A numerical comparison of approximations to the Stokes equations used in ice sheet and glacier modeling

first_imgA computational analysis of the accuracy of different approximations to the Stokes equations for momentum balance used in ice sheet modeling is performed by solving a particular tractable form of the equations appropriate for small perturbations of the ice surface, describing the uniform flow of ice with a Glen rheology on an infinitely long and broad section. The approximants comprise the shallow ice approximation and various schemes for incorporating longitudinal stresses and, in one case, the horizontal gradient of the horizontal plane shear stresses. The simplifications lead to a vertically one-dimensional numerical problem, whose solution can be computed rapidly. The relaxation rate of perturbations as well as other response descriptors for the stable full system and approximants are compared. Compared with the shallow ice approximation, the inclusion of longitudinal stresses increases accuracy at shorter wavelengths, but accuracy is poor at wavelengths around or less than the ice sheet thickness. Even though analysis shows that the horizontal gradients of the horizontal plane shear stresses are of similar magnitude to longitudinal stress effects, computations show, in agreement with glaciological belief, that longitudinal stress effects are more significant and need to be corrected for first in practice. Two schemes, a multilayer scheme and a one-layer scheme, are particularly good and should be investigated further in cases where perturbations from uniformity are large. Some other apparently plausible approximations introduce nonphysical instabilities. New schemes need to be assessed in the way described in this paper before being used in real ice sheet models.last_img read more

Image analysis of color aerial photography to estimate penguin population size

first_imgPenguin populations are potentially sensitive indicators of ecological change in Antarctic and sub-Antarctic marine ecosystems. Aerial photographic surveys provide the most robust method for estimating breeding population size, particularly for large colonies. Obtaining population estimates from aerial photographs is laborious and usually carried out by manually counting individual birds on highly magnified prints. I derived population estimates using computer-based image analysis of digitally scanned color aerial photographs of macaroni penguin (Eudyptes chrysolophus) colonies at Bird Island, South Georgia. I compared automated image analysis with manual counts from the photographic prints and conventional ground counts, highlighting assumptions that contributed to differences in population estimates. The automated image-analysis routines produced estimates that were highly correlated with ground counts, indicating that the technique could be reliably used for large-scale macaroni penguin population surveys.last_img read more

Surface-ocean CO2 variability and vulnerability

first_imgImproved sampling technologies, international observing networks, and data synthesis efforts are providing an unprecedented view of the global patterns and decadal variability of surface-ocean partial pressure of carbon dioxide (pCO(2)) and air-sea CO2 flux. A new observational synthesis for surface-ocean pCO(2) leads to a global ocean net CO2 sink of -1.8 Pg C yr(-1) (+/-0.7); continental margins and estuaries together act as a small net source (similar to 0.15 Pg C yr(-1)). New results on decadal trends indicate recent decreases in efficiency of the ocean to absorb CO2 in the Southern Ocean, North Atlantic, and Equatorial Pacific. Although we do not yet have an estimate of the trend in the global oceanic sink Of CO2, the existing observations may help explain temporal variations in the fraction of anthropogenic CO2 emissions that remain in the atmosphere (airborne fraction). Major vulnerabilities in the future behavior of the ocean-carbon-climate system include the effects of ocean warming, enhanced vertical stratification, strengthening and poleward contraction of westerly winds in the Southern Ocean, and shifts in the biological pump and ecosystem functioning. To address issues of both ocean-carbon variability and vulnerability, a sustained surface-ocean-carbon-observing system needs to be established with improved global spatial coverage and internationally coordinated data synthesis activities. (C) 2009 Elsevier Ltd. All rights reserved.last_img read more

East Antarctic rifting triggers uplift of the Gamburtsev Mountains

first_imgThe Gamburtsev Subglacial Mountains are the least understood tectonic feature on Earth, because they are completely hidden beneath the East Antarctic Ice Sheet. Their high elevation and youthful Alpine topography, combined with their location on the East Antarctic craton, creates a paradox that has puzzled researchers since the mountains were discovered in 19581. The preservation of Alpine topography in the Gamburtsevs2 may reflect extremely low long-term erosion rates beneath the ice sheet3, but the mountains’ origin remains problematic. Here we present the first comprehensive view of the crustal architecture and uplift mechanisms for the Gamburtsevs, derived from radar, gravity and magnetic data. The geophysical data define a 2,500-km-long rift system in East Antarctica surrounding the Gamburtsevs, and a thick crustal root4 beneath the range. We propose that the root formed during the Proterozoic assembly of interior East Antarctica (possibly about 1 Gyr ago), was preserved as in some old orogens5, 6 and was rejuvenated during much later Permian (roughly 250 Myr ago) and Cretaceous (roughly 100 Myr ago) rifting. Much like East Africa7, the interior of East Antarctica is a mosaic of Precambrian provinces affected by rifting processes. Our models show that the combination of rift-flank uplift, root buoyancy and the isostatic response to fluvial and glacial erosion explains the high elevation and relief of the Gamburtsevs. The evolution of the Gamburtsevs demonstrates that rifting and preserved orogenic roots can produce broad regions of high topography in continental interiors without significantly modifying the underlying Precambrian lithosphere.last_img read more

