Seminal role of stratiform clouds in large-scale aggregation of tropical rain in boreal summer monsoon intraseasonal oscillations

Climate Dynamics - Modification of the vertical structure of non-adiabatic heating by significant abundance of the stratiform rain in the tropics has been known to influence the large-scale...     

Self-similar flow behind a spherical shock with varying strength in an inhomogeneous medium

The model of self-similar shock wave produced on account of an instantaneous release of energy in a non-uniform gaseous mass, has been discussed with the equilibrium conditions. The disturbance are headed by a shock of variable strength. This model is of considerable physical interest in sonic booms, phenomena associated with laser production of plasma, high altitude nuclear detonation, supernova explosion and sudden expansion of corona into the interplanetary space.     

Incipient vortex size-dependent evolution of tropical disturbances Part I — Results from a numerical experiment

The present work is concerned with the study of intensification of tropical disturbances with a view to improve prediction and early warning. The tropical disturbances are known to come in sizes (radii) ranging from 100–400 kms. Since the vortices of different sizes give rise to different initial convergence fields and since the subsequent development of the tropical depressions is very sensitive to the initial convergence fields, we argue that the size of the incipient vortex is likely to be an important factor in determining the subsequent development of a tropical disturbance. We have examined the above hypothesis using an axisymmetric model of tropical cyclone. The incipient vortex is introduced by prescribing an initial temperature perturbation with wind in gradient balance. The results show a fairly sharp selection of scale at about 250 km radius. This implies that out of a number of initial disturbances of varying sizes and embedded in the same large scale environment, it is the vortex with about 250 km radius size that will develop to the most severe system. The sensitivity of this selective intensification at this incipient vortex radius to initial perturbation field and the mean thermodynamic state is investigated. Finally, the importance of such a selective scale of intensification for prediction, tracking and early warning of tropical cyclones is emphasized.     

Assessment of glacial lake development and downstream flood impacts of critical glacial lake

Natural Hazards - Glacial lakes are rapidly growing in response to climate change and glacier retreat which may lead to catastrophic socio-economic disasters. The failure of moraine-dammed lakes is...     

Probabilistic Assessment of Earthquake Recurrence in Northeast India and Adjoining Regions

Northeast India and adjoining regions (20°–32° N and 87°–100° E) are highly vulnerable to earthquake hazard in the Indian sub-continent, which fall under seismic zones V, IV and III in the seismic zoning map of India with magnitudes M exceeding 8, 7 and 6, respectively. It has experienced two devastating earthquakes, namely, the Shillong Plateau earthquake of June 12, 1897 (M _w 8.1) and the Assam earthquake of August 15, 1950 (M _w 8.5) that caused huge loss of lives and property in the Indian sub-continent. In the present study, the probabilities of the occurrences of earthquakes with magnitude M ≥ 7.0 during a specified interval of time has been estimated on the basis of three probabilistic models, namely, Weibull, Gamma and Lognormal, with the help of the earthquake catalogue spanning the period 1846 to 1995. The method of maximum likelihood has been used to estimate the earthquake hazard parameters. The logarithmic probability of likelihood function (ln L) is estimated and used to compare the suitability of models and it was found that the Gamma model fits best with the actual data. The sample mean interval of occurrence of such earthquakes is estimated as 7.82 years in the northeast India region and the expected mean values for Weibull, Gamma and Lognormal distributions are estimated as 7.837, 7.820 and 8.269 years, respectively. The estimated cumulative probability for an earthquake M ≥ 7.0 reaches 0.8 after about 15–16 (2010–2011) years and 0.9 after about 18–20 (2013–2015) years from the occurrence of the last earthquake (1995) in the region. The estimated conditional probability also reaches 0.8 to 0.9 after about 13–17 (2008–2012) years in the considered region for an earthquake M ≥ 7.0 when the elapsed time is zero years. However, the conditional probability reaches 0.8 to 0.9 after about 9–13 (2018–2022) years for earthquake M ≥ 7.0 when the elapsed time is 14 years (i.e. 2009).     

Comparative Study of Nucleation by Different Alcoholic Solutions of Benzoin, and Benzoin Dust

Nucleation properties of different alcoholic solutions of Benzoin and Benzoin dust have been studied by varying the seeding temperature in a laboratory Cold Chamber. In the present study, three different alcoholic solutions are being used and these are due to the standard Ethyl alcohol, one higher alcohol i.e. Isopropyle alcohol and one lower alcohol i.e. Methyl alcohol. In addition, Benzil dust and droplets of Acetone and Chloroform have been used to examine if these agents have any ice nucleation ability. It has been observed that all alcoholic solutions of Benzoin have sufficient ice nucleation ability at the negative side of 0^°C, with the maximum concentration occurring at −14.4^°C. In case of Benzoin dust, the nucleation ability is better on the positive side of 0^°C than the negative side. On the other hand, Benzil dust has insignificant nucleation ability and the droplets of two other agents have no nucleation ability at all.Considering the chemical as well as crystalline structure, one can conclude that the presence of OH group in the seeding molecule plays the central role in ice nucleation/hydrate crystal formation. In fact, a number of supplementary experiments like crystal growth in an incubator, XRD analysis and NMR study are performed to identify the nature of crystals formed by Benzoin dust above 0^°C. One can conclude that these are hydrate crystals of Benzoin with water caging around Benzoin crystals.     

