AGU Journal Highlights—13 November 2007

AGU Journal Highlights
13 November 2007

Contact: Peter Weiss
Phone: +1 202 777 7507
E-mail: pweiss@agu.org

Contents

I. Highlights, including authors and their institutions

You may read the scientific abstract for any already-published paper by clicking on the link provided at the end of each Highlight. Or, if links are not active in this email, you can also read the abstract by going to http://www.agu.org/pubs/search_options.shtml and inserting into the search engine the portion of the doi (digital object identifier) following 10.1029/ (e.g., 2007GL030276). The doi is found at the end of each Highlight below. To obtain the full text of the research paper, see Part II.

The following highlights summarize research papers in Geophysical Research Letters (GRL):

  1. Recent damaging fire seasons in the northern Rockies may be due to a decline in storm frequency
  2. Biological controls on carbon uptake and degassing in the ocean
  3. Concrete dams as seismic imaging sources
  4. Current and future U.S. weather extremes and El Ninño
  5. Earthquake-triggered landslides: Regional patterns and their relation to ground motion
  6. Radar signatures of the urban effect on precipitation distribution: A case study for Atlanta, Georgia
  7. Polar mesosphere summer echoes detected at ultrahigh frequency

Journalists and public information officers of educational and scientific institutions (only) may receive one or more of the papers cited in the Highlights (including pre-publication copies of articles listed as “in press”) by sending a message to Peter Weiss at pweiss@agu.org, indicating which one(s). Include your name, the name of your publication, and your phone number. The papers will be e-mailed as pdf attachments.

Members of the public can read the abstract of any published paper by clicking on the doi link in the source section, at the end of the highlight. The full scientific article is available for purchase through a link in the abstract.

The Highlights and the papers to which they refer are not under AGU embargo.

1. Recent damaging fire seasons in the northern Rockies may be due to a decline in storm frequency

Nearly 60 percent of the total increase in forest fires in the western United States during the past three decades occurred in the northern Rockies. Though previous studies attributed this trend to earlier thaw and warmer temperatures, Knapp and Soulé wondered whether recent fire seasons were influenced by shifts in the summer timing and frequency of major midlatitude cyclones (MLCs), a type of low pressure zone. Noting that MLCs can diminish fire activity through both cooler temperatures and higher humidity and precipitation, the authors study data collected since 1900 from eight climate stations in the northern Rockies. They compare these data with wildfire records from 1940 through 2004, finding that timing of the first MLC occurred progressively later each year during the past century. Further, the frequency of MLCs has declined since 1900, with pronounced decreases beginning in the mid-1980s. Thus the trend of later and fewer MLCs may have led to an increase in burned areas in the northern Rockies, and the link between wildfire activity and fluctuating climate patterns may have serious ecologic and economic consequences for western states.

Title:

“Trends in midlatitude cyclone frequency and occurrence during fire season in the Northern Rockies: 1900–2004”

Authors:

Paul A. Knapp
Carolina Tree-Ring Science Laboratory, Department of Geography, University of North Carolina at Greensboro, North Carolina, U.S.A.
Peter T. Soulé
Department of Geography and Planning, Appalachian State University, Boone, North Carolina, U.S.A.

Source:

Geophysical Research Letters (GRL) paper doi:10.1029/2007GL031216, 2007

2. Biological controls on carbon uptake and degassing in the ocean

Paleoclimate studies have revealed a strong correspondence between glacial-interglacial changes in atmospheric carbon dioxide (CO2) levels and global climate during the Pleistocene Epoch (1.6 million to 8,000 years ago). However, whether CO2 fluctuations drive past climate shifts or whether past climate fluctuations drive CO2 fluctuations remains unknown. Temperature decreases can affect the partial pressure of CO2 in the oceans by increasing the amount of gas able to dissolve in the ocean. However, Matsumoto considers the fact that temperature decreases can also reduce the rate of organic carbon production and degradation. In particular, organic carbon degrades more slowly at lower temperatures, thus allowing more of it to be exported to the deep ocean. This biologically mediated temperature effect is nearly comparable to the effect of temperature on gas solubility, implying that the higher ocean temperatures predicted for the future might act as a positive feedback on atmospheric CO2 by reducing the vertical transport of carbon to the deep ocean and thereby increasing CO2 degassing from the ocean.

Title:

“Biology-mediated temperature control on stratospheric pCO2 and ocean biochemistry”

Authors:

Katsumi Matsumoto
Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota, U.S.A.

