Estimating statistics of sky brightness using radio interferometric observations

Radio interferometers are used to construct high resolution images of the sky at radio frequencies and are the key instruments for accessing the statistical properties of the evolution of neutral hydrogen over cosmic time. Here we use simulated observations of the model sky to assess the efficacy of different estimators of the large-scale structure and power spectrum of the sky brightness distribution. We find that while the large-scale distribution can be reasonably estimated using the reconstructed image from interferometric data, estimates of the power spectrum of the intensity fluctuations calculated from the image are generally biased. This bias is found to be more pronounced for diffuse emission. The visibility based power spectrum estimator, however, gives an unbiased estimate of the true power spectrum. This work demonstrates that for an observation with diffuse emission the reconstructed image can be used to estimate the large-scale distribution of the intensity, while to estimate the power spectrum, visibility based methods should be preferred.With the upcoming experiments aimed at measuring the evolution of the power spectrum of the neutral hydrogen distribution, this is a very important result.     

The Persistence of Warps in Spiral Galaxies with Massive Haloes

We study the persistence of warps in galactic discs in the presence of massive haloes. A disc is approximated by a set of massive rings, while a halo is represented by a conventional n -body simulation. We confirm the conclusion of Nelson &38; Tremaine that a halo responds strongly to an embedded precessing disc. This response invalidates the approximations made in the derivation of classical 'modified tilt' modes. We show that the response of the halo causes the line of nodes of a disc that starts from a modified tilt mode to wind up within a few dynamical times. We explain this finding in terms of the probable spectrum of true normal modes of a combined disc–halo system. The key physical point is that in each radial range the halo rapidly aligns with the disc, so calculations based on the assumption that, in the presence of a warped disc, the halo retains a regular spheroidal structure are based on a fatally flawed assumption.     

Skills of different mesoscale models over Indian region during monsoon season: Forecast errors

Performance of four mesoscale models namely, the MM5, ETA, RSM and WRF, run at NCMRWF for short range weather forecasting has been examined during monsoon-2006. Evaluation is carried out based upon comparisons between observations and day-1 and day-3 forecasts of wind, temperature, specific humidity, geopotential height, rainfall, systematic errors, root mean square errors and specific events like the monsoon depressions.It is very difficult to address the question of which model performs best over the Indian region? An honest answer is ‘none’. Perhaps an ensemble approach would be the best. However, if we must make a final verdict, it can be stated that in general, (i) the WRF is able to produce best All India rainfall prediction compared to observations in the day-1 forecast and, the MM5 is able to produce best All India rainfall forecasts in day-3, but ETA and RSM are able to depict the best distribution of rainfall maxima along the west coast of India, (ii) the MM5 is able to produce least RMSE of wind and geopotential fields at most of the time, and (iii) the RSM is able to produce least errors in the day-1 forecasts of the tracks, while the ETA model produces least errors in the day-3 forecasts.     

Optimization of a multiple gauge,regulated river‐system model. A system approach

With increasing demands on limited water resources, regulation of larger river systems continues to increase and so too does the need for accurate water accounting and prediction in these systems. River system models are either calibrated manually or automatically on a reach‐by‐reach basis, i.e. each reach is calibrated as a separate entity with little or no consideration of fluxes at other locations within the river system. While this is a practical approach, simulation errors can propagate downstream to make calibration or prediction difficult at those locations. Likewise parameters may suffer from over‐fitting especially where observations are erroneous. We developed and implemented a system calibration strategy in a portion of the Murrumbidgee River, Australia, where parameters for 11 gauges (36 parameters) were calibrated together. Parameter values, model states and model goodness of fit were compared to reach‐by‐reach calibration. The system calibration produced a better goodness of fit across the whole system relative to reach‐by‐reach calibration. Additionally, model system states were more realistic than reach‐by‐reach optimized models. Over‐fitting was obvious using the reach‐by‐reach method for one reach/gauge in particular. This was avoided with system calibration method, with improved goodness of fit at all gauges downstream of the problem gauge. The results here suggest that the system calibration approach provides more hydrologically consistent states, improved overall fit and avoids over‐fitting at problem gauges. Copyright © 2015 John Wiley & Sons, Ltd.     

Analysis of flood vulnerability and assessment of the impacts in coastal zones of Bangladesh due to potential sea-level rise

The 4th IPCC report highlights the increased vulnerability of the coastal areas from floods due to sea-level rise (SLR). The existing coastal flood control structures in Bangladesh are not adequate to adapt these changes and new measures are urgently necessary. It is important to determine the impacts of SLR on flooding to analyse the performance of the existing structures and corresponding impact to plan for suitable adaptation and mitigation measures to reduce the impacts of floods on coastal zone. The study aims to develop a comprehensive understanding of the possible effects of SLR on floods in the coastal zone of Bangladesh. A hydrodynamic model, which is a combination of surface and river parts, was utilized for flood simulation. The tool was applied under a range of future scenarios, and results indicate both spatial variability of risk and changes in flood characteristics between now and under SLR. Estimated impact on population, infrastructure and transportation is also exposed. These types of impact estimation would be of value to flood plain management authorities to minimize the socio-economic impact.