Analysis and numerical simulation of the meteorological observations at a tropical coastal site Chennai in India

Meteorological measurements were carried out at North Chennai semi rural area during pre-monsoon period as a part of an air quality study program. Analysis of the data showed the effects of coastal terrain namely the land-sea breeze circulation, temperature cooling during the sea breeze, difference in onset times at these sites etc. Sea breeze onset was observed with a sharp turning of the wind from westerly to south easterly associated with rise in wind speed. Advection speed of the front was about 2.0 m s^− 1. A simple mesoscale meteorological model (MAM-I) developed at Kalpakkam for coastal atmospheric dispersion estimation was used to simulate the observed characteristics. All the major features observed could be simulated by the model while significant difference was noticed in sea breeze frontal movement. MAM results were also inter-compared with MM5. There were no significant differences in the estimate of mean parameters by both the models. It is concluded that the simple model, which takes less run time in a desktop PC, is adequate enough for practical application of providing wind field for plume dispersion models at coastal sites.     

A megacity in a changing climate: the case of Kolkata

Projections by the Intergovernmental Panel on Climate Change suggest that there will be an increase in the frequency and intensity of climate extremes in the 21st century. Kolkata, a megacity in India, has been singled out as one of the urban centers vulnerable to climate risks. Modest flooding during monsoons at high tide in the Hooghly River is a recurring hazard in Kolkata. More intense rainfall, riverine flooding, sea level rise, and coastal storm surges in a changing climate can lead to widespread and severe flooding and bring the city to a standstill for several days. Using rainfall data, high and low emissions scenarios, and sea level rise of 27 cm by 2050, this paper assesses the vulnerability of Kolkata to increasingly intense precipitation events for return periods of 30, 50, and 100 years. It makes location-specific inundation depth and duration projections using hydrological, hydraulic, and urban storm models with geographic overlays. High resolution spatial analysis provides a roadmap for designing adaptation schemes to minimize the impacts of climate change. The modeling results show that de-silting of the main sewers would reduce vulnerable population estimates by at least 5 %.     

Deriving Crop Phenology Metrics and Their Trends Using Times Series NOAA-AVHRR NDVI Data

In this study, an attempt has been made to derive the spatial patterns of temporal trends in phenology metrics and productivity of crops grown, at disaggregated level in Indo-Gangetic Plains of India (IGP), which are helpful in understanding the impact of climatic, ecological and socio-economic drivers. The NOAA-AVHRR NDVI PAL dataset from 1981 to 2001 was stacked as per the crop year and subjected to Savitzky-Golay filtering. For crop pixels, maximum and minimum values of normalized difference vegetation index (NDVI), their time of occurrence and total duration of kharif (June-October) and rabi (November–April) crop seasons were derived for each crop year and later subjected to pixel-wise regression with time to derive the rate and direction of change. The maximum NDVI value showed increasing trends across IGP during both kharif and rabi seasons indicating a general increase in productivity of crops. The trends in time of occurrence of peak NDVI during kharif dominated with rice showed that the maximum vegetative growth stage was happening early with time during study period across most of Punjab, North Haryana, Parts of Central and East Uttar Pradesh and some parts of Bihar and West Bengal. Only central parts of Haryana showed a delay in occurrence of maximum vegetative stage with time. During rabi, no significant trends in occurrence of peak NDVI were observed in most of Punjab and Haryana except in South Punjab and North Haryana where early occurrence of peak NDVI with time was observed. Most parts of Central and Eastern Uttar Pradesh, North Bihar and West Bengal showed a delay in occurrence of peak NDVI with time. In general, the rice dominating system was showing an increase in duration with time in Punjab, Haryana, Western Uttar Pradesh, Central Uttar Pradesh and South Bihar whereas in some parts of North Bihar and West Bengal a decrease in the duration with time was also observed. During rabi season, except Punjab, the wheat dominating system was showing a decreasing trend in crop duration with time.     

