3,862 research outputs found

    The sensitivity of oceanic precipitation to sea surface temperature

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    Our study forms the oceanic counterpart to numerous observational studies over land concerning the sensitivity of extreme precipitation to a change in air temperature. We explore the sensitivity of oceanic precipitation to changing sea surface temperature (SST) by exploiting two novel datasets at high resolution. First, we use the Ocean Rainfall And Ice-phase precipitation measurement Network (OceanRAIN) as an observational along-track shipboard dataset at 1 min resolution. Second, we exploit the most recent European Reanalysis version 5 (ERA5) at hourly resolution on a 31 km grid. Matched with each other, ERA5 vertical velocity allows the constraint of the OceanRAIN precipitation. Despite the inhomogeneous sampling along ship tracks, OceanRAIN agrees with ERA5 on the average latitudinal distribution of precipitation with fairly good seasonal sampling. However, the 99th percentile of OceanRAIN precipitation follows a super Clausius–Clapeyron scaling with a SST that exceeds 8.5 % K−1 while ERA5 precipitation scales with 4.5 % K−1. The sensitivity decreases towards lower precipitation percentiles, while OceanRAIN keeps an almost constant offset to ERA5 due to higher spatial resolution and temporal sampling. Unlike over land, we find no evidence for a decreasing precipitation event duration with increasing SST. ERA5 precipitation reaches a local minimum at about 26 ∘C that vanishes when constraining vertical velocity to strongly rising motion and excluding areas of weak correlation between precipitation and vertical velocity. This indicates that instead of moisture limitations as over land, circulation dynamics rather limit precipitation formation over the ocean. For the strongest rising motion, precipitation scaling converges to a constant value at all precipitation percentiles. Overall, high resolutions in observations and climate models are key to understanding and predicting the sensitivity of oceanic precipitation extremes to a change in SST

    High Temperature Expansion for Frustrated and Unfrustrated S=1/2 Spin Chains

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    A computer aided high temperature expansion of the magnetic susceptibility and the magnetic specific heat is presented and demonstrated for frustrated and unfrustrated spin chains. The results are analytic in nature since the calculations are performed in the integer domain. They are provided in the form of polynomials allowing quick and easy fits. Various representations of the results are discussed. Combining high temperature expansion coefficients and dispersion data yields very good agreement already in low order of the expansion which makes this approach very promising for the application to other problems, for instance in higher dimensions.Comment: 13 pages, 8 figures, to appear in Eur. Phys. J. B, minor corrections, correction of a[5] in table A.1.a, discussion of the region of validity added, coefficients available electronically: http://www.thp.uni-koeln.de/~g

    Quiet Sun magnetic fields observed by Hinode: Support for a local dynamo

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    The Hinode mission has revealed copious amounts of horizontal flux covering the quiet Sun. Local dynamo action has been proposed to explain the presence of this flux. We sought to test whether the quiet Sun flux detected by Hinode is due to a local or the global dynamo by studying long-term variations in the polarisation signals detectable at the disc centre of the quiet Sun between November 2006 and May 2012, with particular emphasis on weak signals in the internetwork. The investigation focusses on line-integrated circular polarisation V_tot and linear polarisation LP_tot profiles obtained from the Fe I 6302.5 \AA absorption line in Hinode SOT/SP. Both circular and linear polarisation signals show no overall variation in the fraction of selected pixels from 2006 until 2012. There is also no variation in the magnetic flux in this interval of time. The probability density functions (PDF) of the line-of-sight magnetic flux can be fitted with a power law from 1.17 x 10^17 Mx to 8.53 x 10^18 Mx with index \alpha=-1.82 \pm 0.02 in 2007. The variation of \alpha 's across all years does not exceed a significance of 1\sigma. Linearly polarised features are also fitted with a power law, with index \alpha=-2.60 \pm 0.06 in 2007. Indices derived from linear polarisation PDFs of other years also show no significant variation. Our results show that the ubiquitous horizontal polarisation on the edges of bright granules seen by Hinode are invariant during the minimum of cycle 23. This supports the notion that the weak circular and linear polarisation is primarily caused by an independent local dynamo

    Systematic search for high-energy gamma-ray emission from bow shocks of runaway stars

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    Context. It has been suggested that the bow shocks of runaway stars are sources of high-energy gamma rays (E > 100 MeV). Theoretical models predicting high-energy gamma-ray emission from these sources were followed by the first detection of non-thermal radio emission from the bow shock of BD+43^\deg 3654 and non-thermal X-ray emission from the bow shock of AE Aurigae. Aims. We perform the first systematic search for MeV and GeV emission from 27 bow shocks of runaway stars using data collected by the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope (Fermi). Methods. We analysed 57 months of Fermi-LAT data at the positions of 27 bow shocks of runaway stars extracted from the Extensive stellar BOw Shock Survey catalogue (E-BOSS). A likelihood analysis was performed to search for gamma-ray emission that is not compatible with diffuse background or emission from neighbouring sources and that could be associated with the bow shocks. Results. None of the bow shock candidates is detected significantly in the Fermi-LAT energy range. We therefore present upper limits on the high-energy emission in the energy range from 100 MeV to 300 GeV for 27 bow shocks of runaway stars in four energy bands. For the three cases where models of the high-energy emission are published we compare our upper limits to the modelled spectra. Our limits exclude the model predictions for Zeta Ophiuchi by a factor \approx 5.Comment: 5 pages, 5 figures, 1 table, accepted by A&

    Magnetohydrodynamic flows in ducts with insulating coatings

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    Age and environment affect constitutive immune function in Red Knots (Calidris canutus)

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    We studied subspecies, age and environmental effects on constitutive immune function (natural antibody and complement titres, haptoglobin activity and leukocyte concentrations) in Red Knots (Calidris canutus). We compared C. c. islandica and C. c. canutus in the Wadden Sea and found no difference in immune function between subspecies. However, C. c. canutus on their wintering grounds in Banc d’Arguin had higher natural antibody and lower complement levels than C. c. canutus or C. c. islandica in the Wadden Sea. This suggests that immune function is determined more by the surrounding environment than by subspecies. We also compared age classes in the Wadden Sea and found that first year birds had significantly lower natural antibody levels than adults, but that second year birds no longer differed from adults. Finally, we examined the interaction of age and environment in Banc d’Arguin. We found that first year birds (but not adults) in a low quality habitat had higher leukocyte concentrations than first year birds or adults in a high quality habitat. Differences in available resources and defence needs between environments, and differences among individuals differentially distributed between sites, are likely important contributors to the variation in immune function we report. Future studies, which examine these factors on wild birds, will be important for our understanding of how animals function in their natural environment.