ALBERT

Description: The Polar version 3.1.1 of the Weather Research and Forecasting model (WRF), a high-resolution regional scale model, is used to simulate conditions for the year December 2006 to November 2007. The goal is to compare model output of near-surface and tropospheric variables to observational data sets. The domain mirrors that of the Arctic System Reanalysis (ASR), an assimilation of model fields with Arctic observations being conducted partly by the Polar Meteorology Group of the Byrd Polar Research Center at Ohio State University. A key development in this Polar WRF study is the extension of the seasonal progression of sea ice albedo to the entire Arctic Ocean. The boundary conditions are specified by the NCEP Final global gridded analysis archive (FNL), a 1° × 1° global grid updated every 6 h. The simulations are performed in 48 h increments initialized daily at 0000 UTC, with the first 24 h discarded for model spin-up of the hydrologic cycle and boundary layer processes. Model large-scale variables of atmospheric pressure and geopotential height show good agreement with observations. Spatial distribution of near-surface air temperatures compares well with ERA-Interim despite a small negative bias in the station analysis. Surface dewpoint temperatures and wind speeds show small biases, but model skill is modest for near-surface winds. Tropospheric temperatures and wind speeds, however, agree well with radiosonde observations. This examination provides a benchmark from which to improve the model and guidance for further development of Polar WRF as ASR's primary model.

Description: The forecast atmospheric hydrologic cycle of the Polar version 3.1.1 of the Weather Research and Forecasting model (WRF) is examined for December 2006 – November 2007. The domain is similar to the Arctic System Reanalysis (ASR), an assimilation of model fields and Arctic observations being conducted partly by the Byrd Polar Research Center. Simulations are performed in 48 h increments initialized daily at 0000 UTC, with the first 24 h discarded for model spin-up of the hydrologic cycle and boundary layer processes. Precipitation analysis reveals a negative annual mean bias (−9.4%) in the polar region, with particularly dry station biases reflected in the Canadian Archipelago. Annual mean bias for the midlatitudes is small and positive (4.6%), attributed to excessive precipitation during spring and summer when convective precipitation is dominant. An examination of precipitation within four major Arctic river basins shows large positive biases due to excessive convective precipitation in summer as well, but highlights the Arctic climate's strong dependence on midlatitude precipitation. Nudging the model's boundary layer moisture toward drier conditions decreases convective precipitation improving the prediction. Cloud fraction analysis shows too little cloud cover, supported by an excess in incident shortwave radiation and a deficit in downwelling longwave radiation throughout the domain. The longwave bias is present regardless of the amount of cloud water or cloud ice, demonstrating a need to improve cloud effects on radiation in Polar WRF. This examination provides a benchmark of the forecast atmospheric hydrological cycle of Polar WRF and its use as ASR's primary model.

Description: We examined whether sensitivity to disturbance in marine fish communities was determined by predisturbance trait structure (i.e., composition of ecological traits) or trait redundancy. We used long‐term data from both a temperate and a tropical ecosystem that have experienced large‐scale climatic disturbances. In both ecosystems, we found that increasing dominance by climatically vulnerable traits rendered fish communities more sensitive to disturbance, while communities with higher trait redundancy were more resistant. To our knowledge, this is the first study demonstrating the influence of trait structure and redundancy on community sensitivity over large temporal and spatial scales in natural systems. Abstract Trait diversity is believed to influence ecosystem dynamics through links between organismal traits and ecosystem processes. Theory predicts that key traits and high trait redundancy—large species richness and abundance supporting the same traits—can buffer communities against environmental disturbances. While experiments and data from simple ecological systems lend support, large‐scale evidence from diverse, natural systems under major disturbance is lacking. Here, using long‐term data from both temperate (English Channel) and tropical (Seychelles Islands) fishes, we show that sensitivity to disturbance depends on communities’ initial trait structure and initial trait redundancy. In both ecosystems, we found that increasing dominance by climatically vulnerable traits (e.g., small, fast‐growing pelagics/corallivores) rendered fish communities more sensitive to environmental change, while communities with higher trait redundancy were more resistant. To our knowledge, this is the first study demonstrating the influence of trait structure and redundancy on community sensitivity over large temporal and spatial scales in natural systems. Our results exemplify a consistent link between biological structure and community sensitivity that may be transferable across ecosystems and taxa and could help anticipate future disturbance impacts on biodiversity and ecosystem functioning.

