Towards Reconciling the Large-Scale Structure of Turbulent Boundary Layers in the Atmosphere and Laboratory

A collaborative experimental effort employing the minimally perturbed atmospheric surface-layer flow over the salt playa of western Utah has enabled us to map coherence in turbulent boundary layers at very high Reynolds numbers, \({Re_{\tau}\sim\mathcal{O}(10^6)}\) . It is found that the large-scale coherence noted in the logarithmic region of laboratory-scale boundary layers are also present in the very high Reynolds number atmospheric surface layer (ASL). In the ASL these features tend to scale on outer variables (approaching the kilometre scale in the streamwise direction for the present study). The mean statistics and two-point correlation map show that the surface layer under neutrally buoyant conditions behaves similarly to the canonical boundary layer. Linear stochastic estimation of the three-dimensional correlation map indicates that the low momentum fluid in the streamwise direction is accompanied by counter-rotating roll modes across the span of the flow. Instantaneous flow fields confirm the inferences made from the linear stochastic estimations. It is further shown that vortical structures aligned in the streamwise direction are present in the surface layer, and bear attributes that resemble the hairpin vortex features found in laboratory flows. Ramp-like high shear zones that contribute significantly to the Reynolds shear-stress are also present in the ASL in a form nearly identical to that found in laboratory flows. Overall, the present findings serve to draw useful connections between the vast number of observations made in the laboratory and in the atmosphere.     

On Titan’s Xanadu region

A large, circular marking ∼1800 km across is seen in near-infrared images of Titan. The feature is centered at 10°S, 120°W on Titan, encompasses much of Titan’s western Xanadu region, and has an off-center, quasi-circular, inner margin about 700 km across, with lobate outer margins extending 200-500 km from the inner margin. On the feature’s southern flank is Tui Regio, an area that has very high reflectivity at 5 μm, and is hypothesized to exhibit geologically recent cryovolcanic flows (Barnes, J.W. et al. [2006]. Geophys. Res. Lett. 33), similar to flows seen in Hotei Regio, a cryovolcanic area whose morphology may be controlled by pre-existing, crustal fractures resulting from an ancient impact (Soderblom, L.A. et al. [2009]. Icarus, 204). The spectral reflectivity of the large, circular feature is quite different than that of its surroundings, making it compositionally distinct, and radar measurements of its topography, brightness temperature and volume scattering also suggest that the feature is quite distinct from its surroundings. These and several other lines of evidence, in addition to the feature’s morphology, suggest that it may occupy the site of an ancient impact.     

变泥质岩的深熔作用与具铈（Ce）负异常熔体的成因

对美国加州南Sierra Nevada岩基中一个典型的中生代变质表壳岩及其混合岩带进行了详细的野外观察和元素地球化学研究。研究发现:在持久(约为150Ma)的花岗岩侵位作用下,早白垩世变泥质岩发生达角闪岩相的中高级变质作用和部分熔融,导致Isabella混合岩的形成;浅色体具有和变泥质岩及混合岩近平行的REE分布模式,但浅色体的LREE含量相对较低;和变泥质岩相似,混合岩中的浅色体具有显著的Ce负异常。野外观测、岩相观察及元素地球化学特征表明,浅色体显著的Ce负异常是继承了原岩的Ce负异常特征,而不是由于副矿物(磷灰石、独居石或锆石)的差异溶解或结晶分异作用造成的。早白垩世变泥质岩(浅色体的原岩)主要由泥质及沙质海相沉积物组成,局部夹基性火山灰和火山碎屑,形成于与大陆岛弧密切相关的浅海环境。原岩的Ce负异常反映了较还原的浅海沉积环境。具有Ce负异常浅色体的产出表明,如果俯冲带上的沉积岩在俯冲过程中发生部分熔融作用并且所产生的熔体参与大洋型岛弧岩浆作用,最终可以导致具有Ce异常的基性岩浆生成。     

Analysis of Urban Atmosphere Plume Concentration Fluctuations

Concentration variability in the fast-response tracer dataset for continuous, near-surface, point source releases in the urban core from the Joint Urban 2003 field study is analyzed. Concentration variability for conditionally and unconditionally sampled time series is characterized by probability densities, concentration fluctuation intensity, skewness, and kurtosis. Significant day-night differences in plume dispersion are observed. Relative to daytime, nighttime plumes were more likely to have reduced concentration fluctuation intensities, higher normalized surface concentrations, suppressed vertical mixing, and a greater prevalence of Gaussian-like distributions rather than log-normal or mixed mode distributions. This was in spite of the similar stability and turbulence conditions in the urban core for day and night. The potential roles of flow meander and thermal stability in explaining these differences are examined. Probability densities of concentration are found to be a strong function of fluctuation intensity. There are few differences in probability densities between day and night when classified by fluctuation intensity. There are no appreciable differences between conditional and unconditional probability densities and only small differences between conditional and unconditional sampling statistics relative to the larger differences usually observed in more homogeneous settings. Fluctuation intensity, skewness, and kurtosis are higher for the daytime experiments, and closer to the source, but show little difference between conditional and unconditional results over most of their range of values. The log-normal distribution provides a better overall fit to a broader range of the dataset than the exponential or clipped-normal distributions.     

