Particulate and water-soluble carbon measured in recent snow at Summit, Greenland

Water-soluble organic carbon (WSOC), waterinsoluble particulate organic carbon (WIOC), and particulate elemental carbon (EC) were measured simultaneously for the first time on the Greenland Ice Sheet in surface snow and in a 3-meter snow pit. Snow pit concentrations reveal that, on average, WSOC makes up the majority (89%) of carbonaceous species, followed by WIOC (10%) and EC (1%). The enhancement of OC relative to EC (ratio 99:1) in Greenland snow suggests that, along with atmospheric particulate matter, gaseous organics contribute to snow-phase OC. Comparison of summer surface snow concentrations in 2006 with past summer snow pit layers (2002 – 2005) found a significant depletion in WSOC (20 – 82%) and WIOC (46 – 65%) relative to EC for 3 of the 4 years. The apparent substantial loss of WSOC and WIOC in aged snow suggests that post-depositional processes, such as photochemical reactions, need to be considered in linking ice core records of organics to atmospheric concentrations. Citation: Hagler, G. S. W., M. H. Bergin, E. A. Smith, J. E. Dibb, C. Anderson, and E. J. Steig (2007), Particulate and water-soluble carbon measured in recent snow at Summit, Greenland, Geophys. Res. Lett., 34, L16505, doi:10.1029/2007GL030110

Warming of the Antarctic ice-sheet surface since the 1957 International Geophysical Year

Assessments of Antarctic temperature change have emphasized the contrast between strong warming of the Antarctic Peninsula and slight cooling of the Antarctic continental interior in recent decades. This pattern of temperature change has been attributed to the increased strength of the circumpolar westerlies, largely in response to changes in stratospheric ozone. This picture, however, is substantially incomplete owing to the sparseness and short duration of the observations. Here we show that significant warming extends well beyond the Antarctic Peninsula to cover most of West Antarctica, an area of warming much larger than previously reported. West Antarctic warming exceeds 0.1 °C per decade over the past 50 years, and is strongest in winter and spring. Although this is partly offset by autumn cooling in East Antarctica, the continent-wide average near-surface temperature trend is positive. Simulations using a general circulation model reproduce the essential features of the spatial pattern and the long-term trend, and we suggest that neither can be attributed directly to increases in the strength of the westerlies. Instead, regional changes in atmospheric circulation and associated changes in sea surface temperature and sea ice are required to explain the enhanced warming in West Antarctica

Ice Cores from the St. Elias Mountains, Yukon, Canada: Their Significance for Climate, Atmospheric Composition and Volcanism in the North Pacific Region

A major achievement in research supported by the Kluane Lake Research Station was the recovery, in 2001 –02, of a suite of cores from the icefields of the central St. Elias Mountains, Yukon, by teams of researchers from Canada, the United States, and Japan. This project led to the development of parallel, long (103 – 104 year) ice-core records of climate and atmospheric change over an altitudinal range of more than 2 km, from the Eclipse Icefield (3017 m) to the ice-covered plateau of Mt. Logan (5340 m). These efforts built on earlier work recovering single ice cores in this region. Comparison of these records has allowed for variations in climate and atmospheric composition to be linked with changes in the vertical structure and dynamics of the North Pacific atmosphere, providing a unique perspective on these changes over the Holocene. Owing to their privileged location, cores from the St. Elias Icefields also contain a remarkably detailed record of aerosols from various sources around or across the North Pacific. In this paper we review major scientific findings from the study of St. Elias Mountain ice cores, focusing on five main themes: (1) The record of stable water isotopes (δ18O, δD), which has unique characteristics that differ from those of Greenland, other Arctic ice cores, and even among sites in the St. Elias; (2) the snow accumulation history; (3) the record of pollen, biomass burning aerosol, and desert dust deposition; (4) the record of long-range air pollutant deposition (sulphate and lead); and (5) the record of paleo-volcanism. Our discussion draws on studies published since 2000, but based on older ice cores from the St. Elias Mountains obtained in 1980 and 1996

