation diagnosis of subtropical water vapor response to global warming

Authors: John V. Hurley: Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico, USA;

Joseph Galewsky: Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL042316, 2010
http://dx.doi.org/10.1029/2009GL042316

5. Model gives insights into sprites

A sprite is an electrical discharge similar to lightning, but it occurs in the upper atmosphere (50-90 kilometers – 31-56 miles – in altitude), above large thunderstorms. These large flashes of light, which are triggered in almost all cases by positive lightning discharge between the thundercloud and ground, can span tens of kilometers of altitude. Sprites were first photographed in 1989. High-speed videos in recent years show that they always start in the ionosphere (sometimes out of a wide saucer-shaped halo) and first shoot downward in the form of long ionized filaments called streamers; sometimes they shoot up again later. The downward streamers are bright at their growing tips, and the channel is first dark and becomes bright again after some distance. While observations continue, many features of sprites remain unexplained. To shed light on some of the characteristics of sprites, Luque and Ebert develop simulations that, unlike previous models, included variations in air density along the length of sprite streamers. Their results help to explain some previously unexplained observations associated with sprites, such as light emission from the streamer trail, the emergence of negatively charged upward propagating streamers, the increase in light emissions from the streamer head with air density, and changes in streamer speed.

Title: Sprites in varying air density: Charge conservation, glowing negative trails and changing velocity

Authors: A. Luque: CWI, Amsterdam, Netherlands and Instituto de Astrofísica de Andalucía, CSIC, Granada, Spain;

U. Ebert: CWI, Amsterdam, Netherlands and Department of Physics, Eindhoven University of Technology, Eindhoven, Netherlands.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL041982, 2010
http://dx.doi.org/10.1029/2009GL041982

6. Greenland ice loss spreading northwest

The Greenland ice sheet has been losing mass at a significant rate during the past several years, contributing to global sea level rise. Recent studies show dramatic ice loss along the southeastern coast. Khan et al. combine Global Positioning System (GPS) measurements with measurements from the Gravity Recovery and Climate Experiment (GRACE) satellite to determine that the ice mass loss is accelerating and also spreading into northwestern Greenland. The GRACE satellite, launched in 2002, measures changes in Earth's gravity field and can detect the motions of the Earth's crust that occur when ice melts. GRACE detects this uplift over large regions, while long-term observations from permanent GPS stations can be used to monitor uplift on smaller scales. Acceleration of ice mass loss on the northwestern coast likely started in late 2005, the researchers find. In addition to documenting the spread of ice loss, the results also confirm the consistency between GRACE and GPS measurements, showing that the combination of the two types of measurements provides a useful new approach for scientists studying ongoing ice loss.

See previous press release at: http://www.agu.org/news/press/pr_archives/2010/2010-07.shtml

Title: Spread of ice mass loss into northwest Greenland observed by GRACE and GPS

Authors: Shfaqat Abbas Khan: DTU Space, Department of Geodesy, National Space Institute, Copenhagen, Denmark;

John Wahr: Department of Physics and Cooperative Institute for Research in Environmental Sciences, University of Colorado at Boulder, Boulder, Colorado, USA;

Michael Bevis, Eric Kendrick: School of Earth Sciences, Ohio State University, Columbus, Ohio, USA;

Isabella Velicogna: Department of Earth System Science, University of California, Irvine, California, USA, and Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2010GL042460, 2010
http://dx.doi.org/10.1029/2010GL042460