On the state dependency of fast feedback processes in (palaeo) climate sensitivity

Palaeo data have been frequently used to determine the equilibrium (Charney) climate sensitivity $S^a$, and - if slow feedback processes (e.g. land ice-albedo) are adequately taken into account - they indicate a similar range as estimates based on instrumental data and climate model results. Most studies implicitly assume the (fast) feedback processes to be independent of the background climate state, e.g., equally strong during warm and cold periods. Here we assess the dependency of the fast feedback processes on the background climate state using data of the last 800 kyr and a conceptual climate model for interpretation. Applying a new method to account for background state dependency, we find $S^a=0.61\pm0.06$ K(Wm$^{-2}$)$^{-1}$ using the latest LGM temperature reconstruction and significantly lower climate sensitivity during glacial climates. Due to uncertainties in reconstructing the LGM temperature anomaly, $S^a$ is estimated in the range $S^a=0.55-0.95$ K(Wm$^{-2}$)$^{-1}$.Comment: submitted to Geophysical Research Letter

Pt/CexPr1−xO2 (x = 1 or 0.9) NOx storage–reduction (NSR) catalysts

Model Pt/Ce0.9Pr0.1O2 and Pt/CeO2 NOx storage–reduction catalysts were prepared via nitrate calcination, co-precipitation and carbon-templating routes. Raman spectroscopic data obtained on the catalysts indicated that the introduction of praseodymium into the ceria lattice increased the concentration of defect sites (vacancies), arising from the higher reducibility of the Pr4+ cation compared to Ce4+. For the Pr-promoted samples, H2-TPR profiles contained high temperature bulk reduction peaks which were less pronounced compared with their ceria analogs, indicating that the presence of praseodymium enhances oxygen mobility due to the creation of lattice defects. Under lean-rich cycling conditions, the cycle-averaged NOx conversion of the Pt/Ce0.9Pr0.1O2 samples was in each case substantially higher than that of the Pt/CeO2 analog, amounting to a difference of 10–15% in the absolute NOx conversion in some cases. According to DRIFTS data, a double role can be assigned to Pr doping; on the one hand, Pr accelerates the oxidation of adsorbed NOx species during the lean periods. On the other hand, Pr doping destabilizes the adsorbed NOx species during the rich periods, and the kinetics of nitrate decomposition are faster on Pt/Ce0.9Pr0.1O2, leading to improved catalyst regeneration. These results suggest that ceria-based mixed oxides incorporating Pr are promising materials for NOx storage–reduction catalysts intended for low temperature operation.The financial support of Generalitat Valenciana (predoctoral stay BEFPI/2012), the Spanish Ministry of Economy and Competitiveness (Project CTQ2012-30703), and co-financing by FEDER resources is acknowledged. Partial financial support was also provided by the National Science Foundation and the U.S. Department of Energy (DOE) under award no. CBET-1258742

Demographic analysis of continuous-time life-history models

I present a computational approach to calculate the population growth rate, its sensitivity to life-history parameters and associated statistics like the stable population distribution and the reproductive value for exponentially growing populations, in which individual life history is described as a continuous development through time. The method is generally applicable to analyse population growth and performance for a wide range of individual life-history models, including cases in which the population consists of different types of individuals or in which the environment is fluctuating periodically. It complements comparable methods developed for discrete-time dynamics modelled with matrix or integral projection models. The basic idea behind the method is to use Lotka's integral equation for the population growth rate and compute the integral occurring in that equation by integrating an ordinary differential equation, analogous to recently derived methods to compute steady-states of physiologically structured population models. I illustrate application of the method using a number of published life-history models

