Northwest-southeast Orientation Research Articles

Factors influencing fractures networks within Permian shale intervals in the Cooper Basin, South Australia

The future success of both enhanced (engineered) geothermal systems and shale gas production is reliant on the development of reservoir stimulation strategies that suit the local geo-mechanical conditions of the prospects. The orientation and nature of the in-situ stress field and pre-existing natural fracture networks in the reservoir are among the critical parameters that will control the quality of the stimulation program. This study provides a detailed investigation into the nature and origin of natural fractures in the area covered by the Moomba–Big Lake 3D seismic survey, in the southwest termination of the Nappamerri Trough of the Cooper Basin. These fractures are imaged by both borehole image logs and complex multi-traces seismic attributes (e.g. dip-steered most positive curvature and dip-steered similarity), are pervasive throughout the cube, and exhibit a relatively consistent northwest–southeast orientation. Horizon extraction of the seismic attributes reveal a strong variation in the spatial distribution of the fractures. In the acreage of interest, fracture density is at its highest in the vicinity of faults and on top of tight antiforms. This study also suggests a good correlation between high fracture density and high gamma ray values. The correlation between high fracture density and shale content is somewhat counterintuitive, as shale is expected to have a higher tensile and compressive strengths at shallow depths and typically contain fewer fractures (Lin, 1983). At large depths, however—and due to sandstone diagenesis and cementation—shale has lower tensile and compressive strength than sandstone and is expected to be more fractured (Lin, 1983). A similar correlation has been noted in other Australian Basins (e.g. Northern Perth Basin). Diagenetic effects, pore pressure, stiffness, variations in tensile versus compressive strength of the shale and the sandstone may explain this disparity.

Vagrant Western Red-shouldered Hawks: Origins, Natal Dispersal Patterns, and Survival

We report the results of a 40-year study of the western Red-shouldered Hawk (Buteo lineatus elegans) involving the banding of 2742 nestlings in southern California from 1970 to 2009 (this study) plus 127 nestlings banded in other California studies (1956–2008) and the analyses of 119 records of subsequent recovery from the Bird Banding Laboratory (1957–2009). Of the Red-shouldered Hawks recovered, 109 (91.6%) moved <100 km (short-distance dispersers), while 10 (8.4%) moved >100 km (long-distance dispersers). Three (2.5%), all long-distance dispersers, were vagrants (recovered outside the species' range of residency), and were found 374 to 843 km northeast and south of their banding locations in the Mojave, Great Basin, and Vizcaino deserts. The distribution of directions of short-distance dispersal was bipolar, closely corresponding with the northwest—southeast orientation of the species' range in southern California, while that of long-distance dispersers was mainly to the north. One of 10 long-distance dispersers, a nonvagrant, survived well into the age of breeding (103.0 months), whereas eight of the other nine perished before 14.5 months. The implications of vagrancy for conservation of this resident subspecies are that a relatively small source area can contribute genetic material over a vastly larger receiving area but rarely does so because of high mortality rates. Nonetheless, the movements of vagrants we documented provide evidence for the species' potential to populate new landscapes in response to changing environmental conditions and to maintain genetic heterogeneity within existing populations.

Basement control on past ice sheet dynamics in the Amundsen Sea Embayment, West Antarctica

The development of landscapes and morphologies follows initially the tectonic displacement structures of the basement and sediments. Such fault zones or lineaments are often exploited by surface erosional processes and play, therefore, an important role in reconstructing past ice sheet dynamics. Observations of bathymetric features of the continental shelf of the Amundsen Sea Embayment and identification of tectonic lineaments from geophysical mapping indicate that the erosional processes of paleo-ice stream flows across the continental shelf followed primarily such lineaments inherited from the tectonic history since the Cretaceous break-up between New Zealand and West Antarctica. Three major ice flow trends correspond to different tectonic phases in east–west, northwest–southeast and north–south directions. East–west oriented basement trends correlate with coastline trends and overlay tectonic lineaments caused by former rift activities. Directional trends with northwest–southeast orientation are observed for the glacial troughs of the western embayment outer shelf, the western Pine Island Bay coastal zones, and the inner Pine Island glacial trough and are associated with a distributed southern plate boundary zone of the former Bellingshausen Plate. The north–south trend of the main Pine Island glacial trough and the north–northeast trend of the Abbot Ice Shelf trough on the outer shelf follow the predicted lineation trend of an eastern branch of the West Antarctic Rift System extending from the Thwaites drainage basin northward into Pine Island Bay. An understanding of this context helps better constrain the geometries and sea-bed substrate conditions for regional paleo-ice sheet models.

