AGU journal highlights -- 24 July 2006

Simulating the horizontal wind velocity variance in the upper troposphere

The spectrum of wind velocity variance as a function of horizontal scale in the atmosphere is often referred to as the kinetic energy (KE) spectrum. Noting that various current high-resolution Global Circulation Models (GCMs) perform very differently in terms of their ability to simulate a realistically shallow mesoscale [tens to hundreds of kilometers] kinetic energy spectrum, Takahashi et al. examined the horizontal spectrum of KE in the upper troposphere with the Atmospheric GCM for the Earth Simulator (AFES). By comparing model output with aircraft data, they found that the control version of AFES can spontaneously simulate a realistic kinetic energy spectrum through a decade or more of the mesoscale. Further, they constructed a reliable way to adjust the horizontal diffusion in the model, so that the KE spectrum at all model resolutions converges to realistic values. The authors note that simulating a realistic kinetic energy spectrum down to the smallest resolved scales is important for the model to dissipate motions in a realistic fashion, a prerequisite for having confidence in the model-generated climate variations and its response to imposed climate changes.

Title: Explicit global simulation of the mesoscale spectrum of atmospheric motions

Yoshiyuki O. Takahashi: Graduate School of Science, Hokkaido University, Sapporo, Japan;

Kevin Hamilton: International Pacific Research Center, University of Hawaii, Honolulu, Hawaii, U.S.A.;

Wataru Ohfuchi: Earth Simulator Center, Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan.

Source: Geophysical Research Letters (GL) paper 10.1029/2006GL026429, 2006

Glaciers in Europe's Alps may disappear by 2100

During the last 150 years, many mountain ranges in Europe have lost a significant proportion of glacial extent, with strong acceleration occurring in the past two decades. To quantify past, as well as potentia,l evolution of the area and volume of glaciers within the European Alps in the context of impending climate change, Zemp et al. characterized the system using on-site measurements, remote sensing techniques, and numerical modeling. They found that between 1850 and 1970, Alpine glaciers lost 35 percent of their total surface area; by 2000, almost 50 percent had disappeared. Their estimates also place current glacial volume at only one third of the 1850 value. Using models based on the rate of glacier loss, and predicted temperatures and precipitation levels over the next century, the authors determine that a 3 degree Celsius [5 degree Fahrenheit] warming of summer air would reduce the currently existing Alpine glacier cover by 80 percent. However, if temperatures were to rise by 5 degrees Celsius [9 degrees Fahrenheit], the Alps would be completely free of perennial surface ice by 2100.

[See also AGU Press Release 06-26:]

Title: Alpine glaciers to disappear within decades?

Michael Zemp, Wilfried Haeberli, Martin Hoelzle and Frank Paul: Glaciology and Geomorphodynamics Group, University of Zurich, Zurich, Switzerland.

Source: Geophysical Research Letters (GL) paper 10.1029/2006GL026319, 2006

Quantifying the variation in motion of the Earth's rotation pole on weekly scales

The motion of Earth's rotation pole shows two large trends, one with a period of about 433 days corresponding to Chandler wobble (free Eulerian wobble), and another corresponding to annual oscillations forced by the seasonal displacement of air and water masses. Every 6.4 years, annual and Chandler wobble combine to almost cancel each other out, severely reducing polar motion, which occurred between November 2005 and February 2006. Lambert et al. sought to characterize the small amount of wobble left, a value representing wobble on weekly scales. Using high-precision Earth orientation data for these days, the authors directly observed the very small structures of this weekly wobble for the first time in the history of polar motion observation. They also calculated the polar motion predicted during this time interval from atmospheric and oceanic circulation models, and found that the centimeter-level polar motion displacements during the 2005-2006 winter season were almost fully explained by major pressure events on the continents and on the ocean.

[See also AGU Press Release 06-22:]

Title: Rapid variations in polar motion during the 2005–2006 winter season

S. B. Lambert and V. Dehant: Royal Observatory of Belgium, Brussels, Belgium;

C. Bizouard: Paris Observatory, IERS Earth Orientation Center, Paris, France.

Source: Geophysical Research Letters (GL) paper 10.1029/2006GL026422, 2006

Observations of aurorae validate current theories of wave dispersion in magnetic fields

Alfvén waves transport energy between the magnetosphere and ionosphere in a manner analogous to waves on a string. Alfvén waves are particularly intense on field lines threading auroral displays, indicating a causal relation. But the mechanism by which the electromagnetic energy of the wave is converted to particle kinetic energy exciting the aurora remains a subject of debate. One hypothesis attributes the energy conversion to wave dispersion, caused by electron inertia as the Alfvén wave approaches the dense ionosphere. In the inertial limit, wave energy spreads across the background magnetic field, with phase- and group-velocities oppositely directed. These features have been observed in laboratory plasmas, but have yet to be observed in the natural auroral plasma. Using an intensified narrow-field video system, recording 50 images per second, Semeter and Blixt have provided the first field validations of these conjectures. Their analysis shows the aurora as an expanding amplitude envelope within which fine-scale periodic features evolved. The authors expect that advances in spectral imaging will help facilitate a more systematic investigation of wave dispersion as a mechanism for dissipating magnetospheric energy.

