Amplitude modulation of Atmospheric Boundary Layer in presence of the monsoon low level jet over Indian subcontinent*

The majority of rainfall in India is reported during June, July, August, and September, collectively known as the Monsoon season in the Indian Subcontinent. One of the primary indicators suggesting the onset of this season is the formation of a strong westerly jet at a height of approximately 1 to 2 km from the surface, known as the monsoon low-level jet (MLLJ). This study investigates scale interactions in the atmospheric boundary layer (ABL) during the presence of a strong MLLJ. Data from a weather flux tower, equipped with a sonic anemometer placed 4m above the ground, are analyzed, along with data from a wind profiling radar to identify the presence of the MLLJ. Scales equal to or larger than the ABL height (δ) are considered large scales in the flow. The amplitude modulation coefficient is calculated to quantify the effect of these large scales on the smaller scales within the ABL. Prior studies mention similarity between a laboratory wall jet and ABL with low level jet in terms of mean velocity profile and turbulence kinetic energy budget (Gupta et.al. JFM 2020). Heeding this, amplitude modulation coefficient (R_AM) is also analyzed in a turbulent plane wall jet and compared with the one calculated from the sonic anemometer data near the ABL surface layer. The atmospheric surface layer appears to exhibit same order of magnitude of amplitude modulation as observed in the laboratory wall jet.