Potential source zones for Himalayan earthquakes: constraints from spatial�Ctemporal clusters

The Himalayan fold-thrust belt has been visited by many disastrous earthquakes (magnitude > 6) time and again. This active collisional orogen bordering Indian subcontinent in the north remains a potential seismic threat of similar magnitude in the adjoining countries like India, Pakistan, Nepal, Bhutan and China. Though earthquake forecasting is riddled with all conjectures and still not a proven presumption, identifying likely source zones of such disastrous earthquakes would be an important contribution to seismic hazard assessment. In this study, we have worked out spatio-temporal clustering of earthquakes (Mb ?? 4.5; 1964?C2006) in the Himalayas. ??Point density?? spatial statistics has helped in detecting 22 spatial seismicity clusters. Earthquake catalog is then treated with a moving time-distance window technique (inter-event time 35 days and distance 100 ± 20 km) to bring out temporal clusters by recognizing several foreshock-main shock-aftershock (FMA) sequences. A total of 53 such temporal sequences identified in the process are confined within the 22 spatial clusters. Though each of these spatio-temporal clusters deserves in-depth analysis, we short-listed only eight such clusters that are dissected by active tectonic discontinuities like MBT/MCT for detail study. Spatio-temporal clusters have been used to constrain the potential source zones. These eight well-defined spatio-temporal clusters demonstrate recurrent moderate to large earthquakes. We assumed that the length of these clusters are indicating the possible maximum rupture lengths and thus empirically estimated the maximum possible magnitudes of eight clusters that can be generated from them (from west to east) as 8.0, 8.3, 8.2, 8.3, 8.2, 8.4, 8.0 and 7.7. Based on comparative study of the eight cluster zones contemplating with their temporal recurrences, historical seismic records, presence of intersecting faults and estimated magnitudes, we have guessed the possibility that Kangra, East Nepal, Garhwal and Kumaun?CWest Nepal clusters, in decreasing order of earthquake threat, are potential source zones for large earthquakes (??7.7 M) in future.     

Multistage reaction textures in xenolithic high-MgAl granulites at Anakapalle, Eastern Ghats Belt, India: examples of contact polymetamorphism and infiltration-driven metasomatism

High‐MgAl rocks occur as xenoliths (up to 2 m in diameter) in mafic granulites at a newly discovered locality near Anakapalle. Following an early phase of deformation, ultrahigh‐temperature (UHT) metamorphism and near‐isothermal decompression, the rocks were intruded in a lit‐par‐lit manner by felsic melts (charnockite), which caused local‐scale metasomatism. A subsequent deformation produced isoclinal folds and the distinct gneissic foliation of the charnockite still at granulite facies conditions. The sequence of multiphase reaction textures in the high‐MgAl xenoliths reflects the changes of physico‐chemical conditions during the polyphase evolution of the terrane; UHT metamorphism (stage 1, > 1000°C, c. 10 kbar) is documented by relics of extremely coarse grained domains with the assemblage orthopyroxene (opx)₁ + garnet (grt)₁ + sapphirine (spr)₁ + spinel (spl)₁ + rutile (rt). A subsequent phase of near‐isothermal decompression in the order of 1–2 kbar (stage 2) resulted in extensive replacement of grt₁ and opx₁ megacrysts by lamellar (opx₂ + spr₂) symplectites. The intrusion of felsic melt (stage 3) led to the development of a narrow metasomatic black wall reaction zone (bt + sil + plg₃ + opx_2,3 + rt) at the immediate contact of the xenoliths and in melt infiltration zones to the partial replacement of (opx₂ + spr₂) symplectites by biotite and sillimanite and/or plg₃, mainly at the expense of orthopyroxene, with concomitant coarsening of the intergrowth texture. The subsequent deformation (stage 4) further modified the symplectite textures through polygonization, recrystallization and grain‐size coarsening. The deformation was followed by a period of cooling and decompression (stage 5, c. 800°C, 4–7 kbar) as indicated by local growth of late garnet (grt₅) at the expense of (opx + spr + plg) domains at static conditions. Recently published isotope data suggest that the multistage evolution of the high‐MgAl granulites at Anakapalle followed a discontinuous P–T trajectory that may be related to heating of the crust through magmatic accretion culminating in deep‐crustal UHT metamorphism at 1.4 Ga (stage 1), fast uplift of the UHT granulites into mid‐crustal levels as a consequence of extensional tectonics (stage 2), emplacement of felsic magmas in the Grenvillian (at c. 1 Ga, stage 3) resulting in reheating of the crust to high–T conditions followed by a phase of compressional tectonics (stage 4) and a period of cooling to the stable geotherm (stage 5) still in the Grenvillian.     