21st century climate change in the Middle East

This study examined the performance and future predictions for the Middle East produced by 18 global climate models participating in the Intergovernmental Panel on Climate Change Fourth Assessment Report. Under the Special Report on Emission Scenarios A2 emissions scenario the models predict an overall temperature increase of ~1.4 K by mid-century, increasing to almost 4 K by late-century for the Middle East. In terms of precipitation the southernmost portion of the domain experiences a small increase in precipitation due to the Northward movement of the Inter-Tropical Convergence Zone. The largest change however is a decrease in precipitation that occurs in an area covering the Eastern Mediterranean, Turkey, Syria, Northern Iraq, Northeastern Iran and the Caucasus caused by a decrease in storm track activity over the Eastern Mediterranean. Other changes likely to impact the region include a decrease of over 170,000 km² in viable rainfed agriculture land by late-century, increases in the length of the dry season that reduces the length of time that the rangelands can be grazed, and changes in the timing of the maximum precipitation in Northern Iran that will impact the growing season, forcing changes in cropping strategy or even crop types.     

C2/c pyroxene phenocrysts from three potassic series in the Neogene alkaline volcanics, NE Turkey: their crystal chemistry with petrogenetic significance as an indicator of P–T conditions

Chemical and structural data are reported for C2/c pyroxene phenocrysts collected from three potassic series (Group A: basanite-tephrite, Group B: tephrite-phonolitic tephrite, Group C: alkaline basalt-trachybasalt) of the Neogene alkaline volcanics (NAVs) in northeastern Turkey, in order to investigate the evolution of the magmatic plumbing system and the location of magma chamber(s) with crystallization conditions. The rock series hosting the clinopyroxene phenocrysts show generally porphyritic texture and have a variable phenocryst-rich nature (20–58%), with phenocryst assemblages characterized by cpx ± ol ± plag ± foid ± amp ± bio. The clinopyroxene phenocrysts can be chemically classified as Ti- and Fe³⁺-rich Al-diopsides for Groups A and B (AB-cpxs) and Ti- and Fe³⁺-poor Al-diopsides for Group C (C-cpxs). They have poorly variable composition, clustering in the diopside field. Structurally, the diopside groups have nearly similar a (ranging from 9.73 to 9.75 ?), V _cell (437.2–440.9 ?³), and 〈beta〉 angle values (106.01°–106.23°), but some differences in polyhedral parameters and geometries of the AB-cpxs and C-cpxs have been observed. For example, the AB-cpxs are characterized by larger c (5.27–5.30 vs. 5.25–5.28 ?), V _T (2.27–2.30 vs. 2.23–2.28 ?³), and V _M2 (25.53–25.72 vs. 25.41–25.59 ?³) values and smaller b (8.87–8.88 vs. 8.88–8.91 ?) and V _M1 (11.49–11.63 vs. 11.64–11.83 ?³) values with respect to the C-cpxs. In addition, the AB-cpxs show higher values of V _M2/V _M1 (2.20–2.23) due to large V _M2 and small V _M1 compared to the V _M2/V _M1 ratios of the C-cpxs (<2.19). Such differences in the crystal structure of the AB-cpxs and C-cpxs from the NAVs are partly related to different crystallization pressures, but mostly related to variation in melt composition and, possibly, the influence of other crystallizing mineral phases. In particular, R(M2-O1) and R(M1-O2) (i.e. bond lengths) differences in the clinopyroxenes of different groups support the presence of evolved host rocks with different alkaline character (i.e. silica-undersaturated Groups A–B and silica-saturated Group C). Based on the cpx-geothermobarometry, the crystallization pressures for the C-cpxs are lower than 4.5 kbars, but the AB-cpxs have relatively high-pressure values (5.6–10.6 kbars), suggesting that the AB-cpxs crystallized in higher pressure environments. The relatively higher crystallization temperatures of the AB-cpxs also indicate higher cooling rates. The P–T estimates suggest that the source regions of the clinopyroxene phenocrysts from the NAVs were crustal magma chambers in a closed plumbing system at a moderate- to low-pressure regime.     

Sea-floor spreading rate changes in the South Atlantic

A computer correlation technique was used to deduce the spreading history of the Mid-Atlantic Ridge from 5 magnetic profiles between 28°S and 43°S. In general, several possible histories are indicated for each profile involving changes of spreading rate and faulting, some of which are easily overlooked by the visual method. The only spreading history that was consistent will all the profiles required spreading at approximately 2.2 cm yr^-1 from 11 m.y.b.p. to approximately 5.5 m.y.b.p., followed by a decrease in rate to 1.7 cm yr^-1 relative to the Vine (1966) magnetic reversal model based on the South Pacific. Comparison of the data with other reported spreading rate discontinuities suggests that the South Pacific may be reponsible for the reported spreading rate changes.