Seasonal prediction of summer monsoon rainfall over cluster regions of India

Shared nearest neighbour (SNN) cluster algorithm has been applied to seasonal (June–September) rainfall departures over 30 sub-divisions of India to identify the contiguous homogeneous cluster regions over India. Five cluster regions are identified. Rainfall departure series for these cluster regions are prepared by area weighted average rainfall departures over respective sub-divisions in each cluster. The interannual and decadal variability in rainfall departures over five cluster regions is discussed. In order to consider the combined effect of North Atlantic Oscillation (NAO) and Southern Oscillation (SO), an index called effective strength index (ESI) has been defined. It has been observed that the circulation is drastically different in positive and negative phases of ESI-tendency from January to April. Hence, for each phase of ESI-tendency (positive and negative), separate prediction models have been developed for predicting summer monsoon rainfall over identified clusters. The performance of these models have been tested and found to be encouraging.     

Horizontal trench system effects on leachate recirculation in bioreactor landfills

Horizontal trenches (HTs) are constructed during the waste filling for leachate recirculation in bioreactor landfills. Leachate distribution depends on HT configuration (i.e., spacing between successive HTs), leachate injection rate, modes of injection, and hydraulic properties and MSW heterogeneity. Presently, the effects of these variables on the moisture distribution have not been studied systematically. This study provides a systematic evaluation of the effect of the HTs variables on the moisture distribution and pore fluid pressures. A two-phase flow model is used to model a bioreactor landfill having an HT leachate injection system. It quantifies the effects of the unsaturated hydraulic properties and MSW heterogeneity, trench configuration, leachate injection flux, and mode of injection on hydraulic behaviour. The results show that unsaturated hydraulic conductivity and MSW heterogeneity significantly shape the zone of influence and excess pore pressures. Under heterogeneous-anisotropic conditions, the leachate migrates more laterally and the developed pore-pressures are lower than for homogeneous MSW. A closely-spaced, multi-level, staggered HT system is found to provide uniform and adequate moisture distribution in MSW. An intermittent mode of injection that alternates between the shallow and deep trenches with a higher leachate injection flow rate is found to be effective to control the excess pore pressures.     

Estimation of confidence interval for hydrological design value for some continuous distributions under complete and censored samples

The quantile of a probability distribution, known as return period or hydrological design value of a hydrological variable is the value corresponding to fixed non-exceedence probability and is very important notion in hydrology. In hydraulic engineering design and water resources management, confidence interval (CI) estimation for a population quantile is of primary interest and among other applications, is used to assess the pollution level of a contaminant in water, air etc. The accuracy on such estimation directly influences the engineering investments and safety. The two parameter Weibull, Pareto, Lognormal, Inverse Gaussian, Gamma are some commonly used probability models in such applications. In spite of its practical importance, the problem of CI estimation of a quantile of these widely applicable distributions has been less attended in the literature. In this paper, a new method is proposed to obtain a CI for a quantile of any distribution for which [or the probability distribution of any one-to-one function of the underlying random variable (RV)] generalized pivotal quantities (GPQs) exist for its parameters. The proposed method is elucidated by constructing CIs for quantiles of Weibull, Pareto, Lognormal, Extreme value distribution of type-I for minimum, Exponential and Normal distributions for complete as well as type II singly right censored samples. The empirical performance evaluation of the proposed method evinced that the proposed method has exact well concentrated coverage probabilities near the nominal level even for small uncensored samples as small as 5 and for censored samples as long as the proportion of censored observations is up to 0.70. The existing methods for Weibull distribution have poor or dispersed coverage probabilities with respect to the nominal level for complete samples. Applications of the proposed method in ground water monitoring and in the assessment of air pollution are illustrated for practitioners.     

Teleconnections: Northern hemisphere lower stratospheric geopotential heights and Indian monsoon rainfall

Summary In this study a search for teleconnection is made between the Northern Hemisphere lower stratospheric geopotential heights and Indian Monsoon Rainfall (IMR) through the correlation analysis approach. Monthly grid-point 50 hPa geopotential height data and the June to September IMR are used for the period 1958–1990.Analysis reveal that there are domains over the Northern Hemisphere where the variation of the geopotential heights during the preceding months of the monsoon period are related with the interannual behaviour of the IMR. During January and February significant positive correlations are seen along the lower latitudes (10°–20° N). However, the maximum relationship is seen during March, with high positive correlations over the Canadian sector and negative correlations over the east Asian sector. The correlation configuration weakens considerably, once the ENSO (El Niño Southern Oscillation) cases are excluded, suggesting that the correlation pattern obtained may be a manifestation of the ENSO cycle. Implications of these results in the long range forecasting of IMR are also discussed.With 5 Figures     

Climatology and variability of historical Soviet snow depth data: some new perspectives in snow – Indian monsoon teleconnections

