Spatial and temporal variations in acoustic propagation characteristics at the New England shelfbreak front

The spatial and temporal variability of the acoustic field in the region of a strong coastal shelfbreak front are examined, using the high-resolution environmental data from the 1996-1997 New England shelfbreak PRIMER experiment to provide input to acoustic propagation models. Specifically, the "SeaSoar" undulating conductivity-temperature-depth (CTD) probe across-shelf transects provide 1-km along-track resolution, including the front, during the spring, summer, and winter seasons. These data allow one to study the diurnal and seasonal temporal variation of the acoustic field, as well as the varying spatial structure of the field. Using the RAM parabolic equation code, across-shelf acoustic field structure at 200, 400, and 1000 Hz is studied for various source depths. A number of interesting propagation effects are noted, the most interesting of which are the inhibition of strong acoustic-bottom interaction by the warm shelf water beneath the shelfbreak front and the existence of small-scale ducts near the front, due to offshore transport. Data from the vertical line arrays deployed as part of PRIMER offer partial validation of the predictions made. Specifically, it is seen that the mean received levels are in reasonable accord with propagation calculations made using locally measured bottom properties and the SeaSoar water-column measurements.     

Distribution characteristics and factors influencing microbial communities in the core soils of a seawater intrusion area in Longkou City,China

Microbes live throughout the soil profile. Microbial communities in subsurface horizons are impacted by a saltwater–freshwater transition zone formed by seawater intrusion (SWI) in coastal regions. The main purpose of this study is to explore the changes in microbial communities within the soil profile because of SWI. The study characterizes the depth-dependent distributions of bacterial and archaeal communities through high-throughput sequencing of 16S rRNA gene amplicons by collecting surface soil and deep core samples at nine soil depths in Longkou City, China. The results showed that although microbial communities were considerably impacted by SWI in both horizontal and vertical domains, the extent of these effects was variable. The soil depth strongly influenced the microbial communities, and the microbial diversity and community structure were significantly different (p < 0.05) at various depths. Compared with SWI, soil depth was a greater influencing factor for microbial diversity and community structure. Furthermore, soil microbial community structure was closely related to the environmental conditions, among which the most significant environmental factors were soil depth, pH, organic carbon, and total nitrogen.

     

Optimal modal beamforming of bandpass signals using an undersizedsparse vertical hydrophone array: theory and a shallow-water experiment

Conventional methods for modal beamforming of underwater acoustic signals using a vertical-line hydrophone array (VLA) can suffer significant degradation in resolution when the array is geometrically deficient, i.e., consists of sparsely spaced elements and spans the water column partially or is poorly navigated. Designed for estimating the coefficients of the normal modes, these conventional methods include the direct projection (DP) of the data on the calculated mode shapes and least-squares (LS) fitting of the mode sum to the data. The degradation, in the form of modal cross talk or sidelobes, is a result of an undersampling in depth. This cross talk may be mitigated with the application of proper space-time filter constraints in the case of a pulse transmission. In this paper, a generalized least-squares (GLS) mode beamformer, capable of incorporating physical space-time constraints on the propagation of sound, is presented. The formulation is based on the well-known theorem of Gauss and Markov. Initialized by a model prediction of the basic arrival structure of the normal modes and incorporating, iteratively, refined estimates of the statistics of the modal fluctuations, this GLS technique strives to boost the resolution of a geometrically deficient VLA. The improvement is demonstrated using the VLA data collected during a shallow-water tomography experiment in the Barents Sea. The superiority of the GLS method over the conventional DP and LS methods is evident, providing a high-quality time series of modal arrivals as a function of geophysical time, which, in turn, reveals the dominant time scales of the oceanic processes associated with the Barents Sea Polar Front     

Mass conservation in finite element groundwater models

Numerical mass balance relations are derived for common formulations of the hydraulic and species transport equations, by summing the Galerkin equations. Precise mass balance is demonstrated, provided the Galerkin equation is retained at all boundaries. The effects of quadrature, variable coefficients, transients and irregular geometry are addressed, and numerical experiments verify the algebra.     

