Structure and Evolution of Supergranulation from Local Helioseismology

Supergranulation is visible at the solar surface as a cellular pattern of horizontal outflows. Although it does not show a distinct intensity pattern, it manifests itself indirectly in, for example, the chromospheric network. Previous studies have reported significant differences in the inferred basic parameters of the supergranulation phenomenon. Here we study the structure and temporal evolution of a large sample of supergranules, measured by using local helioseismology and SOHO/MDI data from the year 2000 at solar activity minimum. Local helioseismology with f modes provides maps of the horizontal divergence of the flow velocity at a depth of about 1 Mm. From these divergence maps supergranular cells were identified by using Fourier segmentation procedures in two dimensions and in three dimensions (two spatial dimensions plus time). The maps that we analyzed contain more than 10⁵ supergranular cells and more than 10³ lifetime histories, which makes possible a detailed analysis with high statistical significance. We find that the supergranular cells have a mean diameter of 27.1 Mm. The mean lifetime is estimated to be 1.6 days from the measured distribution of lifetimes (three-dimensional segmentation), with a clear tendency for larger cells to live longer than smaller ones. The pair and mark correlation functions do not show pronounced features on scales larger than the typical cell size, which suggests purely random cell positions. The temporal histories of supergranular cells indicate a smooth evolution from their emergence and growth in the first half of their lives to their decay in the second half of their lives (unlike exploding granules, which reach their maximum size just before they fragment).     

Curie point depth beneath the Barramiya-Red Sea coast area estimated from spectral analysis of aeromagnetic data

The geothermal regime beneath the Barramiya-Red Sea coast area of the Central Eastern Desert of Egypt has been determined by using the Curie point depth, which is temperature dependent. This study is based on the analysis of aeromagnetic data. The depth to the tops and centroid of the magnetic anomalies are calculated by power spectrum method for the whole area. The result of this investigation indicates, two new maps of the Curie point depth (CPD) and the surface heat flow (q) maps of the study area. The coastal regions are characterized by high heat flow (83.6 mW/m²), due to the geothermic nature of the region, and shallow Curie depth (22.5 km), where (CPD) depends on the tectonic regime and morphology in the eastern part of the area. The western portion of the studied area has a lower heat flow (<50 mW/m²) and deeper Curie depth (∼40 km), due to the existence of a large areal extent of negative Bouguer anomaly in the NE-SW direction. In addition to its bordering to the Red Sea margin, such high heat flow anomaly is associated with the increased earthquake swarms activity in the Abu Dabbab area.     

Aquifer modelling of the Ganga–Mahawa sub‐basin,a part of the Central Ganga Plain,Uttar Pradesh,India

The Ganga–Mahawa sub‐basin, which has an area of 1280 km² forms the western part of the Central Ganga Plain in the Moradabad and Badaun districts of western Uttar Pradesh, India. The Bundelkhand granite forms the basement complex, overlain unconformably by the upper Vindhyan sequence, which is further overlain by the Neogene (Middle and Upper) Siwaliks and finally by Quaternary alluvium. Four geomorphological units, the Varanasi older alluvial plain, Aligarh older alluvial plain, terrace zones and the Ganga recent floodplain, abandoned channels, channel scars and meander scars represent various landforms. The hydrogeological cross‐sections indicate the occurrence of a single aquifer down to 120 m. Some influent seepage from the River Ganga could be seen around Gangeswari, but the rest of the River Ganga is effluent. Groundwater‐flow modelling was carried out to assess the degree of Ganga river and aquifer interaction. The River Ganga marks the western boundary; boundaries to the northeast and southeast are set as fixed heads to simulate lateral inflow into and outflow from the sub‐basin respectively. The eastern boundary is simulated as a no‐flow condition. The Mahawa and Badmar rivers are considered to be effluent. The area modelled is covered by a grid of 34 rows×46 columns with three layers, viz., an unconfined aquifer, an aquitard which is underlain by a semi‐confined to confined aquifer. The permeability distribution was inferred from morphometric analysis and pumping tests. Natural recharge due to monsoon rainfall forms the main input. The River Ganga stage data at Ahar, Naora and Ramghat has been used for assigning surface water levels and river bed elevations in the model. Abstraction from all existing deep and shallow tube wells has been assigned as output at various cells. A steady state flow simulation was carried out and calibrated against the June 1986 water level; subsequent transient conditions were calibrated up to May 1995. The computed groundwater balance was comparable to that estimated from field investigations. The aquifer modelling study has attempted to integrate all available information and provided a tool that could be used for predictive simulation. Copyright © 2000 John Wiley & Sons, Ltd.     

Evaluating the performance of a deterministic daily rainfall–runoff model in a low-flow context

The characterization of a stream's low-flow regime is required for ecological purposes, water quality studies and various other water projects. If observed stream flow records are insufficient, low-flow characteristics may need to be estimated from simulated daily stream flow time-series. The model employed should conceptualize low-flow generation mechanisms and surface–subsurface interactions adequately. The ability of the model to simulate low-flow regimes may be assessed by means of various low-flow analysis techniques. This paper illustrates the approach using the example of the physically based, semi-distributed VTI daily rainfall–runoff model. The model has been applied to five perennial headwater catchments in South Africa, which are located in different parts of the country, represent different physiographical environments and are characterized by different baseflow responses. The model simulations are evaluated in terms of both conventional goodness-of-fit criteria and several low-flow measures such as recession characteristics, baseflow volumes, flow duration curves and continuous low-flow events below specified threshold discharges. For all the catchments considered the model has been found to perform successfully in terms of conventional fit statistics and flow duration curves. However, its ability to reproduce recession characteristics and continuous low-flow spells appears to be less satisfactory. This suggests that daily model simulations should be evaluated by low-flow criteria, which are frequently ignored in water resource assessment practices. © 1998 John Wiley & Sons, Ltd.     