 This study presents the monthly climatology and variability of the historical soviet snow depth data. This data set was developed under the bilateral data exchange agreement between United States of America and the former Union of Soviet Socialist Republics. The original data is for 284 stations for periods varying from 1881 upto 1985. The seasonal cycle of the mean snow depth has been presented both as spatial maps and as averages over key locations. The deepest snow (=80 cms/day) areas are found over Siberia (in Particular over 80′–100 ′E, 55′–70 ′N) during March. Over the course of the annual cycle average snow depth over this region changes dramatically from about 10 cms in October to about 80 cms in March. The variability is presented in the form of spatial maps of standard deviation. To investigate the interaction of snow depth with Indian monsoon rainfall (IMR), lag and lead correlation coefficients are computed. Results reveal that the winter-time snow depth over western Eurasia surrounding Moscow (eastern Eurasia in central Siberia) shows significant negative (positive) relationship with subsequent IMR. Following the monsoon the signs of relationship reverse over both the regions. This correlation structure is indicative of a midlatitude longwave pattern with an anomalous ridge (trough) over Asia during the winter prior to a strong (weak) monsoon. As the time progresses from winter to spring, the coherent areas of significant relationship show southeastward propagation. Empirical orthogonal function analysis of the snow depth reveal that the first mode describes a dipole-type structure with one centre around Moscow and the other over central Siberia, depicting similar pattern as the spatial correlation structure. The decadal-scale IMR variations seem to be more associated with the Northern Hemisphere midlatitude snow depth variations rather than with the tropical ENSO (El Nino Southern Oscillation) variability. Received: 16 March 1998 / Accepted: 24 December 1998     

Experimental evidence of gravity waves at 80–90 km altitude from OH night airglow observations

Measurements of intensities at two fixed wavelengths in the OH (7-2) band were carried out at Mt Abu (24.6° N, 76.7° E) from 1973–76 to estimate neutral temperature in the region of 80–90 km altitude. It was observed that on some nights the temperature in this region shows periodic oscillations throughout the night. It was found from statistical analysis that the periods of these fluctuations are comparable to the theoretically predicted periods of internal gravity waves. If the periodic temperature variations observed in this region are attributed to the influence of gravity waves, according to the Hines theory only the magnitude of the horizontal component of wave-induced wind velocity can be computed. Using the measured relative temperature fluctuations it is found that the magnitude is 15–30 m/sec.     

Projected changes in South Asian summer monsoon by multi-model global warming experiments

South Asian summer monsoon (June through September) rainfall simulation and its potential future changes are evaluated in a multi-model ensemble of global coupled climate models outputs under World Climate Research Program Coupled Model Intercomparison Project (WCRP CMIP3) dataset. The response of South Asian summer monsoon to a transient increase in future anthropogenic radiative forcing is investigated for two time slices, middle (2031–2050) and end of the twenty-first century (2081–2100), in the non-mitigated Special Report on Emission Scenarios B1, A1B and A2 .There is large inter-model variability in the simulation of spatial characteristics of seasonal monsoon precipitation. Ten out of the 25 models are able to simulate space–time characteristics of the South Asian monsoon precipitation reasonably well. The response of these selected ten models has been examined for projected changes in seasonal monsoon rainfall. The multi-model ensemble of these ten models projects a significant increase in monsoon precipitation with global warming. The substantial increase in precipitation is observed over western equatorial Indian Ocean and southern parts of India. However, the monsoon circulation weakens significantly under all the three climate change experiments. Possible mechanisms for the projected increase in precipitation and for precipitation–wind paradox have been discussed. The surface temperature over Asian landmass increases in pre-monsoon months due to global warming and heat low over northwest India intensifies. The dipole snow configuration over Eurasian continent strengthens in warmer atmosphere, which is conducive for the enhancement in precipitation over Indian landmass. No notable changes have been projected in the El Niño–Monsoon relationship, which is useful for predicting interannual variations of the monsoon.     

Performance of GCMs for seasonal prediction over India��a case study for 2009 monsoon

The 2009 drought in India was one of the major droughts that the country faced in the last 100?years. This study describes the anomalous features of 2009 summer monsoon and examines real-time seasonal predictions made using six general circulation models (GCMs). El Ni?o conditions evolved in the Pacific Ocean, and sea surface temperatures (SSTs) over the Indian Ocean were warmer than normal during monsoon 2009. The observed circulation patterns indicate a weaker monsoon in that year over India with weaker than normal convection over the Bay of Bengal and Indian landmass. Skill of the GCMs during hindcast period shows that neither these models simulate the observed interannual variability nor their multi-model ensemble (MME) significantly improves the skill of monsoon rainfall predictions. Except for one model used in this study, the real-time predictions with longer lead (2- and 1-month lead) made for the 2009 monsoon season did not provide any indication of a highly anomalous monsoon. However, with less lead time (zero lead), most of the models as well as the MME had provided predictions of below normal rainfall for that monsoon season. This study indicates that the models could not predict the 2009 drought over India due to the use of less warm SST anomalies over the Pacific in the longer lead runs. Hence, it is proposed that the uncertainties in SST predictions (the lower boundary condition) have to be represented in the model predictions of summer monsoon rainfall over India.     

Estimation of probable maximum precipitation for catchments in eastern India by a generalized method

Summary A generalized method to estimate the probable maximum precipitation (PMP) has been developed for catchments in eastern India (80° E, 18° N) by pooling together all the major rainstorms that have occurred in this area. The areal raindepths of these storms are normalized for factors such as storm dew point temperature, distance of the storm from the coast, topographic effects and any intervening mountain barriers between the storm area and the moisture source. The normalized values are then applied, with appropriate adjustment factors in estimating PMP raindepths, to the Subarnarekha river catchment (upto the Chandil dam site) with an area of 5663 km². The PMP rainfall for 1, 2 and 3 days were found to be roughly 53 cm, 78 cm and 98 cm, respectively. It is expected that the application of the generalized method proposed here will give more reliable estimates of PMP for different duration rainfall events.With 5 Figures     

Emission of electromagnetic waves in magnetized plasma in the presence of field aligned currents: A model for occurrence of circular polarizations in the pulsar radiations

The growth rate of electromagnetic waves, in the whistler mode frequency range, in the presence of field aligned currents is calculated. A model for the occurrence of circular polarizations in pulses of pulsars is presented.