Correction method for full-depth current velocity with lowered acoustic Doppler current profiler (LADCP)

A new method is presented to process and correct full-depth current velocity data obtained from a lowered acoustic Doppler current profiler (LADCP). The analysis shows that, except near the surface, the echo intensity of a reflected sound pulse is closely correlated with the magnitude of the difference in vertical shear of velocity between downcast and upcast, indicating an error in velocity shear. The present method features the use of echo intensity for the correction of velocity shear. The correction values are determined as to fit LADCP velocity to shipboard ADCP (SADCP) and LADCP bottom-tracked velocities. The method is as follows. Initially, a profile of velocity relative to the sea surface is obtained by integrating vertical shears of velocity after low-quality data are rejected. Second, the relative velocity is fitted to the velocity at 100–800 dbar measured by SADCP to obtain an “absolute” velocity profile. Third, the velocity shear is corrected using the relationship between the errors in velocity shears and echo intensity, in order to adjust the velocity at sea bottom to the bottom-tracked velocity measured by LADCP. Finally, the velocity profile is obtained from the SADCP-fitted velocity at depths less than 800 dbar and the corrected velocity shear at depths greater than 800 dbar. This method is valid for a full-depth LADCP cast throughout which the echo intensity is relatively high (greater than 75 dB in the present analysis). Although the processed velocity may include errors of 1–2 cm s⁻¹, this method produced qualitatively good current structures in the Northeast Pacific Basin that were consistent with the deep current structures inferred from silicate distribution, and the averaged velocities were significantly different from those calculated by the Visbeck (2002) method.     

A note on internal wavetrains and the associated undulation of the sea surface observed upstream of seamounts

An internal wavetrain, generated by a tidal current in superposition with the Tsushima Warm Current, has been observed by use of an acoustic echo-sounder upstream of the Shichiri-Ga-Sone Seamounts in the East Tsushima Strait of the Japan Sea. The sea surface above the internal wavetrain was simultaneously observed and was found to be undulated at the wavelength of the internal wave.     

The Nadanhada Terrane

Abstract The Nadanhada terrane, a Jurassic disrupted terrane in Heilongjiang Province of China, is principally composed of Permo- Carboniferous limestone and greenstone, Triassic bedded chert and middle Jurassic siliceous shale, all enclosed within younger (presumably Late Jurassic- Early Cretaceous) clastics. Palaeontological and lithological characteristics and structural features of these formations are entirely identical to those of the Mino terrane of the Japanese Islands. Prior to opening of the Sea of Japan, these terranes formed a single superterrane together with the Western Sikhote-Alin terrane. Tectono-stratigraphic terranes very similar to the Nadanhada and Mino terranes are also found in the Ryukyu are, the Philippines and probably in Borneo. All these terranes constituted a belt of accretionary complexes during Late Jurassic and / or Early Cretaceous time along the eastern continental margin of Asia after completion of the Triassic collage of the Chinese continent.     

Deep-circulation current through the Main Gap of the Emperor Seamounts Chain in the North Pacific

The deep-circulation current in the North Pacific carries lower circumpolar deep water (LCDW), which is characterized by high dissolved oxygen and low echo intensity of reflected sound pulses. Using the characteristics of LCDW, we examined a branch current of the deep circulation passing through the Main Gap of the Emperor Seamounts Chain (ESC) by analyzing conductivity temperature depth profiler (CTD) data and data of velocity and echo intensity from a lowered acoustic Doppler current profiler (LADCP), which were obtained along 170°E immediately west of the ESC, along 180°W and 175°W over the northern slope of the Hess Rise, and along 165°W. The velocity and water characteristics showed that the eastern branch current of the deep circulation, which has penetrated into the Northwest Pacific Basin (NWPB) through Wake Island Passage, bifurcates around 30°N, 170°E in the NWPB into the westward main stream and a northward branch current, and that the latter current proceeds along the western side of the ESC and passes through the Main Gap of the ESC, flowing eastward. The current in the Main Gap at 170°E flows southeastward with eastward velocity cores around 4000 dbar and at depths greater than 4800 dbar centered at 5400 dbar. The current in the deeper core is stronger and reaches a maximum velocity of approximately 10 cm s^?1. The eastward current in the Main Gap enters the Northeast Pacific Basin (NEPB) and flows eastward along the northern slope of the Hess Rise. As the current flows downstream, the characteristics of LCDW carried by the current are diluted gradually. To the east of the Hess Rise, the branch current joins another branch current of the deep circulation from the south carrying less-modified LCDW. As a result, LCDW carried from the Main Gap is renewed by mixing with the less-modified LCDW coming from the south. Carrying the mixed LCDW, the confluence flows eastward south of 37°N at 165°W toward the northeastern region of the NEPB, where the LCDW overturns and changes to North Pacific Deep Water (NPDW). NPDW is probably carried by the westward current in the upper deep layer north of 37°N at 165°W.     

Satellite-based assessment of large-scale land cover change in Asian arid regions in the period of 2001–2009

Arid regions in Asia are commonly characterized by rapidly growing populations with limited land resources and varying rainfall frequencies under climatic change. Despite being one of the most important environmental challenges in Asia, the changing aridity in this region, particularly due to large-scale land cover change, has not been well documented. In this study, we used rainfall data and a new land heterogeneity index to identify recent trend in land cover changes in the Asian arid regions. The result indicates a significant decreasing trend of barren lands and an increasing trend of vegetated lands. Although the potential land cover change is commonly believed to be strongly sensitive to rainfall change, such sensitivity has not been observed during the nine-year period (2001–2009) analyzed. Through the analyses of two separate periods (2001–2005 and 2005–2009), the sensitivity of rainfall to land cover change in arid regions is found to be dependent on the initial spatial heterogeneity of vegetated land cover. The approach used and the findings in this study represent an important step toward better understanding of large-scale land cover change in the Asian arid regions, and have the potential to predict future land cover change under various climate change scenarios.     