Evaluation of an ensemble of Arctic regional climate models: spatiotemporal fields during the SHEBA year

Simulations of eight different regional climate models (RCMs) have been performed for the period September 1997–September 1998, which coincides with the Surface Heat Budget of the Arctic Ocean (SHEBA) project period. Each of the models employed approximately the same domain covering the western Arctic, the same horizontal resolution of 50 km, and the same boundary forcing. The models differ in their vertical resolution as well as in the treatments of dynamics and physical parameterizations. Both the common features and differences of the simulated spatiotemporal patterns of geopotential, temperature, cloud cover, and long-/shortwave downward radiation between the individual model simulations are investigated. With this work, we quantify the scatter among the models and therefore the magnitude of disagreement and unreliability of current Arctic RCM simulations. Even with the relatively constrained experimental design we notice a considerable scatter among the different RCMs. We found the largest across-model scatter in the 2 m temperature over land, in the surface radiation fluxes, and in the cloud cover which implies a reduced confidence level for these variables. An erratum to this article can be found at     

Multispacecraft Observations of Magnetic Clouds and Their Solar Origins between 19 and 23 May 2007

We analyze a series of complex interplanetary events and their solar origins that occurred between 19 and 23 May 2007 using observations by the STEREO and Wind satellites. The analyses demonstrate the new opportunities offered by the STEREO multispacecraft configuration for diagnosing the structure of in situ events and relating them to their solar sources. The investigated period was characterized by two high-speed solar wind streams and magnetic clouds observed in the vicinity of the sector boundary. The observing satellites were separated by a longitudinal distance comparable to the typical radial extent of magnetic clouds at 1 AU (fraction of an AU), and, indeed, clear differences were evident in the records from these spacecraft. Two partial-halo coronal mass ejections (CMEs) were launched from the same active region less than a day apart, the first on 19 May and the second on 20 May 2007. The clear signatures of the magnetic cloud associated with the first CME were observed by STEREO B and Wind while only STEREO A recorded clear signatures of the magnetic cloud associated with the latter CME. Both magnetic clouds appeared to have interacted strongly with the ambient solar wind and the data showed evidence that they were a part of the coronal streamer belt. Wind and STEREO B also recorded a shocklike disturbance propagating inside a magnetic cloud that compressed the field and plasma at the cloud’s trailing portion. The results illustrate how distant multisatellite observations can reveal the complex structure of the extension of the coronal streamer into interplanetary space even during the solar activity minimum. Electronic Supplementary Material  The online version of this article () contains supplementary material, which is available to authorized users.     

Neogene and Quaternary coexisting in the geological time scale: The inclusive compromise

Removing the Tertiary and Quaternary Periods whilst conserving the Paleogene and Neogene Periods in The Geological Timescale 2004 caused a storm of protest. One response was to advocate restoring an enlarged Quaternary and consigning the Neogene to a minor role within the Tertiary. Amongst an array of practical, traditional, sentimental and anthropocentric reasons for this response, the one hard-core justification was that the rigidly nested hierarchy of the geological timescale must be preserved.The central objective of this paper is conserving the historically legitimate, Miocene-present, Neogene Period and System. There are two options for conserving the Quaternary concurrently with the Neogene: (i) an inclusive compromise in a flexible hierarchy, and (ii) an upgrading of Pliocene and Pleistocene divisions to the level of epoch.In the inclusive compromise there coexist alternative pathways through the hierarchical ranks. Thus geohistorians and biohistorians have two options for traversing the hierarchy from era to age, as in this example using the hierarchical positioning of the Calabrian Age and Stage:either Cenozoic [era]↔Neogene [period]↔Pleistocene [epoch]↔Calabrian [age],or Cenozoic [era]↔Quaternary [subera]↔Pleistocene [epoch]↔Calabrian [age].We reaffirm that the inclusive compromise is entirely viable. In so doing we (i) challenge the necessity of the rigidly nested hierarchy, which should be capable of a little flexibility; (ii) reject all analogies of the arbitrary and conventional chronostratigraphic hierarchy with three natural biological hierarchies; (iii) reaffirm the integrity of the Neogene extending to the present; and (iv) see no reason to doubt the harmonious coexistence of the two options preserving the Quaternary and Neogene traditions in an orderly working and stable time scale.In the alternative schema conserving the Neogene, divisions of the Pliocene and Pleistocene are upgraded, so that the Late Pleistocene, Early Pleistocene and Late Pliocene Epochs comprise the Quaternary Subperiod, itself equivalent to Late Neogene. The inflexibly nested hierarchy is preserved but the Tertiary is lost.