Ground-penetrating radar measurements of debris thickness on Lirung Glacier, Nepal

first_imgSupraglacial debris thickness is a key control on the surface energy balance of debris-covered glaciers, yet debris thickness measurements are sparse due to difficulties of data collection. Here we use ground-penetrating radar (GPR) to measure debris thickness on the ablation zone of Lirung Glacier, Nepal. We observe a strong, continuous reflection, which we interpret as the ice surface, through debris layers of 0.1 to at least 2.3 m thick, provided that appropriate acquisition parameters were used while surveying. GPR measurements of debris thickness correlate well with pit measurements of debris thickness (r = 0.91, RMSE = 0.04 m) and two-way travel times are consistent at tie points (r = 0.97). 33% of measurements are <0.5 m, so sub-debris melting is likely important in terms of mass loss on the debris-covered tongue and debris thickness is highly variable over small spatial scales (of order 10 m), likely due to local slope processes. GPR can be used to make debris thickness measurements more quickly, over a wider debris thickness range, and at higher spatial resolution than any other means and is therefore a valuable tool with which to map debris thickness distribution on Himalayan glaciers.last_img read more

Strong whistler mode waves observed in the vicinity of Jupiter’s moons

first_imgUnderstanding of wave environments is critical for the understanding of how particles are accelerated and lost in space. This study shows that in the vicinity of Europa and Ganymede, that respectively have induced and internal magnetic fields, chorus wave power is significantly increased. The observed enhancements are persistent and exceed median values of wave activity by up to 6 orders of magnitude for Ganymede. Produced waves may have a pronounced effect on the acceleration and loss of particles in the Jovian magnetosphere and other astrophysical objects. The generated waves are capable of significantly modifying the energetic particle environment, accelerating particles to very high energies, or producing depletions in phase space density. Observations of Jupiter’s magnetosphere provide a unique opportunity to observe how objects with an internal magnetic field can interact with particles trapped in magnetic fields of larger scale objects.last_img read more

Recent decrease of summer sea ice in the Weddell Sea, Antarctica

first_imgIn Austral summer 2016/17 the sea ice extent (SIE) in the Weddell Sea dropped to a near‐record value in the satellite era (1.88 × 106 km2, 56% of the long‐term mean); a large negative seasonal anomaly that persisted in an unprecedented fashion for the following three summers. Various atmospheric and oceanic factors played a part in the change. Ice loss started in September 2016 when the northern Weddell Sea experienced westerly winds of record strength advecting multi‐year sea ice from the region. In late 2016 a polynya over Maud Rise contributed to low SIE over the eastern Weddell Sea. With extensive areas of open water early in the summer, upper ocean temperatures increased by ~0.5° C, with the anomalies persisting in subsequent years. The re‐appearance of the Maud Rise polynya in 2017, high ocean temperatures and storms of record depth kept the summer SIE low.last_img read more