A 700 year record of Southern Hemisphere extratropical climate variability

Annually dated ice cores from West and East Antarctica provide proxies for past changes in atmospheric circulation over Antarctica and portions of the Southern Ocean, temperature in coastal West and East Antarctica, and the frequency of South Polar penetration of El Niño events. During the period AD 1700–1850, atmospheric circulation over the Antarctic and at least portions of the Southern Hemisphere underwent a mode switch departing from the out-of-phase alternation of multi-decadal long phases of EOF1 and EOF2 modes of the 850 hPa field over the Southern Hemisphere (as defined in the recent record by Thompson and Wallace, 2000; Thompson and Solomon, 2002) that characterizes the remainder of the 700 year long record. From AD 1700 to 1850, lower-tropospheric circulation was replaced by in-phase behavior of the Amundsen Sea Low component of EOF2 and the East Antarctic High component of EOF1. During the first phase of the mode switch, both West and East Antarctic temperatures declined, potentially in response to the increased extent of sea ice surrounding both regions. At the end of the mode switch, West Antarctic coastal temperatures rose and East Antarctic coastal temperatures fell, respectively, to their second highest and lowest of the record. Polar penetration of El Niño events increased during the mode switch. The onset of the AD 1700–1850 mode switch coincides with the extreme state of the Maunder Minimum in solar variability. Late 20th-century West Antarctic coastal temperatures are the highest in the record period, and East Antarctic coastal temperatures close to the lowest. Since AD 1700, extratropical regions of the Southern Hemisphere have experienced significant climate variability coincident with changes in both solar variability and greenhouse gase

An automated approach for annual layer counting in ice cores

A novel method for automated annual layer counting in seasonally-resolved paleoclimate records has been developed. It relies on algorithms from the statistical framework of hidden Markov models (HMMs), which originally was developed for use in machine speech recognition. The strength of the layer detection algorithm lies in the way it is able to imitate the manual procedures for annual layer counting, while being based on statistical criteria for annual layer identification. The most likely positions of multiple layer boundaries in a section of ice core data are determined simultaneously, and a probabilistic uncertainty estimate of the resulting layer count is provided, ensuring an objective treatment of ambiguous layers in the data. Furthermore, multiple data series can be incorporated and used simultaneously. In this study, the automated layer counting algorithm has been applied to two ice core records from Greenland: one displaying a distinct annual signal and one which is more challenging. The algorithm shows high skill in reproducing the results from manual layer counts, and the resulting timescale compares well to absolute-dated volcanic marker horizons where these exist

Decadal ocean forcing and Antarctic ice sheet response: Lessons from the Amundsen Sea

Mass loss from the Antarctic Ice Sheet is driven by changes at the marine margins. In the Amundsen Sea, thinning of the ice shelves has allowed the outlet glaciers to accelerate and thin, resulting in inland migration of their grounding lines. The ultimate driver is often assumed to be ocean warming, but the recent record of ocean temperature is dominated by decadal variability rather than a trend. The distribution of water masses on the Amundsen Sea continental shelf is particularly sensitive to atmospheric forcing, while the regional atmospheric circulation is highly variable, at least in part because of the impact of tropical variability. Changes in atmospheric circulation force changes in ice shelf melting, which drive step-wise movement of the grounding line between localized high points on the bed. When the grounding line is located on a high point, outlet glacier flow is sensitive to atmosphere-ocean variability, but once retreat or advance to the next high point has been triggered, ocean circulation and melt rate changes associated with the evolution in geometry of the sub-ice-shelf cavity dominate, and the sensitivity to atmospheric forcing is greatly reduced

Developing optical efficiency through optimized coating structure: biomimetic inspiration from white beetles

Copyright © 2009 Optical Society of America. This paper was published in Applied Optics and is made available as an electronic reprint with the permission of OSA. The paper can be found at the following URL on the OSA website: http://www.opticsinfobase.org/ao/abstract.cfm?uri=ao-48-17-3243 . Systematic or multiple reproduction or distribution to multiple locations via electronic or other means is prohibited and is subject to penalties under law.The recent discovery of brilliant whiteness in ultrathin beetle scales indicated the availability of significant whiteness, brightness, and opacity from limited sample thickness. This is achieved in the beetle through optimization of the packing density of scattering centers in its elytral scales. Here, we directly test and apply this idea to whiteness and brightness in the production and appearance of mineral coatings on paper by varying the scattering center parameters that underpin its optical properties. Through biomimetic design principles, we find that desirably high optical scattering from mineral coatings can be achieved. Commercially, by using appropriately designed coating formulations, this leads to the prospect of equal optical performance using less scattering material

