Intraseasonal zonal wind fluctuations seen through Indian radiosonde observations

Summary Twice-daily radiosonde records for Calcutta, Madras, Nagpur, New Delhi, Trivandrum, and Visakhapatnam have been quality checked and analyzed for intraseasonal activity in the summer monsoon. The records for all six cities end on 30 June 1978 and are 17–27 years long.Spectra were estimated by smoothing periodograms and by using the maximum entropy method (MEM), which entails fitting an autoregressive (AR) model. We show how confidence limits can be placed on an average of MEM spectral estimates using the Central Limit Theorem and how this helps pick the orders of the AR models to be tried. We donot favor use of Akaike's Final Prediction Error in choosing the model order, because it can grossly underestimate the AR model order needed to resolve low frequency peaks.MEM spectra of zonal wind based on thirtieth order AR models have large peaks around 30 day time scales. 10–20 day activity is identifiable but is much less prominent. On average, 3–5 day time scales are part of the spectral continuum and do not have a significant spectral peak.Time-height diagrams of filtered zonal wind show that a deep layer of the troposphere acts in phase on 30–50 day time scales over India. The shortest vertical length scale is over southern India and the longest is over northern India. Vertical phase propagation is irregular except over Nagpur, where it is generally upward. Empirical orthogonal functions indicate that vertically in phase activity has more variance (kinetic energy) than the first baroclinic mode.With 10 Figures