Diurnal variation of monsoon winds during the onset, active-break and withdrawal phases observed at its gateway in India: A case study of 2021

Abstract

Diurnal variation of the low-level jet (LLJ) and tropical easterly jet stream (TEJ) over Cochin – the gateway of Indian summer monsoon – was investigated during the monsoon onset over Kerala (MOK), active-break, and withdrawal phases in the year 2021. Horizontal wind profiles obtained from the 205 MHz wind profiling radar at Cochin University of Science and Technology, India, were analyzed to delineate the core speed and core heights of the LLJ and TEJ. Distinct diurnal variability in wind patterns was observed during various monsoon phases. The LLJ and TEJ core speed and heights exhibited significant diurnal variations, with the LLJ reaching maximum diurnal amplitudes between 2.5 m s−1 (onset phase) and 0.5 m s−1 (break phase). Notably, the LLJ core speed during the break phase was approximately half of that during the active phase. The ratios of the diurnal range to the time mean LLJ core speed varied between 0.19 (active phase) and 1.33 (onset phase). Moreover, the LLJ core height is higher during active compared to break. Similarly, the TEJ core height at Cochin is marginally higher during active compared to that during break. Interestingly, an enhanced TEJ core speed and weakened LLJ were observed during the break phases. The variation of moisture flux exhibited positive correlation with the core speed with 2–3 h lag, significantly influencing the diurnal variability of rainfall. The causative factors behind the diurnal oscillations during various phases of the monsoon are discussed.