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AGU journal highlights -- Sept. 3, 2008

1. Tracking the dust belt

Dust is one of several types of aerosols in the atmosphere that affects climate in still poorly understood ways. Dust scatters and absorbs solar radiation, cooling the atmosphere, but it can also interact with cloud formation and other climate-affecting processes. Launch of the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite in 2006 was aimed at improving understanding of how clouds and aerosols interact. Liu et al. present data from first-year CALIPSO lidar measurements to generate the first snapshot of the global distribution of dust. On the basis of dust's optical properties, they were able to separate data on dust from those of clouds and other aerosols, such as soot. They find that dust is mainly a Northern Hemisphere phenomenon, arising largely from North Africa and the Arabian Peninsula. They also determined that spring is the dustiest season, when some areas are influenced by dust up to 50 percent of the time, and that dust can reach as high as 6 kilometers (4 miles). The authors expect that understanding where and when dust arises and how it moves can help improve model simulations of cloud-aerosol interactions and their effect on climate.

Title: A Height Resolved Global View of Dust Aerosols from the First Year CALIPSO Lidar Measurements

Authors: Dong Liu: Department of Atmospheric Science, University of Wyoming, Laramie, Wyoming, U.S.A.; also at Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei, China;

Zhien Wang: Department of Atmospheric Science, University of Wyoming, Laramie, Wyoming, U.S.A.;

Zhaoyan Liu: National Institute of Aerospace, Hampton, Virginia, U.S.A;

Dave Winker and Charles Trepte: NASA Langley Research Center, Hampton, Virginia, U.S.A.

Source: Journal of Geophysical Research-Atmospheres (JGR-D) paper 10.1029/2007JD009776, 2008;

2. New model better represents global climate effects in Amazon basin

El NioSouthern Oscillation (ENSO) cycles strongly influence the interannual variability of climate, weather patterns, and streamflow in South America's Amazon basin. However, current global climate models (GCMs) are unable to exactly capture this variability, in part because the grids used in GCMs (roughly 2 degrees�� degrees of latitude and longitude) are too large to properly resolve topographical features such as the Andes, which have a narrow width (only 200? kilometers (120-190 miles). Seeking to correct for this, Medvigy et al. use the Ocean-Land-Atmosphere Model (OLAM), a new Earth System Model (ESM), which is capable of simulating regions such as mountain ranges at selectively high resolution while the remainder of the world continues to be simulated at coarser resolution. They find that when the Andes are resolved more coarsely than 100 km (60 mi), the model incorrectly simulates ENSO effects such as the dry anomalies that are observed in the Amazon during ENSO events. In contrast, the model correctly simulates the observed dry conditions provided that the Andes are simulated at resolutions finer than 100 km (60 mi).

Title: Modeling interannual variability of the Amazon hydroclimate

Authors: D. Medvigy, R. L. Walko, and R. Avissar: Department of Civil and Environmental Engineering, Duke University, Durham, North Carolina, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL034941, 2008;

3. Dust sources identified in Australia's Lake Eyre Basin

Covering about 1.17 million square kilometers (0.452 million square miles), Australia's Lake Eyre Basin is an important source of dust for the Southern Hemisphere. But what within the basin is creating the dust? Using data from NASA's Moderate Resolution Imaging Spectroradiometer, Bullard et al. analyze the spatial and temporal variability of dust sources in Lake Eyre Basin. They find that of 529 dust plumes detected, 37 percent originated from dunes and other windblown features, 30 percent originated from floodplains and other alluvial features, and 29 percent originated from ephemeral lakes (only 4 percent of dust plumes originated from the bed of ephemeral Lake Eyre itself). At this subbasin scale, the relative importance of different dust source areas varied primarily in response to sediment supply and availability and thus was not related to wind transport, suggesting that the Lake Eyre Basin is a supply-limited system. The authors expect that their method, if used for other basins, has the potential to bridge the gap between global modeling and field studies and will allow scientists to assess how dust emissions within a basin vary over time.

Title: Sub-basin scale dust source geomorphology detected using MODIS

Authors: Joanna Bullard and Matthew Baddock: Department of Geography, Loughborough University, Leicestershire, U.K.;

Grant McTainsh: Australian Rivers Institute, Griffith School of the Environment, Griffith University, Brisbane, Queensland, Australia;

John Leys: Department of Environment and Climate Change, Gunnedah, New South Wales, Australia

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL033928, 2008;

4. Infrared images reveal groundwater plumes off Hawaii coast

Although most freshwater entering the world's oceans comes from rivers, nutrient inputs into coastal environments via submarine groundwater discharge (SGD) are disproportionately large. Groundwaters typically contain high concentrations of dissolved chemicals, making SGD important to phytoplankton and algae growth patterns and thus critical to healthy aquatic habitats. SGD has also been suggested as stimulating harmful algal blooms and as a pathway for pollution dispersion into the ocean. Noting that human populations are increasingly reliant on resources within coastal zones, Johnson et al. conducted airborne surveys using thermal infrared (TIR) sensors to determine how much SGD is escaping into the waters off the western side of the island of Hawaii. Groundwater is the only significant source of freshwater and new nutrients to this area's coastal ocean. The authors use TIR imagery to pinpoint input locations and detail fine-scale dispersal patterns of groundwater discharging as both diffuse flow and point-source plumes into the region's coastal zone. They further show how nutrient mixing trends can be integrated into TIR sea surface temperature to produce surface water nutrient maps.

