Historical temporal variation in precipitation over Western Himalayan Region: 1857-2006

This study has examined the temporal variation in monthly, seasonal annual precipitation over the Western Himalayan Region(WHR) and the influence of global teleconnections, like the North Atlantic Oscillation(NAO) and Southern Oscillation(SO) Indices on seasonal annual precipitation. The Mann–Kendall non-parametric test is applied for trend detection and the Pettitt–Mann–Whitney test is used to detect possible shift. Maximum entropy spectral analysis is applied to find the periodicity in annual seasonal precipitation. The study shows a non-significant decreasing trend in annual precipitation over WHR for the period 1857-2006. However, in seasonal precipitation, a significant decreasing trend is observed in monsoon and a significant increasing trend in post-monsoon season during the same period. The significant decrease in monsoon precipitation may be due to weakening of its teleconnection with NAO as well as SO Indices mainly during last three decades. It is observed that the probable change of year in annual monsoon precipitation over WHR is 1979. The study also shows significant periodicities of 2.3-2.9 years and of 3.9-4.7 years in annual seasonal precipitation over WHR.     

Laboratory investigation of pullout behavior of hollow and solid shaft helical nail in frictional soil

Acta Geotechnica - Helical soil nails are passive elements installed in the soil which attains its bond strength through skin friction and bearing from helices. The present study examines the...     

Assessment of the APCC coupled MME suite in predicting the distinctive climate impacts of two flavors of ENSO during boreal winter

Forecast skill of the APEC Climate Center (APCC) Multi-Model Ensemble (MME) seasonal forecast system in predicting two main types of El Niño-Southern Oscillation (ENSO), namely canonical (or cold tongue) and Modoki ENSO, and their regional climate impacts is assessed for boreal winter. The APCC MME is constructed by simple composite of ensemble forecasts from five independent coupled ocean-atmosphere climate models. Based on a hindcast set targeting boreal winter prediction for the period 1982–2004, we show that the MME can predict and discern the important differences in the patterns of tropical Pacific sea surface temperature anomaly between the canonical and Modoki ENSO one and four month ahead. Importantly, the four month lead MME beats the persistent forecast. The MME reasonably predicts the distinct impacts of the canonical ENSO, including the strong winter monsoon rainfall over East Asia, the below normal rainfall and above normal temperature over Australia, the anomalously wet conditions across the south and cold conditions over the whole area of USA, and the anomalously dry conditions over South America. However, there are some limitations in capturing its regional impacts, especially, over Australasia and tropical South America at a lead time of one and four months. Nonetheless, forecast skills for rainfall and temperature over East Asia and North America during ENSO Modoki are comparable to or slightly higher than those during canonical ENSO events.     

Coherence Inherent in an Incoherent Synchrotron Radio Source

We show that a partial coherence due to antenna mechanism can be inherently present in any compact synchrotron source, which resolves many long-standing problems in the spectra and variability of compact extragalactic radio sources.     

Global stability of disk-bulge systems: Spiral structure of disk galaxies

The spiral arms of disk galaxies are very sensitive to various morphological properties, such as, the gas content, the disk-to-bulge ratioetc. Here, the stability of self-gravitating annular disks surrounding the central rigid bulge component has been studied in order to explain the transition from the tight spiral arms in Sa galaxies to rather open patterns in Sc galaxies as the central amorphous component diminishes. Smooth spiral patterns are found associated with the dominant (or the fastest growing) modes of the system. When the disk-to-bulge mass ratio is small, a tight pattern results restricted to the inner regions of the disk. This pattern opens up and occupies larger disk areas as the disk component becomes comparable to the bulge. It is found here that the ‘explosive’ instabilities of the global density waves do not occur in the presence of a massive bulge. The growth-rates of the eigen-modes decrease as the disk-to-bulge mass ratio decreases. It is also found that unstable modes of the annular disk can be suppressed by increasing the thermal pressure sufficiently.     