Timescales of crustal melting in the Higher Himalayan Crystallines (Sikkim, Eastern Himalaya) inferred from trace element-constrained monazite and zircon chronology

The petrology and timing of crustal melting has been investigated in the migmatites of the Higher Himalayan Crystalline (HHC) exposed in Sikkim, India. The metapelites underwent pervasive partial melting through hydrous as well as dehydration melting reactions involving muscovite and biotite to produce a main assemblage of quartz, K-feldspar, plagioclase, biotite, garnet ± sillimanite. Peak metamorphic conditions were 8–9 kbar and ~800 °C. Monazite and zircon crystals in several migmatites collected along a N–S transect show multiple growth domains. The domains were analyzed by microbeam techniques for age (SHRIMP) and trace element composition (LA-ICP-MS) to relate ages to conditions of formation. Monazite preserves the best record of metamorphism with domains that have different zoning pattern, composition and age. Zircon was generally less reactive than monazite, with metamorphic growth zones preserved in only a few samples. The growth of accessory minerals in the presence of melt was episodic in the interval between 31 and 17 Ma, but widespread and diachronous across samples. Systematic variations in the chemical composition of the dated mineral zones (HREE content and negative Eu anomaly) are related to the variation in garnet and K-feldspar abundances, respectively, and thus to metamorphic reactions and P–T stages. In turn, this allows prograde versus decompressional and retrograde melt production to be timed. A hierarchy of timescales characterizes melting which occurred over a period of ~15 Ma (31–17 Ma): a given block within this region traversed the field of melting in 5–7 Ma, whereas individual melting reactions lasted for time durations below, or approaching, the resolution of microbeam dating techniques (~0.6 Ma). An older ~36 Ma high-grade event is recorded in an allocthonous relict related to mafic lenses. We identify two sections of the HHC in Sikkim that traversed similar P–T conditions at different times, separated by a tectonic discontinuity. The higher structural levels reached melting and peak conditions later (~26–23 Ma) than the lower structural levels (~31–27 Ma). Diachronicity across the HHC cannot be reconciled with channel flow models in their simplest form, as it requires two similar high-grade sections to move independently during collision.     

Nonstoichiometry and growth of some Fe carbides

Iron carbides containing from 31 to 17 atomic % carbon, with cohenite XRD structure and optical properties, were grown in experiments in Fe–Ni–S–C, Fe–Ni–C, and in Fe–C at 1, 6, and 7 GPa. X-ray cell volumes increase with C content. Compositions listed above vary considerably outside the nominal (Fe,Ni)₃C stoichiometry of cohenite/cementite. Cohenites coexisting with Fe–C liquid are carbon poor. The Eckstrom-Adcock carbide, nominally Fe₇C₃, was found to show compositions from 29 to 36 atomic % C at 7 GPa in Fe–C. Both these materials are better regarded as solutions than as stoichiometric compounds, and their properties such as volume have compositional dependencies, as do the iron oxides, sulfides, silicides, and hydrides. The fraction of C dissolved in cohenite-saturated alloy is found to become smaller between 1 and 7 GPa. If this trend continues at higher pressures, the deep mantle should be easier to saturate with carbide than the shallow mantle, whether or not carbide is metastable as at ambient pressure. At temperatures below the cohenite-graphite peritectic, cohenite may grow as a compositionally zoned layer between Fe and graphite. The Eckstrom-Adcock carbide joins the assemblage at 7 GPa. Phases appear between Fe and C in an order consistent with metasomatic interface growth between chemically incompatible feed stocks. Diffusion across the carbide layer is not the growth rate limiting step. Carbon transport along the grain boundaries of solid Fe source stock at 1 GPa, to form C-saturated Fe alloy, is observed to be orders of magnitude faster than the cohenite layer growth. Growth stagnates too rapidly to be consistent with diffusion control. Furthermore, lateral variations in carbide layer thickness, convoluted inert marker horizons, and variable compositional profiles within the layers suggest that there are local transport complexities not covered by one-dimensional diffusive metasomatic growth. In contrast to many transport phenomena which slow with pressure, at 7 GPa and 1,162 °C, carbide growth without open grain boundaries is faster than at 1 GPa with fast grain boundary channels, again suggesting C transport is less of a constraint on growth than C supply. C supply at 7 GPa is enhanced by graphite metastability and the absence of fast grain boundary channels to divert C into the Fe instead of growing carbide. At both 1 and 7 GPa, the growth rate of carbide is found to systematically vary depending on which of two stock pieces of graphite are used to form the growth couple, suggesting that some property of each specific graphite, like C release rate, possibly from amorphous binder material, may influence the cohenite growth process. At temperatures near and above the cohenite-graphite peritectic at 1–1.5 GPa, complex intergrowths involving Fe–C liquids and extensive thermal migration transport were encountered, eroding the organized spatial resolution, and the range of cohenite compositions found grown below this peritectic from growth couples of crystalline Fe and graphite. The migration of graphite to a position in the metasomatic sequence between liquid and cohenite demonstrates that the solubility of graphite in liquid increases with temperature above the peritectic, whereas the solubility of graphite in cohenite below the peritectic decreases with temperature. The variable solubility of graphite in cohenite, shown by thermal migration, emphasizes that cohenite does have compositional variations.     

