• Volume 100, Issue 2

      June 1991,   pages  105-218

    • Application of principal component analysis to understand variability of rainfall

      R N Iyengar

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      The usefulness of principal component analysis for understanding the temporal variability of monsoon rainfall is studied. Monthly rainfall data of Karnataka, spread on 50 stations for a period of 82 years have been analysed for interseasonal and interannual variabilities. A subset of the above data comprising 10 stations from the coherent west zone of Karnataka has also been investigated to bring out statistically significant interannual signals in the southwest monsoon rainfall. Conditional probabilities are proposed for a few above normal/below normal transitions. A sample prediction exercise for June–July using such a transition probability has been found to be successful.

    • On the gross predictability of tropical monsoonal depressions

      M Sankar-Rao Prashant Goswami H V Nagaraj Upadhyaya

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      The experiments reported here emphasize the importance of observations in the prediction of tropical cyclones. Towards this end a symmetric numerical model in which convection, parametrized by the Arakawa-Schubert (AS) scheme, was adopted. A mean thermodynamical state, which represents the monsoon conditions over the Bay of Bengal, with constant moist static energy for the mixed layer was adopted. Experiments were then done with different initial conditions.

      We found that tropical cyclone development measured by the central pressure was very sensitive to the initial convergence field. In the present state of satellite technology, it was impossible to predict even a gross parameter like the central pressure with an accuracy better than 6 mb for 12 hours. However, it was seen that under a variety of initial conditions the final state characterized by the magnitude of the central low pressure remained practically unaltered. We suggest that, given the necessary conditions for genesis, the final state of the cyclone acts as an attractor (regarding its central pressure) and the diverse initial conditions, under the influence of thermodynamic forcing, will lead to the same final state.

    • High resolution radio refractive index studies in India using airborne microwave refractometers

      S B S S Sarma

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      We discuss high resolution measurements on radio refractive index, in the boundary layer and above the atmosphere in the Indian sub-continent spread over a period of more than a solar cycle by making use of airborne microwave refractometers, operating around 9 GHz. Some atmospheric turbulence parameters comparable with earlier results have also been derived. The results demonstrate sharp gradients of radio refractive index, layer structures and the variability of the boundary layer in time and space.

    • Numerical experiment with modelled return echo of a satellite altimeter from a rough ocean surface and a simple iterative algorithm for the estimation of significant wave height

      Sujit Basu P C Pandey

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      We have simulated the return echo of a satellite altimeter from a rough ocean surface using an analytical formula and have studied its sensitivity with respect to various oceanic and altimeter parameters. Our numerical experiment shows that for normally observed significant wave heights (SWH) the effect of off-nadir angle (ONA) up to 0.5° on the leading edge is not severe. Also, small surface roughness skewness seems to have little effect on the overall shape of the echo.

      Newton’s iterative scheme has been used to retrieve SWH from the mean return waveform without noise and with additive Gaussian noise typical of Seasat and Geosat altimeters. It has been observed that SWH can be retrieved in the presence of noise with an accuracy of ±0.6 m for ONA less than 0.5°. For higher ONA, accurate retrieval requires the use of precomputed look-up table along with our scheme.

    • The warm pool in the Indian Ocean

      P N Vinayachandran S R Shetye

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      The structure of the warm pool (region with temperature greater than 28°C) in the equatorial Indian Ocean is examined and compared with its counterpart in the Pacific Ocean using the climatology of Levitus. Though the Pacific warm pool is larger and warmer, a peculiarity of the pool in the Indian Ocean is its seasonal variation. The surface area of the pool changes from 24 × 106 km2 in April to 8 × 106 km2 in September due to interaction with the southwest monsoon. The annual cycles of sea surface temperature at locations covered by the pool during at least a part of the year show the following modes: (i) a cycle with no significant variation (observed in the western equatorial Pacific and central and eastern equatorial Indian Ocean), (ii) a single maximum/minimum (northern and southern part of the Pacific warm pool and the south Indian Ocean), (iii) two maxima/minima (Arabian Sea, western equatorial Indian Ocean and southern Bay of Bengal), and (iv) a rapid rise, a steady phase and a rapid fall (northern Bay of Bengal).

    • An analysis of Indian tide-gauge records

      P K Das M Radhakrishna

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      The paper presents an analysis of four Indian tide-gauge records. The stations were: Bombay, Madras, Cochin and Vishakhapatnam (Vizag). They were selected because of their reliability.

      There was no evidence of a monotonic rising trend at all four stations. The test by Mann and Kendall (loc. cit.) showed a rising trend at Bombay from 1940 to 1986 and at Madras from 1910 to 1933. The other records did not reveal a significant trend.

      The records reveal evidence of long-period cycles (50–60 year period), with shorter cycles (4.5 to 5.7-year period) riding on them. Spectral peaks corresponding to shorter cycles passed a false alarm probability test at 95% level of significance. The peaks were identified by computing periodograms and by maximizing the entropy of the time series.

      ARIMA models suggest a third order autoregressive model for Bombay and Madras (1953–1986). The remaining records only had a moving average component.

      Monthly tide-gauge data of Bombay reveal a 13.4-month cycle which was statistically significant. This was close to the 14.7-month Chandler wobble. But, an interaction between a 13.4-month and an annual cycle could not fully explain the observed short period cycles.

      Finally, the paper summarizes evidence to indicate that a pattern exists between fluctuations of monsoon rain and relative sea level at Bombay.

    • Removal of sediments from the Alleppey nearshore region during post-mud bank formation

      K K Ramachandran M Samsuddin

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      Formation of mud banks along the southwest coast of India is marked by an unusual calmness of the nearshore area with a large amount of suspensates in the water column and mud deposition. After cessation of the monsoon, the removal of mud from the mud bank region is evident. Sediment dynamics worked out from the textural characteristics suggest resuspension of sediments by waves and subsequent transportation of the suspended load by longshore/coastal currents as the main factors responsible for the removal of sediments from the mud bank region during the post-monsoon season.

    • Static deformation of an orthotropic multilayered elastic half-space by two-dimensional surface loads

      Nat Ram Garg Sarva Jit Singh Sushma Manchanda

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      The transfer matrix approach is used to solve the problem of static deformation of an orthotropic multilayered elastic half-space by two-dimensional surface loads. The general problem is decoupled into two independent problems. The antiplane strain problem and the plane strain problem are considered in detail. Integral expressions for displacements and stresses at any point of the medium due to a normal line load and a shear line load, acting parallel to a symmetry axis, are obtained. In the case of a uniform half-space, closed form analytic expressions for displacements and stresses are derived. The procedure developed is quite easy and convenient for numerical computations.

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