Ice sheet record of recent sea-ice behavior and polynya variability in the Amundsen Sea, West Antarctica

Author Posting. © American Geophysical Union, 2013. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 118 (2013): 118–130, doi:10.1029/2012JC008077.Our understanding of past sea-ice variability is limited by the short length of satellite and instrumental records. Proxy records can extend these observations but require further development and validation. We compare methanesulfonic acid (MSA) and chloride (Cl–) concentrations from a new firn core from coastal West Antarctica with satellite-derived observations of regional sea-ice concentration (SIC) in the Amundsen Sea (AS) to evaluate spatial and temporal correlations from 2002–2010. The high accumulation rate (~39 g∙cm–2∙yr–1) provides monthly resolved records of MSA and Cl–, allowing detailed investigation of how regional SIC is recorded in the ice-sheet stratigraphy. Over the period 2002–2010 we find that the ice-sheet chemistry is significantly correlated with SIC variability within the AS and Pine Island Bay polynyas. Based on this result, we evaluate the use of ice-core chemistry as a proxy for interannual polynya variability in this region, one of the largest and most persistent polynya areas in Antarctica. MSA concentrations correlate strongly with summer SIC within the polynya regions, consistent with MSA at this site being derived from marine biological productivity during the spring and summer. Cl– concentrations correlate strongly with winter SIC within the polynyas as well as some regions outside the polynyas, consistent with Cl– at this site originating primarily from winter sea-ice formation. Spatial correlations were generally insignificant outside of the polynya areas, with some notable exceptions. Ice-core glaciochemical records from this dynamic region thus may provide a proxy for reconstructing AS and Pine Island Bay polynya variability prior to the satellite era.This research was supported by an award from the Department of Energy Office of Science Graduate Fellowship Program (DOE SCGF) to ASC, a James E. and Barbara V. Moltz Research Fellowship to SBD, and by grants from NSF-OPP (#ANT-0632031 & #ANT-0631973); NSF-MRI (#EAR-1126217); NASA Cryosphere Program (#NNX10AP09G); and a WHOI Andrew W. Mellon Foundation Award for Innovative Research.2013-07-2