Prediction of geometrical properties of perfect breaking waves on composite breakwaters

Breaking wave loads on coastal structures depend primarily on the type of wave breaking at the instant of impact. When a wave breaks on a vertical wall with an almost vertical front face called the “perfect breaking”, the greatest impact forces are produced. The correct prediction of impact forces from perfect breaking of waves on seawalls and breakwaters is closely dependent on the accurate determination of their configurations at breaking. The present study is concerned with the determination of the geometrical properties of perfect breaking waves on composite-type breakwaters by employing artificial neural networks. Using a set of laboratory data, the breaker crest height, h_b, breaker height, H_b, and water depth in front of the wall, d_w, from perfect breaking of waves on composite breakwaters are predicted using the artificial neural network technique and the results are compared with those obtained from linear and multi-linear regression models. The comparisons of the predicted results from the present models with measured data show that the h_b, H_b and d_w values, which represent the geometry of waves breaking directly on composite breakwaters, can be predicted more accurately by artificial neural networks compared to linear and multi-linear regressions.     

Models of solar irradiance variations: Current status

Regular monitoring of solar irradiance has been carried out since 1978 to show that solar total and spectral irradiance varies at different time scales. Whereas variations on time scales of minutes to hours are due to solar oscillations and granulation, variations on longer time scales are driven by the evolution of the solar surface magnetic field. Here the most recent advances in modelling of solar irradiance variations on time scales longer than a day are briefly reviewed.     

New exact cosmologies on the brane

We develop a method for constructing exact cosmological solutions in brane world cosmology. New classes of cosmological solutions on Randall–Sandrum brane are obtained. The superpotential and Hubble parameter are represented in quadratures. These solutions have inflationary phases under general assumptions and also describe an exit from the inflationary phase without a fine tuning of the parameters. Another class solutions can describe the current phase of accelerated expansion with or without possible exit from it.     

Tidal and annual variability of the population structure of Eurytemora affinis in the middle part of the Seine Estuary during 2005

Annual variability in abundance and population structure of the copepod Eurytemora affinis was studied in the maximum turbidity zone of the Seine Estuary in 2005. An Eulerian sampling strategy was applied monthly from March to July and from September to December. Chlorophyll a and suspended particulate matter (SPM) concentration, copepod abundance and stage distribution, and phytoplankton abundance were measured in sub-surface and near-bottom water during the ebb phase. Total E. affinis abundance was at a maximum in March and April (>200 × 10³ ind. m⁻³), and decreased from May to September (<25 × 10³ ind. m⁻³). This decrease corresponds to annual increases in temperature, salinity, chlorophyll a concentration and phytoplankton abundance, which was dominated by large diatoms, and decreases in SPM and river discharge. The phenology observed in 2005 was almost two months earlier compared to previous studies in the 1990s, when E. affinis reached maximum abundance in May and June. The low proportion of nauplii (<50%) in the population and high abundance of ovigerous females suggests that low recruitment is probably related to anomalously low temperatures in late winter (<5 °C). Whatever the horizontal position of the population in the estuary, adult and late copepodid stages are distributed in higher salinity than naupliar stages. Overall E. affinis population abundance was driven by parameters that characterize water masses at the tidal scale and by river discharge and chlorophyll a at the annual scale. By integrating the tidal effect, the high-frequency sampling protocol used appears to be optimal for investigating annual variability of planktonic communities in megatidal estuaries.     

Recharge processes during snowmelt: An isotopic and hydrometric investigation

Results from hydrometric and isotopic investigations of unsaturated flow during snowmelt are presented for a hillslope underlain by well-sorted sands. Passage of melt and rainwater through the vadose zone was detected from temporal changes in soil water ²H concentrations obtained from sequential soil cores. Bypassing flow was indicated during the initial snowmelt phase, but was confined to the near-surface zone. Recharge below this zone was via translatory flow, as meltwater inputs displaced premelt soil water. Estimates of premelt water fluxes indicate that up to 19 per cent of the premelt soil water may have been immobile. Average water particle velocities during snowmelt ranged from 6.2 × 10^?7 to 1.1 × 10^?6 ms^?1, suggesting that direct groundwater recharge by meltwater during snowmelt was confined to areas where the premelt water table was within 1 m of the ground surface. Soil water ²H signatures showed a rapid response to isotopically-heavy rain-on-snow inputs late in the melt. In addition, spatial variations in soil moisture content at a given depth induced a pronounced lateral component to the predominantly vertical transport of water. Both factors may complicate isotopic profiles in the vadose zone, and should be considered when employing environmental isotopes to infer recharge processes during snowmelt.     

Temperature and precipitation trend analysis of the Iraq Region under SRES scenarios during the twenty-first century

Theoretical and Applied Climatology - Iraq is classified as the fifth most vulnerable country in the world to decreased water and food availability, extreme temperatures, and associated health...