First satellite-imaging observation of medium-scale traveling ionospheric disturbances by FORMOSAT-2/ISUAL

On the night of May 16, 2007, a satellite limb imager of FORMOSAT-2/ISUAL observed wave-like structures of the 630-nm airglow simultaneously with an all-sky imager deployed at Darwin in Australia. The height of the airglow layer was estimated as 220 km, and the structures were aligned in the northeast-southwest orientation with a wavelength of ∼300 km and propagated toward the northwest with a phase velocity of ∼100 m s−1, showing typical characteristics of the nighttime medium-scale traveling ionospheric disturbances (MSTIDs). We conclude that ISUAL for the first time succeeded in observing the airglow layer altitude and airglow structures modulated by MSTID from space. Such a satellite limb airglow imaging could be a new tool to characterize ionospheric irregularities on a global level.

Investigation of links between Precambrian basement structure and Paleozoic strata in the Fort Worth basin, Texas, U.S.A., using high-resolution aeromagnetic (HRAM) data and seismic attributes

Effective hydraulic fracturing is critical for generating permeability within the Barnett Shale of the Fort Worth basin (FWB). Therefore, knowledge of the nature of the induced and natural fractures, faults, and collapse features that may form conduits to the underlying Ellenburger aquifer is vital. We use coherence and curvature seismic attributes, which are sensitive to faults, fractures, and collapse features, to map sedimentary features. We then integrate high-resolution aeromagnetic (HRAM) data with the seismic attributes extracted along the Ellenburger Formation and the top of basement from the north-central portion of the FWB, thereby linking features in the Precambrian basement to shallower sedimentary structures. HRAM-derived maps, designed to enhance basement structures, confirm that much of the sedimentary faulting is basement controlled. Specifically, attribute lineaments are aligned parallel to HRAM anomaly lineaments, consistent with regional tectonics. The northeast-southwest and northwest-southeast orientations of folds and faults in the sedimentary section parallel the northeast-trending Ouachita orogenic belt and the northwest-trending Muenster arch, which in turn correlate with reactivated Cambrian/late Precambrian basement faults. Mapping such features can aid in the design of the hydraulic fracture program and ability to predict structurally deformed areas of the basin.

Ice-flow pattern and extent of the last Scandinavian Ice Sheet southeast of the Baltic Sea

A review of scientific literature, geological maps and available previous digital data on ice-marginal positions, glacial landforms and sediment distribution are utilised to reconstruct ice streams and lobes, and the maximum (LGM) and subsequent recessional ice-marginal positions of the Scandinavian Ice Sheet (SIS) southeast of the Baltic Sea. This paper presents preliminary results of the ongoing research that aims to build a Geographic Information System (GIS) based model on the extent and timing of the last SIS in the area between its maximum extent and the Baltic Sea. Digitized subglacial bedforms, ice marginal and other glacial landforms and features from published sources are compared and validated against digital elevation model (DEM). This has allowed specifying, revising and questioning the location of the LGM, and to interpret and correlate the post-LGM ice streams with marginal positions. Morphological evidence demonstrates that large ice streams of between 100 and 300km lengths, were in operation ca 2–3ka after the LGM and formed the Middle and North Lithuanian end moraines. Ice streams from the Onega and White Sea basins had high lateral slopes and clearly channelled flow while the ice streams that drained the ice sheet through the Ladoga–Ilmen depression and in the eastern Baltic usually had a fan-shaped flow pattern and morphologically unclear lateral slopes. In elevated areas and highlands that are located between ice streams a crossing of lineated bedforms is common. One set of the crossing lineations on Zemaitia, Eastern Kursa, Vidzeme, Haanja, Latgale and Sudoma highlands that are 200–350km inside of the LGM margin, has northwest–southeast orientation that in general conforms to the direction of ice flow during the LGM and Baltija phases. It is expected that the presented compilation will stimulate discussions and scrutiny of earlier published data and generate ideas for future investigations.

