Characteristics of spectral aerosol optical depths over India during ICARB

Spectral aerosol optical depth (AOD) measurements, carried out regularly from a network of observatories spread over the Indian mainland and adjoining islands in the Bay of Bengal and Arabian Sea, are used to examine the spatio-temporal and spectral variations during the period of ICARB (March to May 2006). The AODs and the derived Ångström parameters showed considerable variations across India during the above period. While at the southern peninsular stations the AODs decreased towards May after a peak in April, in the north Indian regions they increased continuously from March to May. The Ångström coefficients suggested enhanced coarse mode loading in the north Indian regions, compared to southern India. Nevertheless, as months progressed from March to May, the dominance of coarse mode aerosols increased in the columnar aerosol size spectrum over the entire Indian mainland, maintaining the regional distinctiveness. Compared to the above, the island stations showed considerably low AODs, so too the northeastern station Dibrugarh, indicating the prevalence of cleaner environment. Long-range transport of aerosols from tshe adjoining regions leads to remarkable changes in the magnitude of the AODs and their wavelength dependencies during March to May. HYSPLIT back-trajectory analysis shows that enhanced long-range transport of aerosols, particularly from the west Asia and northwest coastal India, contributed significantly to the enhancement of AOD and in the flattening of the spectra over entire regions; if it is the peninsular regions and the island Minicoy are more impacted in April, the north Indian regions including the Indo Gangetic Plain get affected the most during May, with the AODs soaring as high as 1.0 at 500 nm. Over the islands, the Ångström exponent (α) remained significantly lower (～1) over the Arabian Sea compared to Bay of Bengal (BoB) (～1.4) as revealed by the data respectively from Minicoy and Port Blair. Occurrences of higher values of α, showing dominance of accumulation mode aerosols, over BoB are associated well with the advection, above the boundary layer, of fine particles from the east Asian region during March and April. The change in the airmass to marine in May results in a rapid decrease in α over the BoB.     

Spatial heterogeneity in near surface aerosol characteristics across the Brahmaputra valley

In order to examine the spatial variability of the aerosol characteristics across the Brahmaputra valley, a land campaign was conducted during late winter (February 3–March 2) 2011. Measurements of particulate matter (PM, PM₁₀, PM_2.5) and black carbon (BC) concentrations were made onboard an interior redesigned vehicle. The length of the campaign trail stretched about 700 km, covering the longitude belt of 89.97°–95.55°E and latitude belt of 26.1°–27.6°N, comprising 13 measurement locations. The valley is divided into three sectors longitudinally: western sector (R1: 89.97°–91.75°E), middle sector (R2: 92.5°–94.01°E) and eastern sector (R3: 94.63°–95.55°E). Spatial heterogeneity in aerosol distribution has been observed with higher PM₁₀ and PM_2.5 concentrations at the western and middle sectors compared to the eastern sector. The locations in the western sector are found to be rich in BC compared to the other two sectors and there is a gradual decrease in BC concentrations from west to east of the Brahmaputra valley. Two hotspots within the western and middle sectors with high PM and BC concentrations have been identified. The associated physico-optical parameters of PM reveal abundance of PM_2.5 aerosols along the entire valley. High population density in the western and middle sectors, together with the contribution of remote aerosols, leads to higher anthropogenic aerosols over those regions. Spectral Radiation-Transport Model for Aerosol Species (SPRINTARS) slightly underestimates the measured PM₁₀ and PM_2.5 at the eastern sector while the model overestimates the measurements at a number of locations in the western sector. In general, BC is underestimated by the model. The variation of BC within the campaign trail has not been adequately captured by the model leading to higher variance in the western locations as compared to the middle and eastern locations.     

Verification of two hydrological models for real-time flood forecasting in the Hindu Kush Himalaya (HKH) region

Natural Hazards - The Hindu Kush Himalayan region is extremely susceptible to periodic monsoon floods. Early warning systems with the ability to predict floods in advance can benefit tens of...     