Invasion speeds for structured populations in fluctuating environments

We live in a time where climate models predict future increases in environmental variability and biological invasions are becoming increasingly frequent. A key to developing effective responses to biological invasions in increasingly variable environments will be estimates of their rates of spatial spread and the associated uncertainty of these estimates. Using stochastic, stage-structured, integro-difference equation models, we show analytically that invasion speeds are asymptotically normally distributed with a variance that decreases in time. We apply our methods to a simple juvenile-adult model with stochastic variation in reproduction and an illustrative example with published data for the perennial herb, \emph{Calathea ovandensis}. These examples buttressed by additional analysis reveal that increased variability in vital rates simultaneously slow down invasions yet generate greater uncertainty about rates of spatial spread. Moreover, while temporal autocorrelations in vital rates inflate variability in invasion speeds, the effect of these autocorrelations on the average invasion speed can be positive or negative depending on life history traits and how well vital rates ``remember'' the past

Diurnal patterns in lightning activity over South America

Ponencia presentada en 2nd TEA–IS Summer School, June 23rd – June 27nd 2014, Collioure, France.Satellite and ground network observations of lightning flash distribution data are used to examine the diurnal cycle of lightning activity over the tropical and subtropical regions of South America. The results show in the subtropical South America, particularly the area limited by [-25°; -40°] of latitude and [-70°; -50°] of longitude, the time of maximum lightning activity was shifted to nocturnal hours, extending from close to midnight to early morning hours. This behavior can be associated to the peak in MCSs in the morning hours in the region. A close connection between peak time of lightning activity and peak time of precipitation events have been observed by comparing the current results with other published studies. On the other hand, storms over northern Argentina are known as leading Transient Luminous Events (TLE) generators on Earth (Thomas et al., 2007).Fil: Nicora, M. Gabriela. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Investigaciones Científicas y Técnicas para la Defensa. Departamento de Investigaciones en Láseres y sus aplicaciones; Argentina.Fil: Nicora, M. Gabriela. Ministerio de Defensa. Instituto de Investigaciones Científicas y Técnicas para la Defensa. Departamento de Investigaciones en Láseres y sus aplicaciones; Argentina.Fil: Castellano, Nesvit. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Física Enrique Gaviola; Argentina.Fil: Castellano, Nesvit. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; Argentina.Fil: Ávila, Eldo E. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Física Enrique Gaviola; Argentina.Fil: Ávila, Eldo E. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; Argentina.Fil: Bürgesser, Rodrigo E. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Física Enrique Gaviola; Argentina.Fil: Bürgesser, Rodrigo E. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; Argentina.Meteorología y Ciencias Atmosférica

Silicon carbide particulates incorporated into microalloyed steel surface using TIG: microstructure and properties

Surface metal matrix composites have been developed to enhance properties such as erosion, wear and corrosion of alloys. In this study, ~5 µm or ~75 µm SiC particulates were preplaced on a microalloyed steel. Single track surface zones were melted by a tungsten inert gas torch, and the effect of two heat inputs, 420Jmm-1 and 840 Jmm-1,compared. The results showed that the samples melted using 420Jmm-1 were crack-free. Pin-on-disk wear testing under dry sliding conditions were conducted. The effects of load and sliding velocity were used to characterise the performance of the crack-free samples. Microstructural and X-ray diffraction studies of the surface showed that the SiC had dissolved, and that martensite, was the main phase influencing the hardness

A dynamical perspective on atmospheric temperature variability and its response to climate change

The atmospheric temperature distribution is typically described by its mean and variance, while higher order moments, such as skewness, have received less attention. Skewness is a measure of the asymmetry between the positive and negative tails of the distribution, which has implications for extremes. It was recently shown that near-surface temperature in the southern hemisphere is positively skewed on the poleward side of the storm tracks, and negatively skewed on the equatorward side. Here we take a dynamical approach to further study what controls the spatial structure of the near-surface temperature distribution in this region. We employ a tracking algorithm to study the formation, intensity, and movement of warm and cold temperature anomalies. We show that warm anomalies are generated on the equatorward side of the storm tracks and propagate poleward, while cold anomalies are generated on the poleward side and propagate equatorward. We further show that while the perturbation growth is mainly achieved through linear meridional advection, it is the nonlinear meridional advection that is responsible for the meridional movement of the temperature anomalies and therefore to the differential skewness. The projected poleward shift and increase of the temperature variance maximum in the southern hemisphere under global warming is shown to be composed of a poleward shift and increase in the maximum intensity of both warm and cold anomalies, and a decrease in their meridional displacements. An analytic expression is derived for the nonlinear meridional temperature tendency, which captures the spatial structure of the skewness and its projected changes