Description: [1]  Reconstructions of Antarctic paleotopography for the late Eocene suggest that glacial erosion and thermal subsidence have lowered West Antarctic elevations considerably since then, with Antarctic land area having decreased ~20%. A new climate-ice sheet model based on these reconstructions shows that the West Antarctic Ice Sheet first formed at the Eocene-Oligocene transition (33.8–33.5 Ma, E-O) in concert with the continental-scale expansion of the East Antarctica Ice Sheet and that the total volume of East and West Antarctic ice (33.4–35.9 x 10 6  km 3 ) was 〉1.4 times greater than previously assumed. This larger modeled ice volume is consistent with a modest cooling of 1–2 °C in the deep ocean during the E-O transition, lower than other estimates of ~3 °C cooling, and suggests the possibility of substantial ice in the Antarctic interior before the Eocene-Oligocene boundary.

Description: Abstract The presence of hydrogen, most likely in the form of water ice, is well established in Mercury's permanently shaded polar craters. But lower concentrations that may exist away from the poles have not previously been well constrained. In this work we use data from the MESSENGER Gamma‐Ray and Neutron Spectrometer (GRNS) to produce the first map of the absolute hydrogen abundance in Mercury's northern hemisphere. We find a mean abundance of ppm and a latitudinal trend that agrees with earlier results showing enhanced hydrogen contained within Mercury's radar bright craters. Additionally, we observe a mid‐ and low‐ latitude variation in hydrogen abundance that is correlated most strongly with temperature 20 cm beneath Mercury's surface.

Description: Abstract Aim Understanding the processes driving population declines in migratory species can be challenging. Not only are monitoring data spatially and temporally sparse, but conditions in one location can carry over to indirectly (and disproportionately) affect the population in another location. Here, we explore whether remote factors can sequentially, and potentially cumulatively, influence local population fluctuations in declining populations of shorebirds. Location Moreton Bay (Australia) and the East Asian–Australasian Flyway. Methods We use N‐mixture models to account for variable observer effort and estimate yearly population growth rate. We then use least squares regressions to correlate population growth rates with remotely sensed climate anomalies at different migratory stages. From this, we estimate species‐specific climate sensitivity indices and explore whether species which are declining more rapidly, or which rely more heavily on areas undergoing rapid habitat loss, have higher climate sensitivity indices. Results We find that species which rely more on the Yellow Sea during migratory stopover (a region which has undergone severe habitat loss) are more sensitive to rainfall anomalies in their Arctic breeding grounds, suggesting that habitat loss reduces the resilience of shorebirds to climate extremes. Furthermore, species with higher sensitivities to climatic conditions during stopover are also those which are declining quickest, suggesting that declining populations may also be less resilient to climate fluctuations at bottleneck sites. We also observed species‐specific correlations between climate anomalies at all migratory stages and population growth rates, primarily for eastern curlew and lesser sand plover. Main conclusion By applying methods in combination, it is possible to use citizen science data from a single location in a flyway of over 160 sites up to 11,680 km apart, to investigate how different stressors correlate with local population dynamics.