Landslide detection in the Himalayas using machine learning algorithms and U-Net

Landslides - Event-based landslide inventories are essential sources to broaden our understanding of the causal relationship between triggering events and the occurring landslides. Moreover,...     

Quantification of improvements by using AWiFS over WiFS data

The Advanced WiFS sensor of RESOURCESAT- 1 satellite offers significantly improved specifications compared to the WiFS sensor onboard IRS IC, P3 and ID satellites. The improvements are in terms of spatial resolution, radiometry (quantisation levels) and number of spectral bands. In the present study, an attempt has been made to quantify the gains due to these enhanced specifications. The study has been carried out in a predominantly agricultural area. For the study reported here, one set of overlapping data acquired on the same day by WiFS and AWiFS sensors has been selected. This eliminates the need of atmospheric correction/ normalization for comparison. The effect of spatial resolution has been studied by applying ISODATA spectral clustering algorithm with number of clusters set at three different levels, viz., 10, 20 and 30. They are assumed to mimic first, second and third level classification, respectively. Output images were filtered using 3 × 3 majority filter. Homogeneous polygons having area less than 1/2 and 1 pixel of WiFS were recorded. This indicates the minimum loss by using WiFS data. A relative gain of 10 – 15 % is observed due to improvement in spatial resolution. For comparison of radiometry, local variance measure was used. It was observed that local variance is much larger for AWiFS data in comparison with WiFS data. This indicates presence of enhanced local contrast, hence heterogeneity, in AWiFS data over WiFS data. Separability analysis has been carried out to demonstrate improvements due to two additional spectral bands (Green and SWIR).     

Ionospheric response to X-class solar flares in the ascending half of the subdued solar cycle 24

The signature of 11 X-class solar flares that occurred during the ascending half of the present subdued solar cycle 24 from 2009 to 2013 on the ionosphere over the low- and mid-latitude station, Dibrugarh (27.5^°N, 95^°E; magnetic latitude 17.6^°N), are examined. Total electron content (TEC) data derived from Global Positioning System satellite transmissions are used to study the effect of the flares on the ionosphere. A nonlinear significant correlation (R² = 0.86) has been observed between EUV enhancement (ΔEUV) and corresponding enhancement in TEC (ΔTEC). This nonlinearity is triggered by a rapid increase in ΔTEC beyond the threshold value ～1.5 (×10¹⁰ ph cm^?2 s^?1) in ΔEUV. It is also found that this nonlinear relationship between TEC and EUV flux is driven by a similar nonlinear relationship between flare induced enhancement in X-ray and EUV fluxes. The local time of occurrence of the flares determines the magnitude of enhancement in TEC for flares originating from nearly similar longitudes on the solar disc, and hence proximity to the central meridian alone may not play the dominating role. Further, the X-ray peak flux, when corrected for the earth zenith angle effect, did not improve the correlation between ΔX-ray and ΔTEC.     

Anisotropy in volume change behaviour of soils during shrinkage

It has been long recognised that some of the fundamental and engineering properties of soils exhibit a certain degree of anisotropy that eventually dictates their directional geoengineering behaviours. Consideration of the importance of the volume change behaviour of soils during shrinkage and a critical review of the literature suggests scopes for further research for the development of a better understanding of the anisotropy in volume change encountered during soil shrinkage. In this paper, anisotropy in volumetric shrinkage behaviour of soil is depicted with the theory of geometry factor and shrinkage strains. A systematic investigation and analysis on the evolution of geometry factors and shrinkage strains of several geomaterial samples during evaporative dewatering is reported herein. A theoretical framework for evaluating shrinkage geometry factors of a cylindrical soil specimen undergoing volume change during progressive moisture loss is described in this paper. Furthermore, based on experimental and literature data, shrinkage geometry factors of several specimens differing in terms of gradational properties, specimen size, evaporative boundary condition and pore fluid salinity are evaluated and discussed in detail in accordance with the theoretical framework. Linkages between shrinkage process, shrinkages strains and geometry factor are also analysed to underpin the usage of geometry factor and shrinkage strains to characterise anisotropy during soil shrinkage.