Towards Energy Synchronization of Two-Level Voltage Source Converters

Denne masteroppgaven ser på muligheten til å forbedre den faselåste sløyfen (PLL) som blir brukt i industrien i dag. PLLen er en del av kontrollsystemet i en frekvensomformer (VSC) og blir brukt til å synkronisere to frekvenser. En VSC er en kraftelektronisk komponent som omformer likestrøm til vekselstrøm og omvendt for å sikre en sikker flyt av energi. På grunn av det grønne skiftet er det en stadig økning av fornybare energikilder i kraftnettet. En utfordring med dette er at de store sykrongeneratorene i kullkraftverkene forsvinner. Dette skiftet gjør at det totale treghetsmomentet i kraftnettet reduseres, noe som gjør at kravene til de kraftelektroniske komponentene øker. I spesialiseringsoppgaven [1], ble det oppdaget tre utfordringer knytett til den tradisjonelle PLLen. To av disse tre utforningene har blitt forsøkt løst i denne masteroppgaven. Hovedmålet med dette arbeidet er å løse problemene knyttet til ulineariteter i systemet. Dagens PLLer er designet for et linært system, mens systemene de blir anvendt i er mer komplekse. Tre PLLer som tar ulinearitetene til betrakting har blitt designet ved hjelp av Lyapunov teori samt passivitet teori. I dette arbeidet har den angulære frekvensen som PLLen genererer blitt sett på som et kontrollobjekt. Dette gjør at det trengs to forskjellige angulære frekvenser for å beskrive systemet istedenfor en. En frekvens relatert til nettet, mens den andre er relatert til PLLen. Forskjellen mellom de tre forskjellige PLLene designet i denne oppgaven er i utgangspunktet relatert til hvilken angulær frekvens de ulike tilstandene i systemet er referert til. De tre ulike PLLene designet er sammenlignet med en simpel PLL som blir brukt i industrien i dag. De blir sammenlignet med hensyn til atferden de får når en referanseverdi er endret, i tillegg stabiliteten de viser. På grunn av differanse mellom stabilitetsresultatene og simuleringsresultatene har simuleringsresultatene blitt favorisert på bakgrunn av at det er en mer kompleks representasjon av systemet sammenlignet med likningene brukt i stabilitetsutregningene. I tillegg til dette ble den ene PLLen utviklet også til å kanskje ha en løsning på et av de to andre problemene PLLene har i dag. På grunn av dårlig tid ble dessverre ikke mange nok simulering utført, men uansett viser PLLene gode resultater. Dersom flere og mer krevende simuleringer utføres på de ulike PLLene, kan de i beste fall bli en del av industrien. De ulike PLLene designet i dette arbeidet kommer med et stabilitets-sertifikat siden de oppfyller kravene som trengs for at et system skal være globalt asymptotisk stabilt.Substituting traditional fossil energy sources with renewable energy sources will require the use of power electronic converters to interface them to the grid, such as the well-known Voltage Source Converter (VSC). This master thesis investigates the possibility to improve the Phase Locked Loops (PLLs) that are currently used in the industry for grid synchronization of VSCs and are a central part of their control strategy. From a stability viewpoint, the PLLs used today seem to be designed for synchronizing converters with strong ac grids, where the frequency can be considered constant and the grid is practically unaffected by disturbances. However, this standard solution has a below par performance when synchronizing with weak grids, making the system more prone to undesired unstable behavior. In order to mitigate this risk, small-signal stability studies are usually carried out on a case-by-case basis in order to find an appropriate control tuning that renders the system stable. However, as small-signal methods are based on linearization around a nominal operating point, the system inherent nonlinearities are neglected in the design, consequently making the VSC and the rest of the system vulnerable under larger disturbances. Hence, a more robust PLL that takes the nonlinearities of the system into account has to be designed. In the specialization project [1], which served as the preliminary groundwork of this master thesis, three challenges associated to the synchronization of the nonlinear model of the VSC and its PLL design were reported; i.e., two related to two different types of system nonlinearities (one trigonometric and a bilinear product), and one related to the unavailability of a state-variable. The work of this master thesis aims to solve two of these three challenges. More precisely, one of the nonlinearities (of trigonometric nature) has been neglected in the model, and consequently in the control design process, to reduce the complexity of the task. Under this approximation, three alternative PLLs have been designed by means of Lyapunov and passivity theories--in order to take into account the remaining system nonlinearity--and compared with a standard PLL used in industry today. In order to design the PLLs, the nonlinear model of the VSC in state-space representation has been validated. It is worth mentioning that in this modeling phase, the angular frequency provided by the PLL, ω_PLL, is treated as a control variable and hence it is different from the angular frequency of the grid, ω_g. This results in two different angular frequencies of the grid; therefore two different Park matrices have to be used in order to represent the system in the two associated synchronous reference frames. The choice regarding which of the two rotating reference frames is associated to the different state variables is what mainly set appart the three alternative PLLs designed. The passivity-based PLLs showed comparable performance with the traditional PLL, but come with the added advantage of a nonlinear stability certificate. Given that in the design process a simplified model was used, the stability of the different PLLs have also been analysed via time-domain simulations. Finally, the most promising PLL alternative out of the three has been identified and further modified to be able to operate without the unavailable state measurement and without compromising the stability proof. The performance results related to this PLL are very promising. However, due to lack of time and resources more simulations and test have to be conducted in order to make available this new alternative PLL to the industry. Preferably simulations with a more complex system, where the PLLs face more challenging tasks have to be made, and an extension of the proof to include the neglected trigonometric nonlinearity