Description: Restorations in the light of climate change will need to take into account whether or not sources of the dominant plants are adapted to the future conditions at a site. In addition, the effect of these dominants, especially if sourced from outside the local area, on the assembling plant community needs assessment. We investigated how different ecotypes of the tallgrass prairie dominants Andropogon gerardii and Sorghastrum nutans affect assembling prairie communities. Four reciprocal common garden experiments were established across a longitudinal climate gradient characterized by a decrease in aridity in western Kansas (COLBY), central Kansas (HAYS), eastern Kansas (MANHATTAN), and southern Illinois (CARBONDALE). At each site, plots were seeded with ecotypes of A. gerardii and S. nutans sourced from central Kansas (CKS), eastern Kansas (EKS), southern Illinois (SIL), or a mix of all three regional ecotypes (MIX). All plots were also seeded with the same suite of seven subordinate species. Species composition was measured during the fourth year of restoration. The greatest variation between communities occurred at HAYS and CARBONDALE between plots seeded with CKS and SIL ecotypes. At these sites, plots seeded with the local source had the lowest diversity and cover of nondominant species. Compositional variation between plots seeded with different dominant grass ecotypes was found exclusively at CARBONDALE between CKS and SIL plots. Differences between locally seeded plots and plots seeded with a MIX of dominant grass ecotypes were contingent upon site. At CARBONDALE, MIX seeded plots had higher diversity than SIL ecotype plots. Our results indicate that across a wide geographic precipitation gradient, limited but important differences in community assembly occur in restorations established with different ecotypes of the dominant grasses. However, our results also support the use of mixtures of nonlocal ecotypes of dominant grasses in restorations without risk to the assembling plant community. Future studies need to determine the potential for out-breeding effects among seed sources in mixed stands.

Description: [1]  Atmospheric thermal tides are global oscillations in atmospheric fields that are sub-harmonics of a solar day. While atmospheric tides on Earth are mainly relevant in the upper atmosphere, on Mars they dominate temperature variations and winds throughout the atmosphere. Observations and model simulations to date have suggested that the migrating diurnal tide is the predominant mode in the martian atmosphere, and that the semi-diurnal tide is only relevant in the tropical middle atmosphere during conditions of high dust loading. New comprehensive observations by the Mars Climate Sounder in a geometry that allows coverage of multiple local times show that the semi-diurnal tide is a dominant response of the martian atmosphere throughout the martian year. The maximum semi-diurnal amplitude of ∼ 16 K is found at southern winter high latitudes, which makes it the largest tidal amplitude observed in the martian middle atmosphere outside of dust storm conditions. The semi-diurnal tide can be successfully modeled due to recent advances of Mars General Circulation Models (MGCMs) that include the radiatively active treatment of water ice clouds. Tidal forcing occurs through absorption of radiation by aerosols and points to the vertical structure of dust and clouds and their radiative effects as being essential for our understanding of the thermal structure and the general circulation of the martian atmosphere. As with terrestrial GCMs trying to quantify mechanisms affecting climate, future Mars modeling efforts will require microphysical schemes to control aerosol distributions, and vertically and temporally resolved measurements of temperature and aerosols will be essential for their validation.

Description: [1]  Recent studies suggest that semivolatile organic compounds (SVOCs) are important precursors to secondary organic aerosol (SOA) in urban atmospheres. However, knowledge of the chemical composition of SVOCs is limited by current analytical techniques, which are typically unable to resolve a large number of constitutional isomers. Using a combination of gas chromatography and soft photoionization mass spectrometry, we characterize the unresolved complex mixture (UCM) of semivolatile aliphatic hydrocarbons observed in Pasadena, CA (~16 km NE of downtown Los Angeles) and Bakersfield, CA during CalNex 2010. To the authors’ knowledge, this work represents the most detailed characterization of the UCM in atmospheric samples to date. Knowledge of molecular structures, including carbon number, alkyl branching, and number of rings, provides important constraints on the rate of atmospheric processing, as the relative amounts of branched and linear alkanes are shown to be a function of integrated exposure to hydroxyl radicals. Emissions of semivolatile branched alkanes from fossil-fuel related sources are up to an order of magnitude higher than those of linear alkanes, and the gas-phase OH rate constants of branched alkanes are ~30% higher than their linear isomers. Based on a box model considering gas/particle partitioning, emissions and reaction rates, semivolatile branched alkanes are expected to play a more important role than linear alkanes in photooxidation of the UCM and subsequent transformations into SOA. Detailed speciation of semivolatile compounds therefore provides essential understanding of SOA sources and formation processes in urban areas.