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  • 1
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    Ice–Ice Collisions: An Ice Multiplication Process in Atmospheric Clouds (2011)
    American Meteorological Society
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    Publication Date: 2011-02-01
    Description: Ice in atmospheric clouds undergoes complex physical processes, interacting especially with radiation, which leads to serious impacts on global climate. After their primary production, atmospheric ice crystals multiply extensively by secondary processes. Here, it is shown that a mostly overlooked process of mechanical breakup of ice particles by ice–ice collisions contributes to such observed multiplication. A regime for explosive multiplication is identified in its phase space of ice multiplication efficiency and number concentration of ice particles. Many natural mixed-phase clouds, if they have copious millimeter-sized graupel, fall into this explosive regime. The usual Hallett–Mossop (H–M) process of ice multiplication is shown to dominate the overall ice multiplication when active, as it starts sooner, compared to the breakup ice multiplication process. However, for deep clouds with a cold base temperature where the usual H–M process is inactive, the ice breakup mechanism should play a critical role. Supercooled rain, which may freeze to form graupel directly in only a few minutes, is shown to hasten such ice multiplication by mechanical breakup, with an ice enhancement ratio exceeding 104 approximately 20 min after small graupel first appear. The ascent-dependent onset of subsaturation with respect to liquid water during explosive ice multiplication is predicted to determine the eventual ice concentrations.
    Print ISSN: 0022-4928
    Electronic ISSN: 1520-0469
    Topics: Geography , Geosciences , Physics
    Published by American Meteorological Society
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  • 2
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    Incoherent Nature of M2 Internal Tides at the Hawaiian Ridge (2011)
    American Meteorological Society
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    Publication Date: 2011-11-01
    Description: Moored current, temperature, and conductivity measurements are used to study the temporal variability of M2 internal tide generation above the Kaena Ridge, between the Hawaiian islands of Oahu and Kauai. The energy conversion from the barotropic to baroclinic tide measured near the ridge crest varies by a factor of 2 over the 6-month mooring deployment (0.5–1.1 W m−2). The energy flux measured just off the ridge undergoes a similar modulation as the ridge conversion. The energy conversion varies largely because of changes in the phase of the perturbation pressure, suggesting variable work done on remotely generated internal tides. During the mooring deployment, low-frequency current and stratification fluctuations occur on and off the ridge. Model simulations suggest that these variations are due to two mesoscale eddies that passed through the region. The impact of these eddies on low-mode internal tide propagation over the ridge crest is considered. It appears that eddy-related changes in stratification and perhaps cross-ridge current speed contribute to the observed phase variations in perturbation pressure and hence the variable conversion over the ridge.
    Print ISSN: 0022-3670
    Electronic ISSN: 1520-0485
    Topics: Geosciences , Physics
    Published by American Meteorological Society
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  • 3
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    The Second Phase of the Global Land–Atmosphere Coupling Experiment: Soil Moisture Contributions to Subseasonal Forecast Skill (2011)
    American Meteorological Society
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    Publication Date: 2011-10-01
    Description: The second phase of the Global Land–Atmosphere Coupling Experiment (GLACE-2) is a multi-institutional numerical modeling experiment focused on quantifying, for boreal summer, the subseasonal (out to two months) forecast skill for precipitation and air temperature that can be derived from the realistic initialization of land surface states, notably soil moisture. An overview of the experiment and model behavior at the global scale is described here, along with a determination and characterization of multimodel “consensus” skill. The models show modest but significant skill in predicting air temperatures, especially where the rain gauge network is dense. Given that precipitation is the chief driver of soil moisture, and thereby assuming that rain gauge density is a reasonable proxy for the adequacy of the observational network contributing to soil moisture initialization, this result indeed highlights the potential contribution of enhanced observations to prediction. Land-derived precipitation forecast skill is much weaker than that for air temperature. The skill for predicting air temperature, and to some extent precipitation, increases with the magnitude of the initial soil moisture anomaly. GLACE-2 results are examined further to provide insight into the asymmetric impacts of wet and dry soil moisture initialization on skill.
    Print ISSN: 1525-755X
    Electronic ISSN: 1525-7541
    Topics: Geography , Geosciences , Physics
    Published by American Meteorological Society
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  • 4
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    On latency of multiple zonal jets in the oceans (2011)
    Cambridge University Press
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    Publication Date: 2011-09-27
    Description: Most of the nearly zonal, multiple, alternating jets observed in the oceans are latent, that is, their amplitudes are weak relative to the ambient mesoscale eddies. Yet, relatively strong jets are often observed in dynamical simulations. To explore mechanisms controlling the degree of latency, we analyse solutions of an idealized, eddy-resolving and flat-bottom quasigeostrophic model, in which dynamically generated mesoscale eddies maintain and interact with a set of multiple zonal jets. We find that the degree of the latency is controlled primarily by the bottom friction: The larger the friction parameter, the more latent are the jets; and the degree of the latency is substantial for a realistic range of the oceanic bottom friction coefficient. This result not only provides a plausible explanation for the latency of the oceanic jets, but it may also be relevant to the prominent atmospheric multiple jets observed on giant gas planets, such as Jupiter. We hypothesize that these jets can be so strong because of the relative absence of the bottom friction. The mechanism controlling the latency in our solutions is understood in terms of the changes induced in the linear eigenmodes of the time-mean flow by varying the bottom friction coefficient; these changes, in turn, affect and modify the jets. Effects of large Reynolds numbers on the eddies, jets, and the latency are also discussed. © 2011 Cambridge University Press.