Title: Evidence for Alfvén wave dispersion identified in high resolution auroral imagery

J. Semeter: Department of Electrical and Computer Engineering and Center for Space Physics, Boston University, Boston, Massachusetts, U.S.A.;

E. M. Blixt: Department of Physics, University of Tromsø, Tromsø, Norway.

Source: Geophysical Research Letters (GL) paper 10.1029/2006GL026274, 2006

Forecasting hurricane intensity using supercomputers and data from Hurricane Katrina

Although hurricane track forecasts have been steadily improving over the past few decades, progress on hurricane intensity forecasts has been slow, mainly because most general circulation models (GCMs) lack sufficient resolution to simulate near-eye structure and other factors. Recent advances in the capabilities of high-end supercomputers have allowed a few GCMs, including the mesoscale-resolving finite-volume GCM (fvGCM) developed by NASA, to overcome previous modeling failures. Using the fvGCM, Shen et al. modeled Hurricane Katrina, which in late August 2005 underwent two stages of rapid intensification, becoming the sixth most intense hurricane in modern history to have developed over the Atlantic Ocean. Six five-day simulations of Hurricane Katrina at two different scale resolutions (0.125 degree and 0.25 degree) show that data modeled at fine spatial scales more accurately predict actual hurricane intensity, producing calculations of hurricane center pressure with a high degree of accuracy. The fine-scale runs also produce better near-eye wind distributions and a more realistic average intensification rate. The authors expect that such research in predicting hurricane intensity will aid in future disaster mitigation efforts.

Title: Hurricane forecasts with a global mesoscale-resolving model: Preliminary results with Hurricane Katrina (2005)

B.-W. Shen: Laboratory for Atmospheres, NASA Goddard Space Flight Center, Greenbelt, Maryland, U.S.A.; and Science Application International Corporation, Beltsville, Maryland, U.S.A.;

R. Atlas: National Oceanic and Atmospheric Administration's Atlantic Oceanographic and Meteorological Laboratory, Miami, Florida, U.S.A.;

O. Reale and J.-D. Chern: Laboratory for Atmospheres, NASA Goddard Space Flight Center, Greenbelt, Maryland, U.S.A.; University of Maryland, Baltimore County, Baltimore, Maryland, U.S.A.;

S.-J. Lin: National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey, U.S.A.;

J. Chang: NASA Ames Research Center, Moffett Field, California, U.S.A.; Computer Sciences Corporation, Moffett Field, California, U.S.A.;

C. Henze: NASA Ames Research Center, Moffett Field, California, U.S.A.;

J.-L. Li: NASA Jet Propulsion Laboratory, Pasadena California, U.S.A.

Source: Geophysical Research Letters (GL) paper 10.1029/2006GL026143, 2006

New insight into transporting coastal waters offshore

Off the coast of northwest Australia, the dominant surface current is the southwestward (poleward) Leeuwin Current, a flow generated by water flushing through Indonesia during normal conditions of the El Nino/Southern Oscillation system. In June-July 2003, during the tail end of an El Nino event, Brink and Shearman conducted a month-long research cruise to study any resulting weakening in the Leeuwin Current. In the middle of their observation period, the authors noticed that shelf edge alongshore currents reversed northeastward (equatorward) for nine days. A survey of this reversal revealed that the weakening of the Leeuwin Current allowed adjacent water over the broad northwest Australian continental shelf to leak further seaward than normal. The authors propose that the shelf water, salty through evaporative densification, was then caught up in Ekman transports, which pushed flow northeastward. They note that such processes, though little explored, are important mechanisms of transporting coastal waters offshore, which among other things, influences the nutrient budget of the ocean.

Title: Bottom boundary layer flow and salt injection from the continental shelf to slope

K. H. Brink: Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, U.S.A.;

R. Kipp Shearman: College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon, U.S.A.

Source: Geophysical Research Letters (GL) paper 10.1029/2006GL026311, 2006


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