Source:

Geophysical Research Letters (GRL) paper doi:10.1029/2007GL031301, 2007

3. Concrete dams as seismic imaging sources

Estimating how the ground moves in response to an earthquake at specific locations requires knowledge of shallow crustal velocity structure. Because active seismic source investigations are expensive, scientists use natural earthquakes and artificial seismic events such as explosions to extract information about crustal velocities. Passive seismic recordings near large engineered structures often contain vibrational noise as the structure resonates in response to ambient Earth vibrations. Though this noise can often obscure signals from earthquakes, O'Connell notes that the structural noise itself can be treated as a seismic source and used to understand properties and structures within the shallow crust. He observes that seismic noise recorded by a seismograph located close to a concrete gravity-arch dam in northern Montana acted as a virtual seismic source that was recorded by several stations located within 17 kilometers (11 miles) of the dam. By cross-correlating the data from distant stations with the station near the dam, the author constrains shallow crustal velocity structures not readily imaged using earthquake data.

Title:

“Concrete dams as seismic imaging sources”

Authors:

Daniel R. H. O'Connell
William and Lettis & Associates, Inc., Golden, Colorado, U.S.A.

Source:

Geophysical Research Letters (GRL) paper doi:10.1029/2007GL031219, 2007

4. Current and future U.S. weather extremes and El Ninño

In recent years, considerable attention has been paid to how future warming climate will change weather extremes. Noting that patterns of extremes are affected by large-scale atmospheric patterns, Meehl et al. seek to learn more about how temperature and precipitation extremes might exacerbate the disruptive effects associated with El Ninño events over the United States. Using a global coupled climate model representative of the current generation of models, the authors simulate baseline aspects of El Ninño events and teleconnections over North America. This simulation captures well the associated observed patterns of extremes seen in present-day climate. The authors also simulate future El Ninño patterns over the United States and find that among other things, the current pattern of decreasing frost days over the southwestern United States expands northward and eastward. Further, the current pattern of increasing intense precipitation in the southwestern United States expands eastward, and precipitation rates in the southeastern United States intensify. Additionally, current patterns showing decreases in heat wave intensity over many southern states reverse.

Title:

“Current and future U.S. weather extremes and El Ninño”

Authors:

Gerald A. Meehl and Hiayan Teng
National Center for Atmospheric Research, Boulder, Colorado, U.S.A.
Claudia Tebaldi
National Center for Atmospheric Research, Boulder, Colorado, U.S.A.; now at RAND Corp., Santa Monica, California, U.S.A.
Thomas C. Peterson
NOAA's National Climatic Data Center, Asheville, North Carolina, U.S.A.

Source:

Geophysical Research Letters (GRL) paper doi:10.1029/2007GL031027, 2007

5. Earthquake-triggered landslides: Regional patterns and their relation to ground motion

Major earthquakes can cause widespread landslides on nearby slopes. Past research has shown that the number and total volume of landslides triggered by an earthquake, as well as the area affected by landsliding, scale with earthquake magnitude. To investigate further, Meunier et al. document patterns of landsliding associated with large earthquakes on three thrust faults: the Northridge earthquake in California, the Chi-Chi earthquake in Taiwan, and two earthquakes in the Finisterre Mountains of Papua New Guinea. In each case, landslide densities are found to be greatest in the area of strongest ground acceleration and to decay with distance from the epicenter. Specifically for the well-monitored regions of California and Taiwan, the density of earthquake-triggered landslides is linearly and highly correlated with the vertical and horizontal components of peak ground acceleration. On the basis of this, the authors derive an expression for the spatial variation of landslide density dependent on regional parameters. Combined with further work, these findings allow for the possibility of constructing shake maps based on geomorphic observations of earthquake-triggered landslides in noninstrumented regions.

Title:

“Regional patterns of earthquake-triggered landslides and their relation to ground motion”

Authors:

Patrick Meunier, Niels Hovius, and A. John Haines
Department of Earth Sciences, University of Cambridge, Cambridge, U.K.

Source:

Geophysical Research Letters (GRL) paper doi:10.1029/2007GL031337, 2007

6. Radar signatures of the urban effect on precipitation distribution: A case study for Atlanta, Georgia

Urban areas can affect precipitation amount and distribution by enhancing precipitation downwind, altering precipitation rates, and disturbing patterns of convective development, clouds, and lightning. Nevertheless, quantifying disturbances and mapping altered patterns for every urban location can be technically difficult. To demonstrate a potentially easier method of monitoring precipitation in urban areas and to better quantify the urban influence on precipitation, Mote et al. examine summer precipitation distribution in and around metropolitan Atlanta for the years between 2002 and 2006 using a ground-based radar station in Peachtree City, Georgia. They find that areas east of Atlanta had 30 percent more rainfall during summer than areas west of the city. Further, both precipitation amount and frequency were enhanced up to 80 kilometers (50 miles) east of Atlanta's urban core; hourly analysis of radar data showed that precipitation enhancements on the periphery of the urban core were most evident between 1900h and midnight local time. The authors expect that similar studies using ground-based radar for other urban centers will help better quantify the impact of urbanization on local precipitation.