Family member tried to extort NFL star for $5 million, court documents show

first_img FacebookTwitterLinkedInEmailBob Levey/Getty Images(NEW YORK) — A relative of NFL star Tyrann Mathieu allegedly tried to extort him for $5 million, according to federal court documents unsealed on Tuesday. Mathieu, who now plays for the Kansas City Chiefs after stints with Houston and Arizona, was allegedly blackmailed over private information, with the relative saying he would send private information to gossip outlet TMZ if he wasn’t paid. Geourvon Keinell Sears, 20, is described in the criminal complaint as “a family member and heretofore a close associate of Victim #1.”While the victim is unnamed in the criminal complaint — only referred to as a professional athlete with the initials “TM” — Mathieu’s lawyer, Denise White of EAG Sports, confirmed to Kansas City ABC affiliate KMBC-TV that Mathieu was the victim.White said Mathieu is dealing with a “private and personal family issue,” and offered no further comment.Sears allegedly spoke to Mathieu on the phone in late April and demanded the NFL star deposit $1.5 million in the suspect’s bank account by the end of the week, according to the complaint. Mathieu refused and blocked Sears from being able to contact him by phone.But a day after being blocked, the suspect allegedly contacted a friend of Mathieu and said in a text, “5 MILLION BY FRIDAY IM DONE TALKING EMAILING TMZ NOW.” He also provided his bank account information so Mathieu, or his associates, could deposit the money, according to court documents.Sears then allegedly went as far as to send Mathieu’s friends a voice message saying, “I want 1.5 million by Friday or I am going to kill all you all.”Mathieu told police he feared for his and his family’s safety and believed Sears was capable of violence, according to the document.The suspect then spoke to Mathieu’s agent by telephone and said he would “injure TM’s reputation by alleging sexual misconduct” if he didn’t receive the money.Sears was initially expected to make a first court appearance in Louisiana, where he lives, on Thursday, but his lawyer successfully petitioned for 30 more days to review evidence in the case.He will appear in court on June 21.Sears was released on $25,000 bond and ordered to have no contact with Mathieu or his associates, in person or via social media, cellphone or computer. He is being held on home incarceration and with a monitoring device, according to the court order.Mathieu struggled with disciplinary issues while at Louisiana State University, but his play on the field was electric as both a freshman and sophomore. As a sophomore he was named a finalist for the Heisman Trophy, awarded to the best college player in the nation each year.But following the 2011-12 season, the star was dismissed from the team by head coach Les Miles. While the reason was never released by the school, ESPN reported that he had failed multiple drug tests. Mathieu enrolled back in LSU that fall as only a student, but was arrested for drug possession in October 2012 and declared for the NFL draft a month later.Mathieu was drafted in the third round due to concerns about his behavior, but quickly became one of the best safeties in the league paired with his former college teammate Patrick Peterson. He made the All-Rookie team in 2013 and was a first-team All Pro in 2015. He signed a five-year, $62 million deal following that season, but was injured in 2016 and released by Arizona in 2018.He played one season with the Houston Texans before signing with the Chiefs this offseason for three years and $42 million. Copyright © 2019, ABC Radio. All rights reserved. May 22, 2019 /Sports News – National Family member tried to extort NFL star for $5 million, court documents show Beau Lundcenter_img Written bylast_img read more

Donnie Tillman Takes Leave of Absence From Utah Men’s Basketball Program

first_img Tags: Donnie Tillman/Larry Krystkowiak/Pac-12 Sixth Man of the Year/Utah Men’s Basketball Written by Brad James June 7, 2019 /Sports News – Local Donnie Tillman Takes Leave of Absence From Utah Men’s Basketball Program FacebookTwitterLinkedInEmailSALT LAKE CITY-Per news released Friday morning by the University of Utah men’s basketball program, junior forward Donnie Tillman is taking a leave of absence from the program. He will not play for the Utes during the 2019-20 season.Tillman has played in 58 games for the Utes in his freshman and sophomore seasons and averaged career highs in points (10.3) and rebounds (5.3) in 2018-19.This made him the Pac-12’s Sixth Man of the Year.Utah head coach Larry Krystkowiak stated he wishes Tillman the best as he will be near his mother in Detroit as she battles her health issues.last_img read more

New York Giants name rookie Daniel Jones starting QB for Week Three

first_img Beau Lund FacebookTwitterLinkedInEmailPhoto by Allen Kee / ESPN Images(NEW YORK) — The New York Giants are turning to rookie Daniel Jones at quarterback for Week Three, ending Eli Manning’s run as the team’s starter after nearly 15 years.“Eli and I spoke this morning,” head coach Pat Shurmur said in a news release. “I told him that we are making a change and going with Daniel as the starter.”Shurmur said Manning was disappointed by the news, but vowed to continue to be a good teammate. Jones was the team’s first-round selection in this year’s draft, number six overall. The selection was questioned by some experts, who questioned whether his collegiate performance at Duke warranted such a high draft pick. Manning made a franchise-record 232 starts for New York. He won two Super Bowls with the team, beating the New England Patriots both times.“Ultimately, this is a move that I felt was best for this team at this time,” Shurmur said. “This move is more about Daniel moving forward than about Eli.”Copyright © 2019, ABC Radio. All rights reserved. Written bycenter_img September 17, 2019 /Sports News – National New York Giants name rookie Daniel Jones starting QB for Week Threelast_img read more