The Monagas Fold–Thrust belt of Eastern Venezuela. Part II: Structural and palaeo-geographic controls on the turbidite reservoir potential of the middle Miocene foreland sequence

This paper presents a new structural-stratigraphic approach to constrain the reservoir potential of the middle Miocene turbidite systems within the Monagas Fold–Thrust Belt (MFTB) and Maturín Sub-Basin (MSB) of eastern Venezuela. In the frontal anticline structures of the MFTB (Amarilis Area) light hydrocarbons have been produced from these turbidite systems which were deposited in a foreland basin with a complex tectonostratigraphic evolution. In order to predict the location of other analogous reservoirs we used the structural model presented in Part I ( Parra et al., 2010) to developed a palaeo-topographic reconstruction at early-middle Miocene. We have then used this reconstruction to constrain the palaeogeography of the middle Miocene foredeep where the turbidites were deposited. The area considered has 5000 km 2. By middle Miocene four regions are identified: 1) The southern basin margin dipped 1.5–2.5° north; 2) The foredeep axis had a southwest–northeast orientation. Within the foredeep the proto-structures of the MFTB created submerged highs that control the distribution of sediments; 3) The northern basin margin dipped 3–4° south; the coastline was controlled by the Pirital thrust sheet; 4) The main source of sediments was located towards the northwest on the Pirital thrust sheet and Serranía del Interior. Variations in shortening across the strike of the Pirital thrust were accommodated by a lateral ramp which controlled the location of a valley that acted as the main sediment pathway for the sediments that fed the turbidite system. This relationship between the thrust belt geomorphology and the location of turbidite sediment within the foredeep must be considered in order to assess the distribution of the Miocene turbidite reservoirs.

The Bingham Canyon Porphyry Cu-Mo-Au Deposit. II. Vein Geometry and Ore Shell Formation by Pressure-Driven Rock Extension

Copper, gold, and molybdenum in the Bingham Canyon deposit (Utah, United States) show a systematic distribution in grade and metal ratios. Most Cu-Au mineralization follows, both spatially and temporally, the emplacement of the quartz monzonite porphyry (QMP), a southwest-northeast-elongated thick dike intruding along the contact between the premineralization equigranular monzonite stock and surrounding sedimentary rocks. Copper ore grades define the shape of an inverted cup, which is centered on the QMP but has a much broader, near-circular footprint. Several deep root zones surround a barren core occupied by the same lithologic units and intense potassic alteration but insignificant metal tenor. Throughout the deposit, gold to copper ratio is systematically zoned. The distribution of molybdenum resembles that of copper and partly overlaps with it, but the molybdenum ore shell is generally displaced inward and downward from the copper ore shell. Systematic measurement of the abundance and orientation of three major vein types obtained at the pit surface were complemented with unoriented vein density data from drill core logging. Quartz stockwork veins, the earliest and most abundant of the mapped vein types, are related to potassic alteration and major Cu-Au mineralization. Their greatest vein density follows the intrusion of the QMP but extends far beyond, into sedimentary rocks and especially into adjacent parts of the pre-ore equigranular monzonite. Their orientation is predominantly steep, with a variable strike. Quartz-molybdenite veins postdate all intrusions and are less abundant than quartz stockwork veins. They show variable orientation of strikes and a weaker tendency to steep dips. Quartz-pyrite veins with sericitic alteration halos crosscut all intrusions and earlier veins. They are rare within the central high-grade part of the deposit and predominantly occur near and outside the northeastern and southwestern ends of the QMP. They have a strongly preferred orientation parallel to the porphyry dikes, with steep dips and strike directions fanning out radially from the center of the deposit. Repeated cycles of dike intrusions with distinct southwest-northeast orientation, followed by steep stockwork veins with variable strikes distributed over the broad ore shell and a final return to oriented postore veins are interpreted to result from alternation between two stress regimes. A regional, probably weakly transtensional regime controls the emplacement of dikes and postore veins. Ore vein formation and mineralization is controlled by active rock extension (increase in differential stress) in the roof area above a broad region of high fluid pressure in a subjacent magma chamber, rather than by local hydrofracturing caused by fluid exsolution from the porphyries (decrease in effective rock pressure and positive volume change upon magma crystallization). Shallow vein mineralization well above the lithostatic-to-hydrostatic transition is consistent with low (hydrostatic or even vapor-static) pressures of ore deposition indicated by a companion study of fluid inclusions. The process of pressure-driven roof extension favors the accumulation of metals in high-grade ore shells, compared to temperature-driven concepts according to which the porphyry mineralization is spread out vertically by following downward-retracting isotherms in a cooling magmatic-hydrothermal system.

