G K Rangarajan
Articles written in Journal of Earth System Science
Volume 87 Issue 11 November 1978 pp 193-200
Monthly rainfall data from Madras for the period 1813–1976 are spectrally analysed for detection of significant oscillations using the new data adaptive technique of maximum entropy developed by Burg. Spectra for annual, NE monsoon and SW monsoon rainfall are separately presented and compared with the spectra derived from Fourier transform using FFT. The long series of data are divided into several subsets and analysed to determine the stability of the periodicities. The effect of changing the length of PEF on the resultant spectra is also discussed. The analysis reveals that longer period variations (>20 years) in the rainfall is either absent or non-stationary. Solar-cycle periodicity of 11 years is also missing in the rainfall. A quasibiennial oscillation in the annual and monsoon rainfall is observed in all the subsets and in the full series. Stacked spectrum derived from the spectra of monthly rainfall series for 30-year spans is dominated by the annual line and its harmonics. The biennial pulse is absent in the monthly rainfall series.
Volume 90 Issue 1 March 1981 pp 55-62
Mean diurnal variation of
Volume 92 Issue 1 March 1983 pp 5-13
Variation of the strength of recurrent geomagnetic activity, which occurs just before a sunspot minimum, with local time is studied for a network of observatories covering different latitude and longitude zones. For this purpose, hourly averages of horizontal intensity(
Volume 94 Issue 1 March 1985 pp 29-34
Using the techniques of second derivatives, digital filtering and power spectral analysis, the fine structure of the quasi-biennial oscillation (
Volume 101 Issue 4 December 1992 pp 329-338
Wiener filters are derived from the horizontal field data of two adjacent equatorial electrojet stations, Adis Ababa and Trivandrum from several sequences of quiet days. The time invariant property of the filter is established and the filter is applied to conditions marked by afternoon counter electrojet events. The prediction efficiency is shown to be consistently high. Possible uses of this technique in studies related to generating mechanisms of counter electrojet events and the day-to-day variability in electrojet currents are indicated. Inter-relationships and day-to-day variability of different components of the fields at the two stations are highlighted.