Differences in Atmospheric Boundary-Layer Characteristics Between Wet and Dry Episodes of the Indian Summer Monsoon

The differences and similarities in atmospheric boundary-layer (ABL) characteristics, in particular the ABL height, evolution and wind field, between two contrasting episodes of the Indian summer monsoon have been studied using measurements from wind profilers and an instrumented 50-m tower at Gadanki in India. The observed differences are discussed in light of various forcing mechanisms, in particular the effect of soil moisture on the surface energy balance and ABL. The differences in ABL height, its evolution and the wind field between episodes are quite pronounced. Wet episodes not only have a shallower ABL but also the growth is delayed by 1–4 h when compared with that for dry episodes. Abundant soil moisture during the wet episodes (a factor of two greater than during the dry episodes) reduces the buoyancy flux, and thereby not only limits the ABL height but also delays the commencement of ABL growth. The low-level jet (LLJ) is stronger during the dry episodes and has a larger diurnal range than during the wet episodes. The highest occurrence and magnitude of LLJ apparent at a height of 1.5 km during early morning hours shift progressively with height and time till the afternoon, following ABL evolution. The weaker LLJ during the wet episodes is attributed to its southward migration from its mean position (15 \(^{\circ }\) N). Larger signal-to-noise ratio and spectral width values are observed during the early night to midnight, compared to noon-time, when the ABL is buoyantly turbulent.     

Relationship between stable isotope ratios and drop size distribution in tropical rainfall

We carried out simultaneous measurements of drop size distribution (DSD) and stable oxygen and hydrogen isotopic compositions (??¹⁸O and ??D) of rain at the National Atmospheric Research Laboratory (NARL), Gadanki (13.5°N, 79.2°E), southern India, during September?COctober 2006, with the aim of understanding microphysical processes leading to rain formation. The MST radar at NARL was operated continuously during rain events, while rain samples were collected at very short time intervals (<1?h), to capture small changes (>0.2?? and >2??) in their ??¹⁸O and ??D. The slope of the local meteoric water line (??D?C??¹⁸O line), was 8.07?±?0.47, similar to that of global meteoric water line, confirming that the precipitation occurred under isotopic equilibrium, and was unaffected by some anomalous process; further, the evaporation of rain drops at the cloud base was insignificant. Whenever the isotopic variations were larger during a rain event (>2??) there was a significant negative correlation between the ??¹⁸O and DSD. The possible explanation is that larger drops are mostly associated with convective rather than stratiform rain, and ¹⁸O (and D) depletion in convective rain is relatively more. Bin-resolved microphysical models incorporating water isotopologues could benefit by considering drop size spectra, which could improve the match with stable isotope observations of precipitation.     

In-cloud and below-cloud scavenging of aerosol ionic species over a tropical rural atmosphere in India

The temporal variation in concentrations of major water soluble ionic species has been studied from several rain events occurred over Gadanki (13.5 °N, 79.2 °E), located in tropical semi arid region in southern India. The contribution from rain-out (in cloud) and wash-out (below cloud) processes to the total removal of ionic species by rain events is also estimated using the pattern of variations of ionic species within an individual event. A number of rain samples were collected from each rain event during June–November in 2006, 2007 and 2008. On average, nearly 20% of the total NH ₄ ⁺ and non-sea SO ₄ ^2? is removed by in-cloud scavenging, suggesting that their removal by “below cloud” washout is relatively dominant. In contrast Na⁺, Ca²⁺, Mg²⁺, NO ₃ ^? and sea-SO ₄ ^2? are mainly removed by below-cloud scavenging or wash-out process. A significant variation in the acidity was observed within rain events with successive precipitation showing higher acidity at the final stage of the precipitation due to partial neutralization of non-sea SO ₄ ^2? . Overall, greater influence of both terrestrial and anthropogenic sources is recorded in the rain events compared to that from marine sources.     

Preparation of volume table of SAL (Shorea robusta) - an approach using satellite data

Sal (Shorea robusta) is an important forest tree species in north and north-eastern India. Large-scale plantations of this species have been raised there under taungya and coppice system of management. The conventional volume table prepared for high sal forest is referred to infer the volume of production of this species. Earlier workers have used aerial remote sensing data to develop volume tables of this species. In the present study a volume table for sal is developed based on remotely sensed satellite data using a regression technique. A two-step method was developed to estimate mean tree volume from satellite data. In step 1, mean crown diameter — an intermediate variable - was estimated from satellite data. In step 2, the estimated mean crown diameter was used to estimate the mean tree volume. Addition of age of the crop as an independent variable improved the predictive ability of the regression equation.     