Title:

“Radar signatures of the urban effect on precipitation distribution: A case study for Atlanta, Georgia”

Authors:

Thomas L. Mote and J. Marshall Shepherd
Climate Research Laboratory, Department of Geography, University of Georgia, Athens, Georgia, U.S.A.
Matthew C. Lacke
Climate Research Laboratory, Department of Geography, University of Georgia, Athens, Georgia, U.S.A.; now at Air and Radiation Protection Division, Jefferson County Department of Health, Birmingham, Alabama, U.S.A.

Source:

Geophysical Research Letters (GRL) paper doi:10.1029/2007GL031903, 2007

7. Polar mesosphere summer echoes detected at ultrahigh frequency

The polar summer mesosphere is the coldest place within the Earth's atmosphere, with minimum temperatures near 130 kelvins (-143 degrees Celsius). Inhomogeneities in this region give rise to polar mesosphere summer echoes (PMSEs), which are enhanced radar backscatter returns occurring between 80 and 90 kilometers (50 and 56 miles) altitude. Although PMSEs are typically studied at very high frequencies (VHF, 30–300 MHz), Nicolls et al. utilize the new 450-MHz Poker Flat Incoherent Scatter Radar to detect and image PMSEs at ultrahigh frequency (UHF) over Fairbanks, Alaska. The patchy echoes exhibit very little aspect sensitivity, indicative of isotropic backscatter. The findings suggest a scattering mechanism for PMSEs at UHF that is different than that at VHF, which is likely associated with neutral air turbulence and charged ice particles in the mesosphere. Imaging with the Poker Flat radar indicates the presence of long-lived irregularities drifting with background winds along with smaller, more localized structures, possibly associated with developing irregularities and active turbulence. Because PMSEs are associated with the dynamics and temperatures of the region, continued research on this topic should help scientists further understand middle atmospheric climate patterns, the authors say.

Title:

“Imaging of polar mesosphere summer echoes with the 450 MHz Poker Flat Advanced Modular Incoherent Scatter Radar”

Authors:

M. J. Nicolls, C. J. Heinselman, E. A. Hope, S. Ranjan, and J. D. Kelly
Center for Geospace Studies, SRI International, Menlo Park, California, U.S.A.
M. C. Kelley
School of Electrical and Computer Engineering, Cornell University, Ithaca, New York, U.S.A.

Source:

Geophysical Research Letters (GRL) paper doi:10.1029/2007GL031476, 2007

II. Downloading/ordering information for science writers and general public

Journalists and public information officers of educational and scientific institutions (only) who have registered with AGU for direct electronic access to selected research papers may download one or more of the reports cited in the Highlights (including pre-publication copies of articles listed as “in press”) by following the direct-access instructions below. Journalists and public information officers who have not registered for direct access may receive papers by contacting Peter Weiss (pweiss@agu.org, +1 202 777 7507), indicating which one(s). Include your name, the name of your publication, and your phone number. The papers will be e-mailed as pdf attachments.

Direct-Access Instructions: Each journalist or public information officer who has requested direct electronic access to selected AGU papers and received a reply email providing a username and password can download pdf files of one or more of the papers cited in the Highlights as follows: Click on the link at the end of the Highlight regarding the paper of interest. When you activate the dx.doi.org link you should be directed immediately to an abstract. Once there, click on the hyperlink at the top of the abstract, which says "Full Article (Nonsubscribers may purchase for $9.00, Includes print PDF." Clicking on it will take you to a page that asks you to log in. There, click on “click here” of the first option. That takes you to another Web page that asks for the username and password. Enter your username and password, then click on Submit. Now you should find yourself at the HTML version of the full article. It has a navigation bar on the left, with a link at the bottom entitled "PDF for Print". Click on that link and you should see the pdf of the selected paper appear on your screen. If you have any questions or problems with downloading, please contact Peter (pweiss@agu.org, +1 202 777 7507).

Anyone not a member of the press can also access any of the already-published AGU papers in this set of Highlights and purchase them for $9.00 apiece. To do so, follow the Direct Access Instructions above to the stage at which a username and password are submitted. At that point, click on the “Purchase Article” link at the bottom of the Web page.