An Indian monsoon intraseasonal oscillations (MISO) index for real time monitoring and forecast verification

The wet/dry spells of the Indian summer monsoon (ISM) rainfall are governed by northward propagating boreal summer monsoon intraseasonal oscillations (MISO). Unlike for the Madden Julian Oscillation (e.g. RMM indices, Wheeler and Hendon in Mon Weather Rev 132:1917–1932, 2004), a low dimensional real-time monitoring and forecast verification metric for the MISO is not currently available. Here, for the first time, we present a real time monitoring index developed for identifying the amplitude and phase of the MISO over the ISM domain. The index is constructed by applying extended empirical orthogonal function (EEOF) analysis on daily unfiltered rainfall anomalies averaged over the longitudinal domain 60.5°E–95.5°E. The gravest two modes of the EEOFs together explain about 23 % of the total variance, similar to the variance explained by MISO in observation. The pair of first two principal components (PCs) of the EEOFs is named as MISO1 and MISO2 indices which together represent the evolution of the MISOs in a low dimensional phase space. Power spectral analysis reveals that the MISO indices neatly isolate the MISO signal from the higher frequency noise. It is found that the current amplitude and phase of the MISO can be estimated by preserving a memory of at least 15 days. Composite pictures of the spatio-temporal evolution of the MISOs over the ISM domain are brought out using the MISO indices. It is further demonstrated that the MISO indices can be used in the quantification of skill of extended range forecasts of MISOs. Since the MISO index does not rely on any sort of time filtering, it has great potential for real time monitoring of the MISO and may be useful in developing some prediction scheme.     

Application of DEM,Remote Sensing and Geomorphic Studies in Identifying a Recent [or perhaps Neogene?] Upwarp in the Dibru River Basin,Assam, India

The Dibru river basin of Assam is investigated to examine the influence of active structure by applying an integrated study on geomorphology, morphotectonics, subsurface structure, Digital Elevation Model (DEM) using topographic map, IRS 1D LISS III, IRS P6 LISS III, SRTM, seismic and subsurface data. Seismic data reveals existence of an upwarp and an important fault in the basement around the central and eastern parts of the Dibru basin, respectively. The influence of these structures is well observed on all the younger formations inferring their active nature possibly till the Recent Period. Existence of fluvial anomalies, viz. annular drainage pattern, lineaments, abrupt changes in the direction of river course, beheaded stream and valley incision infer role of structural control on the fluvial features of this basin. Most commonly used indices for morphotectonic analysis, viz. basin elongation ratio (Re), transverse topographic symmetry (T), asymmetric factor (AF), valley floor width to valley height ratio (Vf) have been used to identify the evidences of active structures in the area. The values of Re indicated tectonically active, T indicated an asymmetric nature, AF indicated tilting and Vf indicated active incision in the Dibru basin. The DEM, profiles across the valley and superimposed longitudinal profiles of incised channel bed and valley shoulder of the Dibru clearly reveal valley incision by the river. Three large paleochannels located in different parts of the basin had their headwaters towards east at the common source, i.e. the Diyun river. These paleochannels had been resulted when their headwaters avulsed to create new rivers due to affect of the subsurface structures during Recent (or perhaps Neogene?) Period.     

Integrating cellular automata and Markov techniques to generate urban development potential surface: a study on Kolkata agglomeration

Uncontrolled, yet fragmented peripheral urban expansion has emerged as a menace to urban development. To cope with this rapid urban expansion process, identification of the forces responsible for this rapid urban expansion is a pre-requisite, especially when its threats to habitability are taken into consideration. This study tries to evaluate fragmented uncontrolled urban expansion faced by Kolkata using cellular automata-Markov chain. Urban growth patterns, land use/land cover transformations and spatial allocation correspondence with planning strategy is the main theme of this study. Depending upon the driving forces, the study result indicates a bi-directional urban development potential surface, which might be a result of the biased planning initiative along with middle-class residential demand. This simulation result provides evidence for the planning authority to evaluate the effectiveness of spatial allocation and urban expansion trends and provide flexibility to modify the current allocation scenario.