Stability of a finite disc under the influence of a spherical halo

We have studied the stability of finite gaseous discs, against large-scale perturbations, under the influence of spherical, massive haloes. A surface-density distribution consistent with the observed spiral-tracer profiles in disc galaxies is considered for the disc. We find that growing eigenmodes with both ‘trailing’ and ‘leading’ spirals exist in ‘cold’ discs for a wide range of values of the halo mass and its radius. The amplification rates of the unstable modes reduce as the ratio of the mass of the halo to the mass of the disc is increased. A uniform halo is not very effective towards stabilizing the disc against these modes. The results from the present study are consideredvis-a-vis previous studies on the global modes of self-gravitating discs.     

An Analytical Slab Model for the Growth of the Coastal Thermal Internal Boundary Layer Under Near-Neutral Onshore Flow Conditions

Analytical parameterisations of the thermal internal boundary-layer (TIBL) height based on the slab approach are widely used in coastal dispersion models. However, they tend to a singular behaviour when the stability of the onshore flow is close to neutral. Assuming that convective turbulence dominates mixing, we derive a more general analytical model that is valid for both stable and neutral onshore flows. The model is based on the existing framework for the slab approach but involving the Zilitinkevich correction (or the spin-up term). The height variation of the onshore flow lapse rate is accounted for in the model by including an initial TIBL height. An algebraic form of the model also includes the mechanical mixing contribution to the TIBL growth and is, therefore, suitable for use when the overland surface heat flux is small and friction velocity large. The new analytical model is tested with field measurements taken under near-neutral onshore flow conditions. The performance of the model is shown to be better than a commonly used TIBL parameterisation scheme.     

Dispersion Moments of Fumigating Plumes – Lidar Estimates and Pdf Model Simulations

Near-instantaneous vertical scans ofpower plant plumes, sampled by a lidar over three days as part ofthe 1995 Kwinana Coastal Fumigation Study, are analysed to obtainhourly-averaged total dispersion moments up to fourth order, andalso the relative and meander spreads. Two distinct fumigationcases, termed as neutral and stable cases, are observed dependingon whether the plume transport prior to fumigation takes place ina neutral atmosphere, or in a stable region above the neutralatmosphere. Plumes in the stable case are observed to undergoslower fumigation. The mean spreads display contrasting behavioursfor the two fumigating cases, while the vertical skewness shows anegative peak value of about -1 for the neutral case and -2for the stable case in the fumigation zone. The lateral skewnessis positive with a peak value of about unity for both cases, andis attributed to wind directional shear within the thermalinternal boundary layer. The vertical kurtosis is greater than theGaussian value of 3 in the fumigation zone, while the lateralkurtosis is scattered about this value. A recently-developedskewed probability density function model with wind shear isapplied to describe the fumigation data. Overall, the modelsimulates the observed vertical and lateral statistics well up tothe fourth order, except for the lateral skewness and kurtosisvalues in the stable case, which is probably due to the lidar'srange and sensitivity constraints and an inadequate sample size.Although the lateral turbulent diffusion in the model is Gaussian(i.e., zero skewness), the inclusion of wind shear in the model ismanifested in the predicted lateral skewness being greater thanzero in the fumigation zone, and of similar magnitude to the lidardata in the neutral case.     

Landslide hazards and mitigation measures at Gangtok, Sikkim Himalaya

Landslides and other mass movements are serious geo-environmental hazards in the Himalayas. Massive landslides killing tens of thousands of people with catastrophic damages have occurred in the Eastern Himalayan State of Sikkim, which shares common borders with Tibet, Nepal, and Bhutan. This paper describes the investigations carried out on recent landslides in Gangtok, Sikkim, India, with emphasis on the triggering mechanisms that have contributed to the release and creep of natural slopes in the region. It is believed that the intense rainfall in the region not only contributes to rapid erosion and weathering of the rock mass, but also increases the groundwater level that leads to reduction in the stability of natural slopes. A landslide instrumentation programme that includes placement of settlement pillars and piezometers is underway to predict the behaviour of landslides in the area.