Mechanisms of bottom boundary fluxes in a numerical model of the Shetland shelf

Across-slope bottom boundary layer (BBL) fluxes on the shelf-edge connect this region to deeper waters. Two proposed ways in which across-slope BBL fluxes can occur, in regions that have a slope current aligned to the bathymetry, are the frictional veering of bottom currents termed the ‘Ekman drain’ and through local wind-forced downwelling (wind-driven surface Ekman flow with an associated bottom flow). We investigate the variability, magnitude and spatial scale of BBL fluxes on the Shetland shelf, which has a prominent slope current, using a high-resolution (～2 km) configuration of the MITgcm model. Fluxes are analysed in the BBL at the shelf break near the 200 m isobath and are found to have a seasonal variability with high/low volume transport in winter/summer respectively. By using a multivariate regression approach, we find that the locally wind-driven Ekman transport plays no explicit role in explaining daily bottom fluxes. We can better explain the variability of the across-slope BBL flux as a linear function of the speed and across-slope component of the interior flow, corresponding to an Ekman plus mean-flow flux. We estimate that the mean-flow is a greater contributor than the Ekman flux to the BBL flux. The spatial heterogeneity of the BBL fluxes can be attributed to the mean-flow, which has a much shorter decorrelation length compared to the Ekman flux. We conclude that both the speed and direction of the interior current determines the daily BBL flux. The wind does not explicitly contribute through local downwelling, but may influence the interior current and therefore implicitly the BBL fluxes on longer timescales.     

Ecological Quality Status of the soft-bottom communities on the algerian coast: General patterns and diagnosis

Between 1995 and 2001, the soft-bottom communities along the 1180 km of the Algerian coast were sampled in nine gulfs and 12 harbours, providing a total of 655 samples. Eight macrozoobenthos-based biotic indices (S, N, H′, BQI, AMBI, BENTIX, BO2A and ITI) were selected to describe the general patterns of the coastal water quality status and to establish a quality diagnosis for the different zones subjected to anthropogenic pressure (e.g., harbour construction, industrial and urban pollution). Reference values were determined for each of the eight indices selected by analyzing the indices’ parameter distribution. The Ecological Quality Ratio (EQR) was estimated for each index, resulting in an EQR Mean Score and an EQR Bad Score. From these EQR, we defined an EQS for each sample. The agreement between these EQS was analysed using the Kappa method in order to propose a survey strategy for the Algerian coastal waters that would take into account the soft-bottom biological compartment. The results clearly indicate that high and good quality assessments are prevalent in the gulfs, while quality assessments in harbours vary greatly from bad to good. The effect of pollution observed in the harbours can be classified in two main groups, according to when they were constructed and their relative degree of openness to the sea, which permits better water circulation and probably dilutes the pollution.     

Spectral properties of the accretion discs around rotating black holes

We study the radiation properties of an accretion disc around a rotating black hole. We solve the hydrodynamic equations and calculate the transonic solutions of accretion disc in the presence of shocks. Then we use these solutions to generate the radiation spectrum in the presence of radiative heating and cooling processes. We present the effect of spin parameter of the black hole on the emitted radiation spectrum. In addition, attention has also been paid to the variation in energy spectral index with Kerr parameter and accretion rate. We find that spectral index becomes harder as the spin parameter changes from negative (accretion disc is counter-rotating with respect to the black hole spin) to a positive value. Finally, we compute and compare the spectral characteristics due to a free-fall flow and a transonic flow. We notice significant differences in high energy contributions from these two solutions.     

Air transport cruise altitude restrictions to minimize contrail formation

While quantification of the effects of NO_x and water vapor is still at an early stage there is evidence that contrail formation could make a significant contribution to global warming. This paper builds on previous research that analyzed a policy of restricting air transport cruise altitudes to eliminate contrail formation. Our previous work [Transport. Res. D 7(6) (2002) 451], examined altitude restrictions in European airspace and concluded that this could be a beneficial policy for reducing climate change impacts from aviation. Since most of the flights in European airspace are short-haul flights, this paper evaluates the trade-offs between altitude restrictions, fuel burn and journey times for longer haul flights of up to 6000 nm. Our focus is on the North Atlantic and US airspace and we examine potential contrail fraction to determine optimal cruise altitudes for reducing contrail formation. Changes in fuel burn and travel times associated with flight levels of 18,000 and 31,000 ft for different aircraft types are analyzed. We find that, in most cases, CO₂ emission increases would be unlikely to entirely counteract the benefit of possible reductions in contrail formation. For some aircraft types, the percentage increase in emitted CO₂ was found to be strongly dependent on journey length. In general, journey times appear not to be a major issue except for some aircraft types. Our results suggest that reducing aircraft cruise altitudes could be a beneficial policy for mitigating climate change impacts from the aviation sector. This is clearly dependent on aircraft type and the distances traveled, but more importantly on ambient atmospheric conditions which can vary significantly between regions and due to daily variation. This suggests that real time flight planning to minimize contrail formation should be investigated as a possible climate mitigation policy.     