Determination of the fractal dimension of a shore platform profile

Morphological studies of shore platforms have focused on elements like width, gradient and elevation, while mostly ignoring the surface roughness as a morphometric attribute. This paper uses the Roughness-length method to show how the Fractal Dimension of a shore platform profile can be obtained. The higher the value of the fractal dimension, the higher is the roughness of the platform. The studied profile is situated in Houghton Bay on the south coast of Wellington, New Zealand. The fractal dimension of the profile under consideration was found to be 1.314. However, one profile is not sufficient to provide information about the surface roughness of a shore platform. Therefore, a number of profiles should be taken to have an idea about surface roughness.     

On using visibility correlations to probe the H i distribution from the dark ages to the present epoch – I. Formalism and the expected signal

Redshifted 21-cm radiation originating from the cosmological distribution of neutral hydrogen (H  i ) appears as background radiation in low-frequency radio observations. The angular and frequency domain fluctuations in this radiation carry information concerning cosmological structure formation. We propose that correlations between visibilities measured at different baselines and frequencies in radio-interferometric observations be used to quantify the statistical properties of these fluctuations. This has an inherent advantage over other statistical estimators in that it deals directly with the visibilities which are the primary quantities measured in radio-interferometric observations. Also, the visibility correlation has a very simple relation with the power spectrum. We present estimates of the expected signal for nearly the entire post-recombination era, from the dark ages to the present epoch. The epoch of reionization, where H  i has a patchy distribution, has a distinct signature where the signal is determined by the size of the discrete ionized regions. The signal at other epochs, where H  i follows the dark matter, is determined largely by the power spectrum of dark matter fluctuations. The signal is strongest for baselines where the antenna separations are within a few hundred times the wavelength of observation, and an optimal strategy would preferentially sample these baselines. In the frequency domain, for most baselines the visibilities at two different frequencies are uncorrelated beyond  Δν∼ 1 MHz  , a signature which, in principle, would allow the H  i signal to be easily distinguished from the continuum sources of contamination.     

Class-based target identification with multiaspect scattering data

In underwater sensing applications, it is often difficult to train a classifier in advance for all targets that may be seen during testing, due to the large number of targets that may be encountered. We therefore partition the training data into target classes, with each class characteristic of multiple targets that share similar scattering physics. In some cases, one may have a priori insight into which targets should constitute a given class, while in other cases this segmentation must be done autonomously based on the scattering data. For the latter case, we constitute the classes using an information-theoretic mapping criterion. Having defined the target classes, the second phase of our identification procedure involves determining those features that enhance the similarity between the targets in a given class. This is achieved by using a genetic algorithm (GA)-based feature-selection algorithm with a Kullback-Leibler (KL) cost function. The classifier employed is appropriate for multiaspect scattering data and is based on a hidden Markov model (HMM). The performance of the class-based classification algorithm is examined using both measured and computed acoustic scattering data from submerged elastic targets.