Mudbank regime off the Kerala coast during monsoon and non-monsoon seasons

Field experiments conducted in the nearshore ocean to understand the dynamics of mudbank off Kerala, south-west coast of India, are highlighted. Real time monitoring of the nearshore ocean off Purakkad, Kerala was accomplished using pressure transducers for nearshore surface wave measurements, and current sensors for nearshore velocity measurements. Comprehensive information on the spatial structure of mudbank was obtained from aerial surveys. Extensive data collected on surface waves and currents in the nearshore ocean, indicate that the infra-gravity (IG) waves (leaky modes and trapped edge wave modes), and far infra-gravity (FIG) waves coupled with strong shoreline reflections and undertow play an important role in the dynamics associated with the mudbanks off Kerala during the monsoon season. During the non-monsoon season evidence for progressive edge waves in the infragravity frequency band, an energetic gravity wave band and a strong undertow with weak reflections was observed.     

Upper layer circulation,hydrography, and biological response of the Andaman waters during winter monsoon based on in situ and satellite observations

Upper layer circulation, hydrography, and biological response of Andaman waters during winter monsoon are assessed based on the observations carried out onboard FORV Sagar Sampada during January 2009 and November–December 2011. Cool and dry air carried by the moderate winds (6 m/s) from north and northeast indicates the influence of northeast monsoon (NEM) in the area during the observation time. The characteristics of physical parameters and the water mass indicate that the southeastern side is dominated by the less saline water from South China Sea intruded through the Malacca Strait, whereas the northeast is influenced by the freshwater from Ayeyarwady-Salween river system. The western side of the Andaman and Nicobar Islands exhibits similar properties of Bay of Bengal (BoB) water as evidenced in the T-S relation. Circulation pattern is uniform for the upper 88 m and is found to be more geostrophic rather than wind driven. Magnitude of the current velocity varies between 100 and 900 mm/s in November–December 2011 with strong current (900 mm/s) near Katchal and Nancowry islands and 100 and 1000 mm/s in January 2009 recording strong current (1000 mm/s) near the Little Nicobar Island. The Andaman waters are observed as less productive during the season based on the satellite-derived surface chl-a (0.1–0.4 mg/m³) and column-integrated primary productivity (PP) (100–275 mgC/m²/d).     

Tectonic geomorphology,tsunamis and environmental hazards: reference to Andaman-Nicobar Islands

Geomorphic features associated with earthquakes and tsunamis have received wide attention in estimating uplift and subsidence after the tectonic event. Although various techniques are in vogue in estimating the uplift and subsidence after the 2004 Andaman-Sumatra earthquake and subsequent tsunami, remote sensing techniques have been proved to be quite handy to study the geomorphic changes. In the present study, geomorphic changes associated with the destructive event of December 2004 have been analyzed. The emergent and subsident coasts around the smaller islands in the Andaman region have been identified. The coral reef area that has been subjected to uplift or subsidence in some of the islands of the Andaman and Nicobar region is delineated, and the net areal extents of these coral beds have been computed. Of the six islands studied in Andaman region, coral reef of four islands was subjected to uplift, and around two islands the area was subsided. The uplifted area varied from 0.10 to 11 km², and subsidence was about 0.50 km². In Nicobar region, the subsidence of coral reefs was recorded. This study helps to monitor the coastal environments and the destruction due to natural hazards.     

Technical Note Reactive silica and strength of fly ashes

The use of fly ash in geotechnical engineering depends greatly on its pozzolanic reactivity. Though many factors influence the reactivity of fly ash it is well recognized that reactive silica and lime content play a major role. A new, accurate and reliable method for the determination of reactive silica content of fly ash has been established. The reactive silica content, obtained as acid soluble silica in about 2 to 3 N hydrochloric acid, is found to correlate well with unconfined compressive strength of fly ashes. The reactive silica content of fly ash is also important in the stabilization of soils using fly ash. © Rapid Science Ltd. 1998