Formulation of reference solutions for compaction process in sedimentary basins

This paper is devoted to the development of semianalytical solutions for the deformation induced by gravitational compaction in sedimentary basins. Formulated within the framework of coupled plasticity–viscoplasticity at large strains, the modeling dedicates special emphasis to the effects of material densification associated with large irreversible porosity changes on the stiffness and hardening of the sediment material. At material level, the purely mechanical compaction taking place in the upper layers of the basin is handled in the context of finite elastoplasticity, whereas the viscoplastic component of behavior is intended to address creep-like deformation resulting from chemo-mechanical that prevails at deeper layers. Semianalytical solutions describing the evolution of mechanical state of the sedimentary basin along both the accretion and postaccretion periods are presented in the simplified oedometric setting. These solutions can be viewed as reference solutions for verification and benchmarks of basin simulators. The proposed approach may reveal suitable for parametric analyses because it requires only standard mathematics-based software for PDE system resolution. The numerical illustrations provide a quantitative comparison between the derived solutions and finite element predictions from an appropriate basin simulator, thus showing the ability of the approach to accurately capture essential features of basin deformation.     

Why is Indian Ocean warming consistently?

Observations have shown that the Indian Ocean is consistently warming and its warm pool is expanding, particularly in the recent decades. This paper attempts to investigate the reason behind these observations. Under global warming scenario, it is expected that the greenhouse gas induced changes in air–sea fluxes will enhance the warming. Surprisingly, it is found that the net surface heat fluxes over Indian Ocean warm pool (IOWP) region alone cannot explain the consistent warming. The warm pool area anomaly of IOWP is strongly correlated with the sea surface height anomaly, suggesting an important role played by the ocean advection processes in warming and expansion of IOWP. The structure of lead/lag correlations further suggests that Oceanic Rossby waves might be involved in the warming. Using heat budget analysis of several Ocean data assimilation products, it is shown that the net surface heat flux (advection) alone tends to cool (warm) the Ocean. Based on above observations, we propose an ocean-atmosphere coupled positive feedback mechanism for explaining the consistent warming and expansion of IOWP. Warming over IOWP induces an enhancement of convection in central equatorial Indian ocean, which causes anomalous easterlies along the equator. Anomalous easterlies in turn excite frequent Indian ocean Dipole events and cause anti-cyclonic wind stress curl in south-east and north-east equatorial Indian ocean. The anomalous wind stress curl triggers anomalous downwelling oceanic Rossby waves, thereby deepening the thermocline and resulting in advection of warm waters towards western Indian ocean. This acts as a positive feedback and results in more warming and westward expansion of IOWP.     

Wavelet based identification of dominant scales for self-affine road roughness in context of riding comfort in vehicles

We present a simulation based study of multiscale roughness of road surfaces and its effect on riding comfort in vehicles, given the fact that characterization of the measured roughness is important to ensure smooth ride. Self-affine fractals are used to simulate typically measured roughness data. Multiscale characteristics of such surfaces are obtained through varied level of spatial resolutions. The hierarchical nature of the multiscale fractal roughness and their role in affecting the riding comfort is investigated herein. Wavelet transform technique is exploited for multiscale decomposition. Roughness is synthesized from cumulation of scales in a multiscale fashion. Single degree of freedom car model is used for characterizing riding comfort parameters in terms of dynamic response of vehicle suspension system subjected to jerks exerted by rough profiles. The dominant scales of roughness governing the comfort parameters are identified through parametric study. It is shown that not all the scales are equally important in deciding the comfort parameters; rather, these parameters remain nearly intact with the inclusion of only a few initial scales. This facilitates multiscale visualization of roughness data and allows representing the profiles in a precise form by excluding the spurious higher scales. Apparently, it gives an economic estimate of the required resolution in the physical measurement for specific purpose.     

Variability of the TRMM-PR total and convective and stratiform rain fractions over the Indian region during the summer monsoon

Level 3 (3A25) TRMM Precipitation Radar (PR) data are used for 13 years period (1998–2010) to prepare climatology of TRMM PR derived near surface rain (Total rain) and rain fractions for the 4-months duration of Indian Summer Monsoon season (June–September) as well as for individual months. It is found that the total rain is contributed mostly (99 %) by two rain fractions i.e. stratiform and convective rain fractions for the season as well as on the monthly basis. It is also found that total rain estimates by PR are about 65 % of the gauge measured rain over continental India as well as on sub-regional basis. Inter-annual variability of TRMM-PR rain estimates for India mainland and its sub-regions as well as over the neighboring oceanic regions, in terms of coefficient of variability (CV) is discussed. The heaviest rain region over north Bay of Bengal (BoB) is found to have the lowest CV. Another sub-region of low CV lies over the eastern equatorial Indian ocean (EEIO). The CVs of total rain as well as its two major constituents are found to be higher on monthly basis compared to seasonal basis. Existence of a well known dipole between the EEIO and the north BoB is well recognized in PR data also. Significant variation in PR rainfall is found over continental India between excess and deficit monsoon seasons as well as between excess and deficit rainfall months of July and August. Examination of rainfall fractions between the BoB and Central India on year to year basis shows that compensation in rainfall fractions exists on monthly scale on both the regions. Also on the seasonal and monthly scales, compensation is observed in extreme monsoon seasons between the two regions. However, much less compensation is observed between the north BoB and EEIO belts in extreme rain months. This leads to speculation that the deficit and excess seasons over India may result from slight shift of the rainfall from Central India to the neighboring oceanic regions of north BoB. Contribution of stratiform and convective rain fractions have been also examined and the two fractions are found to contribute almost equally to the total rain. Results are further discussed in terms of the possible impact of the two rain fractions on circulation based on possible difference is vertical profiles of latent heat of two types of rain. Substantial differences in the lower and upper tropospheric circulation regimes are noticed in both deficit and excess monsoon months/seasons, emphasizing the interaction between rainfall (latent heat) and circulation.