Title: Aerial infrared imaging reveals large nutrient-rich groundwater inputs to the ocean

Authors: Adam G. Johnson and Craig R. Glenn: Department of Geology and Geophysics, School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, Hawaii, U.S.A.;

William C. Burnett and Richard N. Peterson: Department of Oceanography, Florida State University, Tallahassee, Florida, U.S.A.;

Paul G. Lucey: Hawaii Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, Hawaii, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL034574, 2008;

5. Transient currents after lightning flashes

Within the Earth system, the ocean, land, and ionosphere are highly conductive. Conductivity of the atmosphere is low near sea level but increases rapidly with height up to the ionosphere. This difference gives rise to the global electric circuit (GEC) through the presence of electrical generators in the atmospherethunderstorms, shower clouds, dust storms, and snowstorms. Important contributors to the GEC are cloud-to-ground (CG) and intracloud lightning flashes. CG flash charges are quickly distributed over the surface and thereby contribute to the Earth's total charge and to charging or discharging of the GEC. After this fast process, a slow transient stage occurs, caused by redistribution of charge in the atmosphere. Noting that contributions of transient currents to the GEC are poorly understood, Mareev et al. use a numerical model of the transient electric field due to CG and intracloud flashes. They find that because the slow transients counteract the flashes, only a portion of the charge neutralized by a flash contributes to the GEC, with efficiency depending on the altitudes of the lightning charges.

Title: On the role of transient currents in the global electric circuit

Authors: E. A. Mareev, S. A. Yashunin, and S. S. Davydenko: Institute of Applied Physics, Russian Academy of Science, Nizhny Novgorod, Russia;

T. C. Marshall, M. Stolzenburg, and C. R. Maggio: Department of Physics and Astronomy, University of Mississippi, University, Mississippi, U.S.A.;

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL034554, 2008;

6. Meteor-produced ionization influences radio propagation

Radio communication relies on a region within the ionosphere (the F region, between 250 and 350 kilometers (160-220 miles) in altitude) that reflects and refracts radio waves so that long-distance communication can be achieved. However, at lower altitudes (in the ionosphere's E region, between 90 and 160 km (56-99 mi), small pockets of unusually ionized gas can also reflect radio waves, intermittently allowing radio communication at higher frequencies and to farther distances. Called "sporadic E," these clouds of ionized gas are frequently observed at midlatitudes but are more rarely observed in low latitudes, most likely because signals become obscured by the intense equatorial electrojet, which is a narrow ribbon of current that flows east above the geomagnetic equator during the day. Using data collected from Peru's Jicamarca Radio Observatory, Malhotra et al. develop a new analysis method to locate sporadic E within the low-latitude ionosphere. They compare the structure and characteristics of these equatorial features with their midlatitude counterparts and find a direct relationship between meteor-produced ionization and the formation and evolution of equatorial sporadic E layers.

Title: Effect of meteor ionization on sporadic-E observed at Jicamarca

Authors: Akshay Malhotra, John D. Mathews, and Julio Urbina: Pennsylvania State University, University Park, Pennsylvania, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL034661, 2008;

7. Redefining biologically distinct oceanic provinces

Biogeographic provinces are categories used for comparing and contrasting biodiversity and nutrient cycling among different ocean regions. However, their use is limited due to current methods that subjectively classify provinces. It is also unknown how province boundaries respond to seasonal and climate forcings. The currently used static provinces make it difficult to test hypotheses about how biodiversity and nutrient cycling will change over time. To solve this problem, Oliver and Irwin develop a method to objectively classify global remote sensing data so that they automatically define ocean provinces in both space and time. They find that seasonal patterns in province boundaries reflect well-known oceanic processes, as verified by ship tracks. Further, province distributions in the equatorial Pacific correlate well with El NioSouthern Oscillation patterns. The authors expect that such objective classification will ease cross-province analysis and will allow for more rigorous testing of hypotheses related to ocean dynamics.

Title: Objective global ocean biogeographic provinces

Authors: Matthew J. Oliver: Institute of Marine and Coastal Sciences, Rutgers University, New Brunswick, New Jersey, U.S.A.; now at College of Marine and Earth Studies, University of Delaware, Lewes, Delaware, U.S.A.;

Andrew J. Irwin: Department of Mathematics and Computer Science, Mount Allison University, Sackville, New Brunswick, Canada.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL034238, 2008;


Contact: Peter Weiss
American Geophysical Union

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