Analysis of conveyor belts in winter Mediterranean cyclones

The relevance of the midlatitude conveyor belt model to Mediterranean cyclones (MCs) is examined using data from two winters. Eight MCs, which exhibit typical midlatitude cyclone structure, were scrutinized and their conveyor belts were examined. The analysis was based on satellite imagery, isentropic wind maps, vertical cross-sections of potential and equivalent potential temperatures, and air back-trajectories. The conveyor belts found in the studied MCs were similar to the common features of midlatitude cyclones, except for three aspects. First, the warm conveyor belt was not associated with massive organized cloudiness in five of the eight cyclones since it consisted of dry air originated from the Saharan desert. Second, the anticyclonic branch of the cold conveyor belt was not found in half of the MCs. Third, the dry air intrusion originated north of the cyclone and extended southward around it, unlike its common midlatitudinal northwest–southeast orientation. This is consistent with the relatively small baroclinic vertical-westward tilt of the cyclones analyzed.

Seasonal Variations in the Circulation over the Middle Atlantic Bight Continental Shelf

Abstract Fits of an annual harmonic to depth-average along-shelf current time series longer than 200 days from 27 sites over the Middle Atlantic Bight (MAB) continental shelf have amplitudes of a few centimeters per second. These seasonal variations are forced by seasonal variations in the wind stress and the cross-shelf density gradient. The component of wind stress that drives the along-shelf flow over most of the MAB mid- and outer shelf is oriented northeast–southwest, perpendicular to the major axis of the seasonal variation in the wind stress. Consequently, there is not a significant seasonal variation in the wind-driven along-shelf flow, except over the southern MAB shelf and the inner shelf of New England where the wind stress components forcing the along-shelf flow are north–south and east–west, respectively. The seasonal variation in the residual along-shelf flow, after removing the wind-driven component, has an amplitude of a few centimeters per second with maximum southwestward flow in spring onshore of the 60-m isobath and autumn offshore of the 60-m isobath. The spring maximum onshore of the 60-m isobath is consistent with the maximum river discharges in spring enhancing cross-shelf salinity gradients. The autumn maximum offshore of the 60-m isobath and a steady phase increase with water depth offshore of Cape Cod are both consistent with the seasonal variation in the cross-shelf temperature gradient associated with the development and destruction of a near-bottom pool of cold water over the mid and outer shelf (“cold pool”) due to seasonal variations in surface heat flux and wind stress.