Changes in the natural dynamics of Nothofagus dombeyi forests : population modeling with increasing drought frequencies

Drought-induced episodes of tree mortality can determine forest dynamics and structure, particularly in forests dominated by single species. Short- and mid-term climate projections indicate that strong changes in annual precipitation may strike more often in northern Patagonia. Data for recruitment, growth, and survival of Nothofagus dombeyi tree individuals were collected at several sites across the Nahuel Huapi National Park in Argentina. We combined mathematically all these different demographic stages into an Integral Projection Model to simulate 100-yr projections of simulated stand structure under different frequencies of extreme drought episodes. We projected total basal area and the number of individuals for three different initial stand types (i.e., young, medium, and old) and for varying drought frequencies (i.e., from 1 to 5 drought events every 100 years). Recruitment into the dbh ≥ 10 cm size class under normal conditions (i.e., without drought) was higher than under episodic drought conditions. In addition, survival under normal conditions was higher than under drought conditions, especially for small trees. Differences in growth were also important, with trees growing more vigorously under normal than under drought conditions. Our simulations predicted that N. dombeyi populations would experience a reduction in tree density in the mid-term if, as predicted by the IPCC projections, the frequency of future drought events increased. The simulations also showed that in those cases, young stands should suffer the most. Drought-mediated changes may induce a decline in the development of N. dombeyi forests in the mid- and long term by a drastic reduction in tree density

Latitudinal Patterns in European Seagrass Carbon Reserves: Influence of Seasonal Fluctuations versus Short-Term Stress and Disturbance Events

Seagrass meadows form highly productive and valuable ecosystems in the marine environment. Throughout the year, seagrass meadows are exposed to abiotic and biotic variations linked to (i) seasonal fluctuations, (ii) short-term stress events such as, e.g., local nutrient enrichment, and (iii) small-scale disturbances such as, e.g., biomass removal by grazing. We hypothesized that short-term stress events and smallscale disturbances may affect seagrass chance for survival in temperate latitudes. To test this hypothesis we focused on seagrass carbon reserves in the form of starch stored seasonally in rhizomes, as these have been defined as a good indicator for winter survival. Twelve Zostera noltei meadows were monitored along a latitudinal gradient in Western Europe to firstly assess the seasonal change of their rhizomal starch content. Secondly, we tested the effects of nutrient enrichment and/or biomass removal on the corresponding starch content by using a short-term manipulative field experiment at a single latitude in the Netherlands. At the end of the growing season, we observed a weak but significant linear increase of starch content along the latitudinal gradient from south to north. This agrees with the contention that such reserves are essential for regrowth after winter, which is more severe in the north. In addition, we also observed a weak but significant positive relationship between starch content at the beginning of the growing season and past winter temperatures. This implies a lower regrowth potential after severe winters, due to diminished starch content at the beginning of the growing season. Short-term stress and disturbances may intensify these patterns, because our manipulative experiments show that when nutrient enrichment and biomass loss co-occurred at the end of the growing season, Z. noltei starch content declined. In temperate zones, the capacity of seagrasses to accumulate carbon reserves is expected to determine carbon-based regrowth after winter. Therefore, processes affecting those reserves might affect seagrass resilience. With increasing human pressure on coastal systems, short- and small-scale stress events are expected to become more frequent, threatening the resilience of seagrass ecosystems, particularly at higher latitudes, where populations tend to have an annual cycle highly dependent on their storage capacity