Projection of future world water resources under SRES scenarios: an integrated assessment

Abstract

Changes in water resources availability, as affected by global climate warming, together with changes in water withdrawal, could influence the world water resources stress situation. In this study, we investigate how the world water resources situation will likely change under the Special Report on Emissions Scenarios (SRES) by integrating water withdrawal projections. First, the potential changes in water resources availability are investigated by a multi-model analysis of the ensemble outputs of six general circulation models (GCMs) from organizations worldwide. The analysis suggests that, while climate warming might increase water resources availability to human society, there is a large discrepancy in the size of the water resource depending on the GCM used. Secondly, the changes in water-stressed basins and the number of people living in them are evaluated by two indices at the basin scale. The numbers were projected to increase in the future and possibly to be doubled in the 2050s for the three SRES scenarios A1b, A2 and B1. Finally, the relative impacts of population growth, water use change and climate warming on world water resources are investigated using the global highly water-stressed population as an overall indicator. The results suggest that population and socio-economic development are the major drivers of growing world water resources stress. Even though water availability was projected to increase under different warming scenarios, the reduction of world water stress is very limited. The principal alternative to sustainable governance of world water resources is to improve water-use efficiency globally by effectively reducing net water withdrawal.

Editor Z.W. Kundzewicz; Associate editor D. Gerten     

A pair of Langmuir cells in two laboratory tanks (II) on generation mechanism

Vertical and cross-wind profiles of mean currents were measured systematically in vertical cross-sections of two wind-wave tanks with aspect ratios of order one to study the secondary flow in the tanks. A pair of Langmuir cells turned out to be driven by a close combination of the pressure gradient along the tank and the side-wall effects. That is, part of the adverse pressure gradient produced a parabolic cross-wind profile with the smallest downwind current at the centerline and the largest current along the two sidewalls. As a result, upwelling occurred in the center zone where the return flow was strongest, probably because of the entrainment action of the wind-driven current. In order to compensate for this upwelling, downwelling occurred along the two side-walls from the flow continuity. The resulting vertical circulation formed a pair of Langmuir circulations across the span and served to maintain the parabolic profile formed by the pressure gradient. A positive feedback mechanism is thus found between the primary and secondary circulations through upwelling of the return flow in the center zone. Vertical shears of the span-averaged downwind current measured in two tanks were found to be systematically different from each other. This difference seems to depend on the magnitude of the advective Reynolds stresses in the two tanks.     

Differences in flood hazard projections in Europe – their causes and consequences for decision making

This paper interprets differences in flood hazard projections over Europe and identifies likely sources of discrepancy. Further, it discusses potential implications of these differences for flood risk reduction and adaptation to climate change. The discrepancy in flood hazard projections raises caution, especially among decision makers in charge of water resources management, flood risk reduction, and climate change adaptation at regional to local scales. Because it is naïve to expect availability of trustworthy quantitative projections of future flood hazard, in order to reduce flood risk one should focus attention on mapping of current and future risks and vulnerability hotspots and improve the situation there. Although an intercomparison of flood hazard projections is done in this paper and differences are identified and interpreted, it does not seems possible to recommend which large-scale studies may be considered most credible in particular areas of Europe.

EDITOR D. Koutsoyiannis

ASSOCIATE EDITOR not assigned     

Predicting dam-related downstream geomorphic response with widely available stream gauge data: A case study of the Godavari River Basin,India

Dam-related downstream adjustments of riverbeds are normally investigated by analysing the trend in sediment supply and high flow events during the pre- and post-dam periods. The required data for existing predictive models is not measured at river gauges, which limits the application of these tools. We derived the frequency of sediment-transporting streamflow events (T*) and upstream sediment supply (S*) in the pre- and post-dam periods with widely available gauged records and predicted changes in the downstream riverbed by adapting an existing model. Ten gauging stations in the Godavari River Basin, India, located downstream of dams, were chosen as study sites. Annually surveyed cross-sections at each site validated the accuracy of the predicted dam-related downstream changes. Then, a regression equation (R² = 0.75) was established between T*/S* (independent variable) and changes in the downstream bed elevation (dependent variable) for the Godavari Basin. We recommended that similar local empirical equations be formulated for larger river basins. Models of large-scale rainfall-runoff and sediment transport processes that can account for the influence of dams, such as the Soil & Water Assessment Tool, can be paired with the proposed regression equation to estimate dam-related downstream erosion and deposition with globally available data.     

A study of rock alteration process based on kinetics of hydrothermal experiment

Experimental investigation was made on hydrothermal alteration of sericite in acid solution. It is found that the sericite is changed to pyrophyllite through kaolinite at 270° C and 260° C, and the alteration is considered to be a consecutive reaction involving two steps, from sericite to kaolinite and from kaolinite to pyrophyllite. On the other hand, pyrophyllite is not formed at 190° C and the sericite is altered to kaolinite. The rate of reaction in each step of this hydrothermal alteration is dependent on reaction temperature and chemical composition of the solution. From the results of the quantitative analyses of the experimental products, kinetic constants and related parameters were obtained.By using these numerical values, an alteration process can be followed in a given model. Computed results for some cases of an alteration of sericite in acid solution are presented. Geological phenomena such as rock alteration or weathering are thought to progress in a disequilibrium state and in an open system. An attempt described in this paper is a physicochemical approach to treat an alteration process accompanied with material transfer occurring in an open system from a kientic viewpoint.