Thermal evolution and geometry of the descending lithosphere beneath the SE-Carpathians: An insight from the past

Mantle heterogeneities imaged by seismic tomography in the SE-Carpathians contain information on the present thermal state of the mantle. Based on P-wave seismic velocity anomalies we develop a model of the present mantle temperature beneath the region and combine the model with a model of crustal temperature constrained by heat flow data. The modeled temperatures are assimilated into the geological past using the information on the regional movement in the Early and Middle Miocene. Prominent thermal states of the lithospheric slab descending in the region are restored from its diffuse present state. In Miocene times the slab geometry clearly shows two portions of the sinking body. The northwest–southeast oriented portion of the body is located in the vicinity of the boundary between the East European and Scythian platforms, and this portion of the sinking body may be a relic of cold lithosphere that has traveled eastward. Another portion has a northeast–southwest orientation and is related to the present descending slab. Above a depth of 60 km the slab had a concave thermal shape, confirming the curvature of the Carpathian arc, and a convex surface below that depth. The slab maintained its convex shape until it split into two parts at a depth of about 220 km. We propose that this change in the slab geometry, which is likely to be preserved until the present, can cause stress localization due to the slab bending and subsequent stress release resulting in large mantle earthquakes in the region. Also we hypothesize that either the processes of dehydration and partial melting of the descending lithospheric slab or dragging down of hotter rocks from the adjacent uppermost mantle by the slab are possible causes of the reduction in seismic velocities beneath the Transylvanian Basin.

Spatiotemporal Variability of Hourly Precipitation over the Eastern Contiguous United States from Stage IV Multisensor Analyses

Abstract The statistical character of precipitation events from hourly stage IV analyses is documented for the eastern United States during the cool [December–February (DJF)] and the warm [June–August (JJA)] seasons for the four years of 2002–05. Isotropic e-folding distances and in situ e-folding times are computed for mesh sizes that vary from 4 km (the minimal stage IV pixel size) to 32 km for two thresholds: light (1 mm h−1) and heavy (5 mm h−1) precipitation rates. Marked seasonal variability characterizes the e-folding times. They typically run between 2 and 3 h during winter and 1 and 2 h during summer for light events, and they run an hour shorter for heavy rainfall during both seasons. Spatial decorrelation estimates also reveal considerable seasonal and geographical variability; e-folding distances typically lie between 60 and 180 km during the winter and between 30 and 60 km during the summer for light episodes, and they are approximately a factor of 2 to 3 shorter for heavy events. Anisotropic statistics are estimated by a simple geometric model. Hourly precipitation patterns show a preference for a southwest–northeast orientation during both seasons with greater elongation during the winter. Mean propagation velocities of precipitating systems are faster and are more closely aligned with the dilatation axis during the winter. These statistics should provide useful guidance for diagnosing and improving the spatiotemporal variance characteristics of precipitation for downscaling algorithms and numerical models of hydrometeorological prediction systems.

The Serra da Graciosa A-type granites and syenites, southern Brazil. Part 1: regional setting and geological characterization

The Serra da Graciosa region includes important occurrences of granites and syenites of the Graciosa A-type Province (formerly Serra do Mar Suite), southern Brazil. Using fieldwork, petrography, and remote sensing imagery, we characterize the geology of the plutons in the region. Five individual plutons were recognized. Two correspond to the previously defined Marumbi and Anhangava Plutons. We divide the former "Graciosa Pluton" into three new plutons: Capivari, Órgãos, and Farinha Seca. The plutons are elliptical with northeast-southwest orientation. Two petrographic associations can be recognized: an alkaline association that includes peralkaline and metaluminous hypersolvus alkalifeldspar granites and syenites (Anhangava, Farinha Seca, Órgãos), and an aluminous association composed of metaluminous and weakly peraluminous subsolvus granites (Capivari, Órgãos, Anhangava, Marumbi). Occurrences of each association are limited to one individual pluton or to portions of a pluton, and the age relationships are not well established. Monzodioritic rocks are found marginal to the Órgãos and Farinha Seca Plutons, and interaction with silicic magmas locally produced hybrid quartz syenites (Farinha Seca Pluton). Geothermobarometry indicates emplacement at shallow crustal levels (P = 2 ± 0.6 kbar), and crystallization temperatures within the interval 900-700ºC for the granitic and syenitic rocks, and 1000-750ºC for the monzodioritic rocks.

