Equivalent Potential Temperature Variability over Chennai during 2018-2023: A Spectral and Wavelet Analysis
Athira Sasikumar
*
Post Graduate Department of Physics and Research Centre, Mahatma Gandhi College, University of Kerala, Thiruvananthapuram. India.
Veena Suresh Babu
Post Graduate Department of Physics, All Saints’ College, University of Kerala, Thiruvananthapuram. India.
*Author to whom correspondence should be addressed.
Abstract
Background & Aim: Equivalent potential temperature (θe) integrates atmospheric temperature and moisture and is useful for examining thermodynamic variability in coastal environments. This study investigates the spectral characteristics and dominant periodicities of equivalent temperature over Chennai, a coastal station on the southeast coast of India, during 2018-2023, and to identify periodic variations associated with synoptic-scale and planetary-scale atmospheric processes.
Study Design: The study is based on the spectral analysis of equivalent temperature time-series data using the Fast Fourier Transform technique.
Place and Duration of Study: Chennai, on the southeast coast of India, during the six-year period from January 2018 to December 2023.
Methodology: Both local mechanisms and large-scale atmospheric disturbances affect atmospheric thermodynamic variability over coastal regions. This study uses radiosonde records from 2018 to 2023 to examine the temporal variability of equivalent potential temperature (θe) over Chennai, a tropical coastal region influenced by the Indian monsoon system and the Bay of Bengal. Fast Fourier Transform (FFT) and continuous wavelet transform (CWT) methods were used to analyse seasonal θe time series. Prior to spectral analysis, the time series underwent the necessary preprocessing; wavelet analysis was utilised to investigate the temporal variability and persistence of the main periodicities, while FFT was utilised to identify them. In addition to longer-period variability observed in some seasons and years, the study revealed significant short-period variability, especially in the roughly 2–6-day range. These periodic patterns indicate recurring thermodynamic variability in the coastal atmosphere above Chennai. The results provide observational evidence of temporal change in θe and may improve understanding of intraseasonal and synoptic atmospheric variability over coastal South India. The results are interpreted within the constraints of the six-year, single-station observational record.
Results: Over Chennai, the spectral analysis showed noticeable shorter-period oscillations, with dominant periodicities often falling between two and six days. These frequent variations at the synoptic scale show how convective disturbances and transient weather systems affect the thermodynamic state of the coastal atmosphere. Additionally, the spectral features show how the strength of the identified oscillations varies over time across seasons and years.
Conclusion: The findings show clear periodic variability in equivalent temperature over Chennai, with short-period synoptic-scale oscillations contributing significantly. Fast Fourier Transform analysis provides an efficient method for identifying these atmospheric variations and improving understanding of coastal thermodynamic variability. The results could help in evaluating atmospheric instability, monsoon variability, and related weather disturbances over India's southeast coast.
Keywords: Equivalent temperature, fast fourier transform, spectral analysis, synoptic oscillations, atmospheric variability, chennai, coastal atmosphere, periodicity