    Print ISSN: 0022-1120
    Electronic ISSN: 1469-7645
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Published by Cambridge University Press
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  • 5
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    Quasi-static magnetohydrodynamic turbulence at high Reynolds number (2011)
    Cambridge University Press
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    Publication Date: 2011-06-21
    Description: We analyse the anisotropy of homogeneous turbulence in an electrically conducting fluid submitted to a uniform magnetic field, for low magnetic Reynolds number, in the quasi-static approximation. We interpret contradictory earlier predictions between linearized theory and simulations: in the linear limit, the kinetic energy of transverse velocity components, normal to the magnetic field, decays faster than the kinetic energy of the axial component, along the magnetic field (Moffatt, J. Fluid Mech., vol. 28, 1967, p. 571); whereas many numerical studies predict a final state characterized by dominant energy of transverse velocity components. We investigate the corresponding nonlinear phenomenon using direct numerical simulation (DNS) of freely decaying turbulence, and a two-point statistical spectral closure based on the eddy-damped quasi-normal Markovian (EDQNM) model. The transition from the three-dimensional turbulent flow to a two-and-a-half-dimensional flow (Montgomery & Turner, Phys. Fluids, vol. 25, 1982, p. 345) is a result of the combined effects of short-time linear Joule dissipation and longer time nonlinear creation of polarization anisotropy. It is this combination of linear and nonlinear effects which explains the disagreement between predictions from linearized theory and results from numerical simulations. The transition is characterized by the elongation of turbulent structures along the applied magnetic field, and by the strong anisotropy of directional two-point correlation spectra, in agreement with experimental evidence. Inertial equatorial transfers in both DNS and the model are presented to describe in detail the most important equilibrium dynamics. Spectral scalings are maintained in high-Reynolds-number turbulence attainable only with the EDQNM model, which also provides simplified modelling of the asymptotic state of quasi-static magnetohydrodynamic (MHD) turbulence. © © 2011 Cambridge University Press.
    Print ISSN: 0022-1120
    Electronic ISSN: 1469-7645
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Published by Cambridge University Press
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  • 6
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    Mammoth Extinction and Radiation Dose: A Comment (2011)
    Cambridge University Press
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    Publication Date: 2011-01-01
    Print ISSN: 0033-8222
    Electronic ISSN: 1945-5755
    Topics: Archaeology , Energy, Environment Protection, Nuclear Power Engineering , Geosciences
    Published by Cambridge University Press
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  • 7
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    Development of Radiocarbon Dating Method for Degraded Bone Samples from Korean Archaeological Sites (2011)
    Cambridge University Press
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    Publication Date: 2011-01-01
    Description: The development of radiocarbon dating for degraded bone samples collected at Korean archaeological sites has been successful through the characterization of raw bone C/N ratios and application of an ultrafiltration method. It was found that the C/N ratios of raw bone samples are inversely proportional to the carbon content and residue amount after gelatinization. We have examined a few dozen Korean archaeological bone samples for this study. Well-preserved bone samples are found to be physically dense. The range of C/N ratios of Korean raw bone samples ranged from 3.4 to 74. We found that the C/N ratios of degraded raw bone samples can be used to determine whether 14C samples are acceptable for normal pretreatment processing and eventual dating. The results of this study support that even if the C/N ratio of a degraded raw bone sample is 11, extraction of collagen for bone dating is feasible by a carefully designed ultrafiltration process. Our preliminary 14C dating results of a depth profile of Gunang-gul Cave, an archaeological site in Danyang, Korea, indicate that this site has been either geologically or anthropologically disturbed in the past, with 14C ages ranging from 28,910 ± 200 to 48,090 ± 1050 yr BP. The C/N ratios of the collagen samples of Gunang-gul were determined to be 3.2–3.6. Our study establishes a new guide for the pretreatment of degraded bone samples such as those collected in Korea for 14C dating.
    Print ISSN: 0033-8222
    Electronic ISSN: 1945-5755
    Topics: Archaeology , Energy, Environment Protection, Nuclear Power Engineering , Geosciences
    Published by Cambridge University Press
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  • 8
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    Human-modified landscapes: patterns of fine-scale woody vegetation structure in communal savannah rangelands (2011)
    Cambridge University Press
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    Publication Date: 2011-11-29
    Description: SUMMARYDespite electrification, over 90% of rural households in certain areas of South Africa continue to depend on fuelwood, and this affects woody vegetation structure, with associated cascading effects on biodiversity within adjacent lands. To promote sustainable use, the interactions between anthropogenic and environmental factors affecting vegetation structure in savannahs need to be understood. Airborne light detection and ranging (LiDAR) data collected over 4758 ha were used to examine woody vegetation structure in five communal rangelands around 12 settlements in Bushbuckridge, a municipality in the Kruger to Canyons Biosphere Reserve (South Africa). The importance of underlying abiotic factors was evaluated by measuring size class distributions across catenas and using canonical correspondence analysis. Landscape position was significant in determining structure, indicating the importance of underlying biophysical factors. Differences in structure were settlement-specific, related to mean annual precipitation at one site, and human population density and intensity of use at the other four sites. Size class distributions of woody vegetation revealed human disturbance gradients around settlements. Intensity of use affected the amplitude, not the shape, of the size class distribution, suggesting the same height classes were being harvested across settlements, but amount harvested varied between settlements. Highly used rangelands result in a disappearance of disturbance gradients, leading to homogeneous patches of low woody cover around settlements with limited rehabilitation options. Reductions in disturbance gradients can serve as early warning indicators of woodland degradation, a useful tool in planning for conservation and sustainable development.
    Print ISSN: 0376-8929
    Electronic ISSN: 1469-4387
    Topics: Biology
    Published by Cambridge University Press
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