Baroclinic development within zonally‐varying flows

AbstractPrevious idealized‐modelling studies have shown the importance of across‐jet barotropic shear to the resulting evolution of cyclones, anticyclones, surface‐based fronts, and upper‐level fronts. Meanwhile, many observational studies of cyclones have shown the importance of along‐jet variations in the horizontal wind speed (i.e. confluence and diffluence). This study investigates the importance of these along‐jet (zonal, for zonally‐oriented jets) variations in the horizontal wind speed to the resulting structures and evolutions of baroclinic waves, using idealized models of growing baroclinic waves. An idealized primitive‐equation channel model is configured with growing baroclinic perturbations embedded within confluent and diffluent background flows. When the baroclinic perturbations are placed in background confluence, the lower‐tropospheric frontal structure and evolution initially resemble the Shapiro–Keyser cyclone model, with a zonally‐oriented cyclone, strong warm front, and bent‐back warm front. Later, as the baroclinic wave is amplified in the stronger downstream baroclinicity, the warm sector of the cyclone narrows, becoming more reminiscent of the Norwegian cyclone model. The upper‐level frontal structure develops with a southwest–northeast orientation, and becomes strongest at the base of the trough, where geostrophic cold advection is occurring. In contrast, when the baroclinic perturbations are placed in background diffluence, the lower‐tropospheric frontal structure and evolution resemble the Norwegian cyclone model, with a meridionally‐oriented cyclone, strong cold front, and occluded front. The upper‐level frontal structure is initially oriented northwest–southeast on the western side of the trough, before becoming zonally oriented. Weak geostrophic temperature advection occurs along its length. These results are compared to those from previous observational and idealized‐modelling studies. Copyright © 2007 Royal Meteorological Society

Multiple Precipitation Mechanisms over Mountains Observed by Airborne Doppler Radar during MAP IOP5

Abstract This study uses airborne Doppler radar measurements from the Special Observing Period of the Mesoscale Alpine Programme (MAP) to document the detailed airflow and precipitation structure over the mountainous regions near the border of northeastern Italy and Slovenia as a cold frontal system moved eastward and encountered the eastern Alps on 4 October 1999, during MAP IOP5. In contrast to previously documented MAP cases, the environmental conditions associated with this case are characterized by a deep layer of strong convective instability in the lower troposphere and by a cold, northeasterly continental flow coming down from the mountains (the so-called bora wind) along the southeastern Alps. Over the study region, there are two primary mountain barriers: the Julian Alps, oriented roughly west–east with a peak mountain height of ∼2500 m and a significant variation in terrain height along its length, and the other, the Dinaric Alps, a relatively lower mountain range oriented northwest–southeast, immediately adjacent to the south of the Julian Alps. How these two mountain barriers and the northeasterly continental flow influence the precipitation, and the nature of orographic precipitation developing in a potentially unstable environment, are explicitly addressed. Comprehensive analyses of airborne Doppler radar measurements reveal significant variations of precipitation in terms of its location, intensity, and type over the mountainous region. Particularly, five distinct forcing types of precipitation are evident for this case: slope convergence triggering, upslope triggering, weak slope convergence triggering, gap exit convergence triggering, and slope convergence forced stratiform. These precipitation characteristics are found to be closely related to the environmental thermodynamics, orographic features, and the complicated interactions between the southerly/southwesterly flow, northerly continental flow, and the orography. Details of these important observational aspects are elaborated upon in this paper.

Annual cycle of the mid-tropospheric easterly jet over West Africa

The paper examines the annual cycle of the mid-tropospheric easterly jet (MTJ) over West Africa against the background of many reviews indicating different locations and characteristics of the jet and considering it as a summer feature. NCEP–NCAR reanalysis zonal wind datasets for the period 1971–2000 and upper air datasets over the region are used. The results exhibit realistic spatial structure of the easterly jet. The long-term mean of the datasets suggests that the jet over West Africa is not only a summer feature but can also be found in winter with the same order of magnitude in the wind velocity at the core. The jet axis is located at about lat. 2° N close to the Guinean Coast in winter and at lat. 14° N in summer. The meridional oscillation of the jet suggests that as it advances northward, it maintains an altitude of 700 hPa in winter and transits in mid-spring to 650 hPa and reaches 600 hPa in summer. In the retreat, it displaces to 650 hPa at the end of September rather sharply to reach 700 hPa in October. The jet’s core has been observed to have a northeast–southwest orientation from season to season, covering a longitude of 29° from its southernmost to the northernmost positions.

Isostatic rebound in the northwestern part of the Lake Agassiz basin: Isobase changes and overflow

Based on new GPS elevation data on the Upper and Lower Campbell strandlines in the northwestern part of the glacial Lake Agassiz basin, the trend of isobases representing differential glacio-isostatic rebound in that region is shown to bend and assume a nearly west–east orientation. This differs from the northwest–southeast orientation to the southeast of the study area, which others had projected into the northwestern corner of the Lake Agassiz basin; this means that there was more isostatic depression than previously thought north of ∼ 53°N. The difference in slopes on reconstructed water planes of the Upper and Lower Campbell beaches is less with these isobases, which better reflects the short period of time between their formation. It is likely that the change in orientation of isobases reflects the presence of a thick Keewatin ice center to the north. Our revised west–east isobase reconstruction indicates that when the lake was at the Campbell beach levels, ∼ 9900–9400 14C B.P., Lake Agassiz overflowed across the paleo-divide at Wycherley Lake, Saskatchewan, which controlled flow through the Northwestern Outlet of Lake Agassiz during part of its history; overflow would not have occurred through the Wycherley Lake channels at this time using the old isobase projections.

Gravity waves in the middle atmosphere during the MaCWAVE winter campaign: evidence of mountain wave critical level encounters

Abstract. Falling sphere and balloon wind and temperature data from the MaCWAVE winter campaign, which was conducted in northern Scandinavia during January 2003, are analyzed to investigate gravity wave characteristics in the stratosphere and mesosphere. There were two stratospheric warming events occurring during the campaign, one having a maximum temperature perturbation at ~45 km during 17–19 January, and the other having a maximum perturbation at ~30 km during 24–27 January. The former was a major event, whereas the latter was a minor one. Both warmings were accompanied by upper mesospheric coolings, and during the second warming, the upper mesospheric cooling propagated downward. Falling sphere data from the two salvos on 24–25 January and 28 January were analyzed for gravity wave characteristics. Gravity wave perturbations maximized at ~45–50 km, with a secondary maximum at ~60 km during Salvo 1; for Salvo 2, wave activity was most pronounced at ~60 km and above. Gravity wave horizontal propagation directions are estimated using the conventional hodographic analysis combined with the S-transform (a Gaussian wavelet analysis method). The results are compared with those from a Stokes analysis. They agree in general, though the former appears to provide better estimates for some cases, likely due to the capability of the S-transform to obtain robust estimates of wave amplitudes and phase differences between different fields. For Salvo 1 at ~60 km and above, gravity waves propagated towards the southeast, whereas for Salvo 2 at similar altitudes, waves propagated predominantly towards the northwest or west. These waves were found not to be topographic waves. Gravity wave motions at ~45–50 km in Salvo 1 were more complicated, but they generally had large amplitudes, short vertical scales, and their hodographs revealed a northwest-southeast orientation. In addition, the ratios between wave amplitudes and intrinsic phase speeds generally displayed a marked peak at ~45–50 km and decreased sharply at ~50 km, where the background winds were very weak. These results suggest that these wave motions were most likely topographic waves approaching their critical levels. Waves were more nearly isotropic in the lower stratosphere.

Mapping 3D salt using the 2D marine magnetotelluric method: Case study from Gemini Prospect, Gulf of Mexico

The dominant salt body at Gemini Prospect, Gulf of Mexico, has been analyzed by seismic methods, revealing a complex 3D salt volume at depths 1 to 5 km beneath the mud line. Because of the high contrast in electrical conductivity between the salt and surrounding sediments, Gemini is an attractive target for electromagnetic interrogation. Using a broadband magnetotelluric (MT) sensor package developed at the Scripps Institution of Oceanography, data in the period band of 1 to 3000 s were collected at 42 sites in a series of profiles over Gemini, one of which was directly over a linear ridgelike salt feature striking roughly northwest–southeast and another orthogonal to it. These two profiles reveal that the strongest MT response arises when the electric field is oriented northeast–southwest. We test the suitability of 2D inversion of these data for recovering the true salt structure by examining inversions of both actual data and synthetic 3D MT responses derived from the seismically inferred salt volume. Occam inversions of the northeast–southwest component result in resistivity images that generally agree with the seismic data, whereas inversions of the complementary component yield significantly poorer fidelity. Disagreement is greatest (1–2 km) along the salt sides and base. Depth errors for top of salt are less than 500 m. Although thin, deep salt (&lt;1 km thick at 5 km depth) is not well resolved, the inversions reveal a resistive basement and a shallow subseabed environment rich in electrical heterogeneity that is weakly, if at all, suggested by the seismic data. A notable exception is a correlation between a previously uninterpreted seismic reflector and the base of a shallow resistivity anomaly whose presence is consistent with gas accumulation near the hydrate stability zone.

A Numerical Study on the Effects of Taiwan Topography on a Convective Line during the Mei-Yu Season

Abstract During the morning hours on 23 May 2002, a convective line associated with a mei-yu front brought heavy rainfall along the coast of central Taiwan under favorable synoptic conditions of warm air advection and large convective available potential energy (CAPE) of over 3000 m2 s−2. Doppler radar observations indicated that deep convection was organized into a linear shape with a northeast–southwest orientation along the front about 70 km offshore from Taiwan over the northern Taiwan Strait. The system then moved toward Taiwan at a slow speed of about 4 m s−1. In the present study, the effects of Taiwan topography on this convective line and subsequent rainfall distribution were investigated through numerical modeling using the Nagoya University Cloud-Resolving Storm Simulator (CReSS) at a 2-km horizontal grid size. Experiments with different terrain heights of Taiwan, including full terrain (FTRN), half terrain (HTRN), and no terrain (NTRN), were performed. The control run using full-terrain and cold rain explicit microphysics realistically reproduced the evolution of the convective line and the associated weather with many fine details. Two low-level convergence zones were found to be crucial in the development of this convective line and the subsequent rainfall distribution over Taiwan. The first was along the mei-yu front and forced mainly by the front, but was terrain enhanced off the northwestern coast of Taiwan due to the blocking of air on the windward side of the Central Mountain Range (CMR). After formation, convective cells along this zone propagated southeastward and produced rainfall over the northwestern coast. As the front moved closer to Taiwan, a second arc-shaped convergence zone with a nearly north–south orientation along about 120°E formed ahead of the front between the prevailing flow and near-surface offshore flow induced by the blocking. This second zone was terrain induced, and convection initiated near its northern end was found to be responsible for the rainfall maximum observed near the coast of central Taiwan. Its intensity and position were highly sensitive to terrain height. In the HTRN run where the terrain was reduced by half, a weaker zone closer to the CMR (by about 50 km) was produced, and the rain fell mostly over the windward slope of the terrain instead of over the coastal plain. When the terrain was removed in the NTRN run, no such zone with the correct orientation formed. It was also found that the frontal movement near northern Taiwan was slightly delayed with the presence of terrain, and this affected the timing and